Professional High-Temperature Humidity Instrument

Portable Oxygen Analyzers are critical tools for monitoring oxygen levels in environments where safety or process control depends on precise measurements, from industrial confined spaces to medical settings. However, high-humidity areas—such as wastewater treatment plants, tropical work sites, or outdoor rainy conditions—pose unique challenges that can affect their accuracy. Understanding these challenges, the factors influencing performance, and mitigation strategies is essential for reliable operation.
1. Impact of High Humidity on Measurement Principles
Most Portable Oxygen Analyzers rely on electrochemical sensors, which operate by oxidizing oxygen at a working electrode to generate a current proportional to oxygen concentration. This process is sensitive to environmental conditions, particularly moisture:
Sensor Flooding: In relative humidity (RH) exceeding 90%, condensation can form on the sensor’s permeable membrane, blocking oxygen diffusion. This reduces the amount of oxygen reaching the electrode, leading to underestimated readings (e.g., a true 21% O₂ level might register as 19–20%).
Electrolyte Dilution: Electrochemical cells contain a liquid or gel electrolyte. Excessive moisture can seep through the membrane, diluting the electrolyte and altering its conductivity. This disrupts the redox reaction, causing drift in readings over time—errors may accumulate to ±2% within hours in saturated conditions.
Interference Reactions: Water vapor can react with byproducts of the electrochemical process (e.g., hydrogen peroxide in some sensors), producing false currents that mimic oxygen’s signal. This is more pronounced in sensors without hydrophobic coatings, leading to overestimated oxygen levels (e.g., 22% instead of 21%).
2. Typical Accuracy Ranges in High Humidity
Manufacturers often specify accuracy as ±0.1–2% in "normal" conditions (20–60% RH). In high humidity (80–100% RH), real-world performance degrades:
Short-Term Exposure (1–2 hours): Modern analyzers with hydrophobic membranes may maintain accuracy within ±0.5–1% of the true value. For example, a unit rated ±0.3% at 50% RH might drift to ±0.8% at 95% RH.
Prolonged Exposure (8+ hours): Accuracy can worsen to ±2–3% due to electrolyte dilution or membrane clogging. In extreme cases (e.g., direct contact with mist or rain), errors may exceed 5%, risking incorrect safety decisions (e.g., declaring a confined space safe when oxygen is actually below 19.5%).
Sensor Age Factor: Older sensors (used for 6+ months) are more vulnerable—moisture accelerates membrane degradation, increasing error rates by 1.5–2x compared to new sensors.
3. Key Factors Influencing Accuracy in High Humidity
Sensor Design:
Hydrophobic Membranes: Sensors with PTFE (Teflon) or porous polypropylene membranes resist water penetration. Models like the Dräger X-am 5000 use such membranes, limiting humidity-induced errors to ±0.5% in 90% RH.
Ventilation Systems: Some analyzers (e.g., MSA Altair 5X) feature built-in fans to circulate air and reduce condensation on the sensor, maintaining accuracy in 85–90% RH.
Heated Sensors: Premium models (e.g., Teledyne Oldham GasAlert Micro 5) include heated sensor chambers that keep the membrane above dew point, preventing condensation even in 100% RH.
Calibration Practices:
Frequent calibration in high-humidity environments is critical. A sensor calibrated at 50% RH may drift by 1% when used at 90% RH; re-calibrating under actual humidity conditions reduces this to ±0.3%.
Using humidity-stabilized calibration gases (mixed with 80% RH air) instead of dry gases improves accuracy by accounting for moisture’s impact on sensor response.
Environmental Extremes:
Temperature fluctuations amplify humidity effects. At 30°C and 95% RH, condensation forms faster than at 15°C and 95% RH, increasing error rates by 30–40%.
Exposure to liquid water (e.g., splashes) is more damaging than high vapor—even waterproof-rated analyzers (IP67) may experience temporary sensor failure if submerged, requiring drying and re-calibration.
4. Mitigation Strategies for Reliable Measurements
Pre-Use Preparation:
Condition the analyzer by exposing it to the target humidity for 10–15 minutes before measurement. This allows the sensor to equilibrate, reducing initial drift.
Inspect the membrane for damage or clogging; replace it if wet spots or discoloration are visible.
Operational Adjustments:
Avoid direct contact with mist or water. Use a protective shroud or extend the sampling probe to draw air from a drier layer (e.g., 30cm above a wet surface).
In continuous monitoring, set the device to log readings every 30 seconds instead of real-time mode, allowing the sensor to stabilize between measurements.
Post-Use Maintenance:
After use in high humidity, store the analyzer in a dry environment with desiccants. Some models have a "dry-out" mode that heats the sensor to evaporate residual moisture.
Calibrate weekly instead of monthly in high-humidity applications to correct drift.
5. Performance of Leading Models in High Humidity
Field tests comparing popular portable analyzers in 90–95% RH conditions show:
Dräger X-am 8000: Maintains ±0.5% accuracy for up to 4 hours, thanks to its heated sensor and advanced hydrophobic membrane.
MSA Altair 4XR: Drifts by ±1% after 2 hours but recovers quickly when returned to moderate humidity.
Industrial Scientific Tango TX1: Shows ±0.8% error in steady high humidity but struggles with sudden moisture spikes (e.g., from steam), requiring re-calibration.
6. When to Question Accuracy
Signs that humidity is compromising measurements include:
Erratic readings (e.g., fluctuating between 19% and 23% in stable air).
Slow response times (taking >30 seconds to stabilize, compared to <10 seconds in dry conditions).
Readings that fail to return to 21% when exposed to ambient air after use in high humidity.
Conclusion
Portable Oxygen Analyzers can maintain reasonable accuracy in high humidity (±0.5–1% for short periods) with proper design and maintenance, but performance degrades significantly in prolonged or extreme conditions. Users must select models with hydrophobic membranes and heating features, calibrate frequently under relevant humidity levels, and protect sensors from direct moisture. By addressing these factors, reliable measurements are achievable—critical for ensuring safety in environments where oxygen levels can mean the difference between life and death.

Introduction and application development of flue gas online high temperature and Humidity Analyzer


Foreword


The measurement of flue gas humidity in the exhaust gas of fixed pollution source is mainly to obtain dry-based oxygen content to calculate the actual content of flue gas and gaseous pollution emissions. Because of the high temperature of wet flue gas, the large amount of dust, and the large difference of the flue gas components of the boiler or industrial kiln used in the industrial field such as thermal power plant, petrochemical plant, garbage incineration plant, steel-making plant, etc., in addition, the humidity measurement in the world is a difficult problem, so the on line detection of flue gas humidity is very difficult. In the past few years, the measurement of high temperature and humidity of domestic flue gas was mostly done by hand, that is, according to the requirements of GB/T 16157-1996 "The Method of Determining Particulate Matter and Sampling Gaseous Pollutants in the Exhaust of Fixed Pollution Source", the method of weight method, condensation method and dry-wet ball method were chosen to determine, and the average value was input into the CEMS system. With the development and progress of technology and the national environmental protection more and more attention, at present there are four kinds of on line measurement methods of high temperature and humidity of flue gas in China: 1. Resistance capacitance method 2. Limit current type zirconium oxide principle method 3. Impact injection method (dry and wet ball) 4. Infrared spectrum absorption method.


1.Resistance capacitance method
The resistance-capacitance method used in high temperature and humidity measurement of flue gas in China is the capacitance method. The sensors used in this method mostly use polyimide as humidity sensitive material. The polymer humidity-sensitive capacitance made of this material has good electrical performance, and its dielectric constant and dielectric loss are very small. The dielectric constant of polyimide is 2~3 in the state of complete drying, and the dielectric constant of water molecule is about 80 at 20°C. The revival dielectric constant after adsorption of water molecule is:
εu=εr+aWuεh               (1)
Wu=b(p/p0) εr+ aWuεh       （2）


εu is the relative humidity of u%RH composite dielectric constant, εr is the humidity of 0%RH, the dielectric constant of polyimide film, a, b is the structure constant, εh is the dielectric constant of water adsorbed in polyimide film, Wu is the humidity of u%RH, the mass of water adsorbed by the unit mass of polymer, p/p0 is the relative pressure of water vapor equilibrium. When the macromolecule humidity-sensitive capacitance absorbs the gaseous water molecules in the environment, the dielectric constant of the material is changed, thereby causing the capacitance value to change; the corresponding environmental humidity value is calculated by measuring the capacitance value of the change. At present, the capacitance sensor-polymer humidity-sensitive capacitor used in high temperature and humidity measurement in flue gas adopts a flat capacitor structure, which is mainly composed of a glass substrate substrate, a lower electrode, a polymer humidity-sensitive film, an upper electrode and the like. According to the formula of flat capacitor, the relation between capacitance and relative humidity can be expressed as



Figure 1 Structure Diagram of Polymer Humidity-sensitive Capacitance


The capacitance value of Ch humidity-sensitive capacitance, ε0 is vacuum dielectric constant, S is the area of humidity-sensitive capacitance electrode, D is the distance between humidity-sensitive capacitance electrodes, and also the thickness of humidity-sensitive film. From the formulas (1), (2) and (3), it can be seen that the relationship between the water molecular adsorption capacity of the humidity-sensitive capacitance and the relative pressure of water vapor equilibrium should conform to the Herry adsorption isotherm, that is, the relationship between the capacitance capacity and the relative humidity is linear.
In summary, we can see that the measurement of high temperature humidity reflects the relative humidity in the flue gas by the resistance volume method, according to the definition of relative humidity: The relative humidity can be expressed as the ratio of water pressure to saturated water vapor pressure at a certain temperature and pressure. According to the definition, the relative humidity and temperature are closely related, in practical application, we need the humidity value of the flue gas is the volume ratio, convenient to calculate dry oxygen content in the flue gas. Therefore, in order to calculate the volume ratio of water vapor in the flue gas, it is necessary to measure the ambient temperature of the humidity sensor.
According to the actual use of the flue gas humidity meter of the resistance-capacitance method, the resistance-capacitance method has the characteristics of fast response time, small volume, not easy to be damaged when condensed water. The disadvantage is that the temperature of flue gas cannot exceed 170°C, the higher the temperature, the more easily the data will fluctuate and the volume ratio is less than 6% humidity is not easy to measure. The reason is that the humidity is 0-40% (volume ratio) water gas (more than 30°C), with the temperature (more than 100°C) corresponding saturated water pressure, the larger the relative humidity, the smaller the corresponding capacity change will be. But the range or resolution of the capacitance change is limited. Thirdly, the waste incineration flue gas, metallurgical flue gas and so on often have certain corrosivity, the electrode is easy to fail and the life is very short.


2.Principle method of limiting current zirconia
Limited current type zirconia works by using the principle of zirconia oxygen pump
That is, firstly, the zirconia solid electrolyte is heated to high temperature (more than 350°C), simultaneously, the working voltage is applied to the platinum electrodes on both sides of the zirconia solid electrolyte, the oxygen molecule on the cathode side is catalyzed into oxygen ions and is driven by the applied voltage to 'pump' to the anode. The output current of the sensor does not increase with the increase of the applied voltage when the oxygen concentration in the atmosphere is certain, but reaches a constant value, which is called the limit current value under the oxygen concentration, and is generally called the first limit current platform I1. According to this principle, by placing the limit current sensor in an environment containing water vapor and increasing the applied voltage, a significant limit current value can be measured, commonly known as the second limit current stage I2, although this current value contains oxygen molecules and water molecules that are ionized. The two limiting current values are proportional to the oxygen content in the environment and the oxygen content containing water vapor, respectively. The micro-reaction mechanism of zirconia on cathode and anode is as follows:
cathode side             O2+4e-→2O2-                    (1)
                               H2O+2e-→H2+O2-             (2)
Anode side               O2-→1/2O2+2e-           (3)

Figure 2 Limit current type zirconium oxide sensor structure


According to the gas diffusion hole limit Ficks rule of the sensor, under the assumption that the diffusion coefficients of oxygen and water vapor are equal, the two limit current values can be expressed as follows:



In the formula: F is the Faraday constant 
D is the mixing gas diffusion coefficient
S is the area of the zirconium oxide current-limiting hole (diffusion hole) 
P is the total pressure of the mixture gas
R is the gas constant   
T is the operating temperature of zirconia (K)
L is the length of the gas diffusion hole


The oxygen content in the flue gas can be calculated according to the first limit current, and the humidity in the flue gas can be calculated according to the difference between the second limit current and the first limit current. Therefore, the use of the limit current zirconium oxide principle humidity instrument compared with other principles of humidity instrument has a clear advantage, because its nature is oxygen measurement and in order to measure humidity must measure oxygen, for the user, there is no need to install an oxygen measurement analyzer, a humidity instrument can provide two measurement data at the same time.
From the actual use of the limit current zirconium oxide principle method of humidity meter, the advantages are small volume, high measurement accuracy, flue gas temperature can be used from normal temperature to 500°C, high cost performance. The disadvantage is that the machine can not be started in liquid water or liquid water, and the machine is easy to be poisoned in a flue gas atmosphere containing more silicon dioxide or heavy metals such as arsenic and lead.
In fact, when the catalytic electrode of zirconium oxide is mentioned, the platinum electrode is usually needed for desulfurization because of the high sulfur content of coal in China.However, the front and back ends of the desulfurization tower often need to monitor the flue gas, but if the platinum electrode of zirconia works in the high SO2 content flue gas environment for a long time, the service life will also be greatly affected. Therefore, in practical use, such as desulfurization inlet, natural gas sulfur recovery, waste incineration and so on, the zirconium oxide electrode using platinum electrode is easy to be corroded. In recent years, in order to solve the influence of corrosive atmosphere on the service life of limit current type zirconia, the zirconia sensor adopted by PC18 humidity transmitter of Shanghai Chang Ai Electronic Technology Co., Ltd. has made bold innovation. The catalytic electrode of zirconia has been changed to ceramic electrode, which has solved the application of limit current type zirconia under the conditions of medical garbage incineration, petrochemical industry, VOC etc. at one stroke.





PC18 Humidity transmitter


3.Impact jet method (dry-wet ball)
The basic principle of moisture measurement by dry-wet ball method: A temperature sensor is used to measure the temperature of the flue gas as the temperature of the dry ball, a measuring pool is used to load a certain amount of water, then a temperature sensor is arranged in the measuring pool, and the temperature sensor is required to be arranged under the water surface, and then the flue gas continuously impacts the water surface directly above the temperature sensor of the measuring pool, and the measured temperature is taken as the temperature of the wet ball. According to the heat transfer principle and thermodynamics theory, the following mathematical formula can be derived:

in formula：
relative humidity % 
saturated Water Pressure at Wet Sphere Temperature
saturated water pressure at dry ball temperature
atmospheric pressure
the difference between dry and wet ball temperatures
constant, related to wind speed


According to the above mathematical formula, we can clearly know that the impact injection method to measure the humidity in the flue gas is achieved by indirect measurement of the flue gas temperature. Temperature measurement technology is relatively mature and reliable. Even if the working condition is very bad, the change of temperature sensor is very fast.
According to the practical application situation of natural gas sulfur recovery plant, food processing plant, textile plant, garbage incineration plant, etc., the high temperature humidity instrument of the impact spraying method (dry-wet ball) has long service life (currently has been used for five consecutive years and is still running normally), the measured data is accurate and reliable, has strong adaptability to harsh environment, wide temperature adaptation range and small maintenance. The disadvantage is that the price is high, the volume is large, and the water needs to be added regularly.
Although the impact injection method is the working principle of dry and wet ball, but not the common use of the meteorological department to measure the relative humidity of air humidity instrument, such as Shanghai Chang Ai Electronic Technology Co., Ltd. CI-PC39 is a brand new or even a new innovative design, to achieve the above measurement effect.



CI-PC39 Humidity analyzer


4.infrared absorption method
IR broad-spectrum absorption is based on the principle that the selective absorption of water vapor molecules to specific infrared wavelengths varies with their concentration. However, since 1912 when Fowle first proposed the infrared measurement of humidity, the measurement of humidity is slow due to the wide-spectrum absorption technology limited by traditional infrared absorption technology. With the rapid development of semiconductor laser spectroscopy (DLAS) in the 1990s, the on line high temperature and humidity analyzer for flue gas has been developed. Compared with the traditional infrared absorption absorption spectroscopy, the DLAS technique belongs to the narrow-spectrum absorption, because the spectral width (less than 0.0001nm) of the semiconductor laser source is much smaller than the broadening of the gas absorption line. Each gas molecule has its inherent absorption spectrum, when the emission spectrum of the light source coincides with the absorption spectrum of the gas molecule, the absorption intensity is related to the volume fraction of the gas. Looking at the relevant database, we can find that the absorption of water gas is very strong near the absorption line with the wavelength of 1390nm and there is no obvious interference absorption of other gases. When a semiconductor laser with I0 intensity passes through the gas to be measured, if the light source spectrum covers the absorption spectrum of the gas molecule, the light will decay when passing through the gas. According to the Lambert-Beer law, the relation between the intensity I of the exit light and the intensity I0 of the incident light and the gas volume concentration is:

In the formula (1), I and I0 are the exit and incident light intensity respectively;α(λ)It is the absorption coefficient of medium with unit concentration and unit length at a certain wavelength. C is the concentration of the gas to be measured, L is the optical path.


In order to obtain higher or higher sensitivity and reduce the 1/f noise of laser, DLAS technique usually needs modulation spectrum detection technique. This technique significantly reduces the influence of laser noise on measurement by high frequency modulation. At the same time, a narrow bandwidth band-pass filter can be obtained by setting a larger time constant for the phase-sensitive detector used in phase-sensitive detection technique (detection of harmonic components), thus effectively compressing the noise bandwidth.
When the flue gas high-temperature humidity analyzer developed by DLAS is used for measuring the flue gas, the invention belongs to the non-contact measurement, which can not be caused by sensor poisoning or be interfered by the background gas. The invention has the advantages of fast response time, high accuracy of measurement data, long calibration period and almost no maintenance. The disadvantage is that prices are high.


5.development trend
As we all know, the core components of online analytical instruments in the sensor, our country due to the late start, the basic industry is weak, although after these years of development, domestic analytical instrument manufacturers in mastering the core technology has made great progress, but compared with foreign countries still very obvious gap. Most domestic analytical instrument manufacturers buy foreign sensors and then design the second instrument to enter the market for homogeneous price competition, and it makes people sad that the low-end market of analytical instruments (the high end market is almost monopolized by foreign countries), in the long run, it will affect or delay the development of domestic analytical instrument industry.
At present, due to the price reasons, the online high temperature humidity instrument mainly adopts the resistance capacity method and the limit current type zirconia in the market, the high temperature humidity instrument based on the principle of impact jet method (dry-wet ball) and infrared spectrum absorption method has a very low market share. Shanghai Chang Ai Electronic Technology Co., Ltd., through long-term and unremitting efforts, has independently developed the above-mentioned online flue gas high-temperature humidity instrument limit current type zirconium oxide sensor and the impact injection method of the core components-measuring pool. Some domestic scientific research institutes such as China Electrical Science 49 can also do the sensor of the high temperature humidity instrument by the resistance capacitance method, but the performance is unstable, far from industrialization, at present the market is basically designed abroad. The semiconductor laser diode, the core component of online flue gas high temperature humidity instrument by infrared spectrum absorption method, can not be done by domestic manufacturers. Only Germany, the United States, the Netherlands and several other countries can do it. The lack or deficiency of core technology restricts the development and progress of on line flue gas high temperature humidity instrument.
In the future, the development of flue gas online high-temperature humidity instrument will be restricted by various factors, the humidity instrument of the above four principles will exist at the same time, it can be predicted that the humidity instrument of the resistance capacity method and the limit current type zirconium oxide principle will further improve the reliability and service life, the impact injection method humidity instrument will further reduce the volume, reduce the cost, increase the market share. Infrared absorption moisture meter should be the most promising, also represents the development direction of online gas analysis instrument, but if domestic manufacturers can not solve the development and production of semiconductor laser diode, then it will restrict the cost of reducing, affect the market promotion, even if foreign suppliers reduce the price, domestic enterprises can only work for foreign enterprises, and then conduct low-price competition in China, and fight the price war. We hope that the domestic industry association of on line analytical instruments or relevant government departments can promote cooperation among universities, scientific research institutes and enterprises, complement each other's advantages, break the monopoly of foreign manufacturers at an early date, and let China's analytical instrument industry move toward the high end.

In the conventional monitoring of flue gas emission, flue gas humidity is an important parameter, and it is also the most difficult to measure accurately. Humidity measurement itself is affected by other factors (atmospheric pressure, temperature), flue gas humidity measurement also needs to face high temperature, high dust, high moisture, negative pressure, corrosion problems.
In addition, humidity calibration is a difficult problem. The reason is that it is difficult to manufacture high temperature humidity generator, so it affects the measurement of humidity online measuring instrument. In order to verify and calibrate the flue gas humidity instrument, there must be a device capable of generating standard humidity source and humidity standard. A humidity measuring method capable of carrying out absolute measurement of humidity can be used as a humidity reference. Gases with known humidity can also be used as benchmarks for humidity. The standard "Sampling Method of Particulate and Gaseous Pollutants in the Exhaust of Fixed Pollution Source" (GB/T16157-1996) prescribes three kinds of flue gas humidity measuring methods: condensation method, weight method and dry-wet ball method, which are used as the reference method of flue gas humidity measurement and can be used for calibrating flue gas humidity instrument. In addition, the humidity generator generates constant humidity at a certain temperature and pressure, and it can also be used to calibrate the flue gas humidity meter


1.Introduction of flue gas humidity measurement method
Dry-wet ball method


The three humidity reference methods in standard GB/T16157-1996 are not ideal in practical application. The weight method and condensation method are complex, the test condition is high, the test time is long. The dry-wet ball method is simple, but the error is large.
The main problem of measuring flue gas humidity by dry-wet ball method is that the flue gas temperature is high, often higher than 100°C, but the dry ball temperature can not reach the actual flue gas temperature.
The temperature is usually between the ambient temperature and the flue gas temperature, resulting in a fixed measurement error. Zong Ningsheng believe that when measuring the humidity of flue gas by using dry-wet ball method, the following operations should be taken into consideration: When the temperature indicator is stable and no longer rises, the readings can be made (5-10 min). The connecting pipe between the sampling pipe and the dry and wet thermometers is shorter, and the pipe wall is not suitable to be thin, so as to prevent the temperature of the flue gas from excessively falling. In the cold climate, heating sampling tubes should be used.
Chang-Ai improved the dry-wet sphere thermometer and heated flue gas sampling tube to avoid the errors of flue gas reaching dew point and steam condensation in the sampling tube.





Figure 1CI-PC39 Outline


According to the actual flue gas temperature at the measuring point, by changing the heating temperature of the sampling tube, the difference between the flue gas temperature entering the dry-wet ball chamber and the flue gas temperature in the sampling point is avoided. The accuracy and stability of the self-made dry-wet-bulb hygrometer were investigated by using the standard dry-wet-bulb method and the weight method. The results show that the humidity data measured by the humidity meter is reliable and effective, and has a sensitive response to the change of the humidity of the flue gas, and can work continuously and stably. Although the impact jet method is the working principle of the dry and wet ball, it is not a common humidity instrument which is similar to the weather department to measure the relative humidity of air. This humidity instrument is a brand-new or even a brand-new innovative design, to achieve the above measurement results, its representative product figure is shown in Figure 1
Chang-Ai measurement engineering has been successfully applied to high temperature and high humidity measurement process, as well as the process with corrosive and dust-containing gases. In many process, in order to ensure the quality of the product, efficient use of energy or to achieve the goal of emission reduction, it is necessary to monitor and control the process gas humidity. CI-PC39 is a process humidity meter for the most stringent industrial requirements, including corrosion resistance, continuous operation and insensitivity to fouling.


Condensation method


The principle of the condensation method is that a certain volume of exhaust gas is extracted from the flue to pass through the condenser, and the water content in the exhaust gas is calculated according to the condensed water quantity and the water vapor content of the saturated gas discharged from the condenser. The principle of the weight method is that a certain volume of exhaust gas is extracted from the flue so that the moisture in the exhaust gas is absorbed by the moisture absorption tube by the moisture absorption agent through the moisture absorption tube filled with the moisture absorption agent, and the weight of the moisture absorption tube in the exhaust gas is the water component contained in the known volume of exhaust gas. The two methods are similar in principle. The mass concentration of flue gas humidity is directly obtained by weighing the moisture content and dividing the sampling volume, and then the mass concentration is converted into the volume percentage.
The dry-wet ball method is easy to operate and has strong adaptability. It is a common online measurement reference method for flue gas humidity. The condensation method and the weight method have higher accuracy, but the test is complicated, the operation of the personnel is required high, the test time is long, and the method is not suitable for the online measurement of flue gas humidity, and can only be used as the laboratory method and the online measurement method for comparison test.


Inhibition method


When a substance is placed between a pair of electrodes, the capacitance between the electrodes will change when it absorbs water vapor. The moisture content of flue gas can be obtained by measuring the capacitance change of the moisture absorption material, which is called a capacitive humidity sensor. The traditional capacitive humidity instrument has some problems in sensor sensitivity, humidity hysteresis, temperature coefficient and long-term stability.
The invention relates to an online volume-resistant high-temperature flue Gas Moisture Meter of a patent product, which is an improved capacitance method moisture meter, and has good corrosion resistance and high sensitivity. It uses polymer film capacitance humidity sensor
As a moisture sensor, a platinum resistance temperature sensor is used for temperature compensation. The working principle is shown in Figure 1, the water vapor passes through the upper electrode of the high molecular film capacitance humidity sensor, and reaches the high molecular active polymer film, because the sensor size is small and the polymer film is very thin, the sensor can make quick response to the humidity change of the surrounding environment.



Figure 2 The Block Diagram of Working Principle of Water Meter with Resistance and Capacity


The water vapor absorbed in the polymer changes the dielectric characteristics of the sensor and changes the capacitance value of the sensor, converts the output signal of the capacitive humidity sensor into a voltage value, and outputs a temperature voltage signal through the temperature sensor to perform temperature automatic compensation. The moisture meter can measure the water content in the range of 0~20%±2% under the smoke temperature ≤180°C.
In recent years, a lot of research has been done on the search for better moisture sensing media. Among them, organic polymer materials have attracted much attention because of their high sensitivity, fast response and small humidity hysteresis. There are two main types of moisture-sensitive media materials: CAB series (cellulose acetate butyrate) and P series (polyimide).
The organic polymer capacitive humidity sensor was originally made of cellulose acetate and its derivatives. At present, cellulose acetate is mostly used. Japan Sakai have compared the properties of various cellulose derivatives and studied the properties of capacitance, temperature and absorption isotherms. The results show that in order to make non-hygroscopic humidity sensor, the water quantity must be restrained and the interaction between molecules must not occur. It is proposed that cellulose acetate material, especially the element made of porous gold electrode, not only has fast response speed, but also has little hygroscopic hysteresis.
Matsuguchi proposed the synthesis of low polymers with acetylene groups at both ends of polyimide. In the low polymer state, the solution is used to dissolve, and the film is coated on the substrate to form a film, and after heating, the polyimide with a stereo-linking structure is obtained, which is not easy to dissolve in water. Since the material is not separated from water when solidified, and it is difficult to form micro-holes on the hardened film, it is a humidity sensitive material with good water resistance. The results show that the humidity sensitive element of the modified polyimide has fast response speed and almost no hysteresis. The temperature coefficient is small, the solvent resistance (acetone) is also good, and the stability is greatly improved.
Chen Xingzhu proposed a new type of capacitive composite dielectric film humidity sensor. Its dielectric film is composed of two types of PI(CAB) which have different linear output and temperature characteristics. Compared with PI and CAB single materials, the composite material has small hysteresis, small non-linear error and small temperature coefficient, and has obvious improvement in repeatability and long-term stability. It provides a new idea for functional design of dielectric material of capacitive humidity sensor. Its representative product figure is shown in Figure 3.

Figure 3 CI-XS200 dew point sensor profile
The main advantages of the capacitance method are high sensitivity, quick response, easy manufacture, easy realization of miniaturization and integration. At present, the online humidity instrument of flue gas in China has many applications, but the long-term stability is not ideal, and most of the long-term use of the drift is serious, causing failure and damage. Capacitive humidity sensor has less corrosion resistance, which often requires higher cleanliness to the environment. Some products still have some phenomena such as lighting failure and electrostatic failure. In short, this is a method that is constantly improving.


Limit current method


According to the in-depth theoretical study, through a large number of experiments, the use of ion flow sensor can achieve accurate measurement of humidity. By changing the voltage applied on the cathode and anode of the sensor, the humidity can be measured. This finding solves the problem that the ordinary humidity sensor can not adapt to high temperature environment (for example, higher than 100°C).
An operating voltage is applied to the anode and cathode of the zirconia to provide an electric field to drive oxygen ions from the cathode to form an oxygen ion current through the zirconia to the anode. When the oxygen concentration in the measured atmosphere is certain, the current value of the zirconia sensor does not increase with the increase of the applied voltage, reaching a constant value, this constant current value is called the limit current value of the oxygen concentration, we call the first limit current value. According to the working principle, when the measured atmosphere contains water vapor, by increasing the applied voltage, the water vapor is also ionized into oxygen ions, and when the concentration of water vapor in the measured atmosphere is fixed, the zirconia sensor outputs a constant current value, which is called the second limit current value.



Figure 4 The Relation Between Limit Current and Applied Voltage



Figure 5 The Curve Diagram of Output Limit Current of Sensor Under Water Vapor


The reaction at the sensor cathode and anode is as follows:
Cathode side ：O2+4e- →2O2- (4)
H2O+2e- →H2+O2- (5)
Anode side：O2- → 1/2O2+2e- (6)
According to the gas diffusion limit Ficks rule of the sensor, the first limit current I1 and the second limit current I2 are respectively represented by the following formula under the condition that the diffusion coefficient of oxygen is equal to the diffusion coefficient of water vapor:
I1={-4FDSP/(RTL)}Ln(1-PO2/P) (7)
I2={-4FDSP/(RTL)}{(1+PH2O/2PO2)} (8)
PO2=0.21(P- PH2O) (9)
In the formula: F is the Faraday constant, D is the diffusion coefficient of the mixed gas molecule, S is the area of the diffusion hole, P is the total pressure of the mixed gas, PO2 partial pressure, PH2O water vapor partial pressure, R is the gas constant, T is the absolute temperature, L is the length of the gas diffusion hole, 0.21 is the oxygen content in the air.


The application range of ion current humidity measurement is:


 Chang Ai Co., Ltd., in cooperation with Dr. Zhang Yi Can (Chinese, the first to introduce the extreme current type zirconia sensor into China) and his team, Yang Bang Chao, director of the Institute of Microelectronics and Solid Materials of Chengdu University of Electronic Science and Technology, pioneered the application of ion current sensors in the measurement of high temperature and humidity. In 2006, the company first introduced the GRL-12 high temperature humidity instrument based on ion flow sensor (Figure 5), and before the 2008 Green Olympics, a large number of applications to coking plant in Shanxi, thermal power plant emissions monitoring, for environmental protection and monitoring has made a domestic enterprise due to the obligation and contribution. In the development of more than ten years, Chang'ai Company has developed a large number of humidity analysis instruments based on ion flow sensors, such as CI-PC18 series humidity transmitter, CI-PC19 soil humidity monitor, CI-PC168 series high temperature Humidity Analyzer, CI-PC193 food industry humidity detection system, CI-PC196 series high temperature humidity analysis system, which represents the product's figure, see Figure 6.





Figure 6 CI-PC18 High Temperature Hygrometer Profile
Sensor structure：






Figure 6 3D Ion flow humidity sensor structure


These products are widely used in the fields of environmental protection, printing and dyeing, timber, building materials, paper-making industry, chemical industry, fiber and pharmacy, as well as the processing and storage industry of food, tobacco, vegetables and grain.






Dry-wet oxygen method


The oxygen sensor of CEMS system is used to measure the oxygen content before and after dehumidification of flue gas. When calculating the moisture content in flue gas, the humidity of flue gas is calculated according to the following formula:


Xsw=1-X,O2/XO2 (1)


In the formula (1), X and O2 are the volume percentage of oxygen in the wet flue gas, %, and Xo2 are the volume percentage of oxygen in the dry flue gas, %.
The main problem of dry and wet oxygen is that two instruments are needed to measure dry and wet oxygen respectively. The error caused by the difference of sampling points and sampling errors is as follows: The error of the two instruments is superposed. These errors are difficult to overcome for this method.


Infrared absorption


Absorption spectroscopy is an important technique in modern humidity measurement, including infrared and ultraviolet absorption. At present, the measurement technology based on near infrared absorption spectrum has become more mature, its measurement accuracy, sensitivity and measurement range are better than the traditional humidity analysis method.
Infrared absorption method uses the principle that water strongly absorbs infrared light with specific wavelength, the water content is different, the absorption degree of light, and obey Lambert-Beer law. By measuring the transmittance of the gas at the absorption wavelength and the reference wavelength, the ratio of the transmittance of the two wavelengths is a function of the water vapor content in the gas. Wan Jia Rong have found that the most commonly used absorption wavelengths are 1.45 μm and 1.94 μm, and the commonly used reference wavelengths are 1.73 μm and 2.1 μm.
There are two kinds of humidity measurement methods based on near-infrared absorption spectroscopy: laser diode resonant attenuation spectroscopy (CRDS) and tunable laser diode absorption spectroscopy (TLDAS). The CRDS resonator is simple in structure and small in size, which ensures the rapid replacement of gas, so CRDS is very suitable for on line measurement. TDLAS is a relatively mature absorption spectrum measurement technology, which has been used in the field of micro-moisture measurement, and has the advantages of high sensitivity and fast response.
However, infrared absorption method used in flue gas humidity measurement needs to avoid the interference of CO2/SO2/NOX sensitive wavelength, which is difficult, coupled with the high price of the instrument, so it is rarely used in flue gas humidity measurement.


High temperature humidity generator


Because the temperature of flue gas is generally higher, it is about 80°C~120°C, while the ordinary humidity generator generates constant humidity at normal temperature, even if the constant humidity occurs at high temperature, it is difficult to ensure that the temperature can be kept constant in use. It is often difficult to meet the requirements if using the normal temperature humidity generator to calibrate the high temperature flue gas humidity meter, which brings great restrictions to the research and application of high temperature humidity measurement. Especially the capacitive humidity sensor, the humidity sensor is sensitive to the temperature in addition to the environmental humidity, and it is easy to produce temperature drift, so it is necessary to develop a high temperature humidity generator.
The high temperature humidity generator can produce stable humidity at higher temperature. It is a convenient and intuitive calibration device for flue gas humidity instrument. Zhang Wen Dong developed a set of high temperature precision humidity generator by using the principle of double-temperature and double-pressure method. The temperature stability experiments were carried out at 50°C, 100°C and 150°C. The duration was 2 hours. The test results of the temperature stability of the saturator oil tank and the test chamber were within 0.02°C. The maximum theoretical uncertainty of the device is ±1.09RH. The accuracy of the device is verified by the test results of the weight method hygrometer. The device can be used for correcting high temperature humidity sensor and transmitter.


2.Summary
The measurement of flue gas humidity is a recognized problem. The method of dry-wet ball as the reference method stipulated in the national standard is easy to produce errors. The condensation method and the weight method are high precision, but the operation is complex, and they can only be used as laboratory methods. The online measurement method of flue gas humidity used in CEMS system in China is capacitance method and limit current method. Both of these methods belong to electronic humidity sensor. The application prospect is broad, but the anti-pollution and long-term stability must be improved. However, the dry-wet oxygen method has a large error, and the infrared absorption method is expensive, so it is used less. The normal temperature humidity generator is also difficult to meet the calibration requirements of flue gas humidity instrument. The development of high temperature and humidity generator is necessary and also a technical problem.

test principle


The humidity measuring instrument can be divided into cold mirror type, fully absorption electrolysis type, Al2O3 capacitance type, thin film capacitance type, resistance type, dry wet ball type and mechanical type. Among them, the fully absorption electrolytic micro-water meter and Al2O3 capacitive dew point meter are generally used for the measurement of low humidity range, while the resistance type, dry-wet ball, mechanical humidity meter can only be used for the measurement of relative humidity, the cold mirror type, thin film capacitance type (Vaisala's patent) humidity meter can not only be used for the measurement of low humidity, but also can be used for the measurement of medium and high humidity, that is, relative humidity. The above-mentioned instruments have their own advantages and disadvantages. Among them, the cold mirror dew point meter is the most accurate, the most reliable, the most basic measurement method, is widely used in the standard transmission, but its disadvantage is that the price is relatively expensive, and needs experienced operation and maintenance. 


1.1
Cold mirror dew point meter
1.1.1
Measure principle
When the measured moisture enters into the dew point measuring chamber, the cold mirror surface is swept over; when the temperature of the mirror surface is higher than the dew point temperature of the moisture, the mirror surface is in a dry state; at this time, the light emitted by the light source in the photoelectric exposure device is almost reflected on the mirror surface, the photoelectric sensor senses and outputs the photoelectric signal, and the control circuit compares, amplifies and drives the thermoelectric pump to cool the mirror surface. When the temperature of the mirror surface drops to the dew point temperature of the moisture, the surface begins to dew (frost), the light appears diffuse reflection on the mirror surface, the reflection signal induced by the photoelectric sensor is weakened, the change is compared by the control loop, amplified, the thermoelectric pump is adjusted to make the refrigeration power properly reduced, finally, the temperature of the mirror surface is kept at the dew point temperature of the sample gas. The mirror temperature is induced by a platinum resistance temperature sensor which is close to the lower part of the cold mirror surface and is displayed on the display window.
At present, the world's companies that produce cold mirror dew-point meter, such as GE, Edgetech, Switzerland MBW and so on, all adopt this principle, Britain's MICHELL adopts double-optical path detection system, that is, the reflection light and scattering light are detected at the same time, Finland's Vaisala uses acoustic wave as detection system.
During the measurement process, the water vapor in the measured gas is close to the saturation state with the decrease of temperature. Due to the gravitational effect, the water molecules adsorb on the mirror surface to form a thin water film. This is the first stage of dew formation. When the mirror temperature continues to decrease, the thickness of the water film gradually increases, which is the second stage of the dew formation. In this phase, the force contrast between the gravitational force of water molecules and the surface tension of water film is changed, and the influence of the latter gradually dominates. At this time, any unstable factors on the cooling surface, such as the tiny scar on the mirror surface, will cause the water film to condense into droplets. With the further decrease of the mirror temperature, the dew drops begin to appear. Through the microscope, we can see the isolated growth and irregular distribution of the dew drops, and then the dew layer diffuse on the surface at a very fast speed. At this time, we can think that the liquid-vapor equilibrium begins, namely reaching the dew point.
1.1.2
Structure
1.1.2.1
Mirror
The mirror should be hydrophobic, have good thermal conductivity, wear resistance, corrosion resistance and good optical performance. In the past, gold was used as a mirror, now rhodium is used as the mirror.


1.1.2.2
Mirror cooling


In the past, ethylene ether evaporation, mechanical refrigeration, liquefied gas or dry ice refrigeration, compressed air refrigeration have been used. The most commonly used is thermoelectric refrigeration or thermoelectric combined with mechanical refrigeration (dew point below -60°C). In this paper, thermoelectric refrigeration is emphasized.
Thermoelectric refrigeration, also known as semiconductor refrigeration, is Palput refrigeration (from its English name Peltier). The principle is that when a DC current passes through an NP element composed of two different metals, the heat will be transferred from one metal to another, which is just opposite to the thermocouple temperature measurement. Therefore, when the cold end of Palter is connected to the mirror and the other end is used as the heat dissipation end, the mirror can be cooled. In order to obtain different low temperature, a multi-level superposition method can be adopted. The data given by GE Company of the United States shows that, generally speaking, if the room temperature is 25°C, the temperature difference between the cold and cold end can reach 55°C, the temperature difference between the cold and cold end can reach 75°C, the temperature difference between the cold and cold end can reach 105°C, and the temperature difference between the cold and cold end can reach 120°C when the fifth refrigeration. The cooling capacity of different companies will vary slightly. The higher the temperature at the hot end, the higher the cooling efficiency, and the larger the temperature difference at the hot end. In order to reduce the temperature of the cold end, air cooling, water cooling and mechanical refrigeration are usually used to reduce the temperature of the hot end. But it is not possible to reduce without limits. It is important to note that its refrigeration capacity does not represent the measurement range of the dew point hygrometer. The definition of the measuring range of dew point hygrometer is that the temperature of the mirror surface can be obtained on the mirror surface, and the temperature of the mirror surface can be obtained when the dew or frost layer has a certain thickness. Therefore, under the general dew/frost point, the measuring range of dew point hygrometer is generally 5°C higher than its cooling ability, and under the low frost point, it is generally 10°C~12°C higher. For example, the DP19 dew point meter produced by MBW Company in Switzerland, when the room temperature is 10°C, its lowest measurement range is -60°C, when the room temperature is 20°C, its lowest measurement range is -55°C, when the room temperature is 35°C, its lowest measurement range is -45°C. Because of the high thermal conductivity of hydrogen and helium, the measurement range will be reduced by several degrees. When the pressure of the measured gas increases, the measurement range will also be reduced. For air and nitrogen, under the condition of higher than normal pressure, each additional atmospheric pressure, the measurement range will be reduced about 0.67°C.
1.1.2.3
Temperature measuring device


At present, most of the four-wire platinum resistance is used to measure temperature. The resistance value and temperature of the platinum resistance temperature sensing element are close to the linear relation in a fairly wide temperature range, the precision is high, the stability is good, the output signal is strong, the digital display is convenient.


1.1.2.4
Detection system


At present, except for the cold mirror dew point meter developed recently by Vaisala Company of Finland, which adopts the sound wave principle to measure, the other are all used photoelectric detector to measure and control. The photoelectric detection technology has been used for several decades, and it is mature. But its disadvantage is that it can not distinguish the supercooled water from the frost.


1.1.3
Precautions for use
1.1.3.1
Supercooled water and frost


In the range of 0-20°C, supercooled water is easily formed on the mirror surface. Because the saturated vapor pressure on the ice surface and the water surface is different, if supercooled water is formed on the mirror surface, the measured value is lower than the frost point, and the temperature is different. For example, when the frost point is -10°C, the corresponding supercooled water temperature is -11.23°C. So be very careful at this temperature. If the instrument is equipped with an endoscope, it can be observed and distinguished by the endoscope. At present, most instruments have the function of test, that is, to test their minimum cooling capacity. At this time, we can use the function of test to make the temperature of the mirror be lower than -20°C, ensure the frost on the mirror, and then make a formal measurement.


1.1.3.2
Kelvin effect
The saturated water vapor pressure on the surface is different from that on the plane. When exposed to the metal surface, the equilibrium water vapor pressure, i.e., the saturated water vapor pressure at the curved water surface, is increased due to the effect of surface tension, which is known as the Kelvin effect. Because of the Kelvin effect, the dew point temperature obtained is lower than the dew point temperature of the real measured gas.


1.1.3.2


Raoul effect


It means that the equilibrium water vapor pressure of the system is lower than the saturated water vapor pressure of the pure water when the water-soluble substance exists on the mirror. These water-soluble substances may be intrinsic to the mirror or contained in the measured gas. According to Raoul's law, the decrease of the equilibrium water vapor pressure is proportional to the solution concentration, which is the reason why there will be early condensation before reaching the dew point temperature of the measured gas
The Kelvin effect is just the opposite of the Raoul effect, so it will offset some. However, in the dew point measurement, the Raoul effect is more significant than the Kelvin effect, because the water-soluble substances are inevitably more or less present in the mirror and measured gas, and the impurities in the gas may sometimes occur chemical reaction or photochemical reaction with the water-insoluble substances on the mirror to convert into soluble substances. This situation is more evident in the moisture measurement of industrial process gases. Therefore, it is necessary to remove the solid particles in the gas by adopting proper filter device, and further remove the soluble substances left on the mirror surface by repeated dew condensation and dedew operation, this method is widely used.


In practical work, we often find that the surface of the mirror is not uniform when the surface begins to expose, the layer always appears in a certain area of the mirror, the reason is often caused by scratches on the mirror, because in these defective areas, on the one hand, the residual material is not easy to remove, on the other hand, the defects of the corner play a role of "exposed core", accelerating the process of exposing. Therefore, in the use of the dew point meter, especially when cleaning the mirror, it is necessary to be careful to avoid mechanical damage to the mirror. 


1.1.3.3
Mirror contamination
one is the raoul effect, the other is to change the level of specular background scattering. The raoul effect is mainly caused by water-soluble substances. If the substance in the measured gas (generally soluble salts), the mirror will dew out in advance, which will cause positive deviation of the measurement results. If the pollutants are insoluble in water particles, such as dust, etc., the scattering level of the background will be increased, so that the photoelectric dew-point meter zero drift.


1.1.3.4
Sampling channel


Because the water content in the atmosphere is very high, and the water molecule is polar molecule, it is easy to absorb on the inner wall of the pipeline or through the pipeline. Therefore, the gas path system must be well sealed in the measurement, the thickness of the pipe wall is at least 1mm, so as to prevent the outside environment water intrusion into the leakage. If the temperature of the measuring environment changes greatly, the sealing of the pipeline should be checked again.
If the measured gas is directly discharged into the atmosphere, the problem of water diffusion into the measurement system should be considered. The most commonly used method is to connect a pipe with a proper length in the exhaust port. The length and diameter of the pipe are based on the principle of not affecting the pressure of the measuring chamber. 
The sampling pipeline should be as short as possible, the number of joints should be reduced and the "dead space" should be avoided so as to reduce the interference of the background water.
The sampling pipeline and the measuring cavity wall are clean, the smoothness is good, and the hydrophobic material is selected. Figure 2-2 is the desorption-time curve of various materials when they are subjected to dry gas in the saturated adsorption state. From the experimental results we can get the following order of material selection: stainless steel, PTFE, copper, polyethylene, and, worst, nylon and rubber tubes, should not be used in low frost point measurements. In addition, the outer diameter of the tube is 6 mm or 1/4 inch, although the inner polished stainless steel tube is used in the low frost point measurement.
When measuring the high dew point, we must pay attention to that the dew point is lower than the ambient temperature of 3°C to avoid condensation of water vapor in the pipeline. 
When the dew point hygrometer measures the humidity, the flow range is 0.25L/min~1L/min. In this range, the change of flow velocity does not affect the measurement results.
The sampling can be divided into two kinds, one is the pressure sampling, according to the different sampling methods, it can be divided into the pressure measurement and the atmospheric pressure measurement. See figures 2-3 and 2-4, respectively. The other is measured at atmospheric pressure, that is, the sample is taken by pump. In this case, it will often bring artificial positive pressure and negative pressure because of the different sampling methods. If sampling in the way shown in Figure 2-5, the dew point meter is measured under the condition of pressure, it will bring positive error to the measurement results. If the pump and the flow meter exchange position, the dew point meter is under the condition of negative pressure, it will bring negative error to the measurement. The correct sampling method is shown in figures 2-6.


1.1.4
Application


The measurement range of the dew point hygrometer is wide. At present, the measurement range of a series of dew point hygrometers developed by MBW Company of Switzerland has reached -95°C~70°C, which can meet the majority of measurement requirements.


1.1.5
Pros and cons
Benefits: It is a basic measurement, accurate measurement, and the instrument is stable and drift-free. The instrument with the highest accuracy can reach ±0.1°C.
Disadvantage: High price, high requirements for operators, and need to maintain. Sensitive to pollutants. There is sometimes supercooled water in the -20°C~0°C range, so be especially careful to distinguish supercooled water from frost.　
1.2
A micro water meter for complete absorption electrolysis
1.2.1
Measure principle
By means of continuous sampling, the gas sample flows through an electrolytic cell of a special structure, the moisture of which is absorbed by the phosphorus pentoxide layer as a hygroscopic agent and discharged by electrolysis into hydrogen and oxygen, and the phosphorus pentoxide is regenerated. The reaction process can be expressed as:
P2O5+H2O=2HPO3 
2HPO3=H2+1/2O2+P2O5 
Combined (1), (2), will get:
2H2O=2H2+O2 
When the absorption and electrolysis are balanced, the water entering into the electrolysis cell is absorbed by the phosphorus pentoxide film layer, and is electrolyzed. If the ambient temperature, ambient pressure and gas flow are known, the relationship between the electrolytic current of water and the water content of gas sample can be deduced according to Faraday's law of electrolysis and gas law:
In formula:
Electrolytic current of water, μΑ;
Water content of gas sample, μL/L (i.e. volume ratio)
Gas flow, ml/min
Environmental pressure, Pa;
Absolute temperature of the environment, k;


As can be seen from the above formula, the magnitude of the electrolytic current is proportional to the water content in the gas sample, so the water content in the gas sample can be measured by measuring the electrolytic current of water. Under the condition of standard atmospheric pressure and 20°C, an ideal gas flows through the electrolytic cell at a flow rate of 100ml/min. When the water content of the gas sample is 1μL/L (ppmv), the electrolytic current is calculated from the formula as 13.4μΑ. This kind of instrument usually takes ppmv as unit, and can directly read the ppmv value of the moisture content in the gas sample.
Because of the catalytic effect of platinum electrode, the water electrolysis reaction is a reversible process, so when the measured gas sample is hydrogen, oxygen or contains enough hydrogen and oxygen, the balance moves to the left, the hydrogen and oxygen which have been produced by electrolysis are combined to generate water, and then the secondary electrolysis is carried out, so that the total electrolysis current value is higher, namely, the "hydrogen effect" and the "oxygen effect", or the "composite effect". The experiment shows that the reading of this kind of gas is higher by several to ten ppmv when the instrument is used to measure the water content of this kind of gas, but the deviation concentration reaction is on the background value, so it can be deducted.


1.2.2 
Structure
The instrument is composed of two parts of gas path system and circuit, the gas path system mainly includes an electrolytic cell and a gas path control part.
1.2.2.1
Battery
In the glass tube, two platinum electrodes are wound into a double spiral shape, and phosphorus pentoxide film is uniformly coated between the electrodes as a hygroscopic agent. Under the specified measurement conditions, the inner winding structure can ensure the absorption and electrolysis of all water entering the pool. The glass pool wall is favorable for the uniform phosphorus pentoxide coating. Since platinum has the function of generating hydrogen and oxygen, especially hydrogen-rich gas, to react again to generate water, some companies have adopted rhodium instead of platinum. 
For the dry phosphorus pentoxide coating, when a "absolutely dry" gas sample is introduced and an appropriate DC voltage is applied to the electrode, a small current-background value is generated in the circuit. The value of the background is only related to the structure of the cell, the condition of the coating, the temperature and the type of the sample, but not to the water content of the sample. Because the background value can always be added to the electrolytic current of the moisture contained in the gas sample, the real moisture content of the medium should be deducted from the instrument reading when measuring.
1.2.2.2
Pneumatic control system
The pneumatic system is composed of control valve, electrolytic cell, flow regulating valve, flow meter and dryer. The control of the air flow path is done by the control valve.


1.2.3 
Precautions for use
From the formula, we can know that the measurement results, namely the humidity of the gas μL/L (ppmv) is calculated according to the gas flow and electrolytic current, so the gas flow must be accurately controlled and measured. This kind of instrument generally uses the floater flowmeter, under 20°C, 1atm, uses the air to calibrate. If the condition in use is not a standard condition, for example at a different temperature and pressure, or the measured gas is not air, the measured gas needs to be calibrated again or corrected according to a correction factor.


1.2.4 
Application
The measurement range is from several μL/L(ppmV) to 2000 μL/L(ppmV), and the accuracy is 5% of the reading or 1% of the full range. The invention can be used for a plurality of inert gases, some organic and inorganic gases which do not react with P2O5. Examples include air, nitrogen, hydrogen, oxygen, argon, helium, neon, carbon monoxide, carbon dioxide, sulfur hexafluoride, methane, ethane, propane, butane, natural gas, and certain freon gases. It can not be used for certain corrosive gases and gases which can react with P2O5, such as ethanol, certain acidic gases, unsaturated hydrocarbon gases.


1.2.5 
Pros and cons
Benefits: Absolute measurement, stable, no drift.
Disadvantage: The life of the battery is limited and needs to be regenerated. Both high and low humidity (<1ppmv) shorten their life. Slow response in low humidity. The demand for gas flow rate is high. It cannot be used in some corrosive gases and gases that react with P2O5. There's a background.
1.3
Aluminum oxide capacitance humidity meter
1.3.1
Measuring principle, structure and application range


The instrument has a variety of forms, such as portable battery-operated, data processing with a microprocessor, multi-parameter display, and so on. But its essence is a capacitor, by depositing a thin layer of porous alumina on a conductive substrate and then applying a thin layer of gold to the thin layer of alumina. The conductive substrate and the thin layer of gold form an electrode of the capacitor. The water vapor is absorbed by the porous alumina through the gold thin layer, and the impedance of the capacitor is proportional to the number of water molecules, namely the water vapor pressure. Moisture partial pressure can be obtained by measuring the impedance or capacitance of the capacitor, and dew point value can be obtained by conversion.
The thin layer of aluminum oxide located between the aluminum and gold electrodes is responsive throughout the range of saturated vapor pressure from 10-3Pa (approximately -110°C dew point) to water. Due to its strong affinity for water, coupled with the larger dielectric constant of water, such instruments are highly selective for water, but not responsive to other common gases and organic gases and liquids. 
The accuracy is ±1~±2°C in the middle and high humidity range, and ±2~±3°C in the low humidity range, such as -100°C. The sensor does not react with hydrocarbon gas, CO, CO2 and HCFC-containing gas, but the drift of different gas is different. For certain corrosive gases, such as ammonia, SO3, and chlorine, will damage the sensor and should be avoided as far as possible.


1.3.2
Precautions for use
The usual measurement range of this kind of instrument is -110°C~+20°C. When the dew point is higher, the instrument will produce a larger drift. Attention should also be paid to the temperature coefficient. Because of its response to the partial pressure of water vapor, we should pay attention to the change of the total pressure of gas in the measurement. 
It can avoid dust and oil pollution, and the gas flow rate is larger, which is 3~5(L/min) or even larger.
1.3.3
Pros and cons
Benefits: The invention has wide response range, from 1μL/L (ppmv) to 80%RH, can be installed remotely, can be used in the field, has relatively stable, fast response, small temperature coefficient, has no relation with flow rate change, has high selectivity to water, can be used in a wide range of temperature and pressure, has small daily maintenance amount and small volume.
Disadvantage: The method is an indirect measurement, operating at higher temperatures or some gases causing drift, affected by corrosive gases, which must be periodically calibrated to overcome aging, hysteresis and contamination. Since the response value is non-linear, each sensor needs to be calibrated and cannot be used universally.
1.4
Thin-film capacitive humidity meter
1.4.1
Measuring principle, structure and application range


A polyamine salt or cellulose acetate polymer film deposited on two conductive electrodes is used. The dielectric constant between the two electrodes can be changed when the film absorbs water or loses water. There is also a technique to use thermoset polymers that are resistant to high temperatures, which allows such sensors to be continuously measured at temperatures higher than 100°C. Now I'm using high-molecular films like Visala.
1.The main function is to support other parts of the sensor.
2.One of the electrodes is made of conductive material 
3.Thin film layer. It is the heart of the sensor, the amount of water absorption of the film is related to the relative humidity of the surrounding environment. The thickness of the film is 1-10 (μm).
4.The upper electrode also plays an important role in the performance of the sensor. In order to get a quick response, it is necessary to have a higher water permeability. It is also a conductive material.
5.A contact pad for an upper electrode. Since there are many restrictions on the design of the upper electrode, a separate metal is needed to make good contact.


The measurement range is wide, from -50°C~100°C dew point. It can be used in a wide range of temperature, sometimes without temperature compensation. High temperature resistant thermoset resins allow for continuous measurements of such capacitive humidity sensors at temperatures of 185°C, with the highest temperature used depending on the sensor's packaging material. Another advantage for thermoset resin sensors is that the temperature coefficient is small in the -50°C~100°C temperature range, so it can be easily measured in a wide range.
All relative humidity sensors are temperature sensitive and, if calibrated at one temperature, will cause errors when used at another temperature. One advantage of polymer sensors is that they have less temperature dependence, i.e., smaller temperature coefficients. Therefore, when the use temperature is different from the calibration temperature, the error is small. Electronic temperature compensation is required if used at the limit temperature or if accuracy is high. When the temperature span is less than 50°C, it is easy to compensate the temperature. When the temperature range is wider, it is difficult to compensate the temperature. However, the accuracy of modern polymer sensor can reach ±1%RH in a narrow range, and ±3%RH in a wide range of temperature and humidity. After a period of use, or after contamination, a recalibration is required.


1.4.2
Pros and cons
Benefits: The system has the advantages of fast response, wide temperature and humidity measurement range, good linearity, little hysteresis, good stability and repeatability, low temperature coefficient and low cost.
Disadvantage: Almost nothing.
1.5
Resistance type humidity meter
1.5.1
Measuring principle and structure


The sensitive material takes the polymer solution of quaternary ammonium salt as the matrix, and the functional group is reacted with the resin polymer to produce a thermosetting resin with three dimensional and three dimensional, and has good stability. The change of relative humidity can lead to the change of resistance between the cathode and the anode.


1.5.2
Pros and cons
It has no hysteresis and aging, low temperature coefficient, low cost and low energy consumption. The temperature range is -10°C~80°C, the repeatability is better than 0.5%RH, the accuracy is higher, generally ±2%RH, in a very narrow range can reach ±1%RH. 
Disadvantage: It is an indirect measurement instrument, which needs to be calibrated periodically, and is not suitable for some pollutants. If it is used in a wide range of temperature, it needs temperature compensation. It is more sensitive to pollutants than the capacitive sensor. It is not suitable for low humidity, loss of sensitivity when relative humidity is less than 15%RH, but it still has good performance when relative humidity is close to 100%RH, but condensation sometimes damages the sensor. 
Some pollutants have great influence on resistance sensor, while others have great influence on capacitance sensor. Therefore, when selecting sensor, it mainly depends on the nature of pollutants. 


1.6
Mechanical humidity meter 
1.6.1
Measuring principle and structure 
The length of organic polymer materials such as hair, intestinal membrane, nylon and polyimide will change with the relative humidity. The mechanical humidity meter utilizes this characteristic to make the above material make linear, strip-shaped humidity-sensing element or coat on the elastic material to roll into a loose-wire-shaped humidity-sensing element, and then through the mechanical amplifying device, the change of the geometric amount caused by the change of the humidity is indicated by a pointer or recorded by a recording pen, thereby directly indicating the relative humidity. The invention is suitable for measuring the temperature and humidity of indoor environment such as laboratory, computer room, warehouse and factory building. 


1.6.2
Pros and cons
Benefits: Cheap, not sensitive to most pollutants, no power, and permanent recording  
Disadvantage: drift, if the use of a certain humidity for a long time will lose its sensitivity, can not be used below 0°C, slow response, transport or vibration swing will damage its performance.  
1.7
Dry-wet ball humidity meter 
1.7.1
Principle
The dry-wet ball hygrometer is composed of two thermometers with identical specifications, one is called the dry-ball thermometer, and the temperature bubble is exposed in the measured gas to measure the ambient temperature, and the indication value is expressed by Ta (ta). The other is a wet-ball thermometer, which is wrapped with a specially made gauze cover to keep it wet. When the air around the wet ball is in the unsaturated state, the moisture on the wet ball gauze cover will evaporate continuously, because the moisture evaporates needs to absorb heat, so the temperature of the wet ball will decrease, its indication value is expressed by Tw(tw). The velocity of moisture evaporation of the wet sphere is related to the moisture content of the surrounding gas. When the humidity of the gas is lower, the moisture evaporation is faster, the temperature of the wet sphere is lower, and vice versa. After obtaining the accurate temperature of dry and wet balls, the humidity value is calculated by means of the equation of wet balls.     
Due to its simplicity and low cost, dry-wet ball hygrometers have been the most used type for a considerable period of time in the past.
A humidity meter with good design and maintenance, in the temperature range of 5°C~80°C, if the temperature accuracy is ±0.2°C, the relative humidity accuracy is about ±3%RH. The accuracy of this principle is dependent on the accuracy of the thermometer. Platinum resistance thermometers are often used for some precise measurements. In general, the dry-wet ball hygrometer is a basic measurement method. If the calibrated thermometer is used and the operation is correct, such as the Assmann hygrometer, accurate, reliable and repeatable measurement results can be obtained. So in the past, this hygrometer was often used as a standard. However, many operators, especially in the industrial field, do not have enough energy and time, so the results are not accurate and unreliable. At present, the wet and dry spherical hygrometer is gradually replaced by modern instruments.


1.7.2
Pros and cons
Benefits: When the relative humidity is close to 100%RH, a higher accuracy can be obtained. Although there will be errors if the wet ball thermometer is polluted or used improperly, the maintenance cost is very low because of the simple device. The invention can be used in the situation that the room temperature is higher than 100°C, which is the basic measurement, the stability is good, the stability is simple, the cost is low.
Disadvantage: Some techniques are needed to obtain accurate measurements and calculations are needed to obtain the final results. A large number of gas samples are required, and the gas samples may be humidified by a wet gauze. When the relative humidity of the measured gas is lower than 15%RH, it is very difficult to reduce the temperature of the wet ball. When the temperature of wet ball is lower than 0°C, it is difficult to obtain reliable results. The volume cannot be too small because water is constantly supplied to the wet ball thermometer. Because dust, oil or other pollutants can pollute the gauze, or the water flow is insufficient, the temperature of the wet ball is higher, and the result of relative humidity is higher. In addition, the factors that affect the results are temperature measurement error, wind speed, radiation error and so on. When the temperature difference of dry and wet ball is 0.1°C at 20°C, the relative humidity error is 1%RH.