A new measurement method of oxygen content -- 3D ion current oxygen analyzer
Summary: This paper introduces an advanced ion flow oxygen measuring instrument by describing the principles and characteristics of oxygen measuring methods such as copper ammonia solution absorption method, fuel cell method, magnetic oxygen method, zirconium oxide method and laser method.
Key words: copper ammonia solution, fuel cell, magnetic oxygen, zirconia, laser, ion flow, oxygen meter.
Oxygen content in many industrial production process is a very important indicator, directly affects the production capacity, speed, efficiency and safety of industrial production. Therefore, how to measure the oxygen content more quickly, conveniently, accurately and reliably, so as to control the oxygen content in time is very important. The ion flow method is a new oxygen content measurement method based on this requirement. Compared with the traditional oxygen content measurement method, the ion flow method has many advantages in response speed, stability, instrument price and sensor service life, etc., especially suitable for high content oxygen analysis.
Traditional methods of oxygen content measurement: It includes copper ammonia solution absorption method, fuel cell method, paramagnetic method, zirconium oxide concentration potential method and laser method, etc. The principle and advantages and disadvantages of the method are summarized as follows:
1.1 Copper ammonia solution absorption method
The copper-ammonia solution is prepared by putting the copper wire which is coiled into a spiral shape into a solution which is prepared by the saturated solution of ammonium chloride and ammonia water in a ratio of 1:1. When a gas sample containing oxygen is introduced into an absorption bottle filled with a copper ammonia solution, in the presence of ammonia, copper is oxidized by oxygen in the sample to produce copper oxide (CuO) and copper oxide (Cu2O), and the reaction equation is as follows:
The copper oxide and the cuprous oxide are respectively acted with ammonia water and ammonium chloride to generate soluble high-valence copper salt Cu(NH3)2Cl2 and low-valence copper salt Cu2(NH3)2Cl2. The low-price copper salt absorbs oxygen to be converted into high-price copper salt, and the high-price copper salt is reduced by copper to be converted into low-price copper salt, thus the cycle action is carried out until the oxygen in the gas is exhausted. The oxygen content in the gas (volume percentage concentration) can be obtained according to the reduction of the volume of the gas.
This method is a classical method of oxygen content measurement, which is usually used in arbitration and has low cost. At present, there are still many gas laboratories and detection institutions retaining this method, but it is generally only suitable for measuring the gas samples with oxygen content less than 99.9%. Its disadvantages include the need to prepare solution, winding copper wire, more cumbersome; The whole measurement process needs manual operation, which is not suitable for on line continuous analysis. When other oxidizing gases are contained in the measured gas, the measurement results will be disturbed. As the whole absorption device is all glassware, it is easy to be damaged.
1.2 Fuel cell method
The fuel cell is usually composed of inert metal electrode (cathode) + lead (or graphite) electrode (anode) + electrolyte (divided into acid and alkaline), the cathode and anode are respectively connected with a metal sheet as electrode lead, the electrolyte is overflow on the surface of the cathode through a plurality of round holes on the cathode, a layer of electrolyte thin layer is covered on the surface of the electrolyte thin layer, a polytetrafluoroethylene (PTFE) film which can penetrate gas is covered, the gas sample enters the cathode through the permeation film, the oxygen and the electrolyte react, the generated OH-ions move to the anode under the action of the electric field, and the anode loses electron to generate water. For example, when silver is used as anode material, the chemical reaction equation is as follows:
The current intensity generated by OH-migration is proportional to the oxygen content in the gas sample, and the oxygen content in the gas sample can be obtained by measuring the current intensity generated in the fuel cell.
The method has the advantages that the fuel cell has simple structure, small volume and fast response speed, therefore, the Oxygen Analyzer of the method is very suitable for portable use, and the price is relatively low. However, the fuel cell is a consumption type detector whose life is determined by the total amount of oxygen accumulated through the sensor, and the anode is continuously reacted and consumed in the measurement. Once exhausted, the fuel cell will fail and need to be replaced. The measurement accuracy and stability of the fuel cell oxygen analyzer are poor, especially when used to measure gas samples with more than 90% oxygen content, the monthly drift can reach more than 1%. Furthermore, it is important to note that when a fuel cell is used with an electrolyte that is alkaline, it is not suitable for the analysis of oxygen content in the acidic gas, while when the electrolyte is acidic it is not suitable for the measurement of the alkaline gas.
1.3 Magnetic field act (field mechanical acts)
The measurement of oxygen content by paramagnetic method is based on that oxygen is a paramagnetic substance, and its volume susceptibility can reach k=1062×10-6(C.G.S.M) at 20°C. The volume susceptibility of other gases is much smaller than that of oxygen (except NO), so the analysis of oxygen content by paramagnetic method is always one of the most effective methods.
The magnetic mechanical oxygen analyzer is one of the representative instruments for analyzing oxygen content by paramagnetic method. The oxygen sensor is a pair of quartz glass dumbbell balls filled with nitrogen, the dumbbell balls are wrapped with platinum wires, forming an electric feedback loop, the dumbbell balls are suspended in a magnetic field, and a small reflector is arranged in the middle. The light source inside the instrument emits light beam, which is reflected by a reflector and received by a light detector made of a photosensitive component. When the oxygen molecule exists around the dumbbell sphere, the oxygen molecule moves under the action of the magnetic field, the dumbbell sphere is driven to deflect, the higher the oxygen concentration, the larger the deflection angle, the deflection will drive the reflector, and the light path of the light detector is also deflected. The light detector will detect the deflection and generate an electrical signal. After amplifying by the amplifier, the circuit is formed by the feedback circuit, and the dumbbell returns to the main equilibrium position under the action of the magnetic field. The current value in the circuit is proportional to the oxygen content. The oxygen content in the sample can be obtained by measuring the current value.
The advantages of the paramagnetic method for measuring the oxygen content are that the measurement is basically not affected by the non-measured components in the gas sample (except NO and Xe), can be used for measuring the gas sample with higher oxygen content, and has the advantages of fast response speed and good stability. But this method also has its defects, including the gas sample pretreatment and the measurement environment and so on the higher requirements, the sample pressure, dust, tar, water vapor and so on will affect the measurement results, even damage the sensor, in addition to ensure the horizontal placement of the instrument, avoid vibration, avoid strong magnetic field, the instrument surrounding can not be used for larger power equipment or power line. The paramagnetic oxygen analyzer is more precious, the internal structure is more complex and the price is higher.
1.4 Zirconia concentration potential method
The zirconium oxide tube used in the zirconium oxide concentration potential method is a stable zirconium oxide ceramic sintered body which is formed by the zirconium oxide material mixed with a certain proportion of yttrium oxide or calcium oxide through high-temperature sintering, because of the existence of yttrium oxide or calcium oxide molecule, the oxygen ion hole exists in the cubic lattice of the zirconium oxide, and the zirconium oxide tube is a good oxygen ion conductor at high temperature. Because of this characteristic, at a certain temperature, when the oxygen content in the gas on both sides of the zirconia tube is different, a typical oxygen concentration battery is formed. The whole zirconia tube is tubular, the middle of which is separated by zirconia material, and a layer of porous metal is sintered on both sides of the zirconia as electrodes (platinum is usually used as electrode material). At a certain temperature (600-1400°C), the oxygen molecules on the side with higher oxygen content are adsorbed on the electrode, under the catalysis of platinum, a reduction reaction occurs, and the electrons form oxygen ions, namely:
At the same time, the side electrode is positively charged to become a positive electrode or an anode of an oxygen concentration cell. The oxygen ions migrate to the other side of the zirconium oxide crystal with lower oxygen content through the holes in the zirconium oxide crystal, and the electrons are lost on the platinum electrode to form oxygen molecules, namely:
At the same time, the electrode is negatively charged to become a cathode or cathode of an oxygen concentration cell. The potential is related to the oxygen content in the gas measured by zirconium oxide. It is in accordance with the Nernst equation.
In formula:
E：Oxygen concentration potential (mV)
R：Gas constant 8.3145 J/mol·K
T：273.15 + t (℃)
n：The working temperature (K) of the zirconium oxide probe indicated by absolute temperature is 273.15 + t(°C).
F：Faraday constant, 96485.3365 (C/mol)
P0：Oxygen partial pressure in the reference gas
P1：Oxygen partial pressure in gas to be measured
The equation is the basis for measuring oxygen content in gas by zirconia concentration battery. In the actual measurement, the zirconia tube is heated to 600~1400°C, the reference side of the zirconia tube is filled with the gas with high oxygen content and known oxygen content as the reference gas, such as air (P0=20.6%), while the other side is filled with the gas to be measured, the oxygen partial pressure (P1) in the gas to be measured can be calculated by measuring the concentration battery potential E and the absolute temperature of the zirconia probe, thereby obtaining the oxygen concentration in the gas to be measured.
The method has the advantages of high sensitivity, fast response, wide linear range, good reproducibility and stability. The internal structure of the zirconia oxygen analyzer is simpler than that of the magnetic oxygen analyzer, and is almost not affected by the external environmental conditions such as temperature, vibration, etc., and almost does not need post-maintenance. However, its shortcomings are also obvious, because it is necessary to be at a higher temperature of the electron in the zirconia material to move, so the instrument must be equipped with heating furnace to heat the zirconia tube, which also lead to the zirconia analysis instrument needs a long preheating time to be used normally. And the zirconia method will be affected by the reducing gas in the gas to be measured when measuring the oxygen concentration, which results in the lower measurement result, so it is not suitable for measuring the oxygen concentration in the gas sample with higher content of the reducing gas or the reducing gas, especially when measuring the gas sample with ppm oxygen concentration, it is more necessary to consider the influence of the reducing gas in the sample on the measurement result. In addition, when the oxygen concentration in the gas sample to be measured is higher than the oxygen concentration in the air (20.6%), in addition to using the gas with higher concentration as the reference gas to ensure that the concentration potential is positive, the zirconium oxide detection tank needs to be reformed, thereby greatly improving the cost of the instrument.
1.5 Laser oxygen measuring method
The laser oxygen measuring method is based on the characteristic that oxygen molecules can absorb certain wavelength laser, a fixed wavelength laser beam with known light intensity is generated by a laser diode inside the instrument, the laser beam is injected into a measuring pool filled with the gas sample to be measured, after reflecting back and forth several times between two mirrors on both sides of the measuring pool, part of the light is absorbed by the oxygen in the gas sample, and the remaining light is reflected to the collecting pole and captured.
According to Bill's law, the ratio of the absorbed beam intensity to the original intensity is proportional to the oxygen content in the gas sample:
Ln[I0/I] = S × L × N
In Formula:
I0:original light intensity
I:Residual light intensity absorbed by oxygen in a gaseous sample
S:Absorption Constant of Oxygen to a Specific Wavelength Laser
L:optical path length
N:The number of oxygen molecules on optical path is related to the oxygen content in the sample gas.
Therefore, the oxygen content in the gas sample can be obtained by measuring the original light intensity and the absorbed light intensity. Since the selected laser wavelength is specific, the measurement results are almost unaffected by other gases. Using I/I0 to calculate can almost eliminate the influence of the light intensity, mirror reflectivity and the change of electrical equipment. At present, the price of instruments produced by using this principle is relatively high, and the stability of performance needs to be further improved.
3D ion flow technology
The working principle of the 3D ion flow oxygen sensor is shown in Figure 1.
Platinum electrodes are coated on both sides of the stabilized ZrO2, and the cathode side is joined by a cover with a gas diffusion hole to form a cathode cavity. At a certain temperature, when the two sides of the ZrO2 electrode are added with a certain voltage, the oxygen molecules in the cavity obtain the electron forming oxygen ions (O2-) at the cathode, the O2- moves to the anode through the oxygen vacancy of ZrO2, the electron is released and becomes the oxygen molecule gas to be released, this phenomenon is called an electrochemical pump, so the oxygen in the cathode cavity is continuously pumped out of the cavity by the ZrO2 electrolyte, and the current is formed in the loop. When the mole fraction of oxygen is constant, the voltage increases and the current intensity increases. When the voltage exceeds a certain value, the current intensity reaches saturation, which is the result of the diffusion of oxygen through the small hole into the cathode cavity limited by the small hole. This saturation current is called ionic current. The diffusion mechanism of gas in small holes determines the properties of the sensor. There are two kinds of ion flow in small hole diffusion, namely molecular diffusion and Knudsen diffusion. When the pore diameter is larger than the average diameter of the gas molecule, the ion current IL in the diffusion region is:
In the formula, F-Faraday constant; D—Diffusion coefficient of oxygen molecules in free space; S—the cross-sectional area of the diffusion hole; L—the length of the diffusion hole; C—the mole fraction of oxygen around the sensor; CT—The molar fraction of the entire gaseous substance. When C/CT<1, from formula (1), the ion current value becomes proportional to the mole fraction of oxygen, and the ion current value IL is:
From the formula (2), the ionic current and oxygen mole fraction almost linear. The oxygen mole fraction in the measured gas can be determined according to the output current.
The oxygen supplied to the sensor cathode is controlled with a porous ceramic substrate as a diffusion layer, which uses LSM as a dense diffusion barrier layer with a porous layer-type structure, as shown in Figure 2.
Figure 2 Porous layer oxygen sensor
The ion flow of the porous layer type oxygen sensor is the same as formula (2).
In the formula, F-Faraday constant; Oxygen effective diffusion coefficient in Deff-porous layer. S—cathode area; L-porous layer substrate thickness; C—Oxygen mole fraction around the sensor. From the formula (3), the limit current value of the porous layer oxygen sensor is linear with the oxygen mole fraction.
voltage-current characteristics
The voltage and current characteristics of the sensor are shown in Figure 3 in different oxygen concentration ambient gases.
Figure 3 A Schematic Diagram of Voltage and Current Characteristics of Sensor
The relation curve between the 3D ion current and oxygen concentration is shown in Figure 4.
Figure 4 Curve Plot of Ion Current and Oxygen Concentration
3.Comparison with the "copper ammonia solution absorption method":
The Shanghai Institute of Metrology and Measurement Technology has compared the ion flow oxygen meter produced by Chang Ai with the copper ammonia solution absorption method. The instrument was calibrated with O2 in 24.1%He, and then the "Cu-Ammonia Solution Absorption Method" sent by a company was used to measure the oxygen content of the gas, the instrument showed 97.71%, after a few days, the instrument was measured several times, the display range was between 97.65% and 97.89%. Obviously, it has good repeatability, stability and small error. The instrument can be stabilized for several minutes after being turned on. The sample can be measured for about six minutes.
4.Comparison of several different principles
5.Application of 3D ion flow oxygen analyzer
The series of 3D ion flow oxygen analyzer produced in China was put into market in 2004. In the past 10 years of market practice and use, it has achieved remarkable results. It has a certain market share in the air separation process analysis market, especially in the medical oxygen making industry, and is convinced that it will have a place in the "national standard". It is not only practical in laboratory, portable instrument can be very convenient to be used everywhere, especially in on line analysis can replace "magnetic oxygen".
Wenfeng Iron and Steel, Longhai Iron and Steel, Tangshan Iron and Steel, Shanghai Baosteel Group, Xinjiang Bayi Iron and Steel, Dayangritic Acid, Shanxi Jianbang Group, Shandong Laigang Tianyuan Gas, Henan Shenma Nylon Chemical, Shanxi Lanxing Chemical, Ningbo Linde Gas, Shougang Changzhi Iron and Steel, etc., all of which have used the ion flow oxygen detector, which broke the high content oxygen detection in the air separation process analysis system, has been dominated by the principle of magnetic oxygen, laid a solid foundation for the home products, and made the users of the world favor.
CI-PC84 series oxygen analyzer
Technical parameters:
measurement range: 10%～95%/99.99%，0～40% O2(Please check the nameplate description)
Sensor:New ion flow oxygen sensor
Accuracy: ≤±1%FS
Repeatability: ≤±0.5%FS
Stability:＜±0.5%FS/7d
Response time: T90＜15s
Sensor lifetime:Greater than 5 years (normal use)
Instrument life:Greater than 6 years (normal use)
Dimensions:See Figures 1 to 4
Instrument weight:2kg about 2 kg
Power supply:Power consumption less than 10VA
Ambient temperature: 0～45℃
Environmental humidity: ＜80%RH
Sample flow: 400～600ml/min
Sample pressure: 86～106kPa
nalog output free-set:4-20mA/0-20mA/0-1V/0-5V/0-10V/1-5V
Communications: RS485 (standard)/232 (optional)
Alert output: 2 sets of concentration alarm switch output
