Latest Oxygen Content Measuring Principle: 3D Ion Flow Oxygen Analyzer

It includes the copper-ammonia solution absorption method, fuel cell method, paramagnetic method, zirconia concentration potential method, and laser method. The principles, advantages and disadvantages are briefly described as follows:

Copper-ammonia solution is prepared by placing helical copper wire into a solution mixed with saturated ammonium chloride solution and ammonia water at a volume ratio of 1:1. When oxygen-containing gas sample is passed into the absorption bottle containing copper-ammonia solution, copper is oxidized by oxygen in the sample in the presence of ammonia, producing copper oxide (CuO) and cuprous oxide (Cu2O). The reaction equations are shown as follows:

The generated copper oxide and cuprous oxide react respectively with ammonia water and ammonium chloride to form soluble cupric salt Cu(NH3)2Cl2and cuprous salt Cu2(NH3)2Cl2.The cuprous salt absorbs oxygen and converts into the cupric salt, while the cupric salt is reduced back to the cuprous salt by copper. This cycle continues until the oxygen in the gas is completely consumed. The oxygen content (volume percentage concentration) in the gas can be determined according to the reduction in gas volume.

This is a classic method for oxygen content measurement, widely used for arbitration with low cost. It is still retained in many gas laboratories and testing institutions. However, it is generally only applicable to gas samples with oxygen content below 99.9%. Its disadvantages include the need to prepare solutions and wind copper wire, which is relatively cumbersome; the entire measurement process requires manual operation, making it unsuitable for online continuous analysis; and the presence of other oxidizing gases in the sample gas will interfere with the measurement results. Furthermore, since the entire absorption device consists of glassware, it is prone to breakage.

>> Fuel Cell Method

A fuel cell generally consists of an inert metal electrode (cathode), a lead (or graphite) electrode (anode), and an electrolyte (available in acidic and alkaline types). Each of the cathode and anode is connected to a metal sheet serving as an electrode lead. The electrolyte overflows through numerous round holes on the cathode and forms a thin electrolyte layer on the cathode surface. Covered atop electrolyte this thin layer is a gas-permeable polytetrafluoroethylene (PTFE) membrane. The gas sample passes through the permeable membrane and enters the cathode, where oxygen reacts with the electrolyte. The generated OH⁻ ions migrate to the anode under the action of an electric field and lose electrons at the anode to form water.Taking silver as the anode material as an example, the chemical reaction equation is as follows:

Silver cathode: O₂+2H₂O+4e→4OH-

Lead anode: 2Pb+ 4OH-→2 PbO+2H₂O+4e

Overall cell reaction: 2Pb + O₂→2 PbO

The current intensity generated by the migration of OH− is proportional to the oxygen content in the gas sample. The oxygen content of the sample can be determined by measuring the current generated in the fuel cell.

The advantages of this method are that fuel cells have a simple structure, compact size, and fast response time. Accordingly, oxygen analyzers based on this method are highly suitable for portable use and are relatively inexpensive. Nevertheless, fuel cells are consumable detectors, whose service life depends on the total cumulative amount of oxygen flowing through the sensor. The anode electrode is continuously consumed via chemical reactions during measurement; once depleted, the fuel cell fails and needs replacement. In particular, when measuring gas samples with oxygen content higher than 90%, the monthly drift can exceed 1%. Additionally, it should be noted that fuel cells with alkaline electrolytes are not suitable for analyzing oxygen content in acidic gases, while those with acidic electrolytes are not suitable for measuring alkaline gases.

>> Paramagnetic method (using a magnetic mechanical type as an example)

The paramagnetic method for oxygen measurement is based on the property that oxygen is a paramagnetic substance and its volume magnetic susceptibility reaches k=1062×10−6 (C.G.S.M) at 20 ℃. The volume magnetic susceptibility of other gases (except NO) is far lower than that of oxygen. Accordingly, the paramagnetic method has long been one of the most effective methods for oxygen content analysis.

Magnetic-mechanical oxygen analyzers are one of the representative instruments adopting the paramagnetic method for oxygen content analysis. Platinum wire is wound around the spheres to form an electrical feedback loop. The spheres are suspended in a magnetic field, with a small reflector mounted at their center. A light source built into the instrument emits a beam of light, which is reflected by the reflector and received by a photodetector made of a photosensitive component. When oxygen molecules are present around the dumbbell spheres, they migrate under the influence of the magnetic field and push the dumbbells to deflect. The higher the oxygen concentration, the larger the deflection angle. 

This deflection moves the reflector, causing the light path directed toward the photodetector to deflect as well. The photodetector captures the deflection and generates an electrical signal. After being amplified by an amplifier, the signal passes through the feedback circuit to form a closed loop, which drives the dumbbell back to its primary equilibrium position under the influence of the magnetic field. The current in this loop is directly proportional to the oxygen content. The oxygen content of the gas sample can be determined by measuring this current value.

At the same time, the electrode on this side becomes positively charged and acts as the positive pole or anode of the oxygen concentration cell. Oxygen ions migrate through the vacancies in the zirconia crystal lattice to the opposite side with lower oxygen content, where they lose electrons at the platinum electrode to form oxygen molecules, that is:

Meanwhile, this electrode is made negatively charged, serving as the negative electrode or cathode of the oxygen concentration cell.Accumulation of positive and negative charges on the two electrodes generates a certain electric potential. This potential is correlated with the oxygen content of the gas on both sides of the zirconia and conforms to the Nernst equation:

E: Oxygen concentration potential (mV);

R: Gas constant, 8.3145 J/mol·K;

T: Operating temperature of the zirconia probe expressed in absolute temperature (K) = 273.15 + t (°C);

n: Number of electrons participating in the reaction; for oxygen, n = 4;

F: Faraday constant, 96485.3365 (C/mol);

P0: Partial pressure of oxygen in the reference gas;

P1: Partial pressure of oxygen in the sample gas.

This equation is the basis for measuring oxygen content in gases using the zirconia concentration cell method. In actual measurement, the zirconia tube is heated to 600~1400 ℃. On the reference side of the zirconia tube, a gas with known and relatively high oxygen content such as air (P0​=20.6%) is introduced as the reference gas, while the other side is fed with the gas to be measured. By measuring the concentration cell potential E and the absolute temperature of the zirconia probe, the oxygen partial pressure (P1) of the gas to be measured can be calculated, and consequently the oxygen concentration of the sample gas is obtained.

The advantages of this method include high sensitivity, fast response time, a wide linear range, and good reproducibility and stability. Compared with analyzers in magnetic oxygen method, the oxygen analyzers in zirconia have a simpler internal structure; they are virtually unaffected by external environmental conditions such as temperature and vibration, and require almost no subsequent maintenance. However, it also has obvious drawbacks. Sinceelectrons can only migrate through the zirconia material at relatively high temperatures, the instrument must be equipped with a heating furnace to heat the zirconia tube, which results in a long warm-up time before the instrument can operate normally. Furthermore, when measuring oxygen concentration, the zirconia method is affected by reducing gases present in the sample gas, leading to lower measurement results. Therefore, it is not suitable for determining oxygen concentration in gases containing reducing substances or high levels of reducing gases. This is particularly important when measuring gas samples at the ppm level, where the impact of reducing gases on the measurement results must be carefully considered. In addition, when the oxygen concentration in the sample gas exceeds that of air (20.6%), it is necessary not only to use a gas with a higher concentration as the reference gas to ensure a positive potential difference but also to modify the zirconia detecting cell, which significantly increases the instrument's cost.

The laser oxygen measurement method is based on the property of oxygen molecules to absorb laser light of specific wavelengths. Inside the instrument, a laser diode generates a fixed-wavelength laser beam with known luminous intensity. The beam enters the measurement cell filled with the gas sample to be tested. After reflecting back and forth multiple times between two reflectors on two sides of the measurement cell, a part of the light is absorbed by oxygen contained in the gas sample. The remaining light beam is reflected to the collector and then captured.

According to Beer's law, the ratio of the intensity of the absorbed light beam to the original light intensity is proportional to the oxygen content in the gas sample:

In the equation

I0: Initial light intensity;

I: Remaining light intensity after absorption by oxygen in the gas sample;                  

S: Absorption constant of oxygen for laser at the specific wavelength;

L: Optical path length;

N: Number of oxygen molecules along the optical path, which is related to the oxygen content in the sample gas;

Therefore, the oxygen content in a gas sample can be determined by measuring the initial light intensity and the intensity of the light after absorption. Since the selected laser wavelength is specific, the measurement result is hardly affected by other gases. Furthermore, using the I/I0 ratio for calculations virtually eliminates the effects of variations in light source intensity, mirror reflectivity, and electric apparatus. At present, domestically manufactured instruments adopting this principle are relatively expensive, and their performance stability still needs further improvement.

The operating principle of the 3D ion flow oxygen sensor is shown in Figure 5.

The principle of the 3D ion flow oxygen sensor

Platinum electrodes are coated on both sides of stabilized zirconia. The cathode side is assembled with a cover provided with gas diffusion holes to form a cathode cavity. At a certain temperature, when a certain voltage is applied across the zirconia electrodes, oxygen molecules inside the cavity gain electrons at the cathode to form oxygen ions (O2-). The O2- migrate to the anode through oxygen vacancies in the zirconia, release electrons, and are converted into oxygen molecules that are discharged outward. This phenomenon is known as an electrochemical pump. In this way, oxygen in the cathode cavity is continuously pumped out of the cavity by the ZrO2 electrolyte, generating a current in the circuit. When the mole fraction of oxygen is constant, the current intensity increases with rising voltage. When the voltage exceeds a certain value, the current intensity reaches saturation; this is attributed to the limitation of oxygen diffusion into the cathode cavity through the tiny holes. This saturated current is defined as the ion current. The diffusion mechanism of gas through tiny holes determines the characteristics of the sensor. The diffusion of tiny holes generally involves two types of ionic current behavior: molecular diffusion and Knudsen diffusion. When the diameter of tiny holes is larger than the average diameter of the gas molecules, the ion current value IL in the molecular diffusion region is expressed as:

F—Faraday constant;

D—Diffusion coefficient of oxygen molecules in free space;

S —Cross-sectional area of the diffusion hole;

L—Length of the diffusion hole;

C—Mole fraction of oxygen around the sensor;

CT—Total mole fraction of all gas substances.

When C/CT＜1, it can be seen from Equation (1) that the ion current value is in direct proportion to the mole fraction of oxygen, and the ion current IL is:

As can be seen from Equation (2), there is an almost linear relationship between the ionic current and the molar fraction of oxygen. The molar fraction of oxygen in the gas being measured can be determined based on the magnitude of the output current.

A porous ceramic substrate is adopted as the diffusion layer to control the oxygen supply to the sensor cathode. The dense diffusion barrier layer with a porous layer structure using LSM is shown in Figure 6.

Figure 6. Porous-layer oxygen sensor

The ion current of this porous-layer oxygen sensor is consistent with Equation (2), and the ion current value is given by:

F—Faraday constant;

Deff—the effective diffusion coefficient of oxygen within the porous layer;

S—the cathode area;

L—the thickness of the porous layer substrate;

C—the molar fraction of oxygen surrounding the sensor.

As shown in Equation (3), the limiting current of a porous-layer oxygen sensor has a linear relationship with the oxygen mole fraction.

Voltage & Current Characteristics

The voltage and current characteristics of the sensor in gases with different oxygen concentrations are shown in Figure 7:

The relationship curve between the 3D ion current value and oxygen concentration is shown in Figure 8.

The Shanghai Institute of Measurement and Testing Technology once conducted a comparative test between the"3D ion flow oxygen analyzer" manufactured by"CHANGAI" and the"copper-ammonia solution absorption method."The instrument was first calibrated using oxygen in 24.1% helium. It was then used to test a gas sample provided by an enterprise, whose oxygen content had been determined as 97.78% by the copper-ammonia solution absorption method." The reading of the analyzer was 97.71%. Several repeated measurements were conducted over the following days, with indicated values ranging from 97.65% to 97.89%. Clearly, the instrument demonstrates good repeatability and stability, with minimal error. It generally stabilizes within several minutes after power-on, and each sample measurement takes approximately five minutes. To a certain extent, it can replace the copper-ammonia solution absorption method" for oxygen content determination.

Environmental conditions:

1. Ambient Temperature:23℃；

2. Relative Humidity:80%；

3. Data/Results of Test

Table 2. Comparison of different principles.

Conclusion

References