Advantages of 3D ion flow oxygen analyzer in high content detection

Advantages of 3D ion flow oxygen analyzer in high content detection

Yan Huai Zhi

(Shanghai Chang Ai Electronic Technology Co., Ltd.)

Summary: Over the years, high oxygen content parameters have become the basis of industrial gas/air controllers. At present, the main methods used to measure excess oxygen are copper ammonia solution absorption method, paramagnetic excess oxygen sensor, electrochemical oxygen sensor, zirconia (ZrO2) and so on. This paper introduces seven measuring principles of oxygen and the measurement of high content oxygen environment.

Key words: Copper-ammonia solution absorption method, zirconium oxide, ion current, high content oxygen, magnetic mechanical type.

The common oxygen measuring principles:

1.Copper-ammonia solution absorption method

The copper-ammonia solution is prepared by ammonium chloride, pure copper and ammonia water. When a certain amount of gas (oxygen) is contacted with the copper-ammonia solution, in the presence of ammonia water, the oxygen (O2) reacts with the copper (Cu) to generate copper oxide (CuO) and copper oxide (Cu2O), and the following chemical reactions occur:Copper oxide (CuO) and cuprous oxide (Cu2O) are produced by the action of ammonia water and ammonium chloride respectively, and soluble high-valence copper salt Cu(NH3)2Cl2 and low-valence copper salt Cu(NH3)2Cl are produced. The low-price copper salt absorbs oxygen and turns into high-price copper salt, the high-price copper salt is reduced by copper into low-price copper salt, and the low-price copper salt reacts with oxygen. The cycle action is carried out until the oxygen consumption in the gas is finished, and then the volume percentage concentration of the oxygen in the gas can be obtained according to the reduction of the volume of the gas. As long as enough pure copper exists in the whole process, the chemical reaction can continue.
2.Zirconium oxide concentration battery method

Porous platinum (Pt) electrodes are sintered on both sides of a zirconium oxide electrolyte (ZrO2 tube), and at a certain temperature, when the oxygen concentration on both sides of the electrolyte is different, oxygen molecules on the high concentration side (air) are adsorbed on the platinum electrode and are combined with electrons (4e) to form oxygen ions O2-, which causes the electrode to be positively charged, and the O2- ions are transferred to the Pt electrode on the low concentration side through oxygen ion vacancies in the electrolyte to release electrons, which are converted into oxygen molecules, so that the electrode is negatively charged. The reaction modes of the two electrodes are:

Reference Side：O₂+4e——2O₂-                      Measurement side：2O₂-－4e——O₂ 

Thus, a certain electromotive force is generated between the two electrodes, the zirconia electrolyte, the Pt electrode and the gas with different oxygen concentration on both sides constitute an oxygen probe, namely a so-called zirconia concentration battery. The electromotive force E between two stages is obtained from the Nernst formula:
In E=RT/nFln(P0/P1), E-concentration battery output; n—electron transfer number (4 in this formula); R-ideal gas constant, 8.314 W·S/mol; T—absolute temperature (K); F-Faraday constant, 96500 C; P1—the percentage of oxygen concentration of the gas to be measured; P0—Reference gas oxygen concentration percentage.

The formula is the basis of the oxygen measurement of the zirconium oxide concentration battery. When the temperature of the zirconium oxide tube is heated to 600-1400°C, the gas on the high concentration side is used as the reference gas with known oxygen concentration, such as air (P0=20.60%), the output electromotive force E of the concentration battery and the absolute temperature T of the measured gas are measured, and the oxygen partial pressure (concentration) P0 of the measured gas can be calculated, which is the basic principle of the zirconium oxide concentration battery.

3.Zirconia wide-area

The components of the broadband oxygen sensor have two parts: One is the induction chamber, the other is the pump oxygen.
The sensing chamber, one side of which is in contact with the atmosphere and the other side of which is the testing chamber, contacts with the exhaust through the diffusion hole, just like the ordinary zirconia oxygen sensor, because the oxygen content on both sides of the sensing chamber is different, a electromotive force Us is generated, the ordinary zirconia sensor takes the voltage as the input signal of the control unit to control the air-fuel ratio, but the wide-area oxygen sensor is different from this: Engine control unit to make the oxygen content of the two sides of the induction chamber consistent, keep the voltage value at 0.45V, this voltage is only the reference standard value of the computer, it needs another part of the sensor to complete.
The pump oxygen is connected to the test chamber on one side and the exhaust on the other. The pump oxygen is to use the reaction principle of the zirconia sensor to apply the voltage to the zirconia component (pump oxygen), which will cause the movement of oxygen ions, pump the oxygen in the exhaust gas into the test chamber, so that the voltage value of the two sides of the induction chamber is kept at 0.45V, the voltage applied on the pump oxygen is the signal of the oxygen content that we want. If the mixture is too thick, the oxygen content in the exhaust gas decreases, and the oxygen from the diffusion hole is more, and the voltage of the induction chamber rises. In order to achieve the balance, the engine control unit increases the control current to increase the pump oxygen efficiency and the oxygen content of the test chamber, so that the voltage of the induction chamber can be adjusted to 0.45V; On the contrary, when the mixture is too thin, the oxygen content in the exhaust gas increases. At this time, the oxygen will enter the test chamber from the diffusion hole, and the voltage of the induction chamber will be reduced. At this time, the pump oxygen will be discharged out to balance the oxygen content in the test chamber, so that the voltage of the induction chamber will be maintained at 0.45V. In short, the voltage added on the pump oxygen can ensure that when the oxygen in the test cavity is more, the oxygen in the cavity is discharged, then the control current of the engine control unit is positive; When the oxygen in the cavity is small, oxygen is supplied, and the control current of the engine control unit is negative. The current supplied to the pump oxygen in the above process reflects the excess air content factor in the exhaust gas.
4.Electrochemical
The electrochemical sensor is composed of a metal electrode + lead (or graphite) electrode + electrolyte, the contact metal sheet as electrode lead is respectively connected with the cathode and the anode, and the electrolyte overflow through a plurality of circular holes of the upper surface cathode to form a thin layer of electrolyte. The electrolyte layer is covered with a gas permeable polytetrafluoroethylene (PTFE) film. The sample gas enters the thin layer electrolyte through the permeable membrane and undergoes chemical reaction. For example, when silver is used as the metal electrode, the oxygen in the sample gas carries on the following electrochemical reaction on the electrode:

silver cathode：O₂+2H₂O+4e-→4OH-

lead anode：2Pb+4OH-→2PbO+2H₂O+4e-

battery synthesis reaction：O₂+2Pb→2PbO

The current generated by OH- ions is proportional to the oxygen concentration in the sample gas.

5.Magnetic mechanical type

Any matter can be induced to magnetization under the action of an external magnetic field. The magnetic susceptibility k and relative permeability μr of various materials are also different due to the different structure composition of the matter.
When μr>1,k>0, the matter or gas can be attracted by the magnetic field, called paramagnetic matter. Oxygen is a paramagnetic substance, and its volume susceptibility is k=106.2×10-6 at 20°C. When μr<1, k<0, the matter or gas is repulsive by the magnetic field, called diamagnetic matter. Nitrogen is an diamagnetic substance, and the bulk susceptibility k=-0.34×10-6 at 20°C. Only the magnetic susceptibility of O2 in the various gases is the largest, and the magnetic susceptibility of other gases is very small compared to the bulk magnetic susceptibility of oxygen (except NO). The volume magnetic susceptibility of the mixed gas is mainly determined by the volume magnetic susceptibility of the oxygen and its percentage content. The percentage content of the oxygen in the mixed gas can be obtained as long as the volume magnetic susceptibility k-mixing of the mixed gas can be measured.

The magnetic oxygen meter is based on the principle of the paramagnetism of oxygen and the maximum magnetic susceptibility to analyze the oxygen content in the mixed gas.
The magnetic mechanical sensor consists of a pair of quartz glass dumbbell balls filled with nitrogen, the dumbbell balls are surrounded by a platinum wire, forming an electric feedback loop, the dumbbell balls are suspended in a magnetic field, and a small reflector is arranged in the middle of the dumbbell balls. When there are oxygen molecules around the dumbbell, the molecules push the dumbbell sphere to deflect under the action of a magnetic field. The higher the oxygen concentration, the larger the deflection angle. A precision optical system composed of a light source, a reflector and a photosensitive element will measure this deflection and convert it into an electrical signal. After the signal is amplified by the amplifier, a current loop is formed through the feedback circuit, and under the action of the magnetic field, the dumbbell is forced to return to the original equilibrium position. The current value in this circuit is proportional to the oxygen concentration.
6.Laser

The principle of laser oxygen measurement is: An infrared laser on one side of the transmitter is emitted to a receiver on the opposite side. The measurement technique is based on the difference of the absorption of light by gas molecules. Most gases only absorb light of specific wavelength, and the absorption of light is a direct reflection of the gas content.
The laser wavelength can be obtained by scanning the selected absorption line, and the detected light intensity varies by the laser wavelength due to the absorption of specific gas molecules on the diode laser and the detector. To increase its sensitivity, wavelength modulation technique can be used: When the absorption line is scanned, the laser wavelength is slightly adjusted. The second harmonic signal is used to measure the concentration of the absorbing gas. Since the absorption lines of other gases do not exist at a specific wavelength, there is no direct interference from other gases. The concentration of the measured gas is proportional to the amplitude of the absorption line.
7.Zirconia ion flow

The working principle of the 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 the limit current. The diffusion mechanism of gas in small holes determines the properties of the sensor. There are two limits for small hole diffusion, namely molecular diffusion and Knudsen diffusion. When the pore diameter is larger than the average diameter of the gas molecule, the limiting 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 limit current value is proportional to the mole fraction of oxygen, the limit current value IL is:

From the formula (2), the limiting current and oxygen mole fraction are almost linear. The oxygen mole fraction in the measured gas can be determined according to the output current.
The porous ceramic substrate is used as a diffusion layer to control the oxygen supplied to the cathode of the sensor, and the structure of the porous layer type oxygen sensor is shown in Figure 2.

Figure 2 Porous layer oxygen sensor

The limiting current of the porous layer oxygen sensor is the same as that of the 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.

Measurement of high concentration oxygen

The above-mentioned principles of oxygen concentration measurement are not all used for high-content oxygen measurement. For example, the zirconia has a wide area, the oxygen concentration is about 80%, the sensor maximum current, if the oxygen concentration continues to rise will cause damage to the sensor, and this type of sensor needs to heat the temperature of the zirconia tube to 600-1400°C to accurately measure, has great limitations; The electrochemical sensor belongs to the fuel cell, the internal chemical reaction of the sensor is irreversible, the anode (lead or graphite) is continuously oxidized (become lead oxide or CO2) in the reaction, until the anode is exhausted, just like some fuel is oxidized and burned out, so the life of the electrochemical sensor is related to the concentration of the measured oxygen, the concentration is larger, the consumption of the anode is more, the life of the sensor is shorter and the monthly drift is about 1% when the concentration of the oxygen is higher than 90%.

Therefore, for the measurement of high concentration oxygen, usually use zirconium oxide ion flow, magnetic mechanical, copper ammonia solution absorption method and so on.
Magnetic mechanical oxygen measurement is a mature technology, its main advantages are:

• It is not affected by the variation of non-measured components in the mixed gas
• Rapid reaction
  
• Good stability
  

Main disadvantages:

The pretreatment of sample gas requires higher, pressure, dust, tar, vapor and so on can easily affect the measurement accuracy, even cause sensor damage

Vulnerable to working environment effects such as horizontal, vibration, environmental magnetic field.
In the experiment process, the copper ammonia solution absorption method can be used to change the consumption of copper wire, ambient temperature, ambient pressure and gas components.
The volume percentage of oxygen in the mixed gas measured by the copper-ammonia solution absorption method is independent of the temperature and pressure of the environment, corresponding to the same gas components, and the measured values in different atmospheric environments should be equal. However, when the gas contains other oxidizing gases, it will be more disturbed.
When the zirconium oxide ion flow is adopted to measure the concentration of the high content of oxygen, only the oxygen can be charged in the cathode of the solid electrolyte and pass through the solid electrolyte, and the limit current value is directly proportional to the mole fraction of oxygen, so the sensor has high measurement precision and wide measurement range (0-100%), is not affected by impurities, pressure and ambient temperature, has good stability and low power consumption.
At present, there are few high content oxygen analyzer based on zirconia ion flow type oxygen sensor at home and abroad, only 3-4 companies in the world, such as British Shi Fu Mei, Germany Bille and so on. Because of the high price of this kind of analyzer, it is difficult to be widely used in the field of high content oxygen measurement. Chang Ai Electronic Technology Co., Ltd. based on many years of development and design experience of gas analyzer, introduced a series of zirconia ion flow oxygen sensor based on CI2000-CY, GNL-2100L, SP-980L, GNL-6100 and other high-content oxygen analyzer, not only has the performance of the same kind of products abroad, but also solves the high price problem of this kind of analyzer, provides more choices for domestic and foreign users.

Technical parameters of Chang Ai high content oxygen analyzer:

Measurement range ：10.000～99.999%

Measure accuracy：±2%FS
Respond time：T₉₀≤20 S
Stability：<±1%FS/7d
Test environmental temperature：0～50℃
Test environmental humidity：<80％RH
Sample gas flow ：400～600ml/min
Sample gas pressure ：0.05MPa≤入口压力≤0.2 MPa

Application：

Air separation industry
The chemical and smelting industry
Detection of oxygen concentration in high temperature furnace
Detection of oxygen concentration in the protective gas of the semiconductor
The determination of oxygen concentration in the process of animal and plant culture, vegetable and food processing and storage
The measurement of oxygen concentration in vessels, underground command centers, tunnels, deep wells, civil air defense projects, and urban tunnels, etc

Reference:

Weng Xiao Ping. Improvement of Pretreatment System of Magnetic Mechanical Oxygen Analyzer [J], Baoshan Iron & Steel Co., Ltd. (Shanghai), 201900.
Zhang Hui and Liu Yingshu. Analysis of Factors Affecting Oxygen Determination by Copper-Ammonia Solution Absorption [J], Beijing University of Science and Technology, 2010.
Wu Qiang and Liu Zhong. Research on Extreme Current Oxygen Sensor [A], 49th Research Institute of China Electronics Technology Group.