The use of a completely sealed fuel pool oxygen sensor is currently one of the most advanced oxygen measurement methods in the world. The fuel cell oxygen sensor consists of a highly active oxygen electrode and a lead electrode, immersed in KOH solution. Oxygen is reduced to hydroxide ions at the cathode, while lead is oxidized at the anode.
The analysis methods of trace oxygen mainly include colorimetry, chemical battery method, yellow phosphorous luminescence method, concentration cell method and gas chromatography. Among them, the colorimetric method is an analysis method adopted earlier. It is a method stipulated by national standards. It uses copper ammonia solution for colorimetric analysis. Due to the complexity of the operation, the accuracy is difficult to guarantee, and automatic online analysis cannot be realized. Now it is rarely used. , But it is still an arbitration method. The yellow phosphorous luminescence method uses oxygen and yellow phosphorous oxidative combustion for analysis. It has the characteristics of fast analysis and continuous analysis. However, the yellow phosphorous used in this method is a hazardous chemical, and the product produced is corrosive and has a low detection limit. So it is rarely used now. Here we mainly introduce the chemical cell method, concentration cell method and gas chromatography. 1. Chemical battery law
The trace oxygen analyzer of the chemical battery method refers to the use of the principle of redox battery for trace oxygen analysis. Its sensor (detector) is a chemical cell, which is mainly composed of a cathode, an anode and an electrolyte. The above components are sealed in inert In the case, the oxygen in the measured gas enters the O2 near the cathode of the battery to obtain electrons. The anode is made of lead metal. The electrons are lost and are oxidized. Measure the oxygen content in the gas. The reaction formula is as follows:
O2+2H2O+4e-→40H- cathode
Pb+2OH-→pbo+H2O+2e anode
Total reaction formula 2pb+O2→2pbO
Due to different implementation methods, it can be divided into primary cell method, fuel cell method and Hertz cell method.
Trace oxygen analyzer (fuel cell electrochemical method)
The use of a completely sealed fuel pool oxygen sensor is one of the most advanced oxygen measurement methods in the world. The fuel cell oxygen sensor is composed of a highly active oxygen electrode and a lead electrode, immersed in a KOH solution. Oxygen is reduced to hydroxide ions at the cathode, while lead is oxidized at the anode.
The solution is separated from the outside by a polymer film, and the sample gas does not enter the sensor directly, so the solution and the lead electrode do not need to be cleaned or replaced regularly. The oxygen molecules in the sample gas diffuse through the polymer film into the oxygen electrode for electrochemical reaction. The current generated in the electrochemical reaction is determined by the number of oxygen molecules diffused to the oxygen electrode, and the diffusion rate of oxygen is proportional to the sample gas. Oxygen content, in this way, the size of the sensor output signal is only related to the oxygen content in the sample gas, and has nothing to do with the total amount of gas passing through the sensor. Through the connection of the external circuit, the charge transfer in the reaction, that is, the magnitude of the current, is proportional to the oxygen participating in the reaction. Using this method to measure oxygen can not be affected by the reducing gas in the measured gas, eliminating many sample gas processing systems. It measures oxygen faster than the old “Golden-Lead” galvanic cell, and does not require a long start-up blowing process. The sample gas of the “Golden-Lead” galvanic cell enters the solution directly, which leads to a large amount of maintenance for the instrument. The sample gas of the battery method does not enter the solution directly, and the sensor can work very stably and reliably for a long time. In fact, the fuel cell oxygen sensor is completely maintenance-free. However, during use, frequent calibration is required to ensure the accuracy of its test. The electrochemical oxygen sensor of fuel cell on the market is relatively stable with the sensor of British CITY.
How does the fuel cell oxygen sensor work?
The fuel cell oxygen sensor is an electrochemical sensor that generates a current signal output (uA). The main reason is that the signal from the cathode is proportional and linear to the oxygen partial pressure in the sample gas. The sensor has an inherently good zero, so there is no signal output without oxygen. Oxygen diffuses through the sensor film at the front end while contacting the anode and catholyte to activate the cathode or sensing electrode.
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What is the signal output of the fuel cell oxygen sensor?
Generally specified as the normal value ± 30% in air (20.9% oxygen) at 25°C and 1 atm. The normal value is based on the thickness or diffusivity of the front sensor membrane. ±30% allows the fluctuation of film (layering process) and heat sealing process. The above PCB network converts the signal output from current uA to mA. Component selection can narrow the mA signal output range. The signal output can be affected and compensated by many factors. Except for #15 below, higher or lower signal output has no performance advantage.
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Maximum load of fuel cell oxygen sensor
The sensor does not tolerate reverse current flow into the sensor. *Large load resistance is flat 10 kiloohms, load is not recommended. A linear error will occur when the load exceeds 10 kilohms.
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The influence of temperature change on electrochemical oxygen sensor
The ratio of temperature affecting signal output is 2.54% / °C. The (gradual) change of the ambient temperature can be maintained at ±2% by processing the signal output and adopting a suitable resistor-thermistor temperature compensation network. It takes 45-60 minutes for a step (rapid) change of 15°C to compensate the signal output to a balanced state. For example, an electronic thermistor reacts immediately to compensate for changes in the sensor (sensing membrane and electrolyte), and the sensor reacts at a slower rate. , Such as melting ice. OEM manufacturers are using electronic means to compensate for temperature. The problem of rapid temperature changes in the stable phase is eliminated.
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