Oxygen sensor

2024-11-19 21:00:18
Oxygen sensor overview
All oxygen sensors are self-powered and have limited diffusion. The metal-air battery consists of an air cathode, anode and electrolyte.
The oxygen sensor is simply a sealed container (metal or plastic container), which contains two electrodes: the cathode is a piece of PTFE (polytetrafluoroethylene) coated with an active catalyst, and the anode is a lead block. This sealed container only has a capillary hole on the top, allowing oxygen to pass into the working electrode. The two electrodes are connected to the two pins protruding from the sensor surface through the current collector, and the sensor is connected to the applied device through the two antennae. The sensor is filled with electrolyte solution, so that different kinds of ions can be exchanged between the electrodes (see Figure 1).

Figure 1-Schematic of oxygen sensor.
The flow rate of oxygen into the sensor depends on the size of the capillary pores on the top of the sensor. When oxygen reaches the working electrode, it is immediately reduced to release hydroxide ions:
O2 + 2H2O + 4e- "type =" #_ x0000_t75 "> 4OH-
These hydroxide ions reach the anode (lead) through the electrolyte and undergo an oxidation reaction with lead to produce the corresponding metal oxide.
2Pb + 4OH- "type =" #_ x0000_t75 "> 2PbO + 2H2O + 4e-
The above two reactions generate a current, and the current depends on the oxygen reaction rate (Faraday's law). A known resistance can be connected to measure the resulting potential difference, so that the oxygen concentration can be accurately measured. In the electrochemical reaction, the lead electrode participates in the oxidation reaction, making these sensors have a certain life span. Once all the available lead is completely oxidized, the sensor will stop functioning. The service life of an oxygen sensor is usually 1-2 years, but the service life of the sensor can also be extended by increasing the anode lead content or limiting the amount of oxygen contacting the anode.
Capillary micropore oxygen sensor and partial pressure oxygen sensor
The oxygen sensor produced by City Technology is divided into two types according to the diffusion of oxygen into the sensor. One is a capillary hole on the top of the sensor, and the other is a solid film to allow gas to pass through. The pore sensor measures oxygen concentration, while the solid film sensor measures oxygen partial pressure.
The current generated by the fine-pore sensor reflects the volume percentage concentration of the measured oxygen, regardless of the total gas pressure. But when the oxygen pressure changes instantaneously, the sensor will generate an instantaneous current, and problems will occur if it is not properly controlled. The same problem occurs when the sensor is subjected to repeated pressure pulses, for example, the gas entering the sensor is pumped. The explanation for this phenomenon is as follows:
Pressure transient
When the fine-pore oxygen sensor encounters a sharp pressure increase or decrease, the gas will be forced through the fine-pore grid (large flow). The increase (or decrease) of the gas produces a transient current signal. Once the situation has stabilized and there are no more pressure pulses, the transient is over. Such transients can be alerted by the instrument, so CityTech can work hard to find a solution to reduce the impact of pressure.
All urban technology's fine-pore oxygen sensors use anti-high flow mechanisms, see Figure 2. Basically, a PTFE anti-high-flow membrane can be added to reduce the transient effects of pressure changes. This layer of film is tightly fixed to the fine hole with a metal or plastic cover. This design can greatly reduce the impact of instantaneous signal changes.

Figure 2-Bulk Flow Membrane on Capillary Sensor
However, the transient force generated by some pressure changes exceeds the allowable range of this design, especially the equipment that uses extractive instruments to deliver gas to the sensor. The gas produced by some pumps causes a continuous pressure pulse on the CiTiceL oxygen sensor, artificially enhancing the signal. In this case, it is necessary to design a gas expansion chamber outside the sensor to reduce the pressure pulse to the sensor.
Partial pressure type oxygen sensor
Capillary pores to control gas diffusion are not the only way to control oxygen entering the sensor. We can also use a very thin plastic film to cover the top of the sensor to disperse oxygen molecules before entering the sensor (Figure 3).

Figure 3-Solid Membrane (partial pressure) oxygen sensor
The flow rate of oxygen into the working electrode is determined by the partial pressure of oxygen through the membrane. This means that the output signal of the sensor is proportional to the partial pressure of oxygen in the mixed gas. A change in atmospheric pressure will cause a corresponding change in the sensor output current. If you use extractive gas delivery, you must ensure that the pulse force does not affect the sensor during the design stage of the device.
City Technology produces two types of partial pressure oxygen sensors, AO2 / AO3 (automotive) and MOX (medical), which are solid film type, the response relationship is linear, and the range is 0-100%.
Linear relationship
The signal from the fine pore oxygen sensor is non-linear and has the following relationship with the oxygen concentration (c):
Signal = constant * ln [1 / (1-C)]
In fact, the output of the sensor rises linearly, and the deviation does not occur until the oxygen concentration exceeds 30%, which makes measurement difficult. The linear output of the partial pressure sensor can reach 100% oxygen (or 1.0% oxygen concentration).
temperature
Fine-pore and thin-film oxygen sensors are sensitive to changes in temperature, but they have different degrees of sensitivity.
The effect of temperature on the fine-pore oxygen sensor is relatively small. Usually the temperature from + 20 ° C to –20 ° C will cause a 10% loss of the output signal. In contrast, the temperature has a much greater effect on the thin film oxygen sensor. Gas diffusion through the thin film is an active process. Usually a temperature change of 10 ° C will cause the sensor signal output to double. Thin film oxygen sensors require relatively stable temperature, so many CiTiceLs? Products have built-in thermistors.
Active reserve
When designing any electrochemical sensor, the grid (film or pores) should be used to limit the gas passing rate, while the other stages are significantly faster. Therefore, in order to ensure the speed of the electrochemical reaction, electrode materials with high catalytic activity must be used.
All CiTiceLs? Products use highly active electrodes to make the sensor have high activity reserves, ensuring the long-term stability and low drift of the sensor.
typical application:
Gas-Oxygen Application-Industrial Safety Choice-0-30%;
series
Gas type
Related downloads
Oxygen sensor.pdf
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