As we all know, the data measured by sensors are generally quite accurate. However, the reading accuracy of gas sensors is relatively susceptible to many factors. So, what factors can lead to inaccurate readings of gas sensors? Below, Instrument Control Expert has sorted out the 5 major factors affecting the reading accuracy of gas sensors in detail, hoping to be helpful to you.
Gas sensors are used to measure gas concentration. When a gas is compressed, its relative concentration does not increase, but its absolute concentration will rise. That is to say, the number of molecules of the measured gas contained in a unit volume of space increases. Therefore, under the condition that the relative concentration remains unchanged, as the gas pressure increases, the sensor reading will also rise accordingly.
Then, how to eliminate the influence caused by ambient pressure?First of all, the test fixture should be well-designed. The gas measured by the sensor should not blow vertically towards the top surface of the sensor, but flow parallel to the top surface of the sensor.
Secondly, there should be a pressure relief gap when the gas flows through, and it is best not to seal the fixture. This can ensure that the pressure inside and outside the gas chamber is almost the same, thereby eliminating the influence of external pressure on the test results.
Most gas sensors need to be aged before use, especially electrochemical sensors. An appropriate aging time can make the sensor have a stable output.
During transportation or storage of electrochemical sensors, impurities are likely to be adsorbed on the surface of the electrodes, resulting in phenomena such as high zero point or fluctuating output values when the sensor is just powered on. If the gas test is carried out at this time, the obtained test results will have obvious errors. Therefore, proper aging of the sensor can achieve stable output, thereby reducing its impact on the sensor reading.
The vast majority of gas sensors are very sensitive to ambient temperature and humidity. For example, when the humidity changes significantly (such as moving from a dry air-conditioned environment to an outdoor humid air environment), the water vapor in the air will displace oxygen, which may cause the oxygen reading to drop by as much as 0.5%.
Next, let’s take a look at the specific impacts of ambient temperature and humidity on gas sensors:
The zero point of an electrochemical sensor refers to its signal output in clean air. The zero signal is mostly caused by interference gases in the air or impurities in the sensor electrodes themselves, which trigger chemical reactions and release signals.
As we all know, chemical reactions are significantly affected by temperature. As the temperature rises, chemical reactions become more intense, which leads to a higher zero point, and vice versa.
The impact of ambient temperature and humidity on the sensitivity of gas sensors is similar to the above phenomenon, that is, the higher the temperature in a short period of time, the more intense the chemical reaction, which is likely to affect the accuracy of the sensor output signal during the detection process.
If the sensor is in a high-temperature and low-humidity environment for a long time, the electrolyte is prone to volatilization and drying up, which will restrict the transmission of electrons, increase the internal resistance, slow down the reaction speed, reduce the sensitivity, manifest as the attenuation of sensor sensitivity, and directly affect the service life of the sensor.
Electrochemical sensors have a relatively narrow temperature range, generally from -20℃ to 55℃. The humidity range suitable for long-term use is 15%RH - 90%RH, and the most favorable humidity is 60%RH at 20℃.
To reduce the impact caused by temperature, temperature and humidity compensation is the most direct and effective solution.
For electrochemical sensors, their output current changes linearly with the concentration of the measured gas. Once the concentration of the measured gas changes, the sensor output signal will also change accordingly. For example: when using a benzene sensor to test benzene standard gas, it is often encountered that the signal of benzene gas cannot be detected or the signal is very weak. This is because benzene gas has a higher density than air, so it is easy for benzene gas to sink in the cylinder, resulting in no signal output or very weak output signal during sensor detection.
In addition, gas adsorption will also lead to a decrease in gas concentration. For example, for highly adsorptive gases such as CL₂, SO₂, NH₃, NO₂, HCL, and HF, you may observe that the sensor test output signal is relatively low within a few minutes after opening the valve. Therefore, gas adsorption is also a common factor affecting sensor readings. It is recommended to use polytetrafluoroethylene (PTFE) pipelines for the gas path to reduce gas adsorption. Moreover, at the initial stage of the test, gas should be pre-purged for 5 minutes to exhaust the air in the gas path.
According to statistics, there are more than 20 types of electrochemical sensors. The vast majority of toxic gases are reducing gases. However, the oxidation of reducing gases requires the participation of oxygen, including CO, H₂S, SO₂, NH₃, PH₃, etc. When testing such gases, insufficient oxygen supply will easily affect the sensor signal output, and common phenomena include low sensitivity or the output signal first rising and then falling. Therefore, when using electrochemical sensors to test reducing gases, air should be used as the balance gas as much as possible.
