One Factor Affecting the Reading of the Gas Sensor: Gas Concentration

What factors affect the reading of the gas sensor? This seemingly common problem actually involves a great deal of underlying principles and covers a broad range of topics. When encountering inaccurate gas meter readings, many customers immediately suspect sensor failure. This is not surprising, as gas sensors are not perfect and can indeed experience problems. When faced with such issues, various factors must be checked systematically to identify and resolve the root cause. Avoid prematurely concluding that the sensor itself is faulty.
 
Many factors affect the reading of gas sensors, such as gas concentration, ambient temperature, and humidity, which can be roughly divided into the following categories:

Let's begin with Gas Concentration. There is no doubt that gas sensors are sensitive to the concentration of the gas being measured. The higher the gas concentration, the greater the change in the sensor's output signal. Why use the word "change" instead of "signal"? Because the output signal form of gas sensors varies, and the relationship between signal change and gas concentration also differs. How do the signals of different gas sensors change?

The signal form of a non-dispersive infrared sensor (NDIR) is a voltage output. When the measured gas is absent, the output voltage peak-to-peak value is at its maximum. As the measured gas concentration increases, the output voltage peak-to-peak value decreases accordingly. However, this change is not linearly related to gas concentration; it follows the Lambert-Beer law. When the concentration is low, sensitivity is high, and when the concentration is high, signal sensitivity decreases. The sensitivity at a specific concentration point is expressed by the mathematical formula: dV/dC. In this formula, V represents the peak-to-peak value of the signal, and C represents the gas concentration.
 
The resistance of the catalytic combustion sensor element (commonly referred to as LEL) changes with the concentration of combustible gas. When a detection element, a compensation element, and two fixed resistors form a Wheatstone bridge, the output of the catalytic combustion sensor is expressed as a voltage output. The voltage output changes linearly at low concentrations (<3% vol CH₄) and nonlinearly at higher concentrations. The change in voltage is measured in millivolts (mV).

The signal form of an electrochemical sensor (EC) is current. The output current increases with the rise in the concentration of the measured gas. For an electrochemical sensor of the potential-controlled type (also known as the fixed-point electrolysis method), the output current changes linearly with the concentration of the measured gas; whereas for a galvanic oxygen sensor, the output current follows an approximately linear logarithmic (Ln) curve.
 
The signal form of a photoionization sensor (PID) is current output. When the measured gas is absent, the output current is close to zero. As the concentration of the measured gas increases, the output current rises correspondingly. This change is not linearly related to the gas concentration. When the concentration is low, the sensitivity is high, and when the concentration is high, the sensitivity is low. The sensitivity at a specific concentration point is expressed by the mathematical formula: dI/dC. In this formula, I represents the current signal magnitude, and C denotes the gas concentration.
 
The signal form of a metal oxide sensor (MOS) is a change in resistance, similar to that of an LEL sensor. When the measured gas is absent, the PN junction resistance of the MOS sensor is at its highest. As the concentration of the measured gas increases, the PN junction resistance decreases correspondingly. This change is nonlinearly related to the gas concentration. When the gas concentration is low, the sensitivity is high, and when the concentration is high, the sensitivity is low. The sensitivity at a specific concentration point is expressed by the mathematical formula: dR/dC. In this formula, R represents the PN junction resistance, and C denotes the gas concentration.
 
The signal form of a non-dispersive infrared sensor (NDIR) is voltage output. When the measured gas is absent, the output voltage peak-to-peak value is at its maximum. As the concentration of the measured gas increases, the output voltage peak-to-peak value decreases accordingly. However, this change is not linearly related to the gas concentration; it follows the Lambert-Beer law. When the concentration is low, the sensitivity is high; when the concentration is high, the signal sensitivity decreases. The sensitivity at a specific concentration point is expressed by the mathematical formula: dV/dC. In this formula, V represents the peak-to-peak value of the signal, and C denotes the gas concentration.

The signal form of a photoionization sensor (PID) is current output. When the measured gas is absent, the output current is close to zero. As the concentration of the measured gas increases, the output current rises correspondingly. This change is not linearly related to the gas concentration. When the concentration is low, the sensitivity is high; when the concentration is high, the sensitivity is low. The sensitivity at a specific concentration point is expressed by the mathematical formula: dI/dC. In this formula, I represents the current signal magnitude, and C denotes the gas concentration.