Dec 10, 2021 
Figure 1: A series of gas sensors.
The gas sensor is used to detect specific gases in a certain area and continuously measure the concentration of gases as well. In the areas of coal, petroleum, chemical, municipal, medical, transportation, family and other safety protection, gas sensors are often used to detect the concentration or presence of gases that are flammable, toxic, or their oxygen consumption. Gas sensors are often indispensable in safety systems thanks to the detection of gases around.
From perspectives of the working principle, characteristics, and measuring technology, as well as materials, manufacturing process, the test object, and the field of application, there could constitute different classification standards, deriving complexity of classification systems. It has been especially united on the issue of classification standards, so it is hard to carry on the strict system.
Types of Gas Sensors
Ways to obtain gases
Definition
Diffusion type sensor
The sensor directly contacts the measured gas through natural diffusion.
Inhalation gas sensor
The gas is introduced into the sensor detection element by using the getter pump and other devices. According to the dilute strength of the measured gas, there are full-inhalation type and dilution type.
Table 2: Definitions of diffusion type sensor and inhalation gas sensor.
Gas composition
Use
Single gas sensor
For specific gases.
Compound gas sensor
Simultaneous detection of various gas components.
Table 3: Single gas sensor and compound gas sensor as well as their uses.
Detection principle
Thermal sensor
Electro-chemical sensor
Magnetic gas sensor
Optical gas sensor
Semiconductor gas sensor
Gas chromatography sensor
Table 4: Six types of gas sensors divided by the detection principle.
In addition, gas sensors are also divided into portable gas sensors and fixed gas sensors according to the use methods.
Characteristics of Gas Sensors
Stability
The stability of the basic response of the sensor during the whole working time depends on the zero drift and interval drift. Zero drift refers to the change of output response of the sensor when there is no target gas. And interval drift refers to the response change when the sensor is continuously placed in the target gas, which is represented by the decrease of the output signal of the sensor. In ideal conditions, the occurrence rate of the zero drift is less than 10% per year under continuous operation of the sensor.
Sensitivity
Sensitivity is the ratio of the change in the output to the change in the measured input of the sensor. It depends on the technology used in the sensor’s structure. Most types of gas sensors are designed based on biochemical, electro-chemical, physical, and optical principles. The first thing is to select a sensitive technology that is sensitive enough to the valve limit or the detection of the percentage of the lowest explosion limit for the target gas.
Cross Sensitivity
This can be determined by measuring the sensor response generated by a particular concentration of interfering gas. And the response is equivalent to the sensor response generated by a given concentration of the target gas. This property is important in tracking multiple gases because. An ideal sensor should have high sensitivity and selectivity.
Corrosion Resistance
Corrosion resistance is the ability to be exposed to target gases with large volume. In case of severe gas leakage, the probe should be able to withstand 10~20 times the desired gas volume fraction. Sensor drift and zero correction should be as small as possible after returning to normal operating conditions. The basic characteristics of gas sensors are the sensitivity, selectivity and stability, which are mainly determined through sensors’ materials. The selection of appropriate materials and the development of new materials can optimize the sensitivity of gas sensors.
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