What are the types of gas sensors?

First, semi-conductive gas sensor

This type of sensor accounts for about 60% of the gas sensors, which can be divided into conductive type and non-conductive type according to its mechanism, and the conductive type is further divided into surface type and volume control type.

(1) SnO2 semiconductor is a typical surface type gas sensor, and its sensing principle is that SnO2 is an N-type semiconductor material. When the voltage is applied, the temperature of the semiconductor material rises, and the adsorbed oxygen accepts the electrons in the semiconductor to form O2 or the presence of O2 primitive gases H2, CO and CH4, so that the surface resistance of the semiconductor decreases, the conductivity rises, and the conductivity change is proportional to the gas concentration. NiO is a P-type semiconductor, which is sensitive to O2 due to the decreasing conductance of oxidizing gas. ZnO semiconductor sensors also belong to this type.

a. Conductive sensor elements are divided into surface sensitive type and volume control type. Surface sensitive sensor materials are SnO2+Pd, ZnO ten Pt, AgO, V2O5, metal phthalocyanin, pt-SNO2. Surface sensitive gas sensor can detect all kinds of combustible gases CO, NO2, Freon. The gas sensor of the sensing material Pt-SNO2 can detect the combustible gases CO, H2 and CH4

b. Volume control sensor material for Fe2O8 and TiO2, Co-Mgo - SnO2 body sensor can detect a variety of combustible gases CO, NO2, freon, sensor material Pt - SnO2. Volume control type semiconductor gas sensor can detect liquefied petroleum gas, alcohol, air fuel ratio control, combustion furnace gas tail gas.

(2) The volume control type is the change of electrical conductivity caused by the change of lattice defect, which is proportional to the concentration of gas. Fe2O8 and TiO2 belong to this kind and are sensitive to combustible gas.

(3) hot wire sensor, is the use of thermal conductivity change of the semiconductor sensor, also known as hot wire semiconductor sensor, is in the Pt silk coil coated with SnO2 layer, Pt wire in addition to heating, and the function of detecting temperature change. When the voltage is applied, the semiconductor becomes hot and the surface absorbs oxygen, so that the concentration of free electrons decreases. When the combustible gas exists, due to the combustion consumption of oxygen free electron concentration increases, the thermal conductivity increases with the increase of free electron concentration, and the heat dissipation rate increases correspondingly, so that the temperature of Pt wire decreases and the resistance value decreases. The resistance value change of Pt wire is linear with the concentration of gas. This kind of sensor is small, stable, anti-poison, can detect low concentration of gas, and plays an important role in the detection of combustible gas.

(4) FET field effect transistor gas sensor, PD-FET, non-conductive type. The FET sensor uses Pd to absorb H and z and diffuses to reach the interface of semiconductor Si and Pd, reducing the work function of Pd, which is sensitive to H2 and CO. Non-conductance FET field effect transistor gas sensor is a promising gas sensor because of its small size, easy integration and multi-function.

Two, solid electrolyte gas sensor

This sensor element is ion to solid electrolyte diaphragm conduction, called electrochemical cell, divided into cationic conduction and anionic conduction, is a strong selectivity sensor, the zirconia solid electrolyte sensor has been studied more and reached practical application, its mechanism is to use the potential difference between two batteries on both sides of the diaphragm is equal to the concentration of the battery potential. Stable chromium oxide solid electrolyte sensors have been successfully applied to the determination of oxygen in steel water and the measurement of air-fuel ratio of engine. In order to make up for the deficiency of solid electrolyte conduction, in recent years, a layer of gas-sensitive film is plated on the solid electrolyte to correlate the number of gas molecules in the surrounding environment with the number of movable particles in the medium.

Three, contact combustion type gas sensor

Contact combustion sensor is suitable for the detection of flammable gas H2, CO, CH4. When the combustible gas contacts the surface catalysts Pt and Pd, the combustion and breaking heat are related to the gas concentration. This kind of sensor has a wide range of applications, small size, simple structure, good stability, the disadvantage is poor selectivity.

Iv. Electrochemical gas sensor

There are two kinds of electrochemical gas sensors commonly used:

(1) constant potential electrolytic sensor

The measured gas is ionized under a specific electric field, and the concentration of the gas is measured by the electrolytic current flowing through it. This kind of sensor is highly sensitive and can be selected to change the potential for the detection of pure gas, which plays an important role in the detection of toxic gas.

(2) galvanic gas sensor

In KOH electrolyte solution, Pt-Pb or Ag-Pb electrodes constitute batteries, which have been successfully used for the detection of O2. Its sensitivity is high, but its disadvantage is that water permeation dissipates moisture and electrical poisoning is easy.

Five, optical gas sensor

(1) Direct absorption gas sensor

Infrared gas sensor is a typical absorption optical gas sensor, according to the gas has their own inherent spectral absorption spectrum detection of gas composition, non-dispersive infrared absorption spectrum of SO2, CO, CO2, NO and other gases have a high sensitivity. In addition, UV absorption, non-dispersive UV absorption, correlation spectrophotometry, second derivative, self-modulated light absorption method has high sensitivity to NO, NO2, SO2, hydrocarbons (CH4) and other gases.

(2) photoreactive gas sensor

The photoreactive gas sensor uses the gas reaction to produce color change and cause optical characteristics change such as light intensity absorption. The sensor element is ideal, but the change of gas light sensitivity is limited, and the degree of freedom of the sensor is small.

(3) New sensors for gas optical properties

The optical fiber temperature sensor is of this type. A catalyst is coated on the top of the fiber to react with gas and generate heat. The temperature of the optical fiber changes. The use of optical fiber temperature measurement has reached the practical degree, the detection of gas is also successful. In addition, sensors that use changes in other physical quantities to measure gas composition are constantly being developed. For example, surface acoustic wave sensors also have high sensitivity for detecting SO2, NO2, H2S, NH3, H2 and other gases.