Gas Sensors. How Stuff Works

Gas Sensors are devices which detects the presence of gases in an area. These are used for safety purposes to activate alarm systems as warning measure or to shut down a machinery. Gas detectors are used to detect different kinds of gases like combustible, flammable and toxic gases. These are also find application in the detection of oxygen depletion in an area. As the gas sensors measure a particular concentration of gas, the sensor responds to a calibrated gas and serves as a point or scale. As the level of gas increases above the sensor’s calibrated level, the alarm will be activated. Individual sensors are usually used to detect particular gases, but sensors capable of detecting various kinds gases are also available.

Gas sensors are classified into different types based on their mode of operation. These may be Catalytic type, Oxidation type, Semiconductor type and Infrared type.

Catalytic detector

Catalytic detectors use the basic oxidation principle to detect a combustible gas like LPG. When the gas oxidizes, heat generates and the gas sensor converts this heat in to corresponding electric signals. A standard Wheat stone arrangement is used for this. The output from the gas sensor is equal to the concentration of gas in the area. The heater inside the sensor has two heating elements. One is the active element and the other is the reference element. The active element is placed in the catalyst. When the combustible gas reacts exothermically with the oxygen, temperature rises due to catalytic action. The rise in temperature changes the resistance of the active element. The catalytic reaction takes place on the surface of the catalyst. The reference element is non- responsive to gas and gives the base line signal to compensate atmospheric temperature changes. This prevents false triggering of the alarm system.

Semiconductor gas detector

Its working is almost similar to the catalytic detectors but it uses a heated oxide on which the gas absorbs. The heating material is a thin film of metal oxide such as tin deposited on a silicon slice. When the catalytic oxidation takes place followed by the gas absorption, the electrical resistance of the oxide material changes. This change in resistance is equal to the concentration of gas.

Infrared Gas sensor

Many hydrocarbons absorb IR rays at the wavelength of 3.4 micrometers but water and carbon dioxide are relatively transparent in this region of IR spectrum. The principle of IR gas sensor is based on the absorption of infrared rays when passes through an amount of gas. The absorption of IR rays is equal to the volume of gas. The gas sensor has Sample and a Reference cells. Into the sample cell gas enters but the reference cell allows only air to enter. Two beams of IR rays are directed into the sample and reference cells. The amount of IR rays received by the sample cell and reference cell are compared to detect the presence of combustible gases. If a gas enters between the IR ray source and the detector, the level of IR rays falls in the sample cell.