Semiconductor gas sensors.
Semiconductor gas sensors
Gas sensors made of semiconductor materials are no less popular — semiconductor gas sensors. The operation of this type of sensor is similar to that of catalytic sensors: it is based on the properties of gas absorption by the surface of a heated oxide. This is a thin film of metal oxide on a silicon wafer, for the placement of which the same processes are used as in the manufacture of computer chips. The absorption of a simple gas by the surface of the oxide of the sample gas, followed by catalytic oxidation, leads to a change in the electrical resistance of the oxide material and can be correlated with the concentration of the sample gas. Semiconductor gas sensors are heated on the surface to a constant temperature above 200 — 250 ° C to accelerate the reaction rate and reduce the effects caused by changes in ambient temperature. Semiconductor gas sensors are characterized by simplicity and a sufficient degree of reliability, and can have a high degree of sensitivity. They are widely used in the production of inexpensive household gas detectors. However, in industry they are rather unreliable, since they are not accurate enough in determining individual gases, they are also affected by changes in atmospheric temperature and humidity. They may need to be checked more frequently than other types of sensors because they are known to lose sensitivity if not monitored regularly. They are also slow to respond and recover from exposure to a gas release.
Catalytic Gas Sensors
The most commonly used device today is the catalytic gas sensor– is essentially a modern development of the earlier explosion-proof lamp, since it is based on the principle of gas combustion and its conversion to carbon dioxide and water. Catalytic gas sensors for detecting flammable gases are of the electrocatalytic type. They consist of a miniature sensing element, sometimes also called a ball, a pellistor, or a sigistor. The latter two are registered trademarks of serial devices. They are made of an electrically heated coil of platinum wire, onto which a ceramic substrate, such as aluminum oxide, is first applied, and then a covering outer shell of palladium or rhodium catalyst sputtered onto a thorium oxide substrate. Operating principle of a gas sensorThe principle of this type of sensor is that when the combustible gas/air mixture passes over the surface of the catalyst, combustion occurs and the heat generated increases the temperature of the bead. This in turn causes a change in the resistance of the platinum coil, which can be measured by using the coil as a temperature sensor in a standard temperature bridge circuit. The change in resistance is directly related to the concentration of the gas in the surrounding environment and can be displayed on a measuring instrument or indicator. To ensure temperature stability in changing environments, catalytic gas sensors use thermally matched beads. They are located at opposite ends of the resistance bridge circuit, where the 'sensitive' sensor responds to any combustible gas present, as opposed to the balanced passive, or insensitive, sensor. Passive operation is achieved either by coating the bead with a thin layer of glass or by deactivating the catalyst, so it merely acts as a compensator for any external changes in temperature or humidity. The catalytic gas sensor is best mounted in a robust metal housing behind a flame arrester — this principle meets the safety requirements of the design. This allows the gas/air mixture to penetrate the housing and the highly active sensing element, but prevents the propagation of flame into the surrounding environment. The flame arrester slightly reduces the response speed of the sensor, but in most cases an electrical output reading appears within a few seconds of gas detection. Since the response curve flattens out considerably as it approaches the final reading, the response time is often defined as the time required to reach 90% of its final reading and is therefore known as the T90 value. The T90 value for catalytic sensors is typically 20 — 30 seconds.
Infrared Gas Detectors
The absorption bands of many combustible gases are in the infrared range of the electromagnetic spectrum of light, and the principle of infrared absorption has been used as a laboratory analytical tool for many years. Infrared Gas Detectorrefers to a low-power and compact type of equipment used for industrial gas detectors. These sensors have several advantages over the catalytic type. They are characterized by a very high response speed — less than 10 seconds, low operating costs and simplified monitoring due to the self-diagnosis function of modern microprocessor-controlled equipment. They can also be insensitive to any known «poisons», fail-safe and will operate successfully in inert atmospheres and over a wide range of temperatures, pressures and humidity of the environment. The operating principle of a gas sensor of this type is based on the absorption of two wavelengths in the infrared range, when light passes through a mixture of a sample with two wavelengths, one of which is set to the absorption peak of the gas to be detected, and the other is not. The two light sources pulse alternatively to each other, directing the light along a conventional optical path so that it exits through an explosion-proof «window» and then passes through the sample gas. The beams are then reflected by a retroreflector and return through the sample to the instrument. The detector compares the signal strength of the sample and reference beams and determines the gas concentration by subtraction.