Micro Gas sensor (Conductimetric gas sensors)


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Conductimetric gas sensors measure the concentration of an element in the gas, based on the principle of measuring a change in the electrical resistance of a material upon the introduction of the target gas.

For sensing the gas semiconducting metal oxides are used in the sensors.

Construction and working of Conductimetric gas sensors:

The device consists of a platinum heating coil wound with wire inside a ceramic former on which a thick layer of porous tin oxide is manually painted. The film is then sintered at a high temperature so that the appropriate nanocrystalline structure is formed. The electrical resistance of the sintered film is then measured by a pair of gold electrodes and a basic potential splitter circuit.

Tin oxide devices are operated at various high temperatures and are doped with different materials to improve their specificity. The response of a tin oxide sensor, in terms of its relative conductance Gs / G0, where Gs is the conductance of a gas of fixed concentration and G0 is the conductance in air.

The devices are operated at high temperatures (typically between 300 and 400 °C) for several reasons:

  • The chemical reaction is more specific at higher temperatures
  • The reaction kinetics are much faster, that is, the device responds in just a few seconds

In a simple physical description, the tin oxide behaves like an n-type semiconductor and, therefore, there is an increase in the electron carrier density n, and hence in the electrical conductivity cr, of the material with increased gas concentration where

gas%20sen

where Un is the electron mobility.

There are two basic configurations of a microhotplate; a solid diaphragm (1st pic) and a suspended
bridge (2nd pic) that contains a meandering resistive heater

The first comprises a resistive heater (e.g. platinum) embedded between layers that make up a solid diaphragm or a resistive heater (e.g. doped polysilicon) embedded between layers in a suspended microbridge configuration.