# Micro Gas sensor (Conductimetric gas sensors)

**URL:** <https://forumautomation.com/t/micro-gas-sensor-conductimetric-gas-sensors/5230>\
**Category:** Field Instrumentation\
**Created:** [November 17, 2018, 6:41pm UTC](https://forumautomation.com/t/micro-gas-sensor-conductimetric-gas-sensors/5230 "2018-11-17T18:41:23Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![sivaranjith](https://forumautomation.com/letter_avatar_proxy/v4/letter/s/ba9def/32.png) [@sivaranjith](https://forumautomation.com/u/sivaranjith)\
**Post date:** [November 17, 2018, 6:41pm UTC](https://forumautomation.com/t/micro-gas-sensor-conductimetric-gas-sensors/5230/1 "2018-11-17T18:41:29Z")

</div>

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:

 ![gas%20sensor](https://cdn.forumautomation.com/original/2X/1/1041339d3f164bb06dd7cf8d62499d443d2f4581.png)

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](https://cdn.forumautomation.com/original/2X/d/d1c17d6d08f435ec24f8ff262a68f8336b2ae778.png)

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

 ![gas%20plate](https://cdn.forumautomation.com/original/2X/1/128f702546eea0fb5112add9c3281f0dc584c33e.png)

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.