Riken Keiki GD-3EC, Technical Manual

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11. Appendix 11-1 Detection principle
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11-1-4 Hot-wire semiconductor type
<Sensor overview>
This sensor type detects gas concentrations based on variations in the resistance of a platinum coil integrated
with a metal oxide semiconductor whose resistance changes when it comes into contact with a gas. The high-
sensitivity low-concentration detection gas sensor is configured simply as a detecting element of metal oxide
semiconductor sintered around a platinum coil.
<Sensor structure and principle>

Structure
The sensor consists simply of a detecting element formed of a metal oxide semiconductor sintered around a
platinum coil.

Principle
The resistance (R) of the detecting element is the combined resistance of the semiconductor resistance (RS)
and the platinum coil resistance (RH).
The detecting element is heated to between 300 °C and 400 °C by the platinum coil and maintains constant
resistance. If methane or another such gas comes into contact with the detecting element, oxygen adsorbed
to the surface of the metal oxide semiconductor is released, increasing the number of free electrons inside
the semiconductor and reducing the resistance of the semiconductor. This reduces the resistance of the
entire detecting element. These variations in resistance are measured as a voltage and used to calculate gas
concentrations.
<Detecting element diagram> <Drive circuit>
Constant
current
Detecting element
Semiconductor material
Resistance changes
Sensor output
Indicator
0.5 mm - 1.4 mm
Platinum coil
0.8 mm - 1.4 mm
Metal oxide
semiconductor