Riken Keiki GD-3EC, Technical Manual

Page: 163 / 177

11. Appendix 11-1 Detection principle
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11-1-3 Semiconductor type
<Sensor overview>
This method detects gas concentrations from variations in resistance that occur when a metal oxide
semiconductor comes into contact with the detection target gas. This general-purpose gas detector sensor is
ideal for detecting a wide range of gases, from toxic gases to combustible gases.
<Sensor structure and principle>

Structure
The sensor consists of a metal oxide semiconductor (SnO
2
) formed around a heater coil and alumina tube.
The alumina tube has two gold electrodes at each end to measure semiconductor resistance.

Principle
Oxygen from the atmosphere is adsorbed as O
-
and O
2-
to the surface of the metal oxide semiconductor
heated to between 350 °C and 400 °C by the heater coil. The semiconductor maintains a constant
resistance. If a methane or other such gas comes into contact with this surface and becomes chemically
adsorbed, the oxygen is oxidized and released by the adsorbed O
2-
ions.
The following chemical reaction occurs here at the sensor surface:
CH
4
+ 4O
2-
→ CO
2
＋ 2H
2
O ＋ 8e
-
In other words, as methane gas is adsorbed to the sensor surface removing adsorbed oxygen, the increase
in free electrons inside the sensor reduces the resistance. This variation in resistance can be measured and
used to calculate the gas concentration.
<Sensor element diagram> <Drive circuit>
Heater coil
Gold electrodes
Metal oxide
semiconductor
Lead wires
Alumina tube
Zero calibrator
Indicator
Signal amplifier
Span adjuster
Detecting
element
DC power
source
Heater
Voltage
regulator
H
2
O
H
2
H
2
+ O 2- → H
2
O + 2e -
Load resistance