Riken Keiki GD-84D-EX Series, Operating Manual

Page: 183 / 194

12. Appendix 12-1 Detection principle
183 / 191
12
Appendix
12-1 Detection principle
12-1-1 Electrochemical type
<Sensor overview>
This sensor type detects gas concentrations as current generated when the detection target gas is
electrolyzed on electrodes maintained at constant potential. This is the most effective gas sensor for
detecting toxic gas. Gases can be selectively detected by selecting a specific potential.
<Sensor structure and principle>

Structure
The sensor consists of an electrode (working electrode) formed of a precious metal catalyst on a gas
permeable membrane, a reference electrode, and a counter electrode, all enclosed in a plastic
container filled with electrolyte solution.

Principle
A potentiostat circuit is used to maintain constant potential between the working electrode and reference
electrode. The detection target gas passes through the gas permeable membrane and undergoes
electrolysis reaction on reaching the working electrode. A side reaction also occurs at the counter
electrode so that the same amount of electric current flows through it as that produced at the working
electrode.
Example : In the case of hydrogen sulfide (H
2
S), the following chemical reaction occurs:
Working electrode: H
2
S + 4H
2
O → H
2
SO
4
+ 8H + + 8e - (e - is an electron)
Counter electrode: 2O
2
+ 8H + + 8e - → 4H
2
O
The current generated here is proportional to gas concentrations. Gas concentrations can be
determined by measuring the current flowing through the working electrode.
<Structure diagram>
Electrolyte
Reference
electrode
Catalyst
Working
electrode
Gas
permeable
membrane
Detection
target gas
Potentiostat circuit
Container
Counter
electrode