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15. Detection Principle
15-1. Electrochemical Sensor (CDS-7)
This sensor consists of three electrodes and an electrolyte, and the method adopted here is to
produce electrolytic oxidation with a potentiostat circuit while keeping the working electrode at a
constant potential against the reference electrode. Measuring the current generated here allows
determining the concentration of the gas (e.g. H2S, CO).
The electrolytic reaction of H2S is as follows:
Working electrode: H2S + 4H2O
H2SO4 +8H+ + 8e
Counter electrode: 2O2 + 8H+ + 8e
4H2O
15-2. Hot-wire Semiconductor Sensor (CHS-7-CH)
A small amount of metal oxide semiconductor is deposited on a platinum coil, then the platinum
coil is heated to a high temperature. When reducing (electron donating) gases react with the
surface of the metal oxide, electrons will be donated to the semiconductor in the course of the
reaction. Consequentially, the resistance of the semiconductor decreases as more charge carriers
(electrons) are available. The sensor element (semiconductor on the platinum coil) can be
understood as two resistances in parallel, being part of a bridge circuit. The resistance change of
the semiconductor is read as differential voltage using a bridge circuit. This type of sensor is very
sensitive and can detect combustible or toxic gases at a low ppm or even a ppb level.
15-3. Catalytic Sensor (CHS-7-CS)
Catalytic combustion occurs on the catalytic layer applied on a platinum coil even if the gas
concentration is well below the lower flammable limit (LFL). This causes a rise in temperature
of the platinum coil and increases its electrical resistance. This change is read as a differential
voltage using a bridge circuit. This process enables detection of combustible gases in air up to
the lower explosive limit (LFL).
15-4. Galvanic Cell Sensor for Oxygen detection (COS-7)
The sensor consists of two electrodes, a membrane and an electrolyte.
The electrodes are two different metals, noble metal (Pt, Ag) and base metal (Pb). The noble
metal electrode has contact with air via a Teflon membrane. Connecting load resistance to both
electrodes generates a potential difference, which promotes the following reactions:
Noble metal electrode:
O
2
+ 2H
2
O + 4e
4OH
Base metal electrode:
2Pb
2Pb2
+
+ 4e
As a result, the current proportional to the oxygen concentration in the air flows from the noble
metal electrode to the base metal electrode via the external circuit. Since the electromotive force
changes depending on the temperature, a thermistor is added to compensate for the ambient
temperature variations.
