- 42 -
10. Disposal
When disposing of the detector, treat it as industrial waste in accordance with the
applicable laws and regulations.
11. Detection Principle
Galvanic cell sensor (oxygen)
The sensor consists of two electrodes, a membrane and an electrolyte. To accelerate
reactions of gas in the atmosphere on the electrodes, the membrane is designed to
adhere tightly to the electrodes.
The electrodes are two different metals, noble metal and base metal. The noble metal
electrode has contact with air via a Teflon membrane. Connecting load resistance to
both electrodes generates a potential difference, which speeds up the following
reactions:
Noble metal electrode:
O
2
+ 2H
2
O + 4e
4OH
Base metal electrode:
2Pb
2Pb
2+
+ 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.
Electrochemical sensor (hydrogen sulfide and carbon monoxide)
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. H
2
S, CO).
The electrolytic reaction of H
2
S is as follows:
Working electrode: H
2
S + 4H
2
O
H
2
SO
4
+8H
+
+ 8e
Counter electrode: 2O
2
+ 8H
+
+ 8e
4H
2
O
Catalytic sensor (combustible gas)
Catalytic combustion occurs on the catalytic layer applied on a platinum coil even if the
gas concentration is well below the lower combustion limit. 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 (LEL).
- 41 -
■
Explosion-proof Markings
1. Manufacturing number sticker
2. Explosion-proof sticker
Model is indicated here
3. Battery case manufacturing number sticker
(BP-4000llAL)
(BP-4000llMH)
5. Battery case warning sticker
(BP-4000llAL)
(BP-4000llMH)
4. CE marking sticker
XXXX
QAN Notified Body No.
