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11. Glossary
Term Definition
O2 Oxygen
H2S
Hydrogen sulfide
COMB
Combustible gas
CO
Carbon monoxide
Zero adjustment
(zeroing)
Adjusting the zero point (or 21.0% for oxygen) in clean air.
Clean air: air free from target or interfering gases, and composed of
20.9-21.0vol% oxygen in dry conditions.
Gas atmosphere: Air containing target or interfering gases.
Span adjustment
Adjusting the indicated values by using span gas.
Target gas
Specific gas to be detected, concentration displayed, and used to
trigger alarms.
Detection range
A range of target gas concentrations that can be displayed and
trigger alarms.
Service range
A range of target gas concentrations the detector is able to indicate,
which are usually outside the Detection Range and used only as
reference.
Flameproof enclosure
(explosion-proof enclosure)
Enclosure in which the parts which can ignite an explosive
atmosphere are placed. This enclosure can withstand the pressure
created during an internal explosion of an explosive mixture, and
prevent the ignition of an explosive atmosphere outside the
enclosure.
Intrinsically safe (IS)
structure
Structure tested (e.g. spark test) to not become an ignition source in
a flammable atmosphere due to an electrical spark or hot surface
during normal operation and fault conditions.
Alarm set value
A gas concentration value that is set on a gas detector for alarm
activation.
Operating temperature and
humidity ranges
Ambient temperature and humidity ranges in which the gas
detection and alarm system can operate normally.
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10. Detection Principle
Galvanic cell sensor (Oxygen)
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
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/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. H2S,
CO).
The electrolytic reaction of H2S 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 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 LFL.