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11.
Detection Principle
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 lower explosive
limit (LFL).
Thermal
conductivity sensor
A thermal conductivity sensor is based on the principle that some gases have a different thermal
conductivity from air.
When a gas comes in contact with a heated platinum coil coated with an inert substance (sensor
element), the gas will conduct the heat from the coil more or less efficiently than air. This results in a
change of the temperature of the sensor element, causing a change in the resistance of the platinum
coil. The resistance change is read as differential voltage using a bridge circuit. The differential voltage
is near proportional to the gas concentration.
This type of sensor is limited to detection of gases whose thermal conductivity is different from air but it
can detect gases in the range from 0 to 100 vol%.
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.
This oxygen sensor is effected by atmospheric pressure because of its principle. When the unit is turned
on in clean air at a standard atmospheric pressure (1,013hPa), the reading will be automatically
adjusted to 20.9vol%. The reading will change in accordance with the atmospheric pressure change,
even though the oxygen level does not change.
For example, if the unit is relocated to an elevation of 1,000m above sea level (900hPa), clean air, the
reading will change from 20.9vol% to 18.6vol%. If the unit is turned on, auto zeroing will start and the
reading will be adjusted to 20.9vol%.
To convert this figure to the one at standard atmospheric pressure (1,013hPa), multiply it by the
correction factor (900÷1013=0.89) to obtain the corrected oxygen level, 18.6vol% (20.9 x 0.89).
Atmos. pressure
Correction coefficient
O
2
level (vol%)
