9
WARNING: The methane and CO sensors use catalytically active surfaces which may be
poisoned by air contaminants. These sensors should not be exposed to atmospheres
that contain silicones, halogens and halides, such as chlorides, and volatile
compounds containing lead or antimony. If exposure to atmospheres that adversely
affect the sensors is suspected, the detector should be recalibrated promptly.
5.
INTERFERENCE FROM OTHER GASES OR LIQUIDS
The methane detectors use two sensors:
i)
A catalytic combustion filament calibrated with methane. It is used up to about the
lower flammable limit (5.0% by volume) of methane.
ii)
A thermal conductivity sensor calibrated with methane. It is used from the lower
flammable limit (5.0 vol %) to 100 vol % of methane.
Models VGC-301 and VGC-311 (methane and carbon monoxide detectors) use the sensors
listed above and:
iii)
A three-electrode, electrochemical cell for carbon monoxide.
Models VGO-321 and VGO-322 (methane and oxygen detectors) use the methane sensors
listed above and:
iv)
A two-electrode, electrochemical cell for oxygen.
Gases, or liquids with appreciable vapor pressure, which may interfere with the detection
of methane include substances which can be combusted on the catalytic combustion filament
(examples are ethane, propane, ethylene, propylene, octane, and the like) and substances which
differ in thermal conductivity from air (examples are hydrogen, helium, carbon dioxide, other
hydrocarbons).
Gases or vapors which may interfere with carbon monoxide detection include substances
which can be electrochemically oxidized or reduced on the working electrode of the
electrochemical sensor. Examples are hydrogen, hydrogen sulfide, oxides of nitrogen, alcohols,
and unsaturated hydrocarbons.
Many of the substances that interfere with CO are removed by the flue gas filter (Part No.
FF-005). This filter can be used for both flue gas measurements and for ambient air measurements
of CO whenever the ambient concentration of other oxidizable substances is significant.
