Interference From Other Gases Or Liquids - BT CGA-411 Operation Manual

5.

INTERFERENCE FROM OTHER GASES OR LIQUIDS

Methane detection uses a catalytic combustion filament and a thermal conductivity
sensor, both calibrated with methane. The filament is used up to about the lower flammable limit
(5.0% by volume) of methane and the thermal conductivity sensor from about 5% to 100% of
methane.
Carbon monoxide (CO) and hydrogen sulfide (H
electrode, electrochemical cell. Oxygen detection is carried out electrochemically with a two-
electrode, diffusion membrane electrochemical cell.
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. Also, 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, oxides of nitrogen, alcohols, and unsaturated
hydrocarbons.
Interference in CO Measurements
If the ambient concentration of oxidizable substances is relatively high, it is likely that
CO measurements will be affected. Many of these substances are removed by the flue gas filter
attached to the telescoping metal probe. 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.
CAUTION: DO NOT attach the flue gas filter to the standard probe. It is designed to be
used only with the CO probe (flue probe).
Catalyst Poisons
WARNING: Both the methane sensor and the CO sensor 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.
exposure to atmospheres that adversely affect the sensors is suspected, the
detector should be recalibrated promptly.
S) detection is carried out with a three-
2
12
If