GEM5000 Gas Analyzer
OMGEM5KN4.7.1
Page
49
of
70
8.9.2
Cross-gas effects on methane, carbon dioxide and oxygen
Methane is measured using dual beam infrared absorption. Analyzers are calibrated using
certified methane mixtures and will give correct readings provided there are no other
hydrocarbon gases present within the sample (e.g. ethane, propane, butane, etc.). If
there are other hydrocarbons present, the methane reading will be elevated (never
lower) than the actual methane concentration being monitored.
The extent to which the methane reading is affected depends upon the concentration of
the methane in the sample and the concentration of the other hydrocarbons. The effect
is totally non-linear and difficult to predict.
Note
:
The effect can be reduced by using an H
2
S filter as it can reduce higher
order hydrocarbons. When using the H
2
S filter you will need to increase the
gas sample and clean air purge run-time, as using an H
2
S filter increases the
response time of the analyzer.
Carbon dioxide is measured by infrared absorption at a wavelength specific to carbon
dioxide. Therefore, the carbon dioxide reading will not be affected by any other gases
usually found on landfill sites.
The oxygen sensor is a galvanic cell type and suffers virtually no influence from CO
2
, CO,
H
2
S, NO
2
, SO
2
or H
2
, unlike many other types of oxygen cell.
The infrared sensors will not be ‘poisoned’ by other hydrocarbons and will revert to
normal operation as soon as the gas sample has been ‘purged’.
H
2
S filter:
Instructions for use:
The following diagram shows how to modify the standard water trap and sample tubing
to fit the H
2
S filter.
Sample
Point
Sample
Tubing
Cut the
Sample
Tubing
Water
Trap
Insert
H
2
S
Filter
Gas Port
Connector
Standard Water Trap and Sample Tubing
Modified Water Trap and Sample Tubing with H
2
S Filter