QED LANDTEC GEM5000 - Cross Gas Effects on Chemical Cells; How to Use an H S Filter (Optional)

GEM5000 Gas Analyzer
Page 49 of 102
8.10 How to use an H
2
S filter (optional)
8.10.1 Cross gas effects on chemical cells
Measurements of CO are important in landfill management. The GEM5000 analyzer
incorporates an improved CO measurement.
Measurements of CO can be affected by two other gases that can be found in landfill gas
– hydrogen and hydrogen sulphide.
To reduce the effect of hydrogen, the GEM5000 analyzer uses a technique that is
hydrogen compensated. Hydrogen compensation is achievable up to a level of around
2000ppm. Above this level the CO reading will not be compensated for.
In order to assist the operator the GEM5000 instrument also indicates the level of
hydrogen present as low, medium or high. If a high hydrogen reading is present then the
CO reading may be affected.
The effect of hydrogen sulphide is eliminated by the use of a H
2
S filter.
CO measurement
The CO measurement is sensitive to hydrogen sulphide. The presence of hydrogen
sulphide can cause the CO reading to elevate (not to be the true value due to the
interfering gas). If the presence of hydrogen sulphide is suspected to be causing false CO
readings, then it is recommended that the external hydrogen sulphide filter is used while
obtaining the CO measurement.
The H
2
S filter only needs to be used when you are trying to remove of any possible cross
gas effects H
2
S might have on other gas channels. Do not use the filter on all boreholes.
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 decreases the response time of the analyzer.
 Note: There is an internal H
2
S filter incorporated in the chemical cell that removes
the H
2
S; however this has a limited life span.
The electrochemical cells used to measure H
2
S and CO do suffer from cross-gas effects.
Such effects are not accurately specified. However, the following table may be useful as
a guide; it represents how many ppm would be read by a cell if 100 ppm of the
interfering gas were applied, with no other cross-contaminates being present in the
sample.
GEM5000
Interfering Gas
CO
H
2
S
SO
2
NO
2
H
2
Chemical
Cell
CO
(H2 compensated
100
0 / ~300*
0
0
<1
CO
100
<4 /~300*
0
-20 to +5
<60
H2S
<4
100
20
<-25
<0.2
*after internal filter depleted.
 Note: Other gases could cause cross-gas effects. If you suspect a cross sensitivity
problem please contact Technical Support (800) 968-2026 or email
landtec_support@qedenv.com