Geotech GEM5000 User Manual

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Page 61 of 127
Operating Manual
OMGEM5KN – Iss. 5.1
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Copyright – Geotechnical Instruments (UK) Ltd
GEM5000
suspected to be causing false CO readings, then it is recommended that the external hydrogen sulphide filter is
used whilst obtaining the CO measurement.
The H2S filter only needs to be used when you are trying to remove of any possible cross gas effects H2S might
have on other gas channels. Do not use the filter on all boreholes.
When using the H2S filter you will need to increase the gas sample and clean air purge run-time, as using an H2S
filter decreases the response time of the analyser.
Note: There is an internal H2S filter incorporated in the chemical cell that removes the H2S; however this has
a limited life span.
The electrochemical cells used to measure H2S 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
CO (H
2
compensated)
100
0 / ~300*
0
0
< 1
CO
100
0 / ~300*
0
-20 to +5
< 60
H
2
s
< 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 the
Technical Support Team at Geotech on +44(0)1926 338111.
Cross-gas effects on methane, carbon dioxide and oxygen
Methane is measured using dual beam infrared absorption. Analysers 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 H2S filter as it can reduce higher order hydrocarbons. When
using the H2S filter you will need to increase the gas sample and clean air purge run-time, as using an H2S
filter increases the response time of the analyser.
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.

Geotech GEM5000 Specifications

General

Brand	Geotech
Model	GEM5000
Category	Measuring Instruments
Language	English