Teledyne 8800A - Appendix H: Sample Gas Filter Considerations

8800 Series Trace Moisture Analyzer Instruction Manual Appendices
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Appendix H: Sample Gas Filter Considerations
Sample treatment with pre-filtering may be a necessity in many instances. However the filter
housing and element may affect the dewpoint reading and/or the response time of the system.
Below is a graph comparing measurements from the same calibrated sensor with and without a
filter utilizing a polycarbonate bowl. All other parameters, which may affect the measurement,
were kept constant. Outside of the sample system, a rather hot and humid environment was cre-
ated, to illustrate a worse case condition. As can be seen the filter does not cause problems for
dewpoints greater than -50°C. However the systems' ability to make precise measurements
quickly deteriorates for dewpoints below -55°C.
With this in mind the user should select the appropriate filter construction materials. It is impor-
tant to note that a stainless steel housing filter will not cause measurement errors, however if the
filter element inside the housing is made of hygroscopic fibers, then there will be a delay intro-
duced in the measurement.
Chilled mirror reference instrument MBW (˚C)
Tested Instrument (˚C)
-90
-85
-80
-75
-70
-65
-60
-55
-50
-45
-40
-35
-90 -85 -80 -75 -70 -65 -60 -55 -50 -45 -40 -35
Effect of polycarbonate filter on dewpoint measurement
at 30˚C temperature and ~100%RH. Test gas
flow rate 3LPM. Using XDTsn: 2007 and
XTR-100 sensor sn: 18107
· Data with legend "ALL SS" was taken with an
all stainless steel system.
· Data with legend "Through Filter" was taken with
same system but with the addition of a polycarbonate
bowl filter
Tests performed inside environmental chamber
MBW ˚C
ALL SS ˚C
Through Filter ˚C