PN 903-8797, Rev A
Adjusting the Sample Gas Flow Rate
The gas sample ow rate needs to be properly adjusted for your application.
During each measurement cycle, the solenoid is closed. The main purge ow
through the solenoid is also shut o. During the Purging/Hold Stage, the
solenoid opens and the sample gas ows through the sample system and
Measuring Cell. This stage allows previously condensed liquids to evaporate,
while obtaining a new representative gas sample. During this stage, the gas
purge ow rate must be high enough (e.g.,1.0–5.0 Nl/minute or 2.1–10.6 SCFH)
to completely purge the sample system and Measuring Cell within the allotted
Purge time.
To set the ow rate, use the pressure regulator on the backpan (Figure 2-1). If
this adjustment has been done properly during installation, no further adjust-
ment should be necessary.
If you measure the ow rate at the Vent Line, you will nd that it is sub-
stantially higher during the Purging/Hold Stage because the Purging/
Hold Stage ow rate (1.0–5.0 Nl/minute (2.1–10.6 SCFH) is added to
the trickle purge ow rate (Bypass Valve is open).
Because high ow rates result in increased pressure drops through the
Membrane Filters, do not set the purge ow rate greater than 5.0 Nl/minute
(10.6SCFH). The higher pressure tends to force liquids through the mem-
branes. The purge ow rate setting is especially important if hydrocarbon
and/or water droplets are present in the gas stream. Hydrocarbons have low
surface tension and can be forced through Membrane Filters more easily than
high surface tension liquids such as glycol, alcohols, and water. Liquid carry-
over through the lter can result in measurement problems. It becomes dif-
cult – or even impossible – to detect condensing hydrocarbons when there is
already hydrocarbon liquid present.
The sample ow rate and the Purge time can be easily adjusted. Therefore, it is
relatively easy to obtain a reasonable balance between the two variables. The
rst thing to determine is whether to operate the analyzer with a bypass ow
(or “trickle purge”) during the measuring cycle. Although the solenoid shuts o
the main purge ow during the measure cycle, opening the Bypass Valve will
allow a small ow rate during measurement (approximately one-tenth of the
purge cycle ow). This bypass ow tends to improve accuracy and response
time because there is always a small, but constant ow of sample gas owing
through the sample system and Measuring Cell.
Installation and Start-Up | 2-37
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