Plasma Gas Analyzer
66 02001001A_9
2 TRACE NITROGEN IN HELIUM VERSION
2.1 User Manual
This User Manual applies to both versions of SERVOPRO PLASMA analyzers, i.e. Argon or
Helium. Since the most popular version is Argon, the manual refers to Argon as background
gas. When Helium is the background gas, replace the word Argon by Helium as required.
2.2 Flow Control System
The flow control valve used in the SERVOPRO PLASMA trace Nitrogen analyzer for Helium
background is the same as used in the Argon version. This valve is not a tight shut off valve.
Its purpose is to maintain the flow at the default flow set point, i.e. 75 cc/min. It is possible to
have a little flow (usually not more than 10 cc/min for inlet pressure of 10 psig) with a flow set
point value of 0 cc/min. This is acceptable for normal operation. Note that the plasma will
shut off when the real flow is below 10 cc/min. The analyzer should be operated and
calibrated with a flow set point of 75 cc/min and stable inlet pressure. The recommended
inlet pressure range is between 5 to 15 psig, ideal being 10 psig.
2.3 Leaks
The most critical point in trace Nitrogen analyzer is to avoid the pollution of the sample
stream. The leak integrity of the sampling and calibrating system is critical for the
performance of the analyzer. Be aware that leaks are more critical with Helium than Argon.
Furthermore, the sample must be as dry as possible. You should use a molecular sieve 3A
trap to keep the sample dry.
2.4 Working with Helium
It is a challenge to work with Helium in an on line continuous measurement of ppm N
2
. The
Helium molecule size and weight are so small compared to N
2
that special attention must be
paid to the sampling system in order to get a better stability.
With Argon, the sampling system operating condition may produce good results. The same
condition applied with Helium can lead to unacceptable performance.
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