Michell Instruments QMA401, Руководство пользователя

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QMA401 User’s Manual
Michell Instruments 39
OPERATION
3.8 Sampling Guidelines
The QMA401 Trace Moisture Analyzer is designed to operate in a flowing gas stream
and is suitable for the measurement of the moisture content of a wide variety of gases.
In general, if the gas (in conjunction with water vapor) is not corrosive to the sampling
system and the sensor base metals then it will be suitable for measurement by the
QMA401.
The analyzer is designed to automatically regulate the flow rate. However, the sample
pressure and back pressure must match what is shown on the calibration certificate -
typically 1 barg (14.5 psig) sample pressure with the oulet at atmospheric pressure, and
should be controlled using a high-quality pressure regulator on the gas inlet and a back
pressure regulator on the outlet.
General guidelines to be followed when setting-up a sampling system are as follows:
• Ensure that the sample is representative of the gas under test
To ensure that the sample is representative of the process being monitored, the sample
point should be as close to the critical measurement point as possible. Also, never
sample from the bottom of a pipe where entrained liquids may be drawn into the
instrument’s sample input line.
• Minimize the ‘dead space’ in sample lines
Dead space in sample lines causes moisture entrapment points, increased system
response times or measurement errors as the trapped moisture is released into passing
sample gas, producing an increase in partial vapor pressure.
Avoid the use of too many T-pieces, in-line couplings or other unnecessary pipework.
Sample pipework should, ideally, be specially designed for each application rather than
adapted from that previously installed for another application. Dead space in sample
lines increases response time by holding water molecules which are more slowly released
to the passing gas sample.
• Remove any particulate matter or oil from the gas sample
Particulate matter can damage the sensors. If particulate matter, such as degraded
desiccant, pipe scale and rust are likely to be present in the sample gas, use a particulate
in-line filter. Michell Instruments’ technical sales department can be contacted for advice.