Michell Instruments Pura 2-Wire, User Manual

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Pura 2-Wire/3-Wire/Digital Transmitter
User’s Manual
Michell Instruments 11
OPERATION
3 OPERATION
Operation is very simple assuming the following installation techniques are adhered to:
Sampling Hints
Be Sure the Sample is Representative of the Gas Under Test:
The sample point should be as close to the critical measurement point as possible. Also,
never sample from the bottom of a pipe as entrained liquids may be drawn into the
sensing element.
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Figure 14
Installation Location
Minimize Dead Space in Sample Lines:
Dead space causes moisture entrapment points, increased system response times, and
measurement errors as a result of the trapped moisture being released into the passing
sample gas and causing an increase in partial vapor pressure.
Deadspace
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Figure 15 Indication of Dead Space
Remove Any Particulate Matter or Oil from the Gas Sample:
Particulate matter at high velocity can damage the sensing element and similarly, at
low velocity, they may ‘blind’ the sensing element and reduce its response speed. If
particulate, such as degraded desiccant, pipe scale or rust is present in the sample gas,
use an in-line fi lter, as a minimum level of protection. For more demanding applications
Michell Instruments offers a range of sampling systems (for more information contact
www.michell.com).
Use High Quality Sample Tube and Fittings:
Michell Instruments recommends that, wherever possible, stainless steel tubing and
fi ttings should be used. This is particularly important at low dew points since other
materials have hygroscopic characteristics and adsorb moisture on the tube walls,
slowing down response and, in extreme circumstances, giving false readings. For
temporary applications, or where stainless steel tubing is not practical, use high quality
thick walled PTFE tubing.
Position Transmitter away from Heat Source:
It is recommended, as good instrumentation practice, that the transmitter is placed
away from any heat source to avoid adsorption/desorption.