Moisture Monitor Hints - GE Moisture Image 1 Series Service Manual

June 2003

Moisture Monitor Hints

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GE Panametrics hygrometers, using aluminum oxide moisture
probes, have been designed to reliably measure the moisture content
of both gases and liquids. The measured dew point will be the real
dew point of the system at the measurement location and at the time
of measurement. However, no moisture sensor can determine the
origin of the measured moisture content. In addition to the moisture
content of the fluid to be analyzed, the water vapor pressure at the
measurement location may include components from sources such as:
moisture from the inner walls of the piping; external moisture through
leaks in the piping system; and trapped moisture from fittings, valves,
filters, etc. Although these sources may cause the measured dew point
to be higher than expected, it is the actual dew point of the system at
the time of measurement.
One of the major advantages of the GE Panametrics hygrometer is
that it can be used for in situ measurements (i.e. the sensor element is
designed for installation directly within the region to be measured).
As a result, the need for complex sample systems that include
extensive piping, manifolds, gas flow regulators and pressure
regulators is eliminated or greatly reduced. Instead, a simple sample
system to reduce the fluid temperature, filter contaminants and
facilitate sensor removal is all that is needed.
Whether the sensor is installed in situ or in a remote sampling system,
the accuracy and speed of measurement depend on the piping system
and the dynamics of the fluid flow. Response times and measurement
values will be affected by the degree of equilibrium reached within
system. Factors such as gas pressure, flow rate, materials of
construction, length and diameter of piping, etc. will greatly influence
the measured moisture levels and the response times.
Assuming that all secondary sources of moisture have been
eliminated and the sample system has been allowed to come to
equilibrium, then the measured dew point will equal the actual dew
point of the process fluid.
Some of the most frequently encountered problems associated with
moisture monitoring sample systems include:
•
the moisture content value changes as the total gas pressure
changes
•
the measurement response time is very slow
•
the dew point changes as the fluid temperature changes
•
the dew point changes as the fluid flow rate changes.
Application of the Hygrometer (900-901D1)