Application of the Hygrometer (900-901D1)
A-5
4/9/98
Response Time
The response time of the GE Panametrics standard M Series
Aluminum Oxide Moisture Sensor is very rapid - a step change of
63% in moisture concentration will be observed in approximately
5 seconds. Thus, the observed response time to moisture changes
is, in general, limited by the response time of the sample system
as a whole. Water vapor is absorbed tenaciously by many
materials, and a large, complex processing system can take
several days to “dry down” from atmospheric moisture levels to
dew points of less than -60°C. Even simple systems consisting of
a few feet of stainless steel tubing and a small chamber can take
an hour or more to dry down from dew points of +5°C to -70°C.
The rate at which the system reaches equilibrium will depend on
flow rate, temperature, materials of construction and system
pressure. Generally speaking, an increase in flow rate and/or
temperature will decrease the response time of the sample system.
To minimize any adverse affects on response time, the preferred
materials of construction for moisture monitoring sample systems
are stainless steel, Teflon
®
and glass. Materials to be avoided
include rubber elastomers and related compounds.
Temperature
The GE Panametrics hygrometer is largely unaffected by ambient
temperature. However, for best results, it is recommended that the
ambient temperature be at least 10°C higher than the measured
dew point, up to a maximum of 70°C. Because an ambient
temperature increase may cause water vapor to be desorbed from
the walls of the sample system, it is possible to observe a diurnal
change in moisture concentration for a system exposed to varying
ambient conditions. In the heat of the day, the sample system
walls will be warmed by the ambient air and an off-gassing of
moisture into the process fluid, with a corresponding increase in
measured moisture content, will occur. The converse will happen
during the cooler evening hours. This effect should not be
mistakenly interpreted as indicating that the moisture probe has a
temperature coefficient.
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