6.2 AcuDew Installation: Direct into Process Line
Shaw Moisture Meters recommend the use of a sample system rather than a
direct insertion into the process line. Installation of the AcuDew on the sample
line or sample system allows for easy isolation of the AcuDew during installation
and removal. A sample system further allows for sample conditioning when
necessary.
6.3 Sample Systems
A typical sample system used for compressed instrument air is detailed in section
6.4. Other sample systems are available through Shaw Moisture Meters as well as
custom sample system design.
The process thread on the AcuDew is ¾”-16 UNF straight thread. The process seal
is a Viton O-ring, so pipe thread tape should not be used on the threads. SHAW
offers a standard sensor holder for installation of the AcuDew, typically on a
sample line or sample system. A ¾” NPT adaptor is also offered for installation of
the AcuDew into a ¾” NPT fitting, possibly for a direct insertion type installation.
Most applications are for the moisture measurement of pressurized gas.
Regardless of the installation type, care must be taken during installation,
operation, and decommissioning of the AcuDew Transmitter. Be sure that the
process line (or sample system if installed) is depressurized when installing or
uninstalling the AcuDew Transmitter.
As stated above, the AcuDew Transmitter can be installed directly into the process
line, but this does create problems with access for maintenance and calibration.
It is for these reasons that we recommend that the transmitter be installed in a
bypass, fast loop, or sample system where the transmitter is accessible without
interrupting the main process flow line.
The flow rate, although not critical to the sensor measurement, should be low
enough to avoid abrasion to the sensor surface, without being so low as to extend
the system response time to an unacceptable level. In general, a flow rate of
between 2 - 5 litres/min will give the right balance.
The sensor is a variable capacitor, which is directly affected by changes in partial
pressure of water vapour. These changes are proportional to the dew/frost point
temperature.
Partial pressure of water vapour is directly affected by total pressure and, this
being the case, the instrument will display the dew/frost point temperature at
whatever total pressure the sensor is exposed, therefore care should be taken to
ensure that the sample pressure at the sensor is that at which the dew/frost point
readings are required.
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