You take a reading of the dew point on the three different
waveguide systems and come up with -32°, -250, and
-21°F, respectively. Is the last reading of -21°F a good
reading? The radar equipment manual specified -25°F
as the minimum requirement (at atmospheric pressure).
Using the dew-point conversion chart, all three of the
above readings convert to 4°F, which is well within
tolerance for the radar.
The conversion chart is used to convert the
individual dew-point reading at various pressures to a
known standard.
The sensor probe, which is used to monitor the dry
air, consists of an aluminum strip that is anodized by a
special process to provide a porous oxide layer. Avery
thin coating of gold is evaporated over this structure.
The aluminum base and the gold layer form the two
electrodes of what is essentially an aluminum oxide
capacitor. Water vapor is rapidly transported through
the gold layer and equilibrates (is in equilibrium) on the
pore walls in a manner functionally related to the vapor
pressure of water in the atmosphere surrounding the
sensor.
The number of water molecules absorbed on the
oxide structure determines the conductivity of the pore
wall. Each value of pore wall resistance provides a
distinct value of electrical impedance; which, in turn, is
a direct measure of water vapor pressure.
The monitor unit, as we have indicated, is usually a
complex impedance meter. It applies a low-frequency
signal of less than 100 Hz to the sensor probe. By
measuring the change in this signal amplitude and
phase, caused by the sensor probe, the monitor unit
continuously computes the impedance of the probe and
then displays the value on the unit’s meter as the dew
point.
Most moisture monitors do not require routine
maintenance. If a malfunction should occur, you should
isolate the problem by substituting a sensor probe
(known to be good). (The sensor is extremely fragile
and nothing should touch the aluminum oxide element.)
If the problem is isolated to the monitor unit and not the
sensor probe, you should use the unit’s schematic
diagrams and a multimeter to isolate the problem.
SUMMARY
In this chapter, you were given a brief overview of
electronics dry air and various types of systems. You
have been introduced to a typical shipboard dry air
system and its major components: fluid separator,
telltale oil filter and dump trap, pressure regulator,
dehydrator dust filter, flow limiter, and moisture
monitor. These topics have provided you with the
fundamental knowledge of their operation so that after
some hands-on training you can operate and maintain
them with confidence.
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