SUMMARY

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. 2-9