|Thunder Support Document|
Multiple sensors in the test chamber will sometimes exhibit self heating due to the imbedded electronics in the transmitter housing. It is important to locate the chamber temp probe as close to the RH/Temp probe as possible. Observe inter-comparison of temperature output between sensors.
A manifold fixture is a simple solution to duct the in coming humidity test value over the probes during calibration. The manifold solution will improve temperature uniformity between probes. Locate the chamber temperature probe inside the manifold during calibration.
A similar manifold fixture can be used for calibration of small hand-held devices where the humidity value is displayed on the device. It is possible to open the door for brief periods for adjustment of the device during calibration.
A dew-point hygrometer utilizes a temperature-controlled, highly polished observable surface. In the instrumentís simplest form, crushed ice is slowly added to a liquid in a thin-walled silver container such as a mint julep cup. An accurate mercury bulb thermometer is used to constantly stir the liquid in the cup. When the first sign of condensation (dew) is observed on the outside of the cup, the temperature of the liquid in the cup is read as the dew-point temperature. This method requires that the temperature of the outside surface of the silver cup and the temperature of the liquid in the cup be essentially the same. In actual practice, the temperature of the liquid in the cup will be slightly lower than the outside surface temperature of the cup.
In its most fundamental form, dew point is detected by cooling a reflective condensation surface (mirror) until water begins to condense, and by detecting condensed fine water droplets optically with an electro-optic detection system. The signal is fed into an electronic feed back control system to control the mirror temperature, which maintains a certain thickness of dew at all times.
This is a graphical representation of the flow of a chilled mirror sensor.
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