Chapter 4 — Theory of Operation Page 31
The time required for one complete cycle of operation is 1 second.
During that period, the on-off duty cycle determines the ratio of
wet air to dry air, which, in turn, determines the percent relative
humidity in the test chamber. When the valve is ON, wet air is
generated and flows into the chamber. When the valve is OFF, dry
air flows into the chamber. Figure 11 shows how the wet/dry air
ratio is proportional to the relative humidity.
During the wet air portion of the cycle, the air is thoroughly satu-
rated by an efficient diffuser. It then passes into the test chamber.
During the remainder of the cycle, the dry air enters the chamber,
after having first been completely temperature stabilized so that it
is virtually the same temperature as the water in the jacket
surrounding the chamber. This stabilization ensures that there is
very little temperature gradient between the incoming air and the
chamber itself.
The Humilab chilled mirror controller
A GE General Eastern chilled mirror dew point hygrometer is built
into the Humilab, providing continuous closed loop feedback to
control the relative humidity of the test chamber. This device
provides a fundamental, N.I.S.T. traceable measurement of the
chamber dew point. See Appendix B for more detailed information
on the operation of the chilled mirror hygrometer.
Mounted inside the test chamber is a chilled mirror dew point
sensor, bringing chamber dew point information to the hygrometer.
Figure 11 — Wet/Dry Air Selection
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