Chapter 4 — Theory of Operation
Page
33
General
description of
the chilled
mirror
hygrometer
system
The Humilab includes a general purpose chilled mirror dew point
system that utilizes advanced microprocessor control. It is supplied
with GE General Eastern’s patented PACER® (Programmable Auto-
matic Contaminant Error Reduction). See Appendix B, page 62, for
information on the Pacer feature. The Chilled Mirror Hygrometer
System also provides 4-20mA and 0-5 VDC analog outputs and
RS232C communications. For detailed Chilled Mirror Hygrometer
System Specifications, refer to Appendix A.
The Humilab Chilled Mirror System measures humidity in dew/
frost point or relative humidity (RH). It also measures temperature
in °C or °F. It accepts inputs from the Model 1111H chilled mirror
dew point sensor and the Model T-100E temperature sensor. These
are located within the test chamber.
Chilled mirror
hygrometer
system theory
of operation
The Humilab Chilled Mirror System operation is illustrated in
Figure 12 on page 34, which shows the way in which the chilled
mirror hygrometer detects and measures dew point. The conden-
sate detection mirror is illuminated with a high intensity, solid
state, light emitting diode (LED). A photodetector monitors the LED
light reflected from the mirror. A separate LED and photodetector
combination are used to compensate for any thermally induced
changes in the optical components.
For detailed information on the chilled mirror hygrometer, refer to
Appendix B.
The photodetectors are arranged in an electrical bridge circuit, and
the photo-detector is fully illuminated when the mirror is clear of
dew. As dew forms on the mirror, the photodetector receives less
light due to scattering losses. Since the bridge output current is
proportional to the light received, a large bridge current develops
whenever the mirror is dry. The bridge output is amplified and
used to control the direct current to the thermoelectric cooler,
causing the mirror to cool toward the dew point.
As dew begins to form on the mirror, the optical bridge output is
reduced due to a reduction in detected light. This in turn, causes a
decrease in cooling current. A rate feedback loop within the ampli-
fier ensures critical response, and the system quickly stabilizes at a
condition where a thin dew or frost layer is maintained on the
mirror surface (i.e., the dew or frost point). Thus, the mirror temper-
