GE HumiLab Operator's Manual page 66

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Page
Water-soluble contaminants.
ring salts, which readily dissolve in water, are detrimental to accu-
rate vapor concentration measurement by any condensation
method. These materials readily go into solution with the water
condensate on the mirror surface, then reduce the vapor pressure
in accordance with Raoult's Law. As the concentration increases
with time, the saturation vapor pressure of the liquid solution
decreases.
The Electronics Module responds to lower vapor pressure by
elevating the mirror temperature to maintain a vapor pressure that
is in equilibrium with the partial pressure of the atmospheric water
vapor. The displayed dew point, therefore, drifts upward with
respect to the true dew point. Because the measurement error
increases gradually, it often goes undetected.
To see if dissolved contaminants are affecting dew point measure-
ment, do the following:
1. Note the indicated dew point.
2. Clean the mirror.
3. Balance the detector by initiating a Pacer cycle (see below).
4. Measure the dew point again.
If the new reading is lower than the first reading, there was prob-
ably soluble material in sufficient quantity to cause a measurement
error. Consider decreasing the interval between Pacer cycles (see
"Set balance type" on page 54), since the Pacer is completely effec-
tive in reducing contaminant-induced errors.
Minimizing effects of contaminants - the Pacer® feature.
Clean the mirror when necessary according to the optics cleaning
procedure in Chapter 5. To determine the proper cleaning interval,
take a dew point reading before and after cleaning. Any appreciable
shift indicates that under the same conditions you should clean the
mirror sooner.
GE General Eastern's patented PACER (Programmable Automatic
Contaminant Error Reduction) rebalances the dew point sensor
bridge and determines whether it requires cleaning. In normal
operation the Pacer feature actually reduces contaminants. You can
largely avoid frequent need to clean the mirror by using the Pacer
feature of the Humilab's mirror system. This feature, used periodi-
Material such as naturally occur-
Appendix B — Humilab chilled mirror hygrometer