Supercooled Dew Points; Contamination - GE Optica Operator's Manual

3.9.1 Supercooled Dew Points

Slightly below the freezing point, water can exist in a supercooled liquid state for extended periods of time. Extra care
may be needed when making measurements in the frost point region of 0 to –20°C, because the mirror temperature may
temporarily stabilize at the supercooled dew point, 0.5 to 1°C below the actual frost point.
To assure that the unit is operating in the ice phase within this temperature range, allow the instrument to operate
continuously. Before manually clearing a frost layer, take a reading, and afterwards allow sufficient time to reform a
stable frost layer before taking further readings.

3.9.2 Contamination

3.9.2a Mirror Cleanliness
Proper operation of a condensation hygrometer depends on the condition of the mirror surface. In general, accuracy is
reduced when contaminants accumulate on the mirror.
However, the mirror does not have to be microscopically clean. In fact, the mirror performs best a few hours after
cleaning, when nucleation sites have formed. On an unscratched, freshly cleaned mirror, there are relatively few
nucleation sites on which dew or frost deposits can form, and more time is required to collect a condensation layer at
low frost points. Also, overshoot may occur, which can cause oscillations as the temperature stabilizes.
3.9.2b Particulate Contaminants
Particulate matter that is insoluble in water may accumulate on the mirror surface, but does not affect the instrument
accuracy until the mirror reflectance is reduced substantially. In many cases, particulates improve instrument response
by providing condensation sites.
3.9.2c Water-Soluble Contaminants
Contaminants which readily dissolve in water, such as naturally occurring salts, are detrimental to accurate vapor
concentration measurement by any condensation method. These materials readily go into solution with the water
condensate on the mirror surface, and 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 unit responds to this lower vapor pressure by elevating the mirror temperature in order to maintain a vapor pressure
that is in equilibrium with the partial pressure of atmospheric water vapor. The displayed dew point, therefore, drifts
upward above the true dew point. Because the measurement error increases gradually, it often goes undetected.
To determine whether dissolved contaminants are affecting dew point measurement, perform the following steps:
1. Note the indicated dew point.
2. Clean the mirror.
3. Balance the detector by initiating a
4. Measure the dew point again.
If the new reading is lower than the first reading, it is likely that soluble material was present in sufficient quantity to
cause a measurement error.
Optica™ Operator's Manual
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Chapter 3. Operation
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