Supercooled Dew Points; Contamination - GE OptiSonde User Manual

October 2007

Supercooled Dew Points

Contamination

Mirror Cleanliness
Particulate Contaminants
3-6
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. The D2 has a viewport to enable viewing the mirror
surface.
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.
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.
Operation