October 2007
3-6
Operation
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. The D2 has a viewport to enable viewing the mirror
surface.
Contamination
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.
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.
Summary of Contents for 1111H
Page 1: ...GE Sensing OptiSonde General Eastern Chilled Mirror Hygrometer User s Manual ...
Page 9: ...Chapter 1 ...
Page 15: ...Chapter 2 ...
Page 33: ...Chapter 3 ...
Page 43: ...Chapter 4 ...
Page 57: ...Chapter 5 ...
Page 68: ...Appendix A ...
Page 73: ...Appendix B ...
Page 74: ...Humidity Equations and Conversion Chart Introduction B 1 Vapor Pressure B 1 Humidity B 2 ...
Page 78: ...Appendix C ...
Page 79: ...Configuring the Serial Interface Wiring to a Personal Computer C 1 ...
Page 82: ...Appendix D ...
Page 87: ...Appendix E ...
Page 88: ...Theory of Operation and Glossary Theory of Operation E 1 The PACER Cycle E 4 Glossary E 5 ...