Hygrometer Calibration; Other Hygrometer Applications - GE Optica Operator's Manual

1.5.2 Hygrometer Calibration

The Optica unit can be sent to the National Institute of Standards and Technology (
certification or to any National Standards lab for calibration against their primary humidity standards. A calibrated
instrument can then be used as a transfer standard in local laboratories to calibrate lower echelon instruments.
CAUTION!
Hygrometers used as calibration standards must have the following characteristics:
•
The mirror thermometer must have suitable long-term accuracy (such as that obtained with a platinum
resistance thermometer).

1.5.3 Other Hygrometer Applications

Many GE Measurement & Control Chilled Mirror Hygrometers are used in industrial applications in addition to
metrology. The optical condensation hygrometer is not readily damaged or contaminated by industrial process gases
that can degrade other secondary measurement schemes such as saturated salt and polymer-based sensors. If the sensor
or sampling components should become contaminated with oils, salts, etc., they can be cleaned without harm to the
sensor or impairment to the system accuracy. The performance of the hygrometer can be checked at any time by
heating the mirror above the dew point, causing the dew deposit to evaporate, then reclosing the servoloop and
checking to see that the system cools and returns to the same dew point.
The GE optical condensation sensors cover a wide range of applications limited only by the heat pumping capabilities
of the thermoelectrically-cooled mirror.
At high dew points (up to 100°C), the sensor is limited by the thermal properties of the solid state optical components
as well as the thermoelectric heat pump capacity.
In a typical application measuring sub-ambient dew points, a two-stage thermoelectrically-cooled mirror can reach a
temperature approximately 65°C lower than an ambient (heat sink) temperature of +25°C. The thermoelectric cooler
pumps heat from the mirror into the heat sink. By reducing the temperature of the heat sink with a coolant such as
chilled water, or by applying the sensor in a low-temperature condition such as monitoring of a test chamber, even
lower dew points can be measured. In meteorological applications where the heat sink temperature is considerably
lower, frost points down to –75°C can be monitored.
Four- and five-stage sensors are available for measuring the lowest dew/frost points.
Optica™ Operator's Manual
Field calibration of the Optica is not recommended.
Chapter 1. Features and Capabilities
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