GE Optica, Operator'S Manual

Page: 18 / 150

January 2006
1-6 Features and Capabilities
Other Hygrometer
Applications
Many GE Infrastrucure Sensing 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 Infrastructure Sensing 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.
The PACER Cycle
GE Infrastructure Sensing has developed and patented a
compensation technique called PACER (Programmable Automatic
Contaminant Error Reduction) that is very effective in reducing the
Raoult Effect error associated with soluble contaminants, particularly
for near-ambient dew points. The Optica is equipped with the PACER
cycle as well as AUTO balance as found on earlier models. The user
can choose which self-cleaning and balancing routine to run
depending on the severity of contamination.
The PACER cycle, diagrammed in Figure 1-3 on page 1-7, begins
with a coalescence period, during which the mirror is cooled well
below the dew point of the sample gas, condensing out a large amount
of water.