GE Optica, Operator'S Manual

Page: 57 / 150

Operation 3-13
January 2006
Minimizing the Effects of
Contaminants
The following steps are suggested for maintaining optimum
performance:
•
Use the PACER feature to reduce the effect of contaminants on the
unit’s performance (see
The PACER Cycle on page 1-6).
•
Reduce the gas flow rate to reduce the rate of accumulation of
contaminants on the mirror.
•
Clean the mirror according to the recommended optics cleaning
procedure (see
Cleaning the Sensor Mirror on page 7-1). To
determine the proper cleaning interval for a given set of conditions,
take a dew point reading before and after the cleaning. Any
appreciable shift indicates that under these conditions, the mirror
should be cleaned more often.
Mirror Flooding
If there is an abrupt transition from dry to moist conditions
(particularly when accompanied by a transition from cold to warm
temperatures), the mirror may accumulate an overload of moisture. It
then may take several minutes before the sensor dries out and valid
readings can be obtained. The drying process can be accelerated by
heating the sensor.
Sample Line Maintenance
Contaminated sample lines slow the unit’s response time and can
cause erroneous readings, usually on the high side. Clean the sample
lines as often as necessary. To determine the required cleaning
frequency, take dew point readings before and after cleaning the lines,
sensor cavity, and mirror. If the two readings differ appreciably, the
sampling lines should be cleaned more often. To reduce the rate of
contamination, reduce flow and/or install a filter upstream.
Pressure Effects
If the pressure of the gas is increased or reduced from atmospheric
pressure, but the mixing ratio (moisture content) stays constant, the
dew point is correspondingly increased or decreased. The Optica
displays the dew/frost point at the pressure to which the sensor
chamber is exposed. The sensor location and hookup arrangement can
influence the pressure.
When the pressure at the sensor is different from the process pressure,
the Optica can perform a conversion from the measured pressure to
the desired process pressure (see
Process Pressure on page 3-4 for
details).
Alternatively, the dew point change due to pressure change can be
calculated by using Dalton’s Law and the Smithsonian Tables or a
proper nomograph. Appendix C contains basic data for these
calculations.