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OPERATION
3.3.5
MAXCOOL
The MAXCOOL function over-rides the dew-point control loop and applies maximum
cooling drive to the Peltier heat pump. It can be used:
•
to determine what temperature the mirror can be driven down to with
reference to the sensor body. This temperature is indicated on the display.
•
to determine whether or not the instrument is controlling at the dew point
and whether it is able to reach it. This situation could, for instance, arise
when attempting to measure very low dew points where, possibly due to
a high ambient temperature, the Peltier heat pump is unable to depress
the temperature far enough to reach the dew point.
•
to determine whether the instrument is controlling by switching MAXCOOL
on for a short period and then switching MAXCOOL off. This will depress
the mirror temperature briefl y and when switched off the control loop
should be able to stabilize the mirror temperature at the dew point again.
The MAXCOOL function can be turned on and off using the application software, or by
sending commands to the instrument using the digital communications port.
3.3.6
FAST –Frost Assurance System Technology
Theoretically, it is possible for water to exist as a super-cooled liquid at temperatures
down to -40°C (-40°F). A gas in equilibrium with ice is capable of supporting a greater
quantity of water vapor at a given temperature than a gas in equilibrium with liquid water.
This means that a measurement below 0°C taken over water will read approximately
10% lower than the same measurement taken over ice.
When turned on, the Optidew's FAST system identifi es when the measured dew point
is between -40 and 0°C (-40 and +32°F) and automatically decreases the mirror
temperature until a pre-determined fi lm thickness of condensate is detected. This
ensures the formation of ice on the mirror. The mirror temperature is then increased
to above the initial measured dew point, but maintained below 0°C (+32°F), and the
excess condensate is driven off the mirror. The instrument then continues operation as
normal – once ice has formed it will remain as ice until the temperature is raised above
0°C (+32°F).
FAST can only be enabled or disabled by sending the appropriate command to the
Optidew via the digital communications port. Refer to Appendix B for a complete list of
instrument commands.
16
Optidew Series User's Manual
97430 Issue 2, March 2017
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