Installation and Start-Up | 3-33
Cooling Stage
The Cooling Stage begins immediately after the Purging/Hold Stage is
complete. During this stage, the temperature of the mirror is decreased
linearly at the preset Fast Cooling Rate [FRate, default = 5 °C/min (9 °F/
min)] from its upper (High) setpoint temperature until the mirror temper-
ature reaches a preset temperature, typically 15 °C (27 °F) above the last
determined HCDP temperature of the sample gas. This value is set in the
ApplicCFG : Above menu. After this temperature is reached, the mirror is
then cooled at the preset Slow Cooling Rate [SRate, default = 2 °C (3.6 °F)]
per minute – to its Low setpoint temperature, typically -30 °C (-22 °F).
AMETEK does not recommend changing the Slow Cooling Rate
(SRate). This rate was tested at the factory for optimum HCDP dew-
point sensitivity.
In the initial measuring cycle after power-up or a system reset, the mirror
is cooled at the SRate for the entire stage.
The Low setpoint temperature should be lower than the HCDP tempera-
ture of the sample gas but not more than 60 °C (140 °F) degrees below the
ambient temperature and never lower than -40 °C (-40 °F).
The instantaneous intensities of light impinging on both phototransis-
tors are monitored simultaneously. At 0.25 °C intervals, the light-intensity
values and mirror temperatures are stored into memory. After each data
set is stored, the array of temperature and light-intensity values are ana-
lyzed to determine if a persistent increase in light intensity has occurred
at either phototransistor. This signifies condensation on the correspond-
ing mirror surface – hydrocarbon on the rough surface or water on the
smooth surface.
The Cooling Stage termination (End-of-Run Type) has been set for the
HCDP and the Cooling Stage will terminate:
• When the Hydrocarbon Dewpoint temperature is found (factory-set
default),
or
• When the Low Mirror Temperature setpoint is reached,
or
• If, after 2 minutes, the Thermoelectric Cooler cannot further reduce
the mirror temperature.
!
CAUTION
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