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OPERATION
This cooling causes additional condensation on the mirror, which dissolves water-soluble
matter, and dislodges non water-soluble matter. When the surface is then heated and
the water evaporated, the contamination will cluster together leaving areas of clean
mirror between, which has less overall impact on the optics.
4.4.2
MAXCOOL Function
The MAXCOOL function overrides the dewpoint 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.
•
Whether or not the instrument is controlling at the dewpoint and whether
it is able to reach it. This situation could, for instance, arise when
attempting to measure very low dewpoints where, possibly due to a high
ambient temperature, the Peltier heat pump is unable to depress the
temperature far enough to reach the dewpoint.
•
Whether the instrument is controlling by switching MAXCOOL on for a
short period and then switching back to MEASURE. This will depress the
mirror temperature briefly and when it is switched back to MEASURE the
control loop should be able to stabilize the mirror temperature at the
dewpoint again.
The MAXCOOL function can be turned on by touching the MAXCOOL button on the Main
Screen.
4.4.3
Frost Assurance Technology (FAST)
In carefully controlled laboratory conditions, super-cooled water can exist in temperatures
as low as –54°F/48°C. However, when using a chilled mirror instrument it only occurs
on the mirror at temperatures down to around, -22°F/–30°C.
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 32°F/0°C taken over water will read approximately 10% lower than
the same measurement taken over ice.
There are two modes of FAST operation, enabling and disabling FAST acts on both
modes:
Following DCC: the Optidew makes an initial dewpoint measurement. If the initial
measurement is between +26°F and -22°F (-3°C and -30°C) then the mirror is driven
down to below -31°F/-35°C to ensure the formation of ice on the mirror surface. The
instrument then continues operation as normal.
Dynamic: whenever the measured dewpoint drops below the value set as 'FAST SetP',
the mirror is driven down to below -31°F/-35°C to insure the formation of ice on the
mirror surface. The instrument then continues operation as normal.
Note that Data Hold is active whenever FAST is active.
For further information, see section 3.2
38
Optidew User's Manual
October 2019
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