31
It provides a
primary
, as opposed to a
secondary
measurement of dew point.
Measurement is continuous, accurate and repeatable.
Results are traceable to N.I.S.T., supporting ISO 9000 and military test
requirements.
No hysteresis.
No drift.
Dew point accuracy of +/- 0.2
o
C
These advantages make the chilled mirror sensor the technology of choice for the
Edgetech Instruments Inc. RH Calibrator.
13.2 MIRROR AUTOMATIC BALANCE CYCLE (ABC)
The Automatic Balance Cycle is an important electronic feature of the RH CAL that
allows much longer operation of the system without any maintenance. At least 90 days
is typical. As contamination from the air sample gradually builds up on the mirror
surface, an error in the indicated dew point reading could eventually occur. In order to
eliminate this potential source of error, the system periodically reprograms itself by
correcting for the loss in reflectivity caused by the contaminants on the surface, allowing
the mirror to operate at the actual dew point temperature once again. This is called
b
alancing.
The user should always use the ABC feature, because it greatly minimizes
mirror cleaning requirements.
The ABC cycle first heats the mirror surface above the dew point, causing the
condensate layer to evaporate, leaving only the contamination on the surface. The
amount of light received from the dry mirror is then measured, and a correction in the
servo loop is made, normalizing the system (balancing) and compensating for the
contaminant layer. The balance cycle only takes a few minutes, and at the end of that
period the mirror resumes tracking the actual dew point temperature.
TRACK and HOLD: The Analog Output can be programmed with the keypad to provide
Relative Humidity, Dew Point, or Temperature information. When Dew Point is selected
(or Relative Humidity, which is a function of Dew Point and Temperature), the actual
analog value is temporarily incorrect during the ABC Cycle. Since the mirror
temperature is constantly measured and defined as the dew point, the heating of the
mirror described above is the one time when the mirror temperature is intentionally
not
at the dew point. A keypad programming option allows the user to have the Analog
Output remember the last dew point value
just before
the ABC cycle started, and
hold
that value constant for the few minutes that the balance cycle requires. It then
continues to track the actual dew point temperature (or RH) as before. This is the HOLD
option, which may be the best choice when driving a strip chart recorder or when using
a date acquisition system. If the TRACK option is selected, the resulting positive output
pulse on the analog output during the heating portion of the ABC cycle may be recorded
and used to tell the operator when the cycle occurred.
