Dew Point Monitor
Powerex
•
150 Production Drive
•
Harrison, OH 45030
•
USA
P 1.888.769.7979
•
F 513.367.3125
•
www.powerexinc.com
IN907800AV
•
08/2018
Page 28 of 40
Integrated Dew Point Monitor Maintenance
Sensor Calibration and Adjustment
The sensor is fully calibrated as shipped from the factory (see
calibration sheet included in the main system manual). With the
Daily Auto-Calibration feature (listed below) the sensor may not need
replacement for up to 10 years. The manufacturer recommended
replacement interval is 2 years from system start-up or if sensor
should malfunction.
Replacing the Filter
The sensor is protected with a stainless steel sintered filter. The
filtering grade is 34 um.
The filter is recommended to be changed whenever it looks dirty or
as a part of the periodical maintenance procedure. The recommended
replacement interval depends heavily on the application and can vary
from less than a year to several years. Dirty or corroded filter may
cause increased response times. In case the sensor is used in a drying
system with silica gel (or other drying agent) the dust from the dryer
may collect on the sensor slowing the response but also generating a
micro climate around the sensor altering the dew point at the sensor.
This is easily corrected by changing or cleaning the filter.
Touching the sensor may damage it. Special
attention shall be paid to avoid touching the sensor
when removing and replacing filters.
Daily Auto-Calibration
The Daily Auto-Calibration feature of the sensor is an automatic
procedure which greatly reduces the possible drift in the dry end of
the dew point measurement. It is performed at one hour intervals, and
when the power is switched on. During auto-calibration the sensor
is warmed for a short period (< 1 min) and the sensor capacitance
values are evaluated at the elevated temperature. The possible dry
end drift is then corrected to correspond to the calibrated values.
During the auto-calibration the transmitter outputs the Td value prior
to the procedure. Auto-calibration is carried out only if several criteria
for the measurement environment are fulfilled. This ensures the
reliability of the adjustments, and maintains the excellent long term
stability that the patented technology offers. These criteria include
e.g. a sufficiently stable enough moisture level in the measured
atmosphere. If the conditions are not fulfilled, the auto-calibration
function is postponed until the satisfied conditions are reached.
Sensor Purge
Sensor purge is also an automatic procedure that minimizes the
drift at the wet end readings of the dew point measurement. Sensor
purge is performed once a day or when the power is switched on.
The sensor is heated for several minutes which will then evaporate all
excess molecules out of the sensor polymer. This, together with the
auto-calibration results in a very small drift of the sensor due to the
very linear behavior of the polymer technology.
Sensor Warming in High Humidity
Additionally the sensor has a warming feature which prevents the
sensor and filter from becoming wet in high humidity. High humidity
can be present when the dew point temperature rises close to the gas
temperature. Sensor warming is switched on automatically when the
humidity level in the measured gas increases to a level where dew
can start to form. The advantage of sensor warming is rapid response
of dew point measurement. A wet sensor and filter would otherwise
result in a dew point equal to ambient temperature (i.e. RH=100%). If
the sensor becomes soaked in any way, it will fully recover and return
back to normal operation after it dries out.
