Vaisala DMP248, Operating Manual

Page: 8 / 141

OPERATING MANUAL _______________________________________________________________
4______________________________________________________________________M210492
EN
-
A
The operating principle of the DMP248
The DMP248 transmitter incorporates the DRYCAP
®
sensor which is
optimized to be used in low humidities but has also an excellent
tolerance against condensation. The DRYCAP
®
sensor uses an
operating principle based on changes in capacitance as its thin
polymer film absorbs water molecules together with a combined
temperature measurement with a Pt 100 resistive temperature sensor.
The capacitance of the thin polymer film has a direct response
proportional to RH, but combined with the temperature signal the
response of the DRYCAP
®
sensor is rather proportional to Pw (water
vapour pressure) or to the dewpoint.
While frostpoints (dewpoints below 0 °C) in principle can be
determined by using traditional RH transmitters, it is very difficult
considering the required accuracy at the dry end calibration. As
relative humidity levels approach zero the accuracy rapidly decreases
and the offset soon becomes the largest source of errors when
monitoring frostpoints. Therefore, the focus is to minimize the offset
(error at 0% RH) when monitoring the process gas.
For example to monitor a process with a frostpoint of -40 C and a
temperature of +20 C translates to a relative humidity of 0.55%. An
offset error of -0.2 %RH, which is well within specifications for a
normal RH-transmitter, brings the measured RH down to 0.35%. This
would bring the calculated frostpoint down to -44 C. Thus seemingly
minor offset errors caused by drift or bad calibration translate into
unacceptable frostpoint errors when the RH is low.
To solve the accuracy problem the DMP248 transmitter utilizes a
patented method that automatically adjusts the dry end measurement
in frostpoints. The offset calibration algorithm incorporated into the
DMP248 transmitter uses the fact that the capacitance of a thin film
polymer sensor is proportional to RH as seen in formula 1.
RH RH Gain
Pw
Pws T
out
= +
0
*
( )
(2-1)
where:
RH
0
=output in completely dry state
Pw=water vapor pressure
Pws(T)=temperature dependent water vapor saturation pressure
All manuals and user guides at all-guides.com