ROTRONIC MBW 473 - Manual (Page 37)

MBW473_MANUAL_E_V2.0 33
At equilibrium, the water vapor pressure is assumed to be ho-
mogenous, but the user should determine this by experimenta-
tion according to the type of chamber being tested.Temperature
probes should be positoned according to the need for validation.
The most common arrangement is four probes in the top
corners, four in the bottom corners and a single probe in the
center. For larger chambers, more probes can be added so that
the entire working volume is measured. This allows for the full
spatial volume to be temperature mapped. By combining this
temperature data with the measured dew point, %RH can also
be mapped. Software such as that supplied with the MBW T12
temperature system can be used to display and record the dew point, temperature and calculated
%RH values. Please contact MBW or your local supplier for more information.
RH Generator Validation
Transportable RH Generators provide a convenient and fast
way of calibrating RH instruments. Most use RH sensor con-
trol probes to measure and control the conditions in the gen-
erator
’s test chamber. The standard calibration uncertainty of
the RH generator is limited by calibration drift, linearity and
temperature coefficients of the control probe. With the use of
a dew point mirror, with its inherent stability and precision,
the uncertainty can be significantly improved.
The 473-RP2 is ideal for this application. As shown, the RP2
measuring head is introduced into the generator chamber
through a front access port. The measuring cell of the mirror
is exposed to the moving air within the chamber and measures the temperature and humidity of the
air as it passes over the measuring head. As with climatic chambers, it is assumed that, at equilibrium,
water vapour pressure is homogenous throughout the chamber volume. Provided that the temperature
is stable and uniform, the %RH derived from the dew point and temperature measurement is suitable
for use as a transfer standard for calibration of the generator
’s control probe or as a reference against
which units under test can be compared.
Temperature Effects
Most RH generators are also able to control
temperature. This means that RH probes can be
calibrated for temperature at the same time as %RH.
The RH calibration can be performed at working
temperature. As RH is highly temperature dependent,
temperature effects and gradients, in particular, must be
considered. With the RP2 probe installed into the RH
generator chamber, the Peltier element that is used to
cool the mirror causes heat to be dissipated from the
body of the measuring head. This can cause
temperature gradients within the generator chamber.