ROTRONIC MBW 473 - Manual (Page 38)

34 MBW473_MANUAL_E_V2.0
The user must therefore determine this effect experimentally for the conditions used. In the example
installation shown above, the RP2 measuring head and the chamber door are thermally coupled to
allow for optimum dissipation of heat generated by the measuring head. At equlibrium, the generator
will establish a thermal equilibrium within the complete system so that temperature stability is
optimized and gradients are minimized.
Whereas the RP2 head-mounted temperature probe (PRT) can be used for the measurement of
temperature in most measurement situations, in some chamber conditions a short extension cable
may be advisable to thermally
‘decouple’ the PRT from the measuring head. . Especially at low RH
conditions, where the mirror temperature can be much lower than the chamber temperature, the
heating of the measuring head can cause appreciable temperature errors . Therefore, consideration of
this possible effect must be included within uncertainty budgets. In calibration laboratories where the
lowest possible uncertainties are required, the implementation of continuous multi-point temperature
measurement allows the user to determine gradients dynamically. In such small chambers, four or six
PRTs arranged around the working volume is sufficient according to IEC 60068.
Condensation
If the RP2 probe is installed through the chamber ports of an RH generator working at high tempera-
ture, a thermal gradient through the RP2 may result and the probe may be at a slightly lower tempera-
ture than the chamber. This may cause condensation to form on the outer body of the RP2 probe. As
a result, the measured dew point values will be incorrect. It is even possible that condensation on the
head will cause a short circuit. To avoid such problems, make sure that the RP2 probe is inserted as
far as possible into the chamber volume. More advanced RH generators feature heated doors which
prevent condensation.
Working or Transfer Standard
In the calibration laboratory or workshop, the 473 provides the best possible measurement capability
for a working standard in RH generators. By using the 473 for continuous dew point and temperature
reference measurement, users can achieve small uncertainties and calibrate multiple probes in short
time periods.
When an RH generator is used for on-site calibration, the combination of the RH generator, dew point
mirror and temperature measurement system may not be practical for site engineers as they need to
run calibrations quickly to minimize down-time. Examples of this situation include calibration of instru-
ments used in pharmaceutical production rooms, laboratories, and critical industrial processes. In this
case, the RH generator can be periodically validated using the 473 as a reference standard.
The user should define a working procedure to manage calibration traceability. Maintaining careful
calibration records and history for both the transfer standard and the generator is essential to properly
determine the calibration stability and drift components of uncertainty budgets. For further support,
please contact MBW or your local distributor.