Vaisala HMP155 - Manual (Page 16)

USER'S GUIDE____________________________________________________________________
14 _______________________________________________________________ M210912EN-C
Additional Temperature Probe Option
1206-038
Figure 2 HMP155 with Additional T-probe
The active output version of HMP155 can be ordered with an additional
temperature probe option, see Figure 2 above. When the additional T-
probe is in use, the relative humidity value is calculated based on the T
d
(dew point) value obtained from the humidity probe and the T
a
value
obtained from the T-probe.
When installing HMP155 with T-probe, it is important to make sure that
the humidity probe and the T-probe are installed in the same conditions
in order to get accurate readings. Even a slight difference in temperature
conditions between the T-probe and the humidity probe will result in
false RH readings. The two probes need to be installed so that the
humidity probe does not heat the T-probe, but is close enough for the
probes to share the same environment conditions. When the RH reading
is required, always install the T-probe in the place where you need the
reading from.