Vaisala PTU200 - Manual (Page 42)

U SER ' S G UIDE _______________________________________________________________________
38 ___________________________________________________________________ M210195 EN -A
Humidity
Calibration of the HMP45A-P/D probes should be performed at
regular intervals, depending on the conditions of use and desired
accuracy. The recommended calibration interval is one year.
The HMP45A-P/D probes are easy to maintain, calibrate and adjust.
The probe consists of a probe head and a handle with cable. All
calibration electronics are in the probe head, which can be
disconnected from the handle without disconnecting the wires. The
probe heads are interchangeable.
D W T
pull the probe head
off the handle
adjustment trimmers:
W = wet D = dry
(T for factory use only!)
F IGURE 6-2 Adjustment trimmers and probe head connection/
disconnection
Humidity adjustment
For a high-accuracy two-point adjustment, use a Vaisala HMK15 or
HMK13B calibrator and saturated salt solutions as described in the
respective manuals. If there is no local display, read the outputs via
the serial line.
Leave the calibrator and the probe head in the same space for at least
four hours so that their temperatures have time to equalize. Unscrew
the plastic grid of the probe.
The adjustment is done first for the dry end and then for the wet end
with trimmer potentiometers marked “D” (dry, <50 %RH) and “W”
(wet, >50 %RH). The potentiometers are located under a protective
plug; see
F IGURE 6-2 . Use a ceramic screwdriver with 2.5 mm blade for
adjusting the potentiometers. Note: if zero point is adjusted in
Nitrogen (N
2
), the minimum output signal of 0.008 V corresponds to a
relative humidity of 0.8 %RH.
T ABLE 6-2 Greenspan’s calibration table
Temperature °C 15 20 25 30 35
LiCl %RH *) 11.3 11.3 11.3 11.3
NaCl %RH 75.6 75.5 75.3 75.1 74.9
K
2
SO
4
%RH 97.9 97.6 97.3 97.0 96.7
*) Do not use or store the LiCl solution in temperatures below +18°C as its humidity
equilibrium may change permanently