Chapter 5 _______________________________________________________________Maintenance
VAISALA _______________________________________________________________________ 139
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 calibration is done first for the dry end (<50 %RH) and then for
the wet end (>50 %RH) by adjusting the trimmer potentiometers
marked with
D
and
W
. The potentiometers are located under a
protective plug, see Figure 116 on page 138. Use a ceramic
screwdriver with 2.5 mm blade for adjusting the potentiometers. Note:
if zero point is calibrated in Nitrogen (N
2
), the minimum output signal
of 0.008 V corresponds to a relative humidity of 0.8 %RH.
Table 26
Greenspan’s Calibration
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.
As the D (dry) and W (wet) adjustments may affect each other, re-
check the humidity reading at the low end. If necessary, repeat the
adjustments in both the low and the high humidity points, until the
reading is correct.
Changing the HUMICAP
®
180 Humidity Sensor
Unscrew the filter. Remove the damaged sensor and mount a new
HUMICAP
®
180 humidity sensor in its place. Handle the sensor with
care. Calibrate the probe using a two-point calibration procedure. Note
that if the probe is not calibrated, the accuracy is still better than
±
7 %RH.
Pressure Sensor
Under normal operating conditions, PMT16A Pressure Sensor needs
only a minimum amount of maintenance.
- Keep the pressure port clean.
- Annually, compare pressure values against a calibrated portable
standard.
