Vaisala HMK15 - Manual (Page 27)

Chapter 4 ________________ Instructions for the Calibration of Humidity Instruments
VAISALA _________________________________________________________ 25
- Insert the probe into a suitable hole of the NaCl (or
K
2
SO
4
) salt chamber. The shorter the time the hole stays
open before inserting the probe, the shorter the
stabilization time required.
- Wait until the humidity reading stabilizes; this takes
about 10 … 30 minutes. Note that in high humidities the
risk for errors increases. Therefore, the stabilization time
should be longer (approximately 20 … 40 minutes).
- Read the salt chamber temperature from the thermometer
and then read the closest humidity value from the
calibration table (Greenspan's Calibration Table below,
NaCl or K
2
SO
4
solution).
- Adjust the wet end (WET, gain) to correspond to the
value given in the calibration table.
Greenspan's Calibration Table
1
C LiCl MgCl
2
NaCl K
2
SO
4
0 * 33.7 ± 0.3 75.5 ± 0.3 98.8 ± 1.1
5 * 33.6 ± 0.3 75.7 ± 0.3 98.5 ± 0.9
10 * 33.5 ± 0.2 75.7 ± 0.2 98.2 ± 0.8
15 * 33.3 ± 0.2 75.6 ± 0.2 97.9 ± 0.6
20 11.3 ± 0.3 33.1 ± 0.2 75.5 ± 0.1 97.6 ± 0.5
25 11.3 ± 0.3 32.8 ± 0.2 75.3 ± 0.1 97.3 ± 0.5
30 11.3 ± 0.2 32.4 ± 0.1 75.1 ± 0.1 97.0 ± 0.4
35 11.3 ± 0.2 32.1 ± 0.1 74.9 ± 0.1 96.7 ± 0.4
40 11.2 ± 0.2 31.6 ± 0.1 74.7 ± 0.1 96.4 ± 0.4
45 11.2 ± 0.2 31.1 ± 0.1 74.5 ± 0.2 96.1 ± 0.4
50 11.1 ± 0.2 30.5 ± 0.1 74.4 ± 0.2 95.8 ± 0.5
The numbers on the right in each column indicate the uncertainty of the reference humidity of
the salt in that temperature.
1) Greenspan, L.: Journal of Research of the National Bureau of Standards - A Physis and
Chemistry Vol. 81A, No. 1 January-February 1977, pp. 89-95