VP-4
4
RH SENSOR
fied accurate to within
±
0.3% RH. As mentioned above, it is critical
that the VP-4 sensor be at the same temperature as the salt solution
or large errors in the measured RH occur.
Table 1: Salt Solutions
Equilibrium RH
Salt
Molality (m)
% Saturation
(mol salt/kg water)
25%
LiCl
13.41 m
50%
LiCl
8.57 m
76%
NaCl
6.00 m
92%
NaCl
2.33 m
4.1
RH Sensor Conditioning
If a VP-4 sensor has been exposed to solvents or other chemicals,
the following conditioning procedure may bring the sensor back to
the original calibration state. First, bake the sensor in dry heat at
100 to 105
◦
C for 10 hours. Then re-hydrate the sensors at 20 to
30
◦
C under ˜
75% RH for 12 hours. A 75% RH environment can be
conveniently established by sealing the sensor in a headspace over
saturated NaCl prepared as described above.
RH Sensor Calibration
Decagon offers a service to calibrate VP-4 sensors (contact Decagon
Support at [email protected] for more information). Prior to
shipping the RH sensors are verified over salt solutions at 25%, 50%,
and 76% RH, to ensure that they are properly functioning.
We recommend that VP-4 sensors be re-calibrated every one to two
years under normal use conditions to ensure best possible accuracy.
For safety-critical or especially high accuracy applications, more fre-
quent calibration is recommended. Additionally, if sensors have been
poisoned by chemical exposure and conditioning fails to restore ac-
curate measurements, the sensors should be sent back to Decagon
for evaluation and possible calibration.
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