4
RH SENSOR
VP-4
4
RH Sensor
RH Sensor Stability
Each VP-4 sensor is verified as accurate before leaving our facility.
However, all capacitance RH sensors drift over long periods of ex-
posure to environmental conditions. The VP-4 RH sensor typically
drifts less than 0.5% RH per year. We recommend that VP-4 sensors
be calibrated every one to two years under normal use conditions to
ensure best possible accuracy. To have your VP-4 fully calibrated or
calibrate contact [email protected] and ask about our calibra-
tion service. See Section 4.1 for more information about calibration.
The polymer RH sensing element in the VP-4 can also be “poisoned”
by exposure to volatile organic compounds, solvents, and other chem-
icals.
The effects of exposure to these chemicals can range from
subtle loss of accuracy to catastrophic failure. If you suspect that
your VP-4 has suffered chemical exposure or notice questionable RH
measurements, you can check the sensor accuracy using known RH
conditions.
A convenient method for generating known RH conditions is through
the use of salt solutions. For an initial check, we recommend prepar-
ing a saturated NaCl solution, which has an equilibrium RH of 0.75
(75%). To prepare the salt solution, start with lab grade NaCl and
mix in enough water that there is a thin layer of liquid water present
over a thick slurry of NaCl crystals. The VP-4 sensor can either be
sealed into a chamber or bell jar with the salt solution or the open-
ing of the VP-4 can be sealed into a small chamber that contains the
salt solution. Whatever method is used, 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.
Salt solutions prepared at a wide range of RH are available from
Decagon (see chart below). It is possible to prepare your own solu-
tions using the mixing ratios shown below, but great care and preci-
sion are required to obtain accurate results. It is especially important
that the salt used be pure and dry. Decagon salt solutions are speci-
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