User's Manual

012-06437B
Humidity Sensor
3
Calibration of the Humidity Sensor
For most applications, calibration of the Humidity
Sensor is not required.
If the sensor is to be calibrated with DataStudio or
ScienceWorkshop, one of two methods may be used:
1) "single point" calibration method or 2) "two point"
calibration method. (Each method is described, in
detail, in the DataStudio online help manual and the
ScienceWorkshop software user manual.)
Use of either method requires that the sensor be
exposed to a sample of known relative humidity.
Single Point Calibration
The "single point" calibration method is fairly easy to
implement. The "single point" calibration is a "two
point" calibration (see below) where the output of the
Humidity Sensor is assumed to be 0 volts at 0% RH.
To open the Calibration window, double-click on the
Humidity Sensor icon in the Experiment Setup win-
dow. The other point for calibration is gained by
exposing the Humidity Sensor to an atmosphere of
known relative humidity. When the humidity reading
is relatively constant, enter the known % RH value in
the High Value area in the Calibration window and
click "Take Reading" (DataStudio) or "Read"
(ScienceWorkshop). Click OK to close the Calibration
window.
Two Point Calibration
The "two point" calibration is more difficult because
the sensor must be exposed to two distinctively
different atmospheres of different levels of known
relative humidities to read high and low values for
calibration. This difficulty may be overcome by using
a device known as a saturated salt cell.
A saturated salt cell is an enclosed system containing a
saturated salt solution and air. The air in the closed
system will come to equilibrium at a particular relative
humidity. The specific RH at which the system comes
to equilibrium is based on the ability of the saturated
salt solution to extract or add (through evaporation)
water vapor from or to the air. This is a common tool
for calibration of relative humidity instruments.
The high and low % RH readings should be entered in
the Calibration window, as explained in the "Single
Point Calibration" section.
Constructing a Saturated Salt Cell
1. A one-liter glass or plastic-covered container may
be used as a vessel for a cell.
2. Place distilled or deionized water approximately
one centimeter deep into the vessel.
3. Add enough "salt" to create the desired saturated
solution.
➤ Hint: Excess "salt" should be present (visible
as it settles to the bottom of the vessel) to ensure
saturation of the solution.
A closed cell constructed with NaCl as the excess
"salt" will have a RH of about 75%. A similar con-
structed cell using MgCl will have a RH of about
33%.
The "single point" calibration method may also be
accomplished with a saturated salt cell.
When performing the calibration, the Humidity Sensor
should be placed in the saturated salt cell and allowed
to equilibrate. This could take as long as 30 minutes
or more.
Mounting on an Experimental Apparatus
Use the 1/4-20 screw connector located on the bottom
of the sensor box to secure the Humidity Sensor to an
experimental apparatus (Figure 2). The alignment hole
fits over an alignment pin included on some PASCO
apparatuses.
Figure 2. Mounting connector and alignment hole.
1/4-20 screw
connector
alignment hole