Campbell Scientific HC2S3 Temperature and Relative Humidity Probe - Manual (Page 29)

Appendix A. Absolute Humidity
The HC2S3 measures the relative humidity. Relative humidity is defined by
the equation below:
100
e
e
RH
s
∗ =
(A-1)
where RH is the relative humidity, e is the vapor pressure in kPa , and e
s
is the
saturation vapor pressure in kPa. The vapor pressure, e, is an absolute measure
of the amount of water vapor in the air and is related to the dew point
temperature. The saturation vapor pressure is the maximum amount of water
vapor that air can hold at a given air temperature. The relationship between
dew point and vapor pressure, and air temperature and saturation vapor
pressure are given by Goff and Gratch (1946), Lowe (1977), and Weiss (1977).
Relative Humidity is relative to saturation above water, even below freezing
point. This is why these sensors should not measure 100% RH below zero
degrees C, as described in Section A.1.
When the air temperature increases, so does the saturation vapor pressure.
Conversely, a decrease in air temperature causes a corresponding decrease in
saturation vapor pressure. It follows then from Eq. (A-1) that a change in air
temperature will change the relative humidity, without causing a change
absolute humidity.
For example, for an air temperature of 20°C and a vapor pressure of 1.17 kPa,
the saturation vapor pressure is 2.34 kPa and the relative humidity is 50%. If
the air temperature is increased by 5°C and no moisture is added or removed
from the air, the saturation vapor pressure increases to 3.17 kPa and the relative
humidity decreases to 36.9%. After the increase in air temperature, there is
more energy to vaporize the water. However, the actual amount of water vapor
in the air has not changed. Thus, the amount of water vapor in the air, relative
to saturation, has decreased.
Because of the inverse relationship between relative humidity and air
temperature, finding the mean relative humidity is often not useful. A more
useful quantity is the mean vapor pressure. The mean vapor pressure can be
computed by the datalogger program as shown in the following example.
TABLE A-1. Wiring for Vapor
Pressure Examples
Color Description CR1000
Brown Temperature SE 2
White Relative Humidity SE 1
Yellow Signal Reference
Gray Power Ground
Clear Shield
Green Power 12V
A-1