User manual

A-1
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 vapour pressure in kPa , and e
s
is the
saturation vapour pressure in kPa. The vapour pressure, e, is an absolute
measure of the amount of water vapour in the air and is related to the dew point
temperature. The saturation vapour pressure is the maximum amount of water
vapour that air can hold at a given air temperature. The relationship between
dew point and vapour pressure, and air temperature and saturation vapour
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 vapour pressure.
Conversely, a decrease in air temperature causes a corresponding decrease in
saturation vapour pressure. It follows then from Eq. (A-1) that a change in air
temperature will change the relative humidity, without causing a change in
absolute humidity.
For example, for an air temperature of 20°C and a vapour pressure of 1.17 kPa,
the saturation vapour 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 vapour 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 vapour
in the air has not changed. Thus, the amount of water vapour 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 vapour pressure. The mean vapour pressure can be
computed by the datalogger program as shown in the following example.
Table A-1. Wiring for Vapour Pressure
Examples
Colour Description CR1000
Brown Temperature SE 2
White Relative Humidity SE 1
Yellow Signal Reference
Grey Power Ground
Clear Shield
Green Power 12V