Campbell HC2S3, Руководство по эксплуатации продукта, Страница: 29 / 42

C-1
Appendix C. Absolute Humidity
The HC2S3 measures relative humidity. Relative humidity is defined by the
equation below:
100
e
e
RH
s
∗ =
(A-1)
where RH is relative humidity, e is vapor pressure in kPa , and e
s
is saturation
vapor pressure in kPa. Vapor pressure, e, is an absolute measure of the amount
of water vapor in the air and is related to the dew point temperature. 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 Appendix
C.1 , Measurement Below 0 °C
(p. C-2)
.
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.
CRBasic Example C-1. CR1000 Program that Computes Vapor Pressure and Saturation
Vapor Pressure
'CR1000 program that calculates Vapor Pressure
'Wiring Diagram
'==============
'HC2S3
'Wire CR1000
'Color Function Terminal
'------ -------- --------
'Brown Temperature signal SE2
'White Relative Humidity signal SE1
'Yellow Signal Reference Ground Symbol
'Gray Power Ground Ground Symbol
'Clear Shield Ground Symbol
'Green Power 12V