Campbell Scientific HMP45C Temperature and RH Probe - Manual (Page 16)

Model HMP45C Temperature and Relative Humidity Probe
;Measure the HMP45C relative humidity.
;
04: Volt (Diff) (P2)
1: 1
Reps
2: 5 2500 mV Slow Range ;CR510, CR500 (2500mv); CR23X (1000 mV);
21X, CR7 (5000 mV)
3: 1 DIFF Channel ;Blue wire (1H), white or purple wire (1L)
4: 2 Loc [ RH_pct ]
5: .1 Mult ;See Table 5-2 for alternative multipliers
6: 0 Offset
;Turn the HMP45C off.
;
05: Do (P86)
1: 51
Set Port 1 Low ;Jumper wire from SW12V control to C1
;Orange wire (C1) if older wiring
;Green wire (C1) if using SW12V device
;For CR23X or CR5000 use 59 for SW12V internal
;control port
7. Absolute Humidity
The HMP45C measures the relative humidity. Relative humidity is defined by
the equation below:
100
e
e
RH
s
∗ =
(2)
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).
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. (2) 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.
12