HMP45C Temperature and RH Probe
12
;Measure the HMP45C temperature.
;
03: Volt (Diff) (P2)
1: 1 Reps
2: 5 2500 mV Slow Range ;CR510, CR500 (2500mv); CR23X (1000 mV);
21X, CR7 (5000 mV)
3: 2 DIFF Channel
4: 1 Loc [ T_C ]
5: .1 Mult ;See Table 4 for alternative multipliers
6: -40 Offset ;See Table 4 for alternative offsets
;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
4: 2 Loc [ RH_pct ]
5: .1 Mult ;See Table 5 for alternative multipliers
6: 0 Offset
;Turn the HMP45C off.
;
05: Do (P86)
1: 51 Set Port 1 Low
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 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).
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. (2) 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 vaporise the water. However, the actual amount of water vapour
(50 pagine)
Manymanuals.com
Manymanuals.de
Manymanuals.fr
Manymanuals.it
Manymanuals.pl
Manymanuals.cz
Manymanuals.es
Manymanuals-pt.com
Commenti su questo manuale