Campbell Scientific HMP35C Temperature and RH Sensor - Manual (Page 11)

HMP35C TEMPERATURE AND RH PROBE
9
;Turn the HMP35C on.
;
05:
Do (P86)
1:
41 Set Port 1 High ;Yellow wire (C1)
;Pause 150 milliseconds, before making the measurement, so the
;probe can stabilize on the true relative humidity.
;
06:
Excitation with Delay (P22)
1:
1 Ex Channel
2: 0 Delay W/Ex (units = 0.01 sec)
3: 15 Delay After Ex (units = 0.01 sec)
4: 0 mV Excitation
;Measure the HMP35C relative humidity.
;
07:
Volt (SE) (P1)
1:
1 Reps
2: 25 2500 mV 60 Hz Rejection Range ;CR500 (2500 mV); CR23X (1000 mV)
;21X, CR7 (5000 mV)
3: 4 SE Channel ;Green wire (SE 4), Purple wire (AG)
4: 2 Loc [ RH_pct ]
5: 0.1 Mult ;See Table 5 for alternate multipliers
6: 0 Offset
;Turn the HMP35C off.
;
08:
Do (P86)
1:
51 Set Port 1 Low ;Yellow wire (C1)
8. ABSOLUTE HUMIDITY
The HMP35C measures the relative humidity.
Relative humidity is defined by the equation
below:
RH
e
e
100
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 in
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, the air can hold
more water vapor. 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 meaningless. A
more useful quantity is the mean vapor
pressure. The mean vapor pressure can be
computed on-line by the datalogger (Example
4).