User Manual

HMP60 Temperature and Relative Humidity Probe
reference and power ground are connected to ground at the datalogger. The
signal reference/power ground lead serves as the return path for 12 V. There
will be a voltage drop along this lead because the wire itself has resistance. The
HMP60 draws approximately 2 mA when it is powered. The wire used in the
HMP60 (P/N 18159) has resistance of 26.2 Ω/1000 feet. Using Ohm’s law, the
voltage drop (V
d
), along the signal reference/power ground lead, is given by
Eq. (1).
ft 1000 mV 2.45
ft 1000 6.22 mA 2
=
Ω∗=
∗= RI
V
d
(1)
This voltage drop will raise the apparent temperature and relative humidity
because the difference between the signal and signal reference, at the
datalogger, has increased by V
d
.
7.6 Absolute Humidity
The HMP60 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,
there is more energy available 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 meaningless. A more useful
quantity is the mean vapor pressure. The mean vapor pressure can be
computed on-line by the datalogger. CRBasic dataloggers use the
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