HMP155ATemperature and Relative Humidity Probe
The signal reference (white) and the power ground (black) are in common
inside the HMP155A. When the HMP155A temperature and relative humidity
are measured using a single-ended analog measurement, both the signal
reference and the power ground are connected to ground at the datalogger. The
signal reference and the power ground both serve as the return path for 12 V.
There will be a voltage drop along those leads because the wire itself has
resistance. The HMP155A draws approximately 4 mA when it is powered.
The wire used in the HMP155A (pn 9721) has resistance of 27.7 Ω/1000 feet.
Since the signal reference and the power ground are both connected to ground
at the datalogger, the effective resistance of those wires together is half of 27.7
Ω/1000 feet, or 13.9 Ω/1000 feet. Using Ohm’s law, the voltage drop (V
d
),
along the signal reference/power ground, is given by Eq. (1).
ft 1000 mV 6.55
ft 1000 9.13 mA 4
=
Ω∗=
∗= RI
V
d
(1)
This voltage drop will raise the apparent temperature and relative humidity
because the difference between the signal and the signal reference lead, at the
datalogger, has increased by V
d
. The approximate error in temperature and
relative humidity is 0.56 °C and 0.56% per 100 feet of cable length,
respectively.
7.5 Absolute Humidity
The HMP155A measures relative humidity. Relative humidity is defined by
the equation below:
(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 dewpoint
temperature. The saturation vapor pressure is the maximum amount of water
vapor that air can hold at a given air temperature. The relationship between
dewpoint 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, 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.
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