Campbell Scientific HMP50 Temperature and Relative Humidity Sensor - Manual (Page 13)

HMP50 Temperature and Relative Humidity Probe
6. Long Lead Lengths
Long lead lengths cause errors in the measured temperature and relative
humidity. The approximate error in temperature and relative humidity is
0.52
°C and 0.52% per 100 feet of cable length, respectively.
When long lead lengths are required and the above errors in temperature and
relative humidity are unacceptable, use the HMP45C temperature and humidity
probe.
Understanding the following details are not required for the general operation
of the HMP50 with Campbell Scientific’s dataloggers. The signal reference
and the power ground (black) are the same lead in the HMP50. When the
HMP50 temperature and relative humidity are measured, both the signal
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
HMP50 draws approximately 2 mA when it is powered. The wire used in the
HMP50 (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 2.4 5
ft 1000 6.2 2 mA 2
=
Ω ∗ =
∗ = R I
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
. The approximate error in temperature and
relative humidity is 0.35 °C and 0.35% per 100 feet of cable length,
respectively.
7. Absolute Humidity
The HMP50 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).
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