Campbell EE181, Руководство по эксплуатации продукта

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When long cable lengths are required and the above errors in temperature and relative humidity
are unacceptable, use the HMP155A temperature and relative humidity probe instead.
Understanding the following details are not required for the general operation of the EE181 with
Campbell Scientific data loggers. The signal reference and the power ground (black) are the
same wire in the EE181. When the EE181 temperature and relative humidity are measured, both
the signal reference and power ground are connected to ground at the data logger. The signal
reference/power ground wire serves as the return path for 12 V. There will be a voltage drop
along this wire because the wire has resistance. The EE181 draws approximately 1.2 mA (worst
case) when it is powered. The wire used in the EE181 has resistance of 25.67 Ω/1000 feet. Using
Ohm’s law, the voltage drop (V
d
), along the signal reference/power ground wire, is given by Eq. 1
(p. 13).
Eq. 1
This voltage drop will raise the apparent temperature and relative humidity because the
difference between the signal and signal reference at the data logger has increased by V
d
.
8.4 Absolute humidity
The EE181 measures relative humidity. Relative humidity is defined by the following equation:
Eq. 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). Relative humidity is relative to
saturation above water, even below freezing point. This is why these sensors should not measure
100% RH below zero degrees C, as described in
Measurement below 0 °C (p. 14).
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 (p. 13) that a change in air temperature will change the relative humidity,
without causing a change absolute humidity.
EE181 Temperature and Relative Humidity Probe 13