Campbell HMP60, Руководство по эксплуатации продукта

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Using Ohm’s law, the voltage drop (V
d
), along the signal reference/power ground wire, is given
by Eq. 1 (p. 12).
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
.
8.3 Absolute humidity
The HMP60 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).
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. 12) 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.
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 online by the data logger. CRBasic data loggers use the
VaporPressure() instruction to calculate vapor pressure from temperature and relative
humidity measurements (see CRBasic Example 2 (p. 18)).
HMP60 Temperature and Relative Humidity Probe 12