Vaisala HMP60 Manual download pdf (Page 20)

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

), 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

(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

7.6 Absolute Humidity

The HMP60 measures the relative humidity. Relative humidity is defined by

the equation below:

100

=∗

(2)

where RH is the relative humidity, e is the vapor pressure in kPa , and e

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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Vaisala HMP60 Manual Page 20

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