HMP50 Temperature and Relative Humidity Probe
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
VaporPressure instruction to calculate vapor pressure from temperature and
relative humidity measurements (see Section 7.1). Edlog dataloggers must first
calculate the saturation vapor pressure and then calculate vapor pressure (see
Section 7.2).
TABLE 6. Wiring for Vapor Pressure Examples
Description
Color
CR10(X)
CR1000
Temperature
Black
SE 3 (2H)
SE 1 (1H)
Relative Humidity
White
SE 4 (2L)
SE 2 (2H)
Signal & Power Reference
Blue
G
G
Power
Brown
12 V
12 V
Shield Clear
G
7.1 CR1000 Vapor Pressure Example
The VaporPressure instruction has the following syntax:
VaporPressure ( Dest, Temp, RH )
Where:
Dest—the variable in which the results of the instruction will be stored.
Temp—the program variable that contains the value for the temperature
sensor. The temperature measurement must be in degrees C.
RH—the program variable that contains the value for the relative humidity
sensor. The RH measurement must be in percent of RH.
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Summary of Contents for HMP50
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