Specifications

Section 8. Processing and Math Instructions
8-37
VaporPressure (Dest, Temp, RH)
The VaporPressure instruction calculates the ambient vapor pressure (Vp) from
previously measured values for air temperature and RH. The instruction first
calculates saturation vapor pressure from air temperature using Lowe’s
equation (see SatVP). Vapor pressure is then calculated by multiplying by the
fractional RH:
Vp = SatVp x RH/100
Parameter
& Data Type
Enter
Dest
Variable or
Array
The variable in which to store the results of the instruction.
Temp
Variable
The program variable that contains the value for the temperature sensor. The
temperature measurement must be in degrees C.
RH
Variable
The program variable that contains the value for the relative humidity
sensor. The RH measurement must be in units of percent.
WetDryBulb (Dest, DryTemp, WetTemp, Pressure)
Calculates vapor pressure in kilopascals from the wet and dry-bulb
temperatures in
°C. The WetDryBulb instruction uses an equation given by
List, Robert. J.: 1966, Smithsonian Meteorological Tables, Sixth Revised
Edition, Smithsonian Institution, Washington, D.C., page 365. This equation is
also used by the National Weather Service. The results are in kPa.
V
p
= Svp
wet
- A (1 + B∗T
w
)(T
a
- T
w
) P
V
p
= ambient vapor pressure in kilopascals
Svp
wet
= saturation vapor pressure at the wet-bulb temperature in kilopascals
T
w
= wet-bulb temperature, °C
T
a
= ambient air temperature, °C
P = air pressure in kilopascals
A = 0.000660
B = 0.00115
Although the algorithm requires an air pressure entry, the daily fluctuations are
small enough that for most applications a fixed entry of the standard pressure
at the site elevation will suffice. If a pressure sensor is employed, the current
pressure can be used.
Parameter
& Data Type
Enter
Dest
Variable or
Array
The variable in which to store Vp (kPA).
DryTemp
Variable
The variable containing air temperature (dry-bulb
°C).
WetTemp
Variable
The variable containing wet-bulb temperature (
°C).