Campbell Scientific CR9000X Measurement and Control System - Manual (Page 328)

Section 8. Processing and Math Instructions
VaporPressure (Dest, Temp, RH)
The VaporPressure instruction calculates the ambient vapor pressure (Vp)
from previously measured values for air temperature and RH.
Syntax
VaporPressure( Dest, Temp, RH )
Remarks
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 VAPORPRESSURE PARAMETERS
Dest
Variable
The variable in which to store the results of the instruction.
Temp
Variable
The Temp parameter is the program variable that contains the value for the temperature sensor. The
temperature measurement must be in degrees C.
RH
Variable
The RH parameter is the program variable that contains the value for the relative humidity sensor. The
RH measurement must be in percent of RH.
WetDryBulb (Dest, Temp, WetTemp, Pressure)
The WetDryBulb instruction calculates vapor pressure in kilopascals from the
wet and dry-bulb temperatures in
°C. This algorithm type is used by the
National Weather Service:
Vp = Svpwet - A (1 + B ∗Tw)(Ta - Tw) P
Vp = ambient vapor pressure in kilopascals
Svpwet = saturation vapor pressure at the wet-bulb temperature in kilopascals
Tw = wet-bulb temperature,
°C
Ta = 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 The variable in which to store Vp (kPA).
Temp The variable containing air temperature (dry-bulb °C).
RH The variable containing RH (%).
WetTemp The variable containing wet-bulb temperature ( °C).
Pressure The variable containing atmospheric pressure (kPa).
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