Campbell Scientific CR9000X Measurement and Control System - Manual (Page 299)

Section 8. Processing and Math Instructions
BeginProg
Scan(250,mSec,0,1)
' Main Scan, 1 scan
VoltSE(Sig1(),256,0,5,-1,0,20,1,0.0) 'Measure Each Channel 256 times repeatedly
VoltSE(Sig2(),256,0,5,-2,0,20,1,0.0) 'Measure Each Channel 256 times repeatedly
VoltSE(Sig3(),256,0,5,-3,0,20,1,0.0) 'Measure Each Channel 256 times repeatedly
VoltSE(Sig4(),256,0,5,-4,0,20,1,0.0) 'Measure Each Channel 256 times repeatedly
VoltSE(Sig5(),256,0,5,-5,0,20,1,0.0) 'Measure Each Channel 256 times repeatedly
CallTable(PSDFFT) 'Table runs FFTs on the measurements
NextScan
GetRecord(Sets,PSDFFT,1)
'Retrieve the FFT results
COVSPA (CoVarVal(1),4,129,Sets(517),Sets(1)) 'Perform Spatial Covariances
EndProg
DewPoint (Dest, Temp, RH)
The DewPoint instruction is used to calculate the dew point temperature from
dry bulb temperature and relative humidity measurements in the program.
Syntax
DewPoint ( Dest, Temp, RH )
Remarks
The DewPoint instruction calculates the dew point temperature from previously
measured values of RH and air temperature. While end results may not be quite
as accurate as those from a dedicated dew point sensor, they are acceptable for a
wide range of applications.
Parameter
& Data Type Enter DEWPOINT PARAMETERS
Dest
Variable
The variable in which to store the dew point temperature ( °C).
Temp
Variable
The variable that contains air temperature ( °C).
RH The variable that contains RH (%).
Calculating Dew Point
Measure the relative humidity (RH) and air temperature (T
a
; units °C) with the
appropriate instruction for the sensors you are using.
Dew point temperature is calculated as follows:
1. The saturation vapor pressure (S
vp
; units kPa) is calculated using Lowe’s
equation (see SatVP).
2. The vapor pressure (V
p
; units kPa) is calculated from V
p
= RH ∗ S
vp
/ 100).
3. The dew point (Td; units °C) is calculated from the inverse of a version of
Tetens’ equation, optimized for dewpoints in the range -35 to 50 °C:
T
d
= (C
3
∗ ln(V
p
/ C
1
)) / (C
2
. ln(V
p
/ C
1
))
where: C
1
= 0.61078
C
2
= 17.558
C
3
= 241.88
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