Campbell Scientific HC2S3 Temperature and Relative Humidity Probe - Manual (Page 30)

Appendix A. Absolute Humidity
A-2
CR1000 Program that Computes Vapor Pressure and Saturation Vapor Pressure
'CR1000 program that calculates Vapor Pressure
Public AirTC
Public RH
Public RH_Frac, e_Sat, e_kPa
DataTable(Temp_RH,True,-1)
DataInterval(0,60,Min,0)
Average(1,AirTC,IEEE4,0)
Sample(1,RH,IEEE4)
Sample(1,e_kPa,IEEE4)
EndTable
BeginProg
Scan(1,Sec,1,0)
PortSet(9,1) 'Turn on switched 12V
Delay(0,3,Sec) '3-second delay
'HC2S3 Temperature & Relative Humidity Sensor measurements AirTC and RH:
VoltSE(AirTC,1,mV2500,2,0,0,_60Hz,0.1,-40.0)
VoltSE(RH,1,mV2500,1,0,0,_60Hz,0.1,0)
If RH>100 And RH<103 Then RH=100
PortSet(9,0) 'Turn off switched 12V
'Calculate Vapor Pressure
'Convert RH percent to RH Fraction
RH_Frac = RH * 0.01
'Calculate Saturation Vapor Pressure
SatVP(e_Sat, AirTC)
'Compute Vapor Pressure, RH must be a fraction
e_kPa = e_Sat * RH_Frac
CallTable(Temp_RH)
NextScan
EndProg
A.1 Measurement Below 0°C
The HC2S3 provides a humidity reading that is referenced to the saturated
water vapor pressure above liquid water, even at temperatures below 0°C,
where ice might form. This is the common way to express relative humidity
and is as defined by the World Meteorological Organization. If an RH value is
required referenced to ice, the HC2S3 readings will need to be corrected.
One consequence of using water as the reference is that the maximum humidity
that will normally be output by the sensor for temperatures below freezing is as
follows:
100%RH at 0°C 82%RH at -20°C
95%RH at -5°C 78%RH at -25°C
91%RH at -10°C 75%RH at -30°C
87%RH at -15°C
In practical terms this means that, for instance, at -20°C the air is effectively
fully saturated when the sensor outputs 82%RH.