User manual

Appendix A. Absolute Humidity
A-2
CR1000 Program that Computes Vapour Pressure and Saturation Vapour Pressure
'CR1000 program that calculates Vapour 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 Vapour Pressure
'Convert RH percent to RH Fraction
RH_Frac = RH * 0.01
'Calculate Saturation Vapour Pressure
SatVP(e_Sat, AirTC)
'Compute Vapour 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
vapour 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.