Relative Humidity Calculation - ABB SM500F User Manual

SM500F
Field mountable paperless recorder
E.2 Relative Humidity Calculation
Relative humidity is calculated using the following formula:
VPSw AP

------------------------------------------------------------ -
=
RH 100
Where:
VPSw = Saturation Vapor Pressure at Wet Bulb Temperature
VPSd = Saturation Vapor Pressure at Dry Bulb Temperature
Td = Dry Bulb Temperature
Tc = Wet Bulb Temperature
P = Total Atmospheric Pressure (1000 mbar)
A = Psychometric Constant (6.66 x 10–4)
RH = % of Relative Humidity
A relative humidity calculation requires two inputs, one from a wet bulb sensor and one from a dry bulb
sensor. Both of these inputs are incorporated into the equation as analog.
RH tables are based on the use of an aspirated psychrometer having an air velocity of at least 11.5 feet per
second or 3.5 meters per second across the bulb sensors.
Inputs used for wet and dry bulb measurement must be in the ranges 0 to 100°C or 32 to 212°F. The result
must be set to 0 to 100.0% RH.
Fig. E.1 (page 168) is an example of configuring a math block to perform a relative humidity calculation.
IM/SM500F Rev. Z
  Td Tw 
– –
VPSd
Appendix E – Math Equations
167