Relative Humidity Calculation - ABB ScreenMaster RVG200 Operating Instructions Manual

ScreenMaster RVG200
Paperless recorder

7.13.3 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
Tw = Wet bulb temperature
P = Total atmospheric pressure (1000 mbar)
A = Psychometric constant (6.66 x 10–4)
RH = % of relative humidity
A relative humidity (RH) 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 that has an air velocity of at least 3.5 meters
(11.5 feet) per second across the bulb sensors.
Inputs used for wet and dry bulb measurement must be in the ranges 0 to 100 °C (32 to 212 °F). The result
must be set to 0 to 100.0 % RH.
Fig. 7.20 (page 135) is an example of configuring a math block to perform a relative humidity calculation.
134
  Td Tw 
– –
VPSd
7 Configuration
OI/RVG200-EN