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4.1.3.2 Math Eample-1
Relative Humidity – PR20 Math application
/*How to Calculate Relative Humidity - Theory
Requirement: Two Analog Inputs, Type: RTD
AI1: To measure dry bulb temperature
AI2: To measure wet bulb temperature
First calculate the saturation vapor pressure (E) for both the dry-bulb (Td) and
wet-bulb (Tw) temperatures using the following equations:
Ew = 0.61078*EXP((17.269*Tw)/(Tw+237.3))*(Td-Tw)
Ed = 0.61078*EXP((17.269*Td)/(Td+237.3))*(Td-Tw)
In the above equations the temperatures units are Celsius and the saturation
vapor pressure units are millibars. The function "EXP" is the exponential and not
raising something to an exponent.
Then calculate actual vapor pressure (Ea) using the following equation:
Ea = Ew-0.63*(Td-Tw)
Relative Humidity is then calculated using the following equation:
RH = (Ea/Ed)*100
The units of relative humidity are in percent.
Here is an example of the using the equations:
Assume that your dry-bulb temperature (Td) = 40 C and your wet-bulb
temperature (Tw) = 30 C.
Ew = 0.61078*EXP((17.269*Tw)/(Tw+237.3))*(Td-Tw)
Ew = 0.61078*EXP((17.269*30)/(30+237.3))*(40-30)
Ew = 42.4262 millibars
Ed = 0.61078*EXP((17.269*Td)/(Td +237.3))*(Td-Tw)
Ed = 0.61078*EXP((17.269 * 40)/(40+237.3))*(40-30)
Ed = 73.7416 millibars
Ea = Ew-0.63*(Td-Tw)
Ea = 42.4262 - 0.63*(40-30)
Ea = 36.1262 millibars
RH = (Ea/Ed)*100
RH = (36.1262/73.7416)*100
RH = 48.99 %
74
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