63
promesstec GmbH
I Niedersachsenstraße 4 I D- 48465 Schüttorf I Tel.:+49 (0)5923/ 90 229 0 I Fax:+49 (0)5923/ 90 229 29
E-Mail: [email protected] I Internet: www.promesstec.com
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 %
*/ End of Theory
5 Math channels are required to calculate one RH.
Td = AI1, analog input for dry bulb temperature (PT100)
Tw =AI2, analog input for wet bulb temperature (PT100)
Math1 = EXP((17.269*AI1)/(AI1+237.3))
Math2 = Ed1 = 0.61078*Math1*(AI1-AI2)
Math3 = EXP((17.269*AI2)/(AI2+237.3))
Math4 = Ew1 = 0.61078*Math3*(AI1-AI2)
Ea = Ew - 0.63 * (Td - Tw)
Math5= RH1 = ((Math4-0.63*(AI1-AI2))/Math2)*100