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On a range Tdry bulb=15..40[deg C] and rh=10...90[%]you can calculate the Twet bulb[degC] at P=1006[mbar](=755 Torr)with max relative error~6% with an approximative polynomial model that you can put in a cells. For lower Tdry and rh the relative error will be higher.
Twet bulb= A*Tdry + B*rh + C*Tdry^2 + D*rh^2+ E*Tdry*rh + F
 where
A=5,391260E-01
B=1,047837E-01
C=-7,493556E-04
D=-1,077432E-03
E=6,414631E-03
F=-5,151526E+00
 regards
m777182