AN607

Rev. 1.7

29

Estimating RH with Heating

Equation Development

The Magnus equation for partial pressure of water in air is:

for a given relative humidity RH in percent and temperature in ºC. Typical values for A1, B1, and C1 are 17.625,

243.04, and 610.94, respectively.

If the air is heated, the partial pressure does not change, and the apparent relative humidity drops according to:

or

this can be simplified to:

P

RH

100

----------





C

A

1

T

B

1

T

+



---------------------





=

P

RH



100

----------





C

A

1

T

T

+



B

1

T

T

++



-----------------------------------





=

RH



RH

e

A

1

T



B

1

T

+



---------------------

e

A

1

T

T

+



B

1

T

T

++



-----------------------------------



-----------------------------------------

=

RH



RHe

A

1

–

B

1

T



B

1

T

+

 B

1

T

T

++





--------------------------------------------------------------

=