# Calculating Dew-point from RH & Temp

>>Anyone know the formula for this? Haven't been able to find one on the net that works out when you run the numbers through a psychrometric chart. Any help appreciated!

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mukundpa
Homework Helper
If I remembers it correctly ??

Due point is the temperature at which the moisture present in the air at room temperature is sufficient to saturate it.
As quantity of unsaturated moisture in the air is not changing till the due point, by decreasing temperature, vapour pressure remains constant. Hence

RH = unsaturated vapour pressure at room temp./ saturated V.P.at room temp.
= saturated v.P. at due point/saturated V.P. at room temp.

The experimental values of Saturated vapour pressure at different temperatures are given in the tables.

>>Okay, lesse. Since I know the temp. in the space, I can approximate my saturation vapor pressure (Temp>0 and constant).

The experimental values of Saturated vapour pressure at different temperatures are given in the tables.
>>Do you know by how much?
Then since I know the RH in the space and now know the saturation vapor pressure I can get the unsaturated vapor pressure in the space. But how do I use that data to determine the dew-point temp at which saturated equals actual i.e. RH=100?

mukundpa
Homework Helper
I got a table in an old booK for saturated vapour pressure, see if this can help you..

Temp. in deg C -- SVP in mm of Hg
-10--2.1
-5--3.1
0--4.6
5--6.5
10--9.2
20--17.5
30--31.7
40--55.1
50--92.3
60--149.2
70--233.5
80--355.1
90--525.8
100--760
110--1074.5
120--1489
150--3569
200--11647

Inbetween the two given temperatures it is calculated by finding the rate of increase in that range x increase in temp.
(The image file was too large to be attached)

>>Thanks mukundpa!