# Relative humidity calculations.

1. Nov 21, 2012

### maistral

This is totally pissing me off, I don't know what the heck am I doing wrong.

Alright, so I was given a temperature of 30°C at 30% relative humidity. I have to get the absolute humidity-

So I used Antoine; log(P)=7.96681-1668.21/(228+30); P = 31.6869mmHg.

0.3 x 31.6869 = 9.41517; 9.41517/(760-9.41517) = 1.25x10^-2.

Apparently the correct answer is 7.86x10^-3; and an air-water psychrometric chart says 0.008. What on earth am I doing wrong?

EDIT: Nevermind, I forgot that I have to multiply y 18/29.

Last edited: Nov 21, 2012
2. Nov 21, 2012

### Staff: Mentor

What you calculated was the mole fraction of water vapor. The absolute humidity is defined as the density of water vapor in the air, in units of gm/m3. You need to use the ideal gas law to calculate the absolute humidity: pM/RT

3. Nov 23, 2012

### SteamKing

Staff Emeritus
The Antoine constants you used are also valid from 60C to 150C.
For temps of 0C to 60C, the following constants are used:

A=8.10765
B=1750.286
C=235.0