# Relative humidity calculations.

by maistral
Tags: calculations, humidity, relative
 P: 60 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.
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P: 2,966
 Quote by 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.
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
 HW Helper Thanks P: 4,309 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

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