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Submitted Chris Haslego, Nov 21 2011 11:21 AM | Last updated Nov 21 2011 01:29 PM

Category:	Thermodynamics
Question:	How do I convert relative humidity to absolute humidity?
Keywords:	relative,absolute,humidity
Answer:	Given the relative humidities at various temperatures, calculate the absolute humidities (in grains of water per pound of air) at those temperatures.The absolute humidity of the ambient air is the saturation humidity multiplied by the percentage relative humidity and divided by 100: H_abs = (H_sat )(RH / 100)where:H_abs = absolute humidity at a given tempH_sat = saturation humidity at the same given tempRH = percentage relative humidity at the same given tempFirst, you must calculate the saturation humidity at each of your temperature data points:H_sat = (7000)(18.02 / 28.85)(P_S)/(P_amb - P_S)where:H_sat = grains of water per pound of air7000 = grains per pound18.02 = molecular weight of water28.85 = molecular weight of dry air (taken as 79 vol % nitrogen, 21 vol % oxygen)P_S = vapor pressure of water at temperature TP_amb = ambient pressure(You will need a good set of steam tables to look up the vapor pressure of water at each of your temperature data points)Example calculation of H_sat at 68 ^oF and an ambient atmospheric pressure of 14.696 psia:From the steam tables, the vapor pressure of water is 0.3389 psia at 68 ^oF.H_sat = (7000)(18.02/28.85)(0.3389)/(14.696 - 0.3389) = 103.2 grains of water/lb of airNow if your relative humidity data point at 68 ^oF was say 60%, then:H_abs = (103.2)(60/100) = 61.9 grains of water/lb of airIt is quite easy to set up a spreadsheet to convert your relative humidity data as per the above example ... then all that would be needed is to enter the water vapor pressure values at each of your temperature data points.