How do you calculate relative humidity?

How do you calculate grains per pound from relative humidity and temperature?

• 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: Habs = (Hsat )(RH / 100) where: Habs = absolute humidity at a given temp Hsat = saturation humidity at the same given temp RH = percentage relative humidity at the same given temp First, you must calculate the saturation humidity at each of your temperature data points: Hsat = (7000)(18.02 / 28.85)(PS)/(Pamb - PS) where: Hsat = grains of water per pound of air 7000 = grains per pound 18.02 = molecular weight of water 28.85 = molecular weight of dry air (taken as 79 vol % nitrogen, 21 vol % oxygen) PS = vapor pressure of water at temperature T Pamb = 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 Hsat 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. Hsat = (7000)(18.02/28.85)(0.3389)/(14.696 - 0.3389) = 103.2 grains of water/lb of air Now if your relative humidity data point at 68 oF was say 60%, then: Habs = (103.2)(60/100) = 61.9 grains of water/lb of air It 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. You can also just use calculator apps to do it for you - like EZ Air: http://www.parkssoft.com/ezair.html