No announcement yet.

Humidity equation from Enthalpy, Temperature, and pressure

  • Filter
  • Time
  • Show
Clear All
new posts

  • Humidity equation from Enthalpy, Temperature, and pressure


    How can calculate the Humidity with Enthalpy, Temperature, and pressure given
    Enthalpy= 71.4 KJ/kg, Temperature= 32 C , Pressure= 1013.25 hpa

    I need an equation to calculate the humidity using Enthalpy, Temperature, and pressure

    thanks in advance!

  • #2


    Engineer Joe has worked for a while to come up with a feasible formula working between 0 and 100F to provide an enthalpy value based on dry bulb temperature and relative humidity. It uses a polynomial model to match Psat to temperature in order to avoid referencing a table (I wanted it to be able to work by a simple math steps.) You can see the polynomial in it's own bracket with several terms raised to the 4,3,2,1st order. That's the approximation model. It's >99% accurate between 0 and 100F. I'm posting here so that if someone googles the key terms, maybe this will pop up for them. I've googled this several times before with no results that were any good. So, future HVAC/excel users, here ya go!
     h = 0.24*OAT+(0.6219)*(0.01*(0.000000007401234*OAT^4 - 0.000000493526794*OAT^3 + 0.000071281097208*OAT^2 - 0.000489806163078*OAT + 0.039762055806989)*RH)/(14.7-(0.01*(0.000000007401234*OAT^4 - 0.000000493526794*OAT^3 + 0.000071281097208*OAT^2 - 0.000489806163078*OAT + 0.039762055806989)*RH))*(1061.2+0.444*OAT)
    Entalpy: h is in btu/lb(dry air)
    OAT: Temperature in F
    RH: Relative Humidity as a percentage. (e.g. 40% RH would just be 40, not 0.40)

    Obviously, plugging this into Excel, you would replace references to OAT and RH with cell references

    Hope this will help

    All credit goes to
    Engineer Joe
    If you feel like saying "Thank You" for the help received ...You can click on the "Like" icon ...just underneath ... ... in the bottom right corner ...