Psychrometrics.m

function [Tdb, w, phi, h, Tdp, v] = Psychrometrics (Tdb_in, w_in, P)
    % Modified version of Psychometrics by Tea Zakula
    % MIT Building Technology Lab
    % Tdb (dry bulb temperature) and Tdp(dew point temperature) in C
    % w (humidity ratio) in kg/kg of dry air
    % phi (relative humidity) in %
    % h (enthalpy) in J/kg of dry air
    % v (specific volume) in m3/kg of dry air
    % P (Atmospheric Station Pressure) in Pa

    c_air = 1006;   %J/kg, value from ASHRAE Fundamentals
    hlg = 2501000;  %J/kg, value from ASHRAE Fundamentals
    cw  = 1860;     %J/kg, value from ASHRAE Fundamentals
    P = P/1000;     % convert from Pa to kPa

    Tdb = Tdb_in - 273.15;
    w = w_in;

    % phi calculation from Tdb and w
    Pw = w*P/(0.621945+w);              %partial pressure of water wapor
    Pws = Saturation_pressure(Tdb);
    phi = Pw/Pws*100;

    % enthalpy calculation from Tdb and w
    h = c_air*Tdb+w*(hlg+cw*Tdb);

    % specific volume calculation from Tdb and w
    v = 0.287042*(Tdb+273.15)*(1+1.607858*w)/P;

    % dew point calculation from w
    pw = (P*w)/(0.621945+w); % water vapor partial pressure in kPa
    alpha = log(pw);
    Tdp = 6.54 + 14.526*alpha+0.7389*(alpha^2)+0.09486*(alpha^3)+0.4569*(pw^0.1984); % valid for Tdp between 0 C and 93 C
end

function [Pws] = Saturation_pressure(Tdb)
    T = Tdb+273.15;
    Pws = exp(-(5.8002206e3)/T+1.3914993+-(4.8640239e-2)*T+(4.1764768e-5)*(T^2)-(1.4452093e-8)*(T^3)+6.5459673*log(T)); %in Pa
    Pws = Pws/1000; % in kPa
end