Function DewpointTemperature

src/HumidAirProp.cpp:1263–1331

}

double DewpointTemperature(double T, double p, double psi_w) {
    int iter = 0;
    double p_w = NAN, eps = NAN, resid = NAN, Tdp = 0, x1 = 0, x2 = 0, x3 = NAN, y1 = 0, y2 = NAN, T0 = NAN;
    double p_ws_dp = NAN, f_dp = NAN;

    // Make sure it isn't dry air, return an impossible temperature otherwise
    if ((1 - psi_w) < 1e-16) {
        return -1;
    }
    // ------------------------------------------
    // Iteratively find the dewpoint temperature
    // ------------------------------------------

    // The highest dewpoint temperature possible is the dry-bulb temperature.
    // When they are equal, the air is saturated (R=1)

    p_w = psi_w * p;

    // 611.65... is the triple point pressure of water in Pa
    if (p_w > 611.6547241637944) {
        T0 = IF97::Tsat97(p) - 1;
    } else {
        T0 = 268;
    }
    // A good guess for Tdp is that enhancement factor is unity, which yields
    // p_w_s = p_w, and get guess for T from saturation temperature

    iter = 1;
    eps = 1e-5;
    resid = 999;
    while ((iter <= 3 || std::abs(resid) > eps) && iter < 100) {
        if (iter == 1) {
            x1 = T0;
            Tdp = x1;
        }
        if (iter == 2) {
            x2 = x1 + 0.1;
            Tdp = x2;
        }
        if (iter > 2) {
            Tdp = x2;
        }

        if (Tdp >= 273.16) {
            // Saturation pressure at dewpoint [Pa]
            p_ws_dp = IF97::psat97(Tdp);
        } else {
            // Sublimation pressure at icepoint [Pa]
            p_ws_dp = psub_Ice(Tdp);
        }
        // Enhancement Factor at dewpoint temperature [-]
        f_dp = f_factor(Tdp, p);
        // Error between target and actual pressure [Pa]
        resid = p_w - p_ws_dp * f_dp;

        if (iter == 1) {
            y1 = resid;
        }