k = 0.69502
constant = 0.3751349
zfsD = Op / T / k
topp = 0.75 * (1 + 9 * Exp(-2 * zfsD))
toppZ = 3 * (1 + 3 * (1 - Exp(-2 * zfsD)) / 4 / zfsD)
bott = 1 + Exp(-2 * zfsD)
CHI = (topp / bott + 2 *toppZ / bott) / 3
CHI = CHI * constant * g * g / (3 * (T - th))+tip