k = 0.69502
constant = 0.3751349
zfsD = Op / T / k
topp = 6 * Exp(-zfsD)
toppZ = 6 * (1 - Exp(-zfsD)) / zfsD
bott = 1 + 2 * Exp(-zfsD)
CHI = (topp / bott + 2 *toppZ / bott) / 3
CHI = CHI * constant * g * g / (3 * (T - th))+tip