k = 0.69502
constant = 0.3751349
zfsD = Op / T / k
topp = 6 * (Exp(zfsD) + 4 * Exp(-2 * zfsD))
toppZ = 2 * (9 * Exp(2 * zfsD) - 7 * Exp(zfsD) - 2 * Exp(-2 * zfsD)) / zfsD 
bott = Exp(2 * zfsD) + 2 * Exp(zfsD) + 2 * Exp(-2 * zfsD) 
CHI = (topp / bott + 2 *toppZ / bott) / 3
CHI = CHI * constant * g * g / (3 * (T - th))+tip