The first candidate for ϕ ϕ was the critical compressibility factor Z c = p c v c / ( R T c ) , but because that quantity is difficult to measure accurately, the acentric factor developed by Kenneth Pitzer, ω ω = − − log 10 [ p r ( T r = 0.7 ) ] − − 1 , is more useful. The saturation pressure in this situation is represented by a one parameter family of curves, p r s = p s ( T r , ω ω ) . Several investigators have produced correlations of saturation data for a number of substances, the best is that of Dong and Lienhard, ln ⁡ ⁡ p r s = 5.37270 ( 1 − − 1 / T r ) + ω ω ( 7.49408 − − 11.181777 T r 3 + 3.68769 T r 6 + 17.92998 ln ⁡ ⁡ T r )