Measured data for CH4 I from:
Tickner, A.W., and F.P. Lossing 1951.  The measurement of low vapor pressures by means of a mass spectrometer.  J. Phys. Chem. 55, 733-740.
T(K)     P(bar)
77.65   1.32e-02
74.15   6.58e-03
70.05   2.63e-03
67.15   1.32e-03
64.65   6.58e-04
61.55   2.63e-04
59.35   1.32e-04
57.35   6.58e-05
54.85   2.63e-05
53.05   1.32e-05
51.55   6.58e-06
49.55   2.63e-06
48.15   1.32e-06
