|WTT User's Guide|
ln(p/p°) = ln(pc/p°) + Tc/T ×(a1×τ + a2×τ1.5 + a3×τ2.5 + a4×τ5)
where p° = 1 kPa, pc = critical pressure and τ = 1 - T/Tc
The basis for this functional form is discussed in  and .
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