Hydrocarbon vapors and natural gases
The LMNO Engineering calculation takes the user input temperature and extracts viscosity from the graph on page A-5 in Crane (1988), performing linear interpolation if needed. The input temperature is restricted to the range 0 < temperature < 1000oF.

Other gases
Gas viscosity is computed using Sutherland's formula (Crane, 1988):

    μ = μo*(a/b)*(T/To)3/2

    a = 0.555To + C
    b = 0.555T + C