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I am doing a rather complicated heat transfer calculation in a low pressure(sub- atmospheric) condition. Any idea where I might find the viscosity of water vapor as a function of pressure? There is an estimation expression for thermal conductivity in Perry's 7th Ed. that requires this information. Alternatively, if you know of a source for data on thermal
conductivity of water vapor as a function of pressure that would be even better. Thanks in advance for any suggestions.

The reason you're having trouble finding this information is that thermal conductivity is very weakly dependent on pressure.  Likewise, it is not extremely dependent on temperature either.  Unless your computation involves what I might call "a ridiculous number of significant figures", you should use the thermal conductivity reported for water vapor at 1 bar and 298 Kelvin which is 0.0179 W/(m K).   If you really need a correlation, I'd suggest the following from
"Properties of Gases and Liquids": Conducitivity = .007341 - .00001013*T + .0000001801*T^2 - (9.1 x 10^-11)*T^3
where T is in Kelvin and Conductivity is in W/(m K).  This equation is valid from 273 to 1070 K.  To relate it pressure, you'd have to use an equation of state that converts the pressure and volume to the corresponding temperature, then use the above equation to get the thermal conductivity.  A lot of work for what will be small changes in this value, but I guess it would work.
Good Luck