How to interpolate steam table values between listed temperatures

[j.m.m]

Well, I am studying for my engineering thermodynamics exam, and i don't seem to have written down some of the information about steam tables.

Im fairly confident using them, except i can't remember the exact formulas we use to get the numbers in between the numbers given. For example, i have a question as follows:

Use Rogers and Mayhew to determine specific internal energy change for water from 17.5 degrees Celsius to 86 degrees Celsius.

Well, my problem is the steam tables only give temperatures of 17 and 18, and 85 and 90.
17 bar - Hf 71.3
18 bar - Hf 75.5
85 bar - Hf 355.9
90 bar - Hf 376.9

how can I use this information to get the Hf of water at 17.5 degrees celsius, and 86 degrees celsius??

Thanks in advance

 

[FredGarvin]

Interpolation is your answer. This does make some assumptions, but for relatively small changes it should work fine.

http://www.wikihow.com/Do-a-Double-Linear-Interpolation

 

[j.m.m]

thanks a million

nice and quick reply too