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Conversion confusion: Rankine in ideal gas constant

  1. Mar 21, 2012 #1
    Hi all. I have ran into a problem in converting from imperial to SI units, as follows.

    The ideal gas constant for air is often given in imperial units as

    [tex]R = 1716 \frac{ft*lbf}{slug*°R} [/tex]

    where

    [tex] 1 ft*lbf = 1.356(10^{-3}) kJ [/tex]

    [tex] 1 slug = 14.59 kg [/tex]

    [tex] °R = \frac{9}{5}K [/tex]

    Thus making these substitutions gives

    [tex]R = 1716 \frac{ft*lbf}{slug*°R} = 1716 \frac{(1.356(10^{-3}) kJ)}{(14.59 kg)(\frac{9}{5}K)} = 0.0886 \frac{kJ}{kg*K}[/tex]

    Yet it is quite common knowledge that in reality,

    [tex] R = 0.287 \frac{kJ}{kg*K} [/tex]

    Somewhere, then, in my conversion, there must be an error. I found that if I instead reversed the temperature conversion to be °R = K*5/9, the desired result is given - but how could this be? Surely it is must be true that °R = K*9/5.

    Thanks for the help.
     
    Last edited: Mar 21, 2012
  2. jcsd
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