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When to use absolute or gauge pressure in computations

  1. Jul 17, 2015 #1
    Hi!

    One of our professors told us once that in solving problems, always use absolute pressure in the computations. Then one time, I was solving some problems from Dr. Felder's Principles of Chemical Processes book, I often noticed that the pressures used in computations were not always in absolute. That got me confused and I don't know which one to follow.

    I am guessing that it is a case to case basis, that the type of pressure to be used depends on the problem. In that case, what is the underlying concept that will guide us to correctly select the type of pressure to be used in the computations? Is there a short-hand guide?

    Thanks!
     
  2. jcsd
  3. Jul 17, 2015 #2
    Some short hand guides are possible like, "Always use absolute pressures when applying the ideal gas law."

    But it is hard to come up with a shorthand guide that covers all possible contexts.
     
  4. Jul 17, 2015 #3
    Oh okay, I understand. And just in case, will it be easier to give a guide on when not to use absolute pressure in the computation?
     
  5. Jul 17, 2015 #4

    Maylis

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    Gold Member

    If I remember correctly from my transport class, the professor said use absolute pressure for the pipe momentum balances, and use gauge for everything else. Don't quote me on it though
     
  6. Jul 17, 2015 #5
    I see! Thanks! That'll be handy.
     
  7. Jul 17, 2015 #6
    You need to use absolute pressure when you are dealing with situations in which the compressibility of the material comes into play, and the P-V-T behavior of the material is a factor. If the material is nearly incompressible (like a fluid), the gauge pressure is adequate.

    Here is an example. Remember the deflate-gate scandal. The footballs were supposed to be blown up to 12.5 psi (guage). The head of the chemistry department at Boston Collage claimed that, if the temperature dropped by about 20 F between the beginning of the game and halftime, that would have been enough to explain the observed lower pressure in the deflated footballs. A friend of mine used the ideal gas law to analyze this, and concluded that the BC professor was mistaken (and he got a big laugh about it). The friend's calculation showed that a 20 F drop was not enough to explain the lower pressure. However, my friend forgot to take into account that the measured pressure of the ball was gauge pressure, and that the ideal gas law requires the use of absolute pressure. When he corrected his mistake, he then got results that were in agreement with the BC professor. So the laugh was on him.

    Chet
     
  8. Jul 18, 2015 #7
    Thank you very much for the explanation! I also really appreciate that you gave a wonderful example. I think I understand it better now, particularly with the compressible fluids. But one thing still bugs me though. I noticed you used the word "adequate," which is kinda vague for me because it suggests that there are other options for incompressible fluids. What if I do an energy balance around a system involving an incompressible fluid (say water) that is being transported from point A to point B by creating a pressure difference using a pump, will it give similar (or the same) results if I used gauge pressure to that of using absolute?
     
  9. Jul 19, 2015 #8
    For incompressible fluids (or nearly incompressible fluids) in fluid mechanics applications, you can confidently use gauge pressure. But in thermodynamics applications, like in determining the change in free energy or enthalpy of a liquid phase, you still need to use absolute pressure.

    Chet
     
  10. Jul 20, 2015 #9
    I was thinking, for example in a mechanical energy balance wherein one has to compute for the ΔP/ρ of the fluid, if gauge pressure can be used? And will it affect the results if absolute pressure was used?

    Sorry if I'm asking too many questions. I really highly appreciate your responses!
     
  11. Jul 20, 2015 #10
    Using absolute pressure never gives you wrong results. But, as I said, if you don't need to consider the thermodynamic P-V-T behavior of the fluid, using gauge pressure is fine. When you write ΔP/ρ, the implication is that the fluid is incompressible. Otherwise, if the flow is compressible, you would need to be using the integral of dP/ρ, in which case the P-V-T behavior would need to be considered, and you would have to use absolute pressure.

    Chet
     
  12. Jul 20, 2015 #11
    Okay I think I get it now. Thanks a lot for the help! I really appreciate it! :)
     
  13. Jul 20, 2015 #12

    Maylis

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    Gold Member

    Also, whether you use gauge or absolute pressure makes no difference, since you are calculating a change in pressure
     
  14. Jul 20, 2015 #13
    Not if the density depends on pressure. You need to integrate dp/ρ.

    Chet
     
  15. Jul 29, 2015 #14
    Just to state the obvious, if you're doing any look-ups in the steam tables or thermo property routines, you'd need to be sure you're using absolute pressures.
     
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