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When is gauge pressure absolute pressure?

  1. Oct 4, 2016 #1
    1. The problem statement, all variables and given/known data

    Hi Physics gurus, this question was in my Chem Eng exam, and I can't agree with my lecturer's answer. He makes a LOT of mistakes, so it's hard to know when he's being clever or reallllly dumb.

    The question I have issue with is: "Is the pressure a manometric or an absolute pressure?"

    upload_2016-10-5_3-18-14.png


    [Notes: 1.The tank is described as 'closed'. 2. Mentally remove the 8cm column, that relates to another part of the question where water has collected inside the gauge and causes an error]

    I have answered that it is absolute, since the space above the manometric scale contains a vacuum. That means that the atmosphere is not impinging on the system in any way so is irrelevant to the gauge. His answer is "There is not atmospheric pressure coming in so the pressure is manometric" (a direct quote).

    2. Relevant equations

    I queried the solution by email and he responded with the classic equation:

    Pabs=Pgauge+Patm

    and since we have not applied this formula and added (or his word, 'accounted') atmospheric pressure to the gauge reading then it is gauge pressure only.

    I agree this applies in some cases (specifically where atmospheric pressure is acting on the system), but I believe a more general equation would be:

    Pabs=Pgauge+P1

    where P1 is the pressure acting on the right-hand column. And since there is a vacuum above this column, then

    P1=0 and

    Pabs=Pgauge

    Could we not place this system in a bell jar or a hyprebaric chamber and obtain an unchanged manometer reading?


    3. The attempt at a solution

    This appears to be a pretty definitive measure of absolute pressure. The only way the gauge can read zero (again assuming the 8cm column of water were not present) is if the vessel contains a vacuum. Not a practical system I know, but given the question as asked that is my interpretation.

    Am I missing something fundamental? Or is there some subtle difference between the terms 'manometric pressure' and 'gauge pressure'? Or has my lecturer just misunderstood an utterly basic concept in his field of expertise?
     

    Attached Files:

  2. jcsd
  3. Oct 4, 2016 #2
    I agree with you. The manometer is not measuring gauge pressure. It is reading absolute pressure. So, unless the system is sitting in a vacuum chamber and the right hand leg of the manometer is open at the top, the pressure being measured is absolute. Of course, in this latter situation, if the surrounding atmosphere is vacuum, the absolute pressure and gauge pressure are the same.

    Is this just an argument about terminology, or does this affect a grade on homework or a test?
     
  4. Oct 4, 2016 #3
    Thanks. Yes, it is an exam question so although it is worth few marks I feel it is important to straighten out.

    I suffered in this exam because I (almost) completed all three questions, then the lecturer realised the exam was too hard, so he decided to excise Q3 to be 'fair'... but unfortunately I cut this question short in order to give time to Q3 and lost marks on this one as a result.

    Not impressed! But with final exam coming up, I am not inclined to pursue it further until after semester ends.
     
  5. Oct 4, 2016 #4
    I hear ya Bro.
     
  6. Oct 23, 2016 #5
    Follow-up: I chased the lecturer up again about this and another issue and he now 'agrees' with me. Thankfully my mark has improved from 55 to 75%. Pity about the other students affected..
     
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