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Why does a warm weighing cup weigh less?

  1. Oct 27, 2011 #1
    One of my jobs at work is to determine the amount of oil and grease present in wastewater samples. In the beginning of the procedure I usually take a tray of oil and grease cups out of a 125 degree oven and place it in a desiccator for roughly an hour before weighing on a balance. After extracting the oil and grease out of the water and into the cup it is again dried in an oven and placed in a desiccator to cool before weighing back.

    The first cup in the tray I use as a constant which has to be within + or - 0.0005g of it's initial weight. If I leave the cups cooling too long, my first cup will be up to 2-3mg heavier than where it started. If I pull them too early then my constant is light.

    I realize this is a very basic question but why does this happen? Is it because the heat transferring to the gas surrounding the cup makes the gas rise and therefore exerts less pressure down on the cup when it is hot? When the cup is cold it can't excite the gas away as well?
     
  2. jcsd
  3. Oct 27, 2011 #2
    I did not quite understood the process...is there a pool of wastewater and then you bring an empty cup and dunk into waste water to fill it up? or what?
     
  4. Oct 27, 2011 #3
    the process for extraction doesn't really matter.

    I initially weigh an empty metal cup that has been cooled in a desiccator after being removed from an oven. Later, I take the same cup which has had nothing added, dry it in an oven, cool it in a desiccator and weigh it again on a balance.
     
  5. Oct 27, 2011 #4

    DaveC426913

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    Very difficult to give an answer here.

    My hypothesis is that, as is cools and sits it begins absorbing a tiny amount of water vapour.

    Wait. The cup is metal? OK, that's a salient piece of information. I assumed it was paper.
     
  6. Oct 27, 2011 #5
    yes. I'm pretty sure its stainless steel.

    So if the cup has expanded and therefore less dense it affects the weight?
     
  7. Oct 27, 2011 #6

    DaveC426913

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    OK, I'm starting to understand. Cup 1 is the control. It does not get filled with anything.

    Everything else aside, if you left out all other cups and even the entire oil measuring process, would this one cup still change weight?
     
  8. Oct 27, 2011 #7

    DaveC426913

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    No.

    It may be less dense because it has expanded but that has not changed its weight.
     
  9. Oct 27, 2011 #8
    yes the control does change and you are correct to think it doesn't get put through any extraction process. it just sits there in the tray. It is only heated in the oven and cooled in a desiccator.
    so basically, in order for me to get a weight within 0.0005g of its initial weight I have to weight it back at the same temperature.
     
  10. Oct 27, 2011 #9

    DaveC426913

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    What would happen if you did your entire process with only the control cup and no oil at all?

    I'm trying to eliminate variables.
     
  11. Oct 27, 2011 #10
    the control cup doesn't ever get anything added to it.

    I weigh it then heat it up then cool it and weigh it again.
     
  12. Oct 27, 2011 #11
    how long does it take you to weigh the cup? could the temperature of the cut be affecting the operation of the scale?...just reaching here...
     
  13. Oct 27, 2011 #12
    the balance has sliding doors on it so the cup sits inside of an enclosed box. It only takes 10-20 seconds for me to get the cup out of the desiccator and onto the balance.
     
  14. Oct 27, 2011 #13
    my point is whether the balance is a mechanical balance with some kind of delicate spring that might change its coefficient should it get warmer or something line that.
     
  15. Oct 27, 2011 #14
    tell me if this is the wrong way to think about it?

    If you could weigh a constant mass of gas at temp 35 degrees then that same mass of gass would weigh less at 40 degrees because it has expanded and is trying to rise.

    Why would this not work for a metal?

    I dunno im just confused now..
     
  16. Oct 27, 2011 #15
    its an analytical balance. the tray is not anywhere near its inner workings.
     
  17. Oct 27, 2011 #16
    hhhmmm...when you are looking for 0.0005g, that does not sound too crazy, after all.

    It should actually be possible to rule that in or out...all you need to do is find the density of air and find out how much volume of air weighs 0.0005g....is it possible that the cup increased in volume that much? Be careful, though, the volume of the empty cup does not include its hollow inside, in this case, it is totally submerge in air and the volume is only the thickness of the cup time its total surface area...
     
  18. Oct 27, 2011 #17

    DaveC426913

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    Yes. But it is still in the vicinity of the other cups with oil and such. I'm trying to deduce what would happen if that were completely eliminated as a variable.
     
  19. Oct 27, 2011 #18
    just to give some examples:


    typical initial weight is 20.3020g
    a cold cup can weigh as high as 20.3055g
    a warm cup as low as 20.2985g
     
  20. Oct 27, 2011 #19
    it is true that the control is heated in an oven along with the other cups sitting in a tray. This is to drive off all the moisture. Then they are all placed together in a tray in a desiccator. I am pretty sure I would get the same results by using the control all by its lonesome.
     
  21. Oct 27, 2011 #20

    DaveC426913

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    I think the issue is that you may have actually stumbled onto a bona fide error. I don't believe it should be lighter when warmer, i.e. there may be an error in the procedure that's contaminating your results.
     
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