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How can I obtain an apparent specific gravity, of a plastic bag? Answered

I need to verify an Apparent Specific gravity of a plastic bag- like the type used for a food vacuum sealer. I'm not sure how to go about it. I think I have all the needed items; scale, water bath, even a vacuum sealer if needed. I just can't find the method, or the formula used to calculate. I even called the manufacturer of the bag I'm using, and they had no idea how they came up with their data. Thank you for any ideas you can share with me. Jodie

3 Replies

klee27x (author)2009-07-13

I'd cut out a small section of the material. Then I'd use a very small lab scale to weigh it to within a fraction of a gram. Then roll it up and place it in a thin, graded cylinder filled with water and a drop of dish soap. Lightly manipulate it with tweezers to get it all the way submerged and all the air bubble out without making suds The soap will help get the air bubbles out. No string needed, cuz it'll stick to the walls. If you use a tank large enough to take a whole bag that is not carefully rolled up, you won't be able to discern any change in water level.

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klee27x (author)klee27x2009-07-13

The actual formula is (DENSITY)/(DENSITY of WATER). Since water weighs 1g/mL, all you have to do is calculate the density of your bag in g/mL then drop the units.

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kelseymh (author)2009-02-26

For the record, I presume you know the definition of specific gravity.

"Apparent" specific gravity means you're dealing with something that has trapped air, and so the bulk density of the object is lower than the density of small samples of the raw material.

the easiest way to do something like this would be to seal up your bag, getting "as much" air out as is possible/reasonable. Weigh the sealed bag on a precision scale (i.e., down to milligrams if possible). Use Archimedes' method -- submersion in a water tank, e.g. by anchoring it with a fine thread to the bottom -- to get the volume, then just divide to find the density.

This technique will be subject to substantial measurement uncertainty, so you should to several trials with the same sealed bag and average the results, and you should also probably make several samples and do (multiple) measurements on each one.

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