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I am trying to do density determinations on a large number of individual rock samples. I know the standard dry weight - wet weight methods but have an idea that might be better.

I can buy a completely water proof digital scale. My question is this " can accurate wet weight values be obtained by submerging the scale, zeroing it, and then weighing the rock sample on the submerged scale. The rock would be completely submerged as well.

I cannot fugure out the fatal flaw in this method, but suspect that there is one.

Any help with this question would be greatly appreciated.


Bob Marvin

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This is just a different way of implementing the dry weight - wet weight method, isn't it? – DJohnM Jun 15 '13 at 19:53

Assuming the rock sinks to the bottom of a water container, you can easily measure the volume of the piece of rock. If your container is cylindrical or rectangular (to make the calculation easy), then simply measure the added water height times the cross-sectionnal area of the cylinder. This would be the volume of the rock. Then, simply weight it, and there you go.

That is if what you're looking for is simply the density of the rock.

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Note that this only gives you the average mass density of the rock. Also, if the number of samples is very large, I suggest labeling them clearly. Then first weigh all the samples before individually submerging them completely. And just another tip: if the rock size allows it, use a sufficiently precise measuring cup filled with some accurately known volume of water. That way, you needn't measure any heights or areas, you can simply read off the volume difference. – Wouter Jun 15 '13 at 19:46
I cannot imagine how one might be interested in the density field of a rock ;) – Mathusalem Jun 15 '13 at 19:50
Merely pointing it out. It is indeed unlikely the OP is interested in the density field of the rocks, but I can imagine such a case. E.g. if you want to determine if a sample contains traces of metals (though you'd probably use easier experiments for that) or to gather information about some other structural property. But the main thing I wanted to stress is that one should generally keep this in mind: total mass over total volume gives the average density. :) – Wouter Jun 15 '13 at 20:01

I think the weight your method yields would be that of the rock minus the weight of water displaced by its volume. Personally, I would place the rock in a container brimful of water, then weigh the displaced water. The rock density would then be its dry weight divided by the weight of the displaced water.

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In response to your specific question:the submerged scale method would be as good as any other in measuring the "wet" weight of the rock sample. The pan would appear to the scale to weigh less, but that would be eliminated by zeroing the scale after it is submerged.

Be sure that the scale is "rated for submerged use", not just "water-proof", or "water-resistant", or "usable in damp conditions". It would need to be zeroed every time you made a measurement, since the buoyancy of the pan would vary with the density of the water.

If there's a fatal flaw, we're both missing it...

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