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Author Topic: Testing densities (split from Re: Black & White Hound Dog)  (Read 2173 times)

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Offline tropdevin

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Re: Testing densities (split from Re: Black & White Hound Dog)
« Reply #20 on: October 07, 2008, 07:28:57 AM »
Here are a couple of images of my 'high tec' set up.  It is in practice accurate and consistent.

a. A 'knife edge' support which can be moved along (as can the scales) so the scales read near their maximum. But don't move it during a set of measurements!  The knife edge may look like a piece of meccano - which it is - but it has a sharp ridge on the top.  The scales read to 1 gram, and up to 2000g, so I try to have readings in the 1900 - 2000g range.  That way the error is 1 in 2000, not 1 in 300 or whatever. And I do other things to improve accuracy, such as repeat measurements.
b. A rigidly fixed pivot.
c. The paperweight suspended in a wire cage.  Pontil upwards so it does not trap air. The item is first weighed in air above the water, then the water is raised around it - so no part of the measuring system is moved. I raise the jug and stand it on a support, making sure that the paperweight is not touching the sides or base. If necessary I pour in a little additional water.

Regarding upthrust on the wire cage - yes, there must be a very small effect, but too small for me to measure.  You can see from the image that very little wire will be under water.


Alan  (The Paperweight People

"There are two rules for ultimate success in life. Number 1: Never tell everything you know."

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Offline krsilber

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Re: Testing densities (split from Re: Black & White Hound Dog)
« Reply #21 on: October 07, 2008, 08:32:13 PM »
Thanks, Alan!  Now I can see it!  And you're right, there's not much wire at all that gets immersed.  Just out of curiosity, have you (or has anyone) compared measurements using your system vs. the water displacement one using the same 'weights?  It would be interesting to know whether there are consistent differences in the two ways of finding density if two systems are commonly being used to measure them.

Oh, re: the stocking question...just saw this again from Kev's post, "If an absorbant material is used, such as with some types of "net bags", then it is possible that enough water can be absorbed up the material such that the weight of the material outside the main water compromises the calcualtions."  That's another good point, the capillary action of whatever's used to hold the things.  The few strands of wire definitely seems a better option!

I wonder how much altitude affects measurements of a weight in water.  Would someone in Denver get the same reading as someone in Denmark?

(Yes, I'm a frustrated scientist, inflicting myself on the GMB.)

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