axehandle
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Placement of very accurate scales.
From my school chemistry lab when I was much younger than I am now, I remember that the very accurate scales (don't think they were digital but
can't see that as making any difference) were placed on concrete pillars that extended below the building some distance down into the grown since
they according to the teacher "wouldn't work otherwise".
I don't remember their accuracy but would assume it being in the 1/1000 gramme range or better (?).
Do modern scales with a similar accuracy still need to be placed on such fundaments or are they "smarter" (say averaging between hundreds of
measurements per second or similar)?
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vulture
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All analytical scales nowadays have a "bubble" that allows the user to place them exactly level .
This is necessary because these scales depend on gravitation pull and any deviation can cause misreadings.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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axehandle
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Aha! So that's the reason, silly me always thought it had something to do with vibrations in the building.
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Mephisto
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Absorbing vibrations is very important, if the scales have the precision you said. Therefore most analytical scales are on special balance tables like
these:
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neutrino
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Do these vibration/balance problems occur in mechanical balances?
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chemoleo
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Yah, they occur too.
Our analytical scale in the lab, going down to 0.0001 g or so, varies MASSIVLEY as soon as the door of the box is open. Also, it is indeed placed onto
a solid stone foundation, which rests on hard rubber. At this resolution, all sorts of effects come in, predominantly air currents.
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vulture
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| Quote: |
At this resolution, all sorts of effects come in, predominantly air currents.
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No shit. Simply exhaling in the direction of the thing can cause misreadings of 0,005g. 
I remember preparing YBaCuO (0,0001g accuracy needed) superconductor with these old (hanging basket, mechani-optical reading) scales. That was a major
pain in the ass because of air currents, the moving basket which had to come to rest after putting something on, etc etc. 
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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mick
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If you are weighing that accurate you have got to be very careful of static on electronic balances especially with gloves on. I got one of those old
brass two pan balances (Ovetling) as an ornament. The service card goes back to 1948. Onyx bass, agate pivot, a pendulum to level it, mahogany case +
glass, lead weighs to balance the front sliding door. I have lost the rider which could take it down to 0.00 of a gram. They were stuck in a cupboard
where I used to work cause everything was going electronic. I offered to buy them but got them for free (one for my mom and
dadjavascript :icon(' '
). I handed in my notice the following week
javascript :icon(''
mick
I still cannot work out these smiley faces but it is close enough
Typo
[Edited on 12-9-2004 by mick]
[Edited on 12-9-2004 by mick]
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unionised
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When our lab moved about 10 years ago nobody thought about the balances much. After all, thy were just on the ordinary heayv timber benches. Then we
installed them on the new modern benches and they wouldn't settle at all. We had to brace the benches to make them rigid enough. Oddly, the
balance that was least affected was the microbalance (100ng resolution). Probably because it was on a vibration damping plinth.
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