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Forgive my ignorance but I've never found an answer to this question....In the spirit of independent confirmation/research replication, should chemists operating a different altitudes above sea level factor in their specific value of gravitational force when measuring the mass of substances via generic spring scales found in most labs? For example when measuring out 50g of NaCl needed for an experiment, would't a chemist working atop the Colorado Rockies using the exact same spring scale as a chemist working in New Orleans technically pour out a larger quantity of NaCl (that still reads 50g per scale) vs. the NOLA chemist due to a lower value of gravitational force?

Larken1111
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  • Closely related: http://physics.stackexchange.com/questions/6074/do-we-take-gravity-9-8-m-s%c2%b2-for-all-heights-when-solving-problems-why-or-why – dmckee --- ex-moderator kitten Nov 03 '14 at 02:20
  • (a) why would this only affect "generic spring scales"? (b) don't most labs use digital scales (i.e., not spring scales)? – Kyle Kanos Nov 03 '14 at 02:24
  • @KyleKanos (a) Beam balance scales don't depend on the specific value of g. (b) Electronic scales work with a similar principle to spring scales so they would be affected just as much. – Asaf Nov 03 '14 at 02:36
  • @AsafParis-Mandoki: Your response (b) is exactly the reason I asked my (a) (and (b), for that matter). – Kyle Kanos Nov 03 '14 at 02:39
  • @KyleKanos I believe the question is stated like that to adress a specific case where the value of g matters and not because he/she thinks is the most prevalent type of scale. – Asaf Nov 03 '14 at 02:46
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    @AsafParis-Mandoki: ...when measuring the mass of substances via generic spring scales found in most labs** appears to be OP thinking this is the most prevalent type of scale. – Kyle Kanos Nov 03 '14 at 02:47
  • Isn't this why you calibrate your scale first? I have 4 different digital scales that can all be calibrated and 0 spring scales. This isn't a problem in my apartment so I can't imagine that this is a problem in modern labs. – Brandon Enright Nov 03 '14 at 02:54
  • While I can't speak to the design of laboratory analytic balances, the kinds of cheap digital scales you buy for the kitchen or the bathroom are based on strain gauges, which is to say that they are "spring" scales. – dmckee --- ex-moderator kitten Nov 03 '14 at 03:02
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    @dmckee - The slightly more precise analytical scales used in chemistry labs (4 digits and more) are usually based on a force-balanced design: they measure the current required to keep the scale in the unloaded position. This gets rid of all nonlinear effects, and all you need to do is measure the current. If you drive the current from a DAC, you don't even need a separate ADC... It's called an electromagnetic force restoration sensor – Floris Nov 03 '14 at 04:50

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Scales that measure to better than 1% need calibrating to local gravity which depends on latitude, and to a lesser extent, local geology. Due to the equatorial bulge, objects near the equator weigh 0.5% less than those at the poles

Electronic scales normally come with 2 masses, typically their full and half range, and a calibrate mode.

Martin Beckett
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The radius of the Earth is so big, compared to differences of altitudes here in the surface, that $g$ doesn't vary much for everyday applications. So, mostly, it's ok to use the standard gravity. Precision experiments, however, may require finer calibration.

From Wikipedia: "an increase in altitude from sea level to 9,000 metres (30,000 ft) causes a weight decrease of about 0.29%."

André Chalella
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    Right. So when you have scales that measure to 1 part in $10^5$, you need to calibrate when you take them upstairs to the 10th floor of the building... or move to a different latitude. – Floris Nov 03 '14 at 04:53
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I work in a lab where we have 6 analytical scales which all have strain gauges to determine the mass. We calibrate them once a day, but one one of them frequently self calibrates several times a day.

Analytical balances rarely are beam balances now since they require more maintenance and the weights and beam needed to be locked down if it was to be moved. They are probably more expensive to construct than strain gauge balances. To minimize the effect of strain on the beam, the weights were removed from the side the material was placed upon to keep the mass on the beam the same.

LDC3
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  • Are you sure your analytical scales use strain gauges? Most analytical balances use the more accurate force restoration sensor... – Floris Nov 03 '14 at 04:54
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    @Floris You're right, after watching the youtube video on the difference between a strain gauge balance and a force restoration balance, all the balances we have are force restoration. – LDC3 Nov 03 '14 at 05:21