Electronics > Projects, Designs, and Technical Stuff
mg scale - interesting effect
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JBeale:
Somewhat tangential to electronics but... I wanted to build a FMES seismometer and the instructions called for a 0.05M K2HPO4 solution as part of the electrolyte, implying an accurate scale to weigh out a small amount of the dry salt to add to the solution.

Amazon has 50g x .001 (milligram) scales around $17 and it turns out, no surprise, that the last digit has no real meaning, although the firmware tries to make it appear realistic when near zero or a standard test weight value.  So I stepped up to a $43 ebay "200x0.001g Analytical Balance Scale" with a possibly-effective-looking boxy draft shield around the pan.  At first this appeared to work, BUT the reading would randomly jump from 0.000 g up to several hundred mg, then return to zero, even with the draft shield in place and the top-load hole covered.  It would also consistently sense the presence of my hand as a ~ 100mg force even just hovering 5 cm above the shield without touching anything. Some tests showed it was not stray light, not vibration and (maybe) not RF. I think it is either electrostatic force or 60 Hz capacitive pickup through the plastic case. I simply wrapped aluminum foil around three sides and the top except for the loading hole, and that completely fixed the problem.  FWIW I then tested it for a while sequencing through 100 mg, 0 mg and 110 mg test weights and so far this scale is repeatable at the +/- 2 mg level although its firmware also "cheats" within 10 mg of zero to hide the zero drift.
T3sl4co1l:
Heh, what do you expect, they aren't going to do EMC susceptibility tests at that price.  Or if they do, their pass criterion is "didn't catch fire". ::)

Those are usually build on a load cell with a very precise (sub-microvolt) amplifier to sense it; it doesn't take much interference to upset the measurement.  A well designed one will be insensitive to AC, or well enough filtered to not mind ambient noise.  Or it could equally well be noise in the digital logic, leading to flipped bits (which could explain the seemingly jumpy response).

Yeah, if foil is making it behave, that's not a terrible solution. Tape it down and make it permanent? :-DD

Tim
Kleinstein:
Sensitive scales can react to some odd effects. However 100 mg is a lot - the odd things happen at 1 mg  or less.

The hand for example could cause a local convection flow from it's heat and this can look like some weight.
Electrostatics can also be tricky, especially in a dry environment, when glass looses it's surface conductivity.
Temperature gradients and resulting air currents are tricky too.
Paper can also be tricky, as is can change it's humidity and this way gain or loose weight.
Depending on the construction variations in air pressure (e.g. wind gusts and closing doors) can be a problem too - even for larger weights.

I remember a precision scale having some +0 and -0. So the interval for 0 reading was twice as large as it should have been. So it was best used with some offset. One could also see this as a mild form of cheating.
helius:
Do you think the EM susceptibility could be improved by coating the glass shields with a conductive layer (say ITO or the like)?
Alex Eisenhut:
They used to make mechanical balances that could resolve milligrams
http://collection.sciencemuseum.org.uk/objects/co13992/torsion-balance-to-weigh-up-to-500-mg-balance
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