Electronics > Projects, Designs, and Technical Stuff
mg scale - interesting effect
coppercone2:
what material is best to handle those polished weights? like the expensive E or whatever grade ones. (well the small ones anyway, that you might not want to handle with gloves
Ceramic? Polished Steel? Ivory? Some kind of felt lining?
JBeale:
I now use metal tweezers to handle my ultracheap weight set. I don't know how to really minimize mass transfer, but I think that's micrograms or below, not few-milligrams like this scale resolves.
I did another set of measurements, this time I used two different "20g" weights A and B. The A unit was about 8 mg heavier than the B unit.
I weighed A, back to zero, then B, then A&B together, then removed B to measure A again (shown as "A2"), then returned to zero. I did all this 10 times with results shown below.
You can see there is some load cell nonlinearity as A&B together always measure slightly higher than the sum of their separate weights.
On average it was 6 mg more, which as a fraction of the total weight is 0.015% so it's small but measurable. The scale does not offer multi-point calibration AFAIK, but if you have a reference mass of similar value, you can mostly calibrate this away.
--- Code: ---200 g x 0.001g no-name ebay scale 25-March-2019
trial mass mass both mass A2-A1 nonlinearity
# A1 B1 A&B A2 drift (A&B)-(A1+B1)
1 20.029 20.021 40.052 20.026 -0.003 0.002
2 20.027 20.021 40.055 20.03 0.003 0.007
3 20.029 20.019 40.052 20.028 -0.001 0.004
4 20.029 20.02 40.052 20.028 -0.001 0.003
5 20.027 20.022 40.056 20.03 0.003 0.007
6 20.029 20.02 40.057 20.03 0.001 0.008
7 20.028 20.018 40.051 20.025 -0.003 0.005
8 20.027 20.022 40.057 20.03 0.003 0.008
9 20.028 20.021 40.055 20.03 0.002 0.006
10 20.026 20.02 40.054 20.03 0.004 0.008
AVG 20.0279 20.0204 40.0541 20.0287 0.0008 0.0058
MIN 20.026 20.018 40.051 20.025 -0.003 0.002
MAX 20.029 20.022 40.057 20.03 0.004 0.008
Range 0.003 0.004 0.006 0.005 0.007 0.006
--- End code ---
Conrad Hoffman:
I use a single 50 gram weight at work that was about $800. The tweezers that came with that are useless and would cause you to drop the weight. I handle it with Kimwipes and have considered making a plastic fork that would slip under the head. The tweezer problem seems to be traditional as my little Ohaus weight set at home came with similarly useless metal tweezers about 50 years ago.
ebastler:
--- Quote from: coppercone2 on March 24, 2019, 03:06:32 am ---For 50G a decent American scale would have about 3-4 times more material in the Part Being Weighed and the Load Scale. It looks like junk unless the industry changed (doubtful). It also uses the chassis as stiffener. The chassis of a real scale is wobbly compared to the 'anvil'. Your talking a heavy glass plastic part not some little sheet of whatever and some dull zinc assembly.
--- End quote ---
How would that help the precision? I should think that a lightweight and stiff internal mechanism (i.e. high internal mechanical resonance frequencies), combined with a heavy weighing table with dampened, soft suspension (to suppress any floor-coupled vibrations above a very low resonance frequency) would be desirable?
JBeale:
Typical vibration can be mostly low-pass filtered out but I think the concern is static deflection. I think the load cell wants a perfectly rigid base, and any flexure in the mounting adds an additional shear, torque and/or levelling change to the loadcell leading to a non-ideal response. The thinner the mounting surface is, the more it will flex under load. I don't know about plates or webbed structures but IIRC the deflection of a rectangular beam is proportional to the inverse cube of the thickness, so twice as thick = 8x less deflection.
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