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mg scale - interesting effect
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David Hess:
Load cells are constructed with a geometry to make them insensitive to side loads and such.  This is especially a problem for those intended for platform scales and they may be mechanically or electronically trimmed to provide the proper response under all conditions.

This is easy enough to test for yourself; what is the reading with the same weight place at different and extreme points on the platform?
JBeale:

--- Quote from: David Hess on March 26, 2019, 06:04:43 pm ---Load cells are constructed with a geometry to make them insensitive to side loads and such.  This is especially a problem for those intended for platform scales and they may be mechanically or electronically trimmed to provide the proper response under all conditions.
This is easy enough to test for yourself; what is the reading with the same weight place at different and extreme points on the platform?

--- End quote ---
My "100g" test mass reads on average 100.098 at the front edge of the scale, 100.104 in the middle and 100.110 at the back edge running through the front-middle-back sequence four times, with a repeatability of 2 mg at each location.  So the effect is small but it is above the noise level.  When I did my previous experiments I tried to get the masses as close to center as possible.
SeanB:
Those eBay mass sets are reasonable, the one I got went through a verification traceable to the SI kilogram, and actually was well within the required specifications for accuracy. The Ohaus set is quite stable, and has been that way for decades as well, because they only get handled with plastic tweezers for the small masspieces (and those have a measured mass down into the nanogram range for the foil pieces, and the 1g is known to 4 decimals as 1.0000g, with an error of 80ug) and the larger masspieces I have a set of cotton gloves and cloths used to wipe them before use, and the gloves for handling them. They are stable as well, because they are handled with care, and not scraped or dropped. They were used for the certification as well, later date (previous week instead of 6 months for the ones on the van) and had been certified by the same national laboratory.

For massmeters the cheap ones are really only 12 bit converters, as the 16 bit ADC's inside are too noisy at DC, and even massive oversampling is not going to make them any good, the reference drifts too much, and the internal IC temperature changes too much as well, they are just relying on digital clamping to show a stable zero, while internally it is drifting slowly.

Only way to get better accuracy is to power it on for a few hours in a constant temperature environment, and then run your tests. Hard as most will power off after 10 minutes or so. You need a commercial or laboratory massmeter, as those in general will have selectable power behaviour, and in general most will stay on 24/7, so they are the most stable. As well they will have a better ADC inside, probably 18 bits, with much better references, and much better drift performance as well, as in service mode ( however you enable it, varies from manufacturer to manufacturer, but in most modern ones either a set of jumpers on the PCB or a key sequence, thankfully the days of bespoke dongles are seemingly over) you can often get the raw ADC value without zero compensation, so you can see the slow drift.

But yes, for your once off application probably better to weigh out 10 times the mass, weigh out the mass of distilled water (more accurate than volumetric measures in most cases, especially without calibrated glassware) 10 times as much and dissolve, then discard the rest, or use it as fertiliser for the garden. 
texaspyro:

--- Quote from: coppercone2 on March 26, 2019, 03:27:57 am ---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?

--- End quote ---

Mine are made of polished Unicorn horn.   
David Hess:

--- Quote from: SeanB on March 26, 2019, 11:34:17 pm ---For massmeters the cheap ones are really only 12 bit converters, as the 16 bit ADC's inside are too noisy at DC, and even massive oversampling is not going to make them any good, the reference drifts too much, and the internal IC temperature changes too much as well, they are just relying on digital clamping to show a stable zero, while internally it is drifting slowly.
--- End quote ---

If that kind of design is common, then current engineers need to study how it was done 30 years ago.

Reference drift is irrelevant when you use do the conversion ratiometrically.   Then a 7815 or 7805 will be fine because the drift and flicker noise cancel out during the measurement.

Integrate the input over a whole number of 0.1 seconds to cancel out 50/60 Hz line frequency noise; forget about using a sampling converter unless it is part of an integrating converter and even that is a bad idea.

The cheap ones use 12 bit converters because they are the most common high resolution converter available in microcontrollers.  They are terrible for this application though with perhaps 10 bits of linearity at best.
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