Comparison of Electronic Scales
*3% SG Accuracy means the lightest gem or mineral, in grams/carats, that you can measure specific gravity on the scale, and still get an SG with reasonable accuracy, defined here as within 3% of actual. See Table below for more details.
Use the following table to help answer that question. It shows the Theoretical Specific Gravity Accuracy for a typical gemstone w/SG of 3.50, such as Diamond. The development of the table is based upon the very real premise that as you approach a scale's minimum Readability, a specific gravity measurement becomes increasingly less accurate.
The top row of the table below lists Readabilities, in grams/carats, of 4 increasingly sensitive scales. The left column represents declining % degrees of accuracy from actual SG. The numbers in the body of the table are in carat weights of increasingly smaller stones, going down.
For example, look at the colored cell that reads .21. An electronic scale that offers Readability down to .001 grams/.005 carats will theoretically produce specific gravity measurements on stones weighing as light as .21 carats, with 3% accuracy. Another way of saying this is that on an electronic scale with Readability of .001g, a diamond that has an SG of 3.50 and weight of .21 carats would theoretically measure from 3.40 to 3.61, or 3% either way from 3.50. It thus follows that heavier stones will give more accurate SG measurements, while lighter stones that "push" the minimum readability of a scale will produce less accurate SG measurements. Read more below the table.
The same scale with readability down to .001g could also be used for stones weighing down to .07 carats, although with 9% accuracy. So you can see that in choosing an electronic scale for SG measurements, this table can be very helpful. For most gems, a scale with readability of down to .01g is ineffective for SG measurements (although adequate for most minerals and larger gem rough). Rather, a scale with readability of .002g is a minimum practical requirement, with SG's reasonably accurate (3%) to .41 carats. An electronic scale with readability to .001g is preferred, and offers the best value for the money, although for stones as lightweight as .02 carats, an analytical scale reading down to .0001g is required.
Remember that as with most Specific Gravity measuring devices, for very lightweight stones, you can always weigh 2 or more of them together so that their combined weight is heavy enough to be "sensed" by the device, providing you know that the group of stones are all from the same source.
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