USSR said:
The Quarter is not a good candidate for use as a weight check. For one, it's weight is completely outside of normal powder drops, whereas a dime falls nicely between handgun drop weights and most rifle drop weights. Also, there is not the variance of designs, thereby resulting in weight variances, that is found with Quarters. For guys using balance beam scales such as myself, the dime works well. Just MHO.
The problem is an electronic scale calibrated by a single weight will alter its load cell output multiplier (gain slope), not its offset like a pan balance will. So it's the percent of error, not the absolute error that matters, and if you look at standard check weights, you see they have a smaller absolute error as the weights get smaller, too, and for the same reason.
I grabbed a fist full of dimes out of my car's tray (always packed with coins) and started weighing them. They varied from 34.11 grains to 35.52 grains, or about a 4% spread. The quarters measured above had about a 3.7% spread, so, for randomly chosen coins it is probably a wash as to which one you use to calibrate an electronic scale. A beam balance is another matter. On that, the error will be an offset, so there the fact that 4% of a dime's weight is less than 3.7% of a quarter's weight will be an advantage.
To try to tighten the dime calibration idea for you, I selected thirty of only newer and sharper looking dimes and weighed those. That narrowed the spread to about 2.5% of the mean, so you can cut the absolute error not quite in half by sticking to recently minted, unworn-looking dimes, and that is certainly worthwhile. For that thirty, I got:
High:
35.52 grains
Low:
34.64 grains
Mean:
35.03 grains
Standard deviation:
0.21 grains
Extreme spread:
0.88 grains.
The
U.S. Mint says the target weight for new dimes is 2.268 grams (35.00 grains), and that certainly agrees well with my result. For quarters it is 5.670 grams (87.5 grains) which isn't too far from my result for quarters, either, though it is less precise than what I got for dimes, both in absolute and percentage-wise terms).
A couple of lessons seem apparent to me from this. One is to select only the new and sharp looking dimes. Weigh 30 and pick one out of the thirty that is closest to average and keep that for your calibrator. As I mentioned earlier, you can take it to the pharmacist and ask for weight confirmation. If his scale doesn't have grains (it should) the conversion factor with more than enough decimal places to go from grams to grains is to multiply by 15.43236.
As a general FYI, most of the check weights sold by powder scale makers seem to be class 6, below.
NC FNS,
Gravity is greatest where mass is greatest, less about 0.3% for the earth's spin at the equator, less another 0.2% for the equatorial bulge, so about 0.5% less weight at the equator than at the poles. So a 40-grain charge would be off about 0.1 grains low at the pole and 0.1 grains high at the equator using an uncalibrated absolute weight scale like a spring scale. But the mass effect is also in play, and gravity actually varies a total of about 0.7% over the whole earth's surface. NASA has
maps of just the mass gravity from the GRACE satellite readings (the satellite isn't rotating with the equator so that centrifugal effect is absent from the maps). At the top of a big mountain, where there is mostly rock under you, gravity is greater than over the Marianas Trench, where there is mostly water under you (rock is denser than water, which why it sinks).
The great thing about a balance is, it doesn't matter what the gravity is. It just compares the mass on one side of the poise to the mass on the other side. So it actually measures mass and not weight. The same thing happens with your electronic scale when you use a check weight to calibrate it. The check weight weighs less at the equator so the scale winds up compensating for the gravity difference via the check weight.
). But I do set it up (RCBS/Ohaus 10-10 balance scale - yes, made in China) and make sure the balance is "floating" freely - i.e. I can see it smoothly rocking back-n-forth as it settles to zero. Sometimes, I'll see - as it nears zero - it abruptly stops (it's a very subtle thing to see). That tells me it's time to wipe the fulcrum with a dry Q-tip.