# Is weighing coins accurate?

I frequently deposit coins into a bank, after counting them with a mechanical counter. When each tube is full, that's the right amount for the bank's coin bag. The bank weighs the bag to check it's got enough coins in it. Sometimes they say there is one short, so I checked the machine, and found that a lot of coins have worn thinner, so it can be +/- one coin in a tube. Surely this means the weight could be +/- one coin too? Thinner coins are lighter by about the same amount that they provide less height in the coin pile in the tube.
Found a possibility. Perhaps they don't lose much weight from wear (or the face would have worn off), but the newer ones are thicker, hence the proportion of newer and older coins makes counting them by height of a pile inaccurate:
Found a possibility. Perhaps they don't lose much weight from wear (or the face would have worn off), but the newer ones are thicker, hence the proportion of newer and older coins makes counting them by height of a pile inaccurate:
It looks to me as though the mass is the tightly controlled parameter: . The thickness varies depending on whether or not the coins are plated steel, a change that mainly happened in 1992.
I wonder if you have a lot of counterfeits.
I've found with 10p coins in particular that you can no longer check that you've got a pounds worth by comparing the heigh of piles because they vary depending on the mix of old and new coins.
The new ones apparently the same as the old ones, though - so you can still 'count' them by weight.
Ah well not my problem. I give the bank bags of coins and sometimes they're 10p out, which their scale detects. At least I know they're not fiddling me.
If you don't want to count them, and can no longer rely on measuring the height of a pile, why not weigh them yourself before taking them to the bank to make sure you have the right amount in each bag?
I've got some electronic scales with a resolution[1] of 1 gram. A 10p piece weighs about 6 grams, so I can easily detect adding or removing a single coin.
[1] This doesn't guarantee an accuracy of +/- 1 gram, of course so, if I regularly paid bags of coins into the bank, I'd make up and label a bag of washers or something which was the right weight for each type of coin, and keep those to calibrate the scales prior to each weighing session.
I don't have such scales, and don't see the point in buying them just to save the bank teller having to deduct 10p from my deposit amount.
Why would you have to calibrate them each time? If they have a resolution of 1 gram, I would think it should distinguish 6g ok? And I've never known electronic scales require (or have the ability to let you) calibration every time.
Resolution and absolute accuracy are not the same thing - not to mention repeatability. Scales like mine currently only cost a tenner, and are not going to be to military spec.
I can't find a specification anywhere, but their accuracy could well be +/- 1% of full scale which - since they measure up to 3Kg - could be +/- 30 grams. It's likely that the calibration changes a bit with temperature. When I mentioned calibration, I didn't mean that I could adjust the relationship between weight and reading, but simply that I could weigh something of the "correct" weight, note what the scales said - and then use that figure for checking my bags of coins.
How do you find that correct weight first?
Easy! Step 1: *Count* (rather than guess!) the requisite number of coins (e.g. 50 x 10p) and put them in a bag. Step 2: Weigh them and note the reading. Step 3: Make up a bag of washers - or sand - anything! - to weigh exactly the same, and label it as your "standard".
When you next need to weigh some 10p's, weight the 'standard' first, and note the reading - which may not be exactly the same as last time if the scales have drifted a bit in the meantime.
Use that new reading as a reference against which to check your bags of 10p's.
Did I *really* need to spell it out?!
I regretted posting that as soon as I hit send.