A shop scale is a legal instrument, tested with certified weights and sealed, and mine passes every time. Put anything from the top shelf of the cold cabinet on it and it reads forty grams over, and the same thing moved to the next shelf reads true.
I run a farm shop, and the scale on the counter is the one thing in it the law cares about.
How a shop scale is kept honest.
It is a trade approved instrument. Trading standards test it with certified weights, and if it reads them true they seal it, and the seal means nobody has adjusted it since. I test it myself every morning with a two kilo check weight, and I write the result in a book, because that is what you do.
It reads the check weight true. It has read the check weight true every morning for nine years.
Anything that has been on the top shelf of the cold cabinet weighs forty grams more on that scale than it does anywhere else.
Carrots, cheese, a bag of apples, a joint. Take it off the top shelf, put it on the scale, and it reads forty grams over what the same item reads on the scale at the other counter, or on my kitchen scale, or on the certified scale in the trading standards officer's van.
Take it off the scale, put it on the middle shelf for a minute, put it back on the scale: true.
I want to be clear that this is not the scale, and I know that because I have had the scale replaced. Twice. Different makes.
What has been checked.
Trading standards have been three times because I asked them to. Certified weights on the scale: true. A one kilo weight that had sat on the top shelf for ten minutes: one thousand and forty grams, on their scale as well as mine, which was the visit where the officer stopped writing.
Temperature. A cold item does not weigh more, but a cold item can pick up condensation, and condensation is water and water is weight. Forty grams of water on a bag of carrots would be running off it, and these are dry. And a one kilo brass weight does not condense forty grams of anything in ten minutes in a cabinet at four degrees.
The cabinet. It is a standard upright chiller, and the shelves are the same shelves, and the top one is the same distance from the fan as the bottom one is from the floor. Replaced in 2020 with a new cabinet. Same shelf, same forty grams.
Magnetism, static, a draught on the pan. The scale is on the other side of the shop from the cabinet, twelve feet away.
How long it lasts.
This is the measurement that I have and nobody else does, because it needed a morning and a stopwatch.
An item off the top shelf reads forty over on the scale. Leave it on the pan and watch. It reads forty over for about eleven minutes and then, over the space of thirty seconds, it comes down to true, and stays true.
Put it back on the top shelf for a minute and it is forty over again.
Whatever it is, the top shelf gives it and eleven minutes takes it away, and it weighs forty grams, always forty, on a bag of apples or a kilo of brass.
What that means for the shop.
I sell nothing off the top shelf by weight. That is the rule and it has been for six years, and it is a nuisance, because it is the shelf at eye level and it is where the good stuff wants to go.
Trading standards know. The officer wrote it up as an unexplained environmental influence on weighed goods, which is a phrase he had to invent, and the file says the scale is compliant and the trader has taken appropriate steps.
What I do.
I test the scale, I write it in the book, and I keep the top shelf for things with a price on them rather than a weight.
The one thing I have changed is that I do not put my hands on that shelf when I stock it.
I use the tongs from the deli counter, which looks odd, and a customer has asked me about it, and I said hygiene.
The reason is that in 2021 I stood on the scale myself, with my hands empty, having just stocked the top shelf, for no reason except that it was a quiet afternoon and I had had a thought.
I weighed what I weigh. Then I put my hands on the top shelf for a minute and stood on the scale again, and I weighed forty grams more, and I stood there for eleven minutes watching it, and the forty grams did not come off me the way it comes off a bag of carrots. It took an hour and a half.