On 02/03/2018 20:56, NY wrote:

So are you saying that the scales only look for an increase in weight
(and don't care about the size of the increase) to denote that an item
(not necessarily the one whose bar code has just been scanned) has been
placed in the bag?

yes.
Can you imagine the error rate of thousands of products and just how
difficult it would be. Even the humidity in the shop would affect the
weight of the boxes.
How about the tub of ice cream I bought yesterday that had half an inch
of frost on it that would weight tens of grams more than it should.


I shall have to test that theory when I'm next shopping by scanning one
item and putting a different item on the bagging scale. I think you are
wrong but I'm happy to be corrected. As regards prepacked fresh items, I
presumed that the bar code of every item was different to encode not
just the item's ID but also its weight.

Most of them are preprinted and are the same. The reduced bar codes are
unique so they know the price and item in some supermarkets. As are the
deli items.

Some supermarkets have turned the checkout scale off altogether as it
was annoying the customers a lot.