Thank you very much Tudor, for the tip of using coins to verify the accuracy of a scale. Coins are made to tight tolerances, else they were useless in vending machines.
Loading coins onto my digital scale, I found that my 500g scale with a division of 0.1 g was mostly spot on, with an occasional deviation of +0.1g. This is typical behaviour of a digital measuring instrument, often accuracy is stated as +-x% of reading +-1 on last digit. It appears that x is small. I'm impressed by this cheap Chinese made scale. The reading is also independent on where the load is placed on the weighing plate.
So I'm confident these weights for the PGI-x20/CLI-x21 cartridges are accurate to within +-0.1g, and that the weights also apply to the PGI-x25/CLI-x26 cartridges, as the windowed and the opaque cartridges are very similar.
The weights are for new Canon OEM cartridges with the orange clip and the vent sealing tape removed:
So Ant, I recommend that you get one of these inexpensive digital scales. If you don't need to weigh objects weighing more than 100g, like some large HP cartridges, a scale with a load capacity of 100g and a division of 0.01g is even better.
