Weigh-scales accuracy

[SussexMan]

Many thanks for all these helpful suggestions. A couple of other observations. I recall one Angling Times writer describing how lifting Avon scales by the casing gave slightly different readings from those when lifting via the Avon's handle. Thanks for the 1 pint = 1lb 4oz guide but if others use it, do be sure to use a UK pint measure (the USA pint measure is slightly different...).
Weighing fish divides the angling audience into 2 camps, it seems; one camp loathes the (sometimes innocent) overweighing method of some when, say, not deducting the soaking-wet net/sling, the other camp regards 'fussing' over the exact weight as laughable. The topic gets very heated at times!

[JAA]

Most old brass scale weights are pretty much bang on - on the underside there's generally a hole which is filled with lead to 'trim' them to quite decent accuracy.

[Barbelbonce]

I have an array of cast iron weights from my mother's kitchen (Balance) scales and have used many of the options discussed above, to check Salter spring balances through the range all the way to 40lb.
I was astonished to discover just how accurate the scales are.
I very rarely weigh fish, if ever in more recent times, and for my purposes, the Salters are accurate enough. Giving an indication. Good enough for me.
Mike

[SussexMan]

For anyone catching big fish (eg pike, cod, conger) there is a secondary problem with the accuracy of scales of the spring-balance variety. For the roach angler, it's simplicity itself to obtain a couple of one-pound weights, drop them into a plastic bag and the 2 pound will register accurately. With 40 lbs Avon scales, just for argument's sake, weighing 10 pounds of weights might reveal an accurate reading, but the adding a second lot of 10 pounds might reveal an under(or over) weighing. This would mean your 7lb barbel was accurately weighed, but your 15lb barbel (I dream...) would be misweighed.
I've just conducted an experiment with water, carefully weighing out 16 pints of water into a previously weighed plastic bucket. The water was measured using one of those Pyrex-style glass kitchen measuring jugs. 16 pints of water is equivalent to 20 lbs (many thanks, Old Man River, for the tip...) but was not what the Avons reported. The dilemma is that it is impossible to tell if the Pyrex jug is 'off' in its accuracy, or the Avons are. Before anyone else tries it, I can tell you that any kitchen scales (digital or otherwise) all suffer the very same inaccuracy risk, particularly when multiplied up to 20 pound plus weighings.
All of the above was the reason for wishing it was possible to visit a Weights & Measures office. These public officials had to have really precisely calibrated weighing devices because the 'fiddles' that tradesmen and shopowners could pull might involve only a fiddle of 1 ounce in every 3 pounds (say) but over the month/year, that would earn the tradesman/shop a hefty financial killing.

[Olly]

The Weights & Measures Office is where scales are tested in order to claim a record - mine were done there for the captor to claim his record Golden Orfe. 1/2oz out they were!