On Sat, 11 May 2019 22:38:42 +0100, alan_m wrote:

On 11/05/2019 19:54, Commander Kinsey wrote:

I know the multimeter is accurate, I've tested it on all sorts of things.

The only way you will know that your multimeter is accurate is by having
it calibrated against a known standard, ideally specified to 10x better
than your meter.

Or by using it to measure a known current on a bulb I know draws a certain current.

At 0.65A a high end Fluke multimeter @ £200+ will give 0.65A +/- 0.01A
on a typical £50 multimeter £50 it will give 0.65A +/-0.09A

Without a specification for what you have purchased, you may have meter
capable of measuring fairly accurately currents in the range 100A to
1000A but not capable of 0 to 10A measurements with any degree of accuracy

Typically you may be trying to measure and compare a 0.65A current with
measuring equipment with a total uncertainty of measurement of around 0.6A

The 8.5A was measured both with one of those energy efficiency meters
on my house's meter tail, and by knowing what devices were running.

So your comparison standard is something British Gas used to give away
for free, possible so inaccurate that it hasn't got a published
specification for its current measuring capability.

"and by knowing what devices were running"