Digital scale giving different weight for same coin

[HeavyT 7]

I have bought a brand-new digital scale, which I have calibrated. Unfortunately, I seem to have a problem every time I check a coin. I know sovs are meant to be 7.98 grams but it might show as 8 grams when put it on the scale, and when I weigh the same coin a minute later it came up as 8.2 grams! If I try it again, it shows 8.1!

The scale is on a flat and steady surface but I can't understand why I would get different readings each time. Has anyone ever come across a similar problem with a new scale giving different readings for the same item? I'm planning to return the scale but thought I'd check here first to see if anyone has any suggestions.

[Rob 2,996]

If they use varying capacitance to give the reading then the conductivity of the surfaces will have an effect. I have a set here which can give a slightly unstable reading if not used for a long time. Either taking off the tray and wiping it or physically pushing down as far as it will go a few times seems to clear it. Not very scientific, but it seems to work.

[bagerap 706]

Does your balance work to two decimal points?  It sounds as though it only works to one. If that is the case, it's choosing to either round up or round down each time.

[Nutsaboutcoins 105]

Mine gives poor results when the battery starts to go dead.

[secret santa 2,013]

I have a cheap Chinese digital scale (called Professional-Mini) and today I weighed a penny that I bought a while ago and was advertised as weighing 13.43 gm. However, on my scale it weighs 13.33 gm. Is this sort of difference to be expected on different scales or does this suggest that my scales may be inaccurate ? 

Does anyone know what the most reliable (in terms of weight) modern coin could be used to test the accuracy of my scale ? I don't have anything of reliably known weight to my knowledge.

[Rob 2,996]

Buy a set of weights. They only cost a couple quid for half a dozen different weights up to say 100g. Alternatively take a new sovereign and use that. A RM error in the weight would be a bit of a curveball, but that can be eliminated by weighing two or three sovereigns to check for consistency.

[secret santa 2,013]

Thanks Rob - I'll go for the set of weights - hopefully they're cheaper than 2 or 3 sovereigns ! Can you suggest a source ?

[Rob 2,996]

There are some on ebay at 3.99 all in.