Thanks Geoff and glad to be here.
A large portion of my work involves weights and measures so was interested to see how you were measuring your water and weighing your vessels as was hoping to learn something, but to respond to your important question.
A clinical set of scales rated at Class III are defined as such in that the measurements taken over time can be used(in conjunction with other factors) to diagnose patients and prescribe drugs, and the tolerances we work to for this class of scale are:
0-500 Divisions +/- 1 Div
>500-2000 Div +/- 2 Div
>2000 Div +/- 3 Div
As a side note have attached a png showing the tolerances allowed for various classes of weights to help with your Q below...
What tolerances do those august (money-making) bodies allow, Neil?
But anyway (and more importantly), what tolerances are permitted clinically for patient weighing scales?