This times a million. Do not just make the device accurate, make sure it is very hard for a motivated individual to trick it into being inaccurate, in either direction. Please excuse the terminology, was learned from very elderly engineers, but above foolproof (stupidity), there is twatproof (deliberate misuse) and cuntproof (educated malice). Fully cuntproofing is very hard, but any device like this should be at least fully twatproofed, with at least some attempt at cuntproofing.