Quickcar tire pressure gauge, liquid filled

More interesting stuff

'The most accurate instruments available for measurement of pressures above the range where manometers may be used is the Deadweight Tester.This type of tester operates on the principle of balancing a known mass against the force exerted by an unknown pressure on a piston of a known area.When an exact balance is achieved,the unknown pressure P is equal to mass M of the weights divided by the area A of the piston,according to the formula P=F/A.' PDF link with calibration machine diagram

not bad eh?
Quote they must use an known sized air-chamber/piston and a set of scales...