Igor Šušić "There is no water ...

Igor Šušić "There is no water meter that is 100% accurate".

Not now there isn't but the 1824 reciprocating piston meter is, or became, the most perfect example of the most perfect technology - positive displacement. reciprocating piston has a flexible seal between the piston and cylinder so there is virtually no slip flow, just what could pass through the four way valve.

As positive displacement meters the only way for flow to pass was to move the piston. When there was not enough pressure drop to move the piston the piston stopped and flow was stopped by the piston.

Positive displacement is the principle  behind ball and piston meter provers, the Bell prover for gases and the flying meniscus and flying bubble methods for low flows. They are used to calibrate all types of meter.

Between 1824 and 1871 over 300 new meter patents were awarded in the quest for a simple cheap and compact positive displacement meter. Many were improvements to existing designs.

The PD meter registered consumption was the truer consumption. The reciprocating piston meter today is virtually unchallenged for its performance.

Rotary piston and nutating disc did not advance after the 1880s. New POD meter designs came such as Tylors reciprocating piston of 1888 and the IMO birotor of 1934 - the most accurate water meter ever.

Then PD meters lost the market to the unchanged rotary piston and nutating disc. The only reason I can see is the "Equability" concept. This allowed the volumetric meters to be judged as giving a reliable proxy measurement. This appears in the UK 1942 water act. It allows the simple and cheap volumetric meters and the inferential meters to take over the market. Solid state are also only proxy meters.

This is my question: How is equability tested and verified? (Someone changed to title of my original post). 

Of course, with a true PD meter as simple and cheap as the volumetric meters, the whole market could change again and just as dramatically. That the PD meters are complex and expensive is simply history. Indeed, the Lewis Nash Crown meter could ahve been that. mechanically simple that it has complex geometry was the problem but the Kent NSM innovation of the 1960s could have solved that problem and delivered a simple cheap true PD meter.