From 1824 on the search was for a PD meter that is simple, compact and cheap. That hasn't been found. Yet. The volumetric meters have been a near miss being only semi-positive. Calibration shift and the effects of wear are a function of the slip flow. The lower the slip flow the less the effects of wear. The reciprocating piston meter today, with its flexible seal between piston and cylinder, and in its core applications (petrol and diesel), will operate for 12milliom litres with no "significant" change in accuracy... and in that application "significant" is probably far less than for a water meter. The volumetric meters probably have very little clearance flow but suffer because of the open flow paths (through the piston wall or disc) created over 20-30% of the cycle, are responsible for significant unregistered flow, the non-linearity etc. and thus vulnerable to wear or anything else that affects "slip flow". I do think solid state meters are perhaps misleading as to the significance of claimed advantages. For example, the lower flowrate. If it was a volumetric meter with such a low flow it would be fine, a clear advantage but when one takes into account the blockage  ratios of the different technologies volumetric meters affect the native flow profile while the solid state does not. The PD meter, for example, below the starting flow rate stops registering but it also stops the flow. In the volumetric meters below the starting flow the flow rate through the open flow paths is significantly reduced. In solid state meters the flowrate below the starting flow i only marginally less than the starting flow.

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From 1824 on the search was for a PD meter that is simple, compact and cheap. That hasn't been found. Yet. The volumetric meters have been a near miss being only semi-positive.

Calibration shift and the effects of wear are a function of the slip flow. The lower the slip flow the less the effects of wear.

The reciprocating piston meter today, with its flexible seal between piston and cylinder, and in its core applications (petrol and diesel), will operate for 12milliom litres with no "significant" change in accuracy... and in that application "significant" is probably far less than for a water meter.

The volumetric meters probably have very little clearance flow but suffer because of the open flow paths (through the piston wall or disc) created over 20-30% of the cycle, are responsible for significant unregistered flow, the non-linearity etc. and thus vulnerable to wear or anything else that affects "slip flow".

I do think solid state meters are perhaps misleading as to the significance of claimed advantages. For example, the lower flowrate. If it was a volumetric meter with such a low flow it would be fine, a clear advantage but when one takes into account the blockage  ratios of the different technologies volumetric meters affect the native flow profile while the solid state does not. The PD meter, for example, below the starting flow rate stops registering but it also stops the flow. In the volumetric meters below the starting flow the flow rate through the open flow paths is significantly reduced. In solid state meters the flowrate below the starting flow i only marginally less than the starting flow.