Andy Godley : My point here ...

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Andy Godley :

My point here is that:

PD meters (e.g. reciprocating piston) Complex, large, expensive but where the registered consumption = true consumption. This kept them in the market from 1824 till sometime in the 20th Century (1934? the IMO bi-rotor patented and "the most accurate water meter ever")

Inferential meters (direct plumbing): simple, compact, cheap  and, in direct plumbing when first introduced, the registered consumption = true consumption but in indirect plumbing a very significant difference between registered consumption and true consumption.

The rotary piston meter (semi-positive, patented in the 1860s) is simple, cheap, compact and, in direct plumbing the registered consumption is the true consumption but in indirect plumbing there is still a significant difference between registered consumption and true consumption.

In the 19th cent. inferential meters were referred to as "rotary" meters and hence direct plumbing as the "rotary" market. Tylors,introduced the "rotary piston" meter as the "British Patent Rotary water Meter". It competed with the inferential meters due to its lower flow rates (and other weaknesses of inferential meters) but not then the PD meters in the indirect plumbing market. Tylors continued to evolve the reciprocating piston meter (new design patented in 1888) for indirect plumbing.

Unchanged since the 1870s (Nash patented improvements) while PD meters continued to evolve

at some point in the 20th cent. Rotary piston finally displaced all the PD meters.

All i see as different is the "Equability" concept which I see appears in the 1942 water act (and in many standards since) This enables the rotary piston meter to take over from PD meters despite the registered value not being the true consumption.

My question is about the validity of the Equability Concept, how it is defined and tested.

Or is it just a convenient fiction that allows the simple cheap compact meters to take over from the expensive complex bulky PD meters which otherwise should ahve been dispalced in the 1860s or 1870s?

PS Static//Solid state meters also depend on the Equability concept. More so, perhaps than is realised.

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Regarding equitability, there are discussions in the various water meter standards committees about how to implement the requirement in the Measuring Instruments Directive (MID) that "The meter shall not exploit the MPE or systematically favour any party" in a meaningful test. As meter error curves are a function of flow-rate, should this be based on a typical consumption pattern (how should that be defined as there is so much variability?) or is it over time (what time period etc). It's not an easy question to answer.

Quite frankly, as far as meter technology is concerned, I don't see us turning the clock back 150 years and starting again! However, if you have a meter that provides improved accuracy, particularly at low flows and is economically viable and meets various other criteria outlined on the previous post,  then we would be delighted to include that in our next test programme. We have used pd meters as reference meters on a number of our test rigs - they are mechanical and do wear - in who's favour? 

I think we need to be looking at some of the questions that are being raised on solid state meters and ensuring that these are captured within the tests in the various standards for metrological approval. That the meter should not systematically favour the customer or water provider is an important aspect of this.

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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.

 

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