Water Meter Accuracy Measurment

Question

How is the accuracy of water meters tested?
Was it proven or is it convenient for utilities to adopt the volumetric meters and retain the inferential meters which are simple and cheap instead of the expensive meters which show the real consumption?
The idea of equability is that volumetric meters (oscillating piston and nutating disc) and the inferential meters (single jet and multi-jet) will show a registered consumption that is not the true consumption but that by the equable measurement.
The registered consumption can be used as a proxy for the true consumption without any consumers being significantly advantaged or disadvantaged by the reading.
For example, if the meter registers 100 m3 then the unregistered consumption may be 5 m3 and it will be, within acceptable limits, the same 5 m3 unregistered consumption for all consumers.

Muhammad Alfalah Fauzi · Answer

Technically, you need to consider the following factors in determining water meters which is also related to its accuracy and capability of measuring "real consumption":
The first is to determine your average/operating flowrate on your installation (commonly known as nominal flowrate or Q3). You need also logged data to know the flow profile of the installation, which flowrate consumes the most volume and spends most of the time.
The second is the Ratio (Q3/Q1), where Q3 is the average flow rate (operating flow rate) on the installation with +/- 2% of error margin and Q1 is the minimum flow to be measured by the water meters to be between +/- 5% of the error margin. Note that starting flow rate (Q start) is another point of measurement which is below Q1 with no error margin (could be more than +/- 5%) and not included in the metrological error of measurement. The bigger the Ratio (R), the better the measurement range and its capability to measure lower flowrate. For example, common mechanical meters have a Ratio (R) between R50 - R160, but current solid state meters could be up to R300 - R400 which is better to measure low flow rates.
The third one is the "Typical Error Curve". The typical error curve is unique for each meter which depends on the design, operating principle, materials, and many other technical factors. Here, you need to deeply analyze which flowrates operate the most and its typical error curve on that flowrate. For example, if 99% of your consumption is resulted by flowrate on Q3, then you need to find a meter with near-zero percent error on Q3 (or adjust the measurement to near-zero percent of error because some meters allow you to adjust the measurement).
Last but not least, current solid state meters (ultrasonic, electromagnetic) have enhanced their capability of measuring constantly near zero percent (+ 0.2% for example) which means the meter error is constantly between + 0.2% between all flowrates (no undulation of typical error curve like mechanical type meters).

Jon Watson · Answer

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.

Andy Godley · Answer

I'm not quite sure of your point here. The choice of a meter for any application is always a compromise between many different factors - including performance, cost, reliability, expected service life etc etc. The rotary piston meter we tend to use in the UK has evolved as the (current) meter of choice because it does a pretty good job of balancing these compromises. It is relatively cheap, will last typically 15 years (admittedly with some degradation of performance but that happens with any mechanical meter), is reasonably robust, it has good low flow sensitivity (which is important where much water supplied is stored in tanks with ballcocks to control inflows) but also has a wide measurement range  - wider than most other meter types. This minimises the number of different sized meters required to cover properties with different consumptions, making installation simple. Importantly the impact of mechanical wear tends to be in the customers' favour, i.e. the meters tend to under-record over time. This means that there is no need for regulation around replacement tor expensive testing programmes driven by consumer protection that there is in countries that use technologies that are likely to over-record. We also like putting meters in boundary boxes underground and outside - few other meter types have been developed as manifold fitting.
Whether these considerations hold up as solid state meter types begin to penetrate the market remains to be seen...

Kurt Rasmussen, P.Eng. · Answer

I work for the government water and wastewater utility in our province here in Canada and we use 2 meters only.  Sensus meters on pipelines 5/8" to 1" (outside diameter) and magnetic meters (specifically Rosemount mag meters) for flows on pipelines exceeding 1" up to 36".  We have spent much time and resources researching the best meters for our customers and after years of testing we have decided that the mag meters are the only way to go for high flows and the Sensus meters for the small flows.
The accuracy of the mag meters that we use have a volumetric accuracy of ±0.25% with a turndown range of 13:1 to 40:1 with a ±0.5% accuracy.  These are very high quality meters.
The Sensus meters for the small flows (5/8" to 1") the accuracy is ±1.5% on normal flows and on low range flows the accuracy is ±3%.  Normal flows = 0.04  m3/hr to 12.5 m3/hr and low flows = 0.025 m3/hr to

Jon Watson · Answer

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.

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