User:Jon M Watson/sandbox

Water metering began with the inferential meters, mostly referred to as "rotary meters" in the 19th century.

Positive displacement meters entered the market with Thomas Kennedy's 1824 patent reciprocating piston meter, a benchmark of water meters for the next 110-120 years. Over this time many other PD meters were designed and introduced with varying success but perhaps the last important design was the Swedish IMO bi-rotor patented in 1934 and manufactured in the USA by the Pitsburg meter company in 1939 and claimed to be the most accurate water meter ever manufactured.

Positive displacement meters were displaced by volumetric meters, the semi-positive rotary/oscillating piston and nutating disc meters which took over from some time in the 1940s.

Though often erroneously referred to as positive displacement, they are only semi-positive being positive for about 70-80% of the cycle but with open flow paths over the remaining 20-30% through the two transitions. These open flow paths are why they perform so badly compared to true PD meters except, perhaps, in so far as they are a more efficient design with a lower starting flow. True PD meters are very accurate and linear, even with a mechanical register and a fixed meter factor for all flow rates. Today they find application in demanding industrial applications where accuracy and linearity are key requirements. The reciprocating piston meter is used to meter fuel into vehicles for example. In theory positive displacement is the most perfect of all flow technologies being perfectly accurate and linear at all achievable flow rates.and with the reciprocating piston meter and IMO bi-rotor being the most perfect examples and coming closest to delivering the theoretical ideal. When the meter stops registering, because there is no open flow path from inlet to outlet, flow stops. This means that though the native flow profile might be to have significant consumption at low flow rates diminishing to zero, in practice the operation of the meter forces all consumption to take place only within the measurement range.

The volumetric meters are far from ideal though in terms of water metering are reasonably accurate, but not when compared to true PD meters and more importantly they are not as linear as could be desired..

Water meters are examples of "transaction meters".

The key "must have" property of a transaction meter is that the registered quantity is the true consumption. It can be appreciated that inferential meters, even as crudely designed as at the beginning of the 19th century, delivered this "true consumption" measurement in early direct plumbing where consumption took place only at flow rates within the meter's measurement range, but were totally unsuited to indirect plumbing.

The Reciprocating piston meter and other PD meters were capable of handling direct plumbing but their real value was in indirect plumbing (where a gravity tank was used and maintained full using a float valve) where a significant proportion of consumption would normally take place at low flows diminishing to zero. They did not replace inferential meters because they were generally mechanically complex, bulky, high pressure drop, relatively high starting flow and expensive whereas inferential meters, though not very accurate, were mechanically simply, compact, low pressure drop and cheap.

The Volumetric meters were introduced firstly in the UK with the rotary (piston) meter in the 1860s by Tylors of London as their "British Patent Rotary Water Meter". This was at a time when inferential meters were becoming less satisfactory in the more advanced direct plumbing installations, for example, due to the Guest and Chrimes 1854 patented regulating tap replacing the typical "carrot" style quarter turn tap used previously and thus shifting consumption into the lower flowrates where inferential meters were unsuited but where volumetric meters had an advantage in their superior low flow performance.

It is only with the ending of the PD meters that volumetric meters took over indirect plumbing but it now means that no water meter delivers the "true consumption". All, including solid state, have a staring flow below which some significant consumption can occur without causing registration. The advantage volumetric meters have over PD meters is that they are simple, compact, low pressure drop and cheap. Important features with respect to the 1945 Rural Water Supply Act in the UK which was designed to extend potable water supplies to even the most remote rural areas and where even a workman's cottage could be supplied. This act originates following on from the 1934 health act which had relevance to public health issues caused by the ongoing use of contaminated ground water as drinking water.

The ideal meter would be a simple compact, low pressure drop low starting flow but cheap positive displacement meter. This would be ideal for both direct and indirect plumbing and suitable for rural water supplies. One meter that came close to meeting this ideal was Lewis Nash's Crown Meter, a mechanically simply, compact positive displacement meter with low pressure drop, low starting flow and which was successful for a period. It is this meter which earned Nash credit for inventing the "rotary piston meter", that is a meter capable of handling both the rotary (inferential) meter market (direct plumbing) and the piston meter market (indirect plumbing).In the US, to avoid confusion with what Europeans refer to as the rotary piston meter, they use the term oscillating piston meter instead. The problem with the Crown meter was that though mechanically simple, it used complex geometry which made it difficult and expensive to manufacture in the 19th century. Nash tried several times to reduce costs and control variances as various patents demonstrate but in the end he failed to deliver a price competitive design.

Note: The US water metering market, despite being primarily direct plumbing, began with the Worthington designed reciprocating piston meter and was dominated by positive displacement meters till the nutating disc meter began to take over. Worthington's meter was patented in about 1855, about the time when inferential meters were less well suited to direct plumbing and before the advent of the volumetric meters which began to take over from inferential meters in some markets.

The only other significant designs were the volumetric meters but despite every attempt, including by Nash who devoted considerable attention to them, no way was found to convert them from semi-positive operation to positive. Were they to be made fully positive they can confidently be expected to outperform every other meter including solid state and even with a mechanical register and fixed meter factor. ,However, the prevailing view is that they are semi positive due to an inherent or intrinsic design limitation. The last significant design changes were those of Lewis Nash with his two rotary/oscillating piston meter patents of the 1870s and his two patents of the 1880s for the nutating disc meters.

(jon Watson: Jon.watson@viscoanalyser.com)