home > technical > the failings of mechanical meters
For some considerable time, there has been a constant debate, as to the validity and accuracy characteristics of mechanical meters and the use of new words, such as ‘volumetric’ and ‘electronic’ have tended to confuse the potential buyer even further.
However, the following is an attempt to clarify the truth of mechanical metering technology and how it compares to electronic metering:-
All water meters sold for measuring small-scale commercial or domestic consumption, and most of the larger meters sold for district metering applications, have been based on traditional mechanical principles and fall into two types:
These meters rely on the water being channelled through a jet, or jets, which imparts thrust on a paddle wheel, causing it to turn. Jet meters are often referred to as velocity meters because the rate of flow is directly dependant upon the velocity of the water passing through the jet. The rotation of the paddle wheel is transferred by mechanical gears to an odometer, which displays a reading. This type of meter is called ‘inferential’ because the precise volume of water is not measured – only the number of rotations of the wheel is measured. The volume of water passed through the meter is inferred from the number of paddle wheel rotations. This product is regarded as inferior to volumetric meters, due to the limited measuring range and poor accuracy.
Meters of this type overcome the problems of inferential systems, by allowing a known volume of water to pass through the meter at each stroke or oscillation of a piston. These meters are more accurate than jet meters, as they rely on positive displacement of water, as opposed to an inferred volume. However, the tight tolerance, inherent within the piston design, makes this type MORE susceptible to mechanical wear and jamming of components by grit and particulates in the water flow.
It is acknowledged by most Water Utilities that impurities, such as grit or air can get into the network and cause problems with mechanical water meters. Over many years, manufacturers of mechanical water meters have developed techniques for coping with meter damage by impurities, but all accept that mechanical meters eventually (depends upon the volume of air and particulates in the water supply) lose accuracy to an unacceptable degree necessitating a regular replacement cycle – usually 3 or 4 times over a 20 year period. The regular replacement is the final cost the Utility must bear but the ‘under-registration’ of flow caused by the mechanical meters is progressive and will cause the Utility to incur income losses that are totally unacceptable, as it may well reach under-registration of 25% in the first year.
An additional problem with mechanical meters is translating readings into electronic data signals, required by automatic meter reading (AMR) systems. Again, manufacturers have developed methods of doing this, but they all involve some form of electro-mechanical system, which wears with time and is prone to mechanical failure.
Many Manufacturers use the description ‘electronic’ for meters that are, in fact, mechanically operated, but may use some form of electronic register to display readings or transfer data. These are not truly ‘electronic meters’ and they suffer from the drawbacks of mechanical wear and tear, described previously.
True ‘electronic meters’, have no moving parts and measure the flow of water by a form of non-mechanical means. They are intrinsically more accurate than mechanical meters, over a longer useful life.
In many parts of the world, the principle challenge facing water Utilities is the ability to operate on a firm, financial basis. As most Utilities depend upon metering to establish the usage of customers, it is essential that metering is accurate, ensuring bills are accurate and regularly sent, resulting in the minimum of disputes and charges collected. All this is vital to the financial income to the Water Utility.
Correct metering also plays a fundamental role in managing the assets of the Utility by providing real-time, accurate information on water losses, rather than relying on statistical techniques and assumptions.
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