Designing for Reliability

It is generally accepted among design engineers that reducing the number of moving parts in a device increases its reliability and life expectancy. Fusion Meters of Chesterfield, England has taken that belief to the limit: the company's new System Equity water meter has no moving parts at all. Achieving that goal, however, required extensive materials evaluation and testing, product and mould design studies, and the use of sophisticated injection moulding technology.

As a starting point, keep in mind that the basic specifications of the System Equity meter include a 20-year life cycle. That is considerably longer than conventional water meters now in use. Traditional meters also suffer from wear-related inaccuracy and under-recording of water volume, stalling of mechanical components by grit and sand, and over-recording of water volume caused by the presence of air. Each of those problems is also eliminated by Fusion's new design.

Fluidic Oscillation

To measure water volume, the System Equity meter uses a flow transducer, a device employing a flow sensing technique based on the principle of fluidic oscillation. Fluidic oscillation provides an extremely high level of accuracy in measuring fluid, in this case water. Fusion engineers first created an initial design for the fluidic oscillator measuring device that would be the main internal component of the meter. They then took it to the Ticona Technical Center in Milton Keynes, England. Following testing and evaluation that included FEA analysis and Moldflow simulation, a decision was made to mould the part in acetal copolymer (POM), specifically the noncoloured C9021 grade of Ticona's Hostaform.

Other materials were considered. Polypropylene was a possibility in the early stages but was deemed not stiff enough. Even if wall sections were thickened to achieve the required stiffness, the PP could not assure the required dimensional stability. POM's ability to hold dimensional stability and to resist creep over the long term, such as a 20-year minimum life cycle, complemented its ability to meet the initial design tolerances such as +.03/.00 mm on the part's 39.10 mm diameter. ABS was also evaluated and eliminated for not having sufficient long-term fatigue resistance to withstand the continuous oscillation of water within the flow chamber. The only thing that does move in this meter is the water passing through it. A development tool was made and evaluation parts were moulded at the Ticona Technical Center for further testing.

Processing Follows Form and Function

Fusion makes the transducers in a single cavity, direct-feed mould on an Arburg 80-ton injection machine. Two metallic pins are automatically loaded and positioned in the open mould by a robot. These pins, which are then en-capsulated using insert moulding technology, function as electrodes detecting the changes in the electromotive force of the flowing water. An energy field is created by permanent magnets on either side of the flow chamber. The magnets, which are strong enough to eliminate the need for electrical power to the electrodes, are fitted into slots and then sealed in using hot plate welding. Some of the wall sections in this area must be less than 1 mm thick to assure that the energy field remains constant.

The transducer's subsequent assembly includes ultrasonic welding to outer fittings previously moulded in a 30 percent glass-reinforced grade of Hostaform POM. That assembly is then welded into the outer housing. The meter's outer housing, which of course must also meet the 20-year life cycle specification, was likewise the subject of intense material research. Critical properties needed include stress crack resistance, stiffness, and scratch resistance, plus UV resistance when the unit will be located outside in a hot, sunny climate. It was discovered that the same grade of acetal copolymer used to make the transducer, only coloured opaque white, could be used for the housings. For sunny climates, a light-stabilized grade of that material, Hostaform POM C9021 LS, is used.

An intrinsic part of the material selection process was consideration of the requirements of the United Kingdom's Water Research Center (WRC). The WRC tests all materials for taste, extraction of metals, water appearance, growth of micro-organisms, and extraction of any substances that could be a public health concern. A manufacturer like Fusion Meters has to not only use materials tested and given an approval number by WRC, but also submit the completed product for testing in accord with the Water Fittings Bylaws Scheme. The product will only be tested if all the construction materials are WRC approved, and if it passes, it goes in the WRC directory used by suppliers and purchasers of water-contact products. The POM materials used by Fusion are among many Ticona materials on the WRC-approved list.

The System Equity meter, in addition to undergoing extensive testing of its water-measuring accuracy, is also strenuously tested for pressure resistance. Although normal operating water pressure is generally 15 bar, the meter is tested up to 50 bar to assure reliable operation.

Contact Information

Fusion Meters
Ms. Jackie Roberts
Smeckley Wood Close
Chesterfield Trading Estate
Chesterfield S41 9PZ, UK
Tel: +44 (1246) 456658
Fax: +44 (1246) 261001

Ticona UK Ltd.
Mr. Graham Wright
Michigan Drive, Tonwell
Milton Keynes MK15 8HQ, UK
Tel: +44 (1908) 513456
Fax: +44 (1908) 513410
E-mail: [email protected]