Measuring and testing – Volume or rate of flow – Combined
Patent
1996-01-16
1998-08-04
Noland, Thomas P.
Measuring and testing
Volume or rate of flow
Combined
73272A, 73273, 73431, 324156, 439271, G01D 1126, G01F 1514
Patent
active
057896720
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to vapour barriers and particularly to methods and arrangements for passing electrical terminals through a glass or other wall, in vapour tight fashion.
In the most important example, the present invention relates to metering assemblies in which there is a dual requirement for a transparent window--enabling viewing of an internal number wheel or other display--and an electrical terminal passing through the transparent window--enabling a connection to be made between remote meter reading equipment and circuitry within the meter.
The example will be taken of a water meter, a traditional form of which comprises a bronze or copper housing having water inlet and outlet, a metering unit within the housing, a number wheel display and a glass cover through which the display can be read. There is more recently a requirement for such meters to be read electronically and circuitry has been developed which, in combination with the number wheel display, provides an electronic representation of the meter reading. Water meters are typically provided in pits, basements or other inaccessible locations and the electronic encoding facility enables the meter to be wired to a more conveniently located reading unit. This may take the form of a simple, slave display. More usually, the reading unit enables remote interrogation through communication with a portable reader unit or communication over telephone, radio or other links with a central reading station.
It is a requirement of the market that, in addition to the remote reading facility, the water meter should continue to have a number wheel or other display directly viewable at the meter. The meter housing must therefore continue to include a transparent window.
Because water meters are often mounted in pits or other constricted locations, it is only from the top that access to the meter can be guaranteed. It is therefore necessary or desirable for electrical connection to be made through the transparent window. Prior proposals have achieved this by forming the transparent window from plastics material, moulded about the electrical terminals. This solution appeared to offer important advantages. Established moulding techniques were available for moulding the plastics material around the terminals so as to support the terminals and to provide a vapour tight seal. In addition, correctly chosen plastics materials can provide greater resistance to breakage than glass.
In a typical example of these prior proposals, a polycarbonate cover has been developed, the shape of the cover being generally identical with a traditional glass cover but the plastics cover having integrally moulded terminals.
Severe problems have, however, been encountered with electrically encoding water meters having polycarbonate or other plastics covers. High rates of electronic circuit failure have been experienced and, at the lower temperatures at which water meters are frequently required to operate, condensation on the inside of the cover has interfered with manual reading.
One cause of these problems is believed to be the relatively higher vapour porosity of polycarbonate as compared with glass. Also, the difference in coefficients of thermal expansion of metal and plastics can provide vapour transmission paths as the meter assembly is subject to temperature variations. In many cases, these temperature variations can be extreme. The cyclic thermal expansion and contraction within the polycarbonate itself, can also promote leaking of water vapour through the cover.
Considerable work has gone into overcoming these problems. One approach is to replace a simple polycarbonate cover by a laminated plastics construction designed to offer superior moisture barrier properties. This is not, however, believed to be a complete solution since the problem of differential thermal expansion and contraction will remain. It has been suggested that the risk of electronic circuit failure through vapour incursion could be reduced by coating or potting of the circuitry. Ho
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Rogers Ivor Thomas
Samuel Wayne David
ABB Kent PLC
Noland Thomas P.
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