Measuring and testing – Volume or rate of flow – Expansible chamber
Patent
1994-11-21
1996-10-22
Chilcot, Richard
Measuring and testing
Volume or rate of flow
Expansible chamber
G01F 308
Patent
active
055678769
DESCRIPTION:
BRIEF SUMMARY
This invention relates to fluid devices and in the most important example to volumetric fluid meters.
Typically, a volumetric water meter comprises an eccentric piston oscillating within a chamber which has inlet and outlet ports. The piston moves circumferentially about the axis of the chamber and sweeps a known volume as fluid passes from the inlet to the outlet port. A magnetic or other external connecting element is rotationally driven by the piston through a drive coupling which conventionally takes the form of a peg or post on the piston engaging a radial arm of the connecting element.
Because of the nature of the fluid forces acting upon the piston, the angular velocity of its circumferential movement is not constant but varies with the angular rotation. Typically, the angular velocity will vary between one maximum and one minimum in each cycle, the maximum and the minimum being diametrically opposed.
This angular velocity modulation can create severe operating difficulties. For example, the driven radial arm can become disengaged from the piston drive peg, when the angular velocity of the piston reduces, as a result of the angular momentum of the connecting element, and of the register or other elements to which it is magnetically coupled. The piston drive peg will then re-engage the radial arm when the piston angular velocity increases. This cyclical disengagement and re-engagement can lead to problems of:
If the magnetic coupling is lost, it may possibly not be re-established until the water flow drops substantially to zero. This loss of coupling of course results in an understatement of the water flow, which may be significant.
Certain of these problems have been identified previously and reference is directed to U.S. Pat. No. 4,738,136 which proposes a solution. In the arrangement disclosed in this prior art reference, the radial arm is made convex at the point of driving engagement with the piston drive post, the arc of curvature of the contact surface being essentially complementary to the arc of curvature of the drive post. In this way, it is suggested that the hitherto point contact between the piston drive post and the radial arm is replaced by a more extensive area of contact. It is explained that in addition to dissipating the energy of re-engagement over a wider contact area, this arrangement provides a cushioning effect in that a significant volume of liquid must be squeezed out of the space between the post and the convex surface before contact can be re-established. Similarly, it is hypothesised, the arm is less likely to lose contact with the post because of the need for liquid to be pulled into the space between the convex surface and the post as the two disengage.
It is believed that, whilst this prior art arrangement represents an improvement over what has gone before, it is not an ideal solution to the problems outlined above. It is clear, for example, that the variations in torque which are the cause of the tendency for disengagement, remain undiminished. Whilst the viscous effects described in the prior art reference may be of some assistance, it is not believed that they can be relied upon to solve the specific problems which this invention addresses.
It is an object of one aspect of the present invention to provide an improved fluid device in which the problem of non-uniform angular velocity and the consequential tendency to lose engagement, is tackled directly and in a way which offers a more comprehensive solution.
Accordingly, the present invention consists in one aspect in a fluid device comprising a cylindrical chamber with an axis and having a fluid inlet port and a fluid outlet port; a piston eccentrically disposed within the chamber and having a cylindrical piston wall in sealing engagement with an interior surface of the chamber such that circumferential movement of the piston about said axis sweeps a fixed volume of fluid passing from the fluid inlet port to the fluid outlet port, the angular velocity of the piston in said circumferential movement varying with the
REFERENCES:
patent: 1238203 (1917-08-01), Selby
patent: 1973670 (1934-09-01), Star
Holmes-Higgin Ian J.
Rogers Ivor T.
ABB Kent PLC
Chilcot Richard
Patel Harshad
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