Magnetic flow meter providing quasi-annular flow

1. A magnetic flow meter for measuring the flow rate of a fluid through a tubular meter body having an axis, the apparatus comprising:

at least one electromagnet operable to provide a magnetic field extending along a selected field direction perpendicular to the axis;

at least one support member extending from an inner surface of the tubular meter body to an inner tube disposed coaxial with the meter body so as to define an inner flow path within the inner tube and a quasi-annular outer flow path disposed between the inner tube and the tubular meter body;

at least one pair of electrodes, the two electrodes of each at least one pair disposed on opposite sides of the at least one supporting member so as to be spaced apart from each other along a line perpendicular to both the axis and to the selected field direction, each at least one pair of electrodes defining a respective voltage generating path extending circumferentially through the quasi-annular region from one electrode in the at least one pair thereof to the other; and

measurement circuitry operable to measure a respective voltage generated along a respective at least one voltage generating path and to generate therefrom an output representative of the flow rate of the fluid through the tubular meter body.

2. The magnetic flow meter of claim 1 wherein the inner tube comprises an electrically conducting internal surface.

3. The magnetic flow meter of claim 1 wherein the inner tube comprises an upstream end shaped to control a ratio of fluid flowing through the inner tube to fluid flowing through the quasi-annular outer flow path.

4. The magnetic flow meter of claim 1 further comprising an obstruction within the inner tube, the obstruction acting to reduce an effective internal diameter of the inner tube.

5. The magnetic flow meter of claim 1 comprising a plurality of support members arranged so that the only entirely electrically insulating support member intersecting one of the at least one voltage generating paths is the support vane on which the pair of electrodes associated with that path are mounted.

6. A magnetic flow meter for measuring the flow rate of a fluid through a tubular meter body having an axis, the apparatus comprising:

two electromagnets having substantially identical coils sharing a single core, each of the electromagnets independently operable to provide a respective magnetic field extending along a selected field direction perpendicular to the axis;

at least one support member extending from an inner surface of the tubular meter body to a flow obstructing body disposed coaxial with the meter body to define a quasi-annular outer flow path disposed between the flow obstructing body and the tubular meter body;

at least two pairs of electrodes, the two electrodes of each pair disposed on opposite sides of the at least one supporting member so as to be spaced apart from each other along a line perpendicular to both the axis and to the selected field direction, each pair of electrodes defining a respective voltage generating path extending circumferentially through the quasi-annular region from one electrode in each one pair to the other; and

measurement circuitry operable to measure at least one voltage generated along a respective at least one voltage generating path to generate therefrom an output representative of the flow rate of the fluid through the tubular meter body.

7. The magnetic flow meter of claim 6 , further comprising synchronization circuitry for operating the two substantially identical coils in unison so as to produce substantially twice the magnetic flux produced by either one of them operated alone.

8. The magnetic flow meter of claim 6 wherein the flow obstructing body comprises an axially disposed tube.

9. The magnetic flow meter of claim 6 wherein the flow obstructing body comprises an axially disposed tube having an electrically conductive internal surface.

10. The magnetic flow meter of claim 6 wherein the flow obstructing body comprises an axially disposed tube and an obstructing member attached to the axially disposed tube and acting to reduce an effective internal diameter thereof.

11. The magnetic flow meter of claim 6 wherein the flow obstructing body comprises a streamlined body configured so as to prohibit axial flow of the fluid therethrough.

12. The magnetic flow meter of claim 6 wherein the single core is disposed external to the meter body.

13. The magnetic flow meter of claim 6 wherein the single core is disposed within the flow obstructing body.

14. A magnetic flow meter for measuring the flow rate of a fluid through a tubular meter body having an axis, the apparatus comprising:

two electromagnets comprising substantially identical coils wound on a common core, the two electromagnets operable to provide respective magnetic fields extending along a selected field direction perpendicular to the axis;

at least one support member extending from an inner surface of the tubular meter body to an inner tube disposed coaxial with the meter body so as to define an inner flow path within the inner tube and a quasi-annular outer flow path disposed between the inner tube and the tubular meter body;

two pairs of electrodes, the two electrodes of each pair spaced apart from each other along a line perpendicular to both the axis and to the selected field direction, each pair of electrodes defining a respective voltage generating path extending circumferentially through the quasi-annular region from one electrode in each pair thereof to the other; and

measurement circuitry operable to measure respective voltages generated along the two voltage generating paths and to generate therefrom an output representative of the flow rate of the fluid through the tubular meter body.