Electromagnetic water current meter with staggered stick out electrodes

An electromagnetic water current meter includes a cylindrical transducer body of circular cross-section with two pairs of electrodes, each pair of electrodes rotated 90.degree. around the transducer axis with respect to the other pair. Each electrode extends beyond the surface of the body of the transducer and each pair of electrodes lie in a plane perpendicular to the longitudinal axis of the transducer. The planes of each of the pairs of electrodes are offset with respect to each other preferably by a distance equal to the diameter of the transducer. Each electrode is preferably of the form of a truncated cone.

FIELD OF THE INVENTION 
The present invention relates to electromagnetic water current meters. 
BACKGROUND OF THE INVENTION 
Electromagnetic water current meters are known to the art and typically 
include an electromagnetic coil mounted in a transducer body which may be 
of cylindrical form preferably with circular cross-section. Mounted at or 
near the surface of the transducer body are plural electrodes from which 
voltages are sensed by circuitry which processes the voltages and produces 
a signal related to water current velocity. It has been known for some 
time that under certain conditions it is desirable for the electrodes to 
project beyond the surface of the body of the transducer, i.e., to 
stick-out. See for example, my article in OCEANS '76 entitled 
"Electromagnetic Water Current Meter" presented at the Second Annual 
Conference of the Marine Technology Society and the Institute of 
Electrical and Electronics Engineers (September 15, 1976). See also "The 
Theory of Induced Voltage Electromagnetic Flow Meters", Bevir appearing in 
the Journal of Fluid Mechanics, Volume 43, part III, pages 577-590 (1970); 
"Measurements of Turbulent Fluctuation and Reynolds Stresses in a Tidal 
Current", Bowden et al, appearing in the Proceedings of the Royal Society 
(London) Series A, Volume 237 pages 422-38 (1956); "Electrodes for 
Magnetic Flow Meters", Gray, appearing in Water and Sewage Works (London), 
page R-93 (August 1972); "The Measurement of Sea Water Velocities by 
Electromagnetic Induction", Guelke et al, appearing in the Journal of the 
Institution of Electrical Engineers, Volume 94 part II, page 71 (1947); 
"The Theory of Electromagnetic Flow Measurement", Shercliff page 90 
(Cambridge University Press, New York 1962); "A Two Component 
Electromagnetic Ship's Log", Tucker et al, appearing in the Journal of the 
Institute of Navigation, Volume 23, Page 302 (1970); "Electromagnetic 
Current Meters", Tucker, appearing in the Proceedings of the Society of 
Underwater Technology, (London) Page 53 (1972); and, "Electromagnetic Flow 
Metering", Webb, appearing in Institute of Technology, ISA March 1974. 
Briefly stated, it is known that ideal flow going past a cylindrical 
electromagnetic water current meter can yield perfect cosine response. 
Ideal flow means that the fluid flow is described by potential theory, 
i.e., the flow has no vorticity. At the same time, it is also well known 
that real flow past a cylinder or sphere can be described in terms of two 
regions, the boundary layer region which is very thin next to the sensor 
or transducer wall and the potential flow region which exists everywhere 
except for this thin boundary region. Thus, by providing protruding 
electrodes, the sensitive area of the electrode extends beyond the 
transducer wall, and the boundary layer and into the region of potential 
flow for accurate measurement thereof. Inasmuch as the transducer is 
designed to sense fluid flow regardless of direction, and hence, has two 
pairs of electrodes, each pair spaced by 90.degree., if the fluid flow is 
directed directly at one of the diameters on which a pair of electrodes is 
located, that pair of electrodes will sense voltages proportional to the 
flow, and the other pair of electrodes will sense no flow at all. However, 
if the flow is not directed directly at a diameter on which a pair of 
electrodes exist, and this of course is typically the case, then a 
protruding electrode wil be in the wake of another electrode. The presence 
of the wake will disturb the velocity flow past the electrode which lies 
within it, resulting in less than ideal operation of the transducer. 
It is therefore, one object of the present invention to provide 
electromagnetic water current meter having a transducer with stick-out 
electrodes while reducing the possibility that one of the electrodes will 
lie in the wake of another. 
SUMMARY OF THE INVENTION 
The present invention meets the foregoing objects, and other objects which 
will become clear hereinafter, by providing an electromagnetic water 
current meter with a transducer of circular cross-section having two pairs 
of electrodes, each pair of electrodes lying on a diameter of the 
transducer body, each of the diameters being rotated 90.degree. with 
respect to the other, each pair of electrodes lying in a plane 
perpendicular to the longitudinal axis of the transducer with the planes 
being staggered or offset from each other.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a side-view of a cylindrical transducer of circular cross-section 
for an electromagnetic flow meter. In many respects, the transducer is 
conventional. Included in the transducer (but not illustrated) is an 
electromagnet for generating a magnetic field. The transducer includes two 
pairs of electrodes, one pair 14 and 14', and a second pair 15 and 15' 
(electrode 15' being hidden in the view of FIG. 1). Each pair of 
electrodes lies on a diameter of the circular cross-section of the 
cylinder, and the diameters of the electrodes are rotated 90.degree. with 
respect to each other. Each of the electrodes is a stick-out electrode 
including a sensitive surface area 16. The stick-out electrodes may be 
(although not necessarily) in the form of a truncated cone (as 
illustrated) wherein the cone includes an angle of from 20.degree. to 
30.degree. for example. The flanks of the cone are insulated by epoxy or 
other similar material so as to leave only a sensitive area 16 for sensing 
flow velocities. The sensitive area of the electrode 16 is arranged to 
extend beyond the boundary layer. For a typical transducer having a 3/4 
inch diameter, the electrodes may protrude, for example, 1/16 inch beyond 
the surface of the transducer. 
In order to improve the performance of the transducer, each pair of 
electrodes lies in different plane perpendicular to the longitudinal axis 
11 of the transducer. For example, the electrode pair 15-15' lies in a 
plane 17, and the electrode pair 14-14' lies in a different plane 19. The 
planes 17 and 19 are each perpendicular to the axis 11 and are separated 
by a distance D. In order to point out the problem and the manner in which 
staggering of the electrodes, as shown in FIG. 1, overcomes that problem 
reference is now made to FIGS. 2A through 2D. 
FIG. 2A is a cross-section of the transducer 10 showing all the electrodes, 
in schematic form. When the transducer 10 is placed in its intended 
environment, and the flow has the direction indicated by the arrow 
labelled "Flow" in FIG. 2A, an electrode pair 14-14' will have voltages 
induced therein proportional to the velocity of the flow, and the 
electrode pair 15-15' will not sense any flow velocity, since there is no 
component of flow parallel to the axis 15-15'. Rotating the transducer 10 
by 90.degree. produces the same result except that now the electrodes 
15-15' read the flow velocity while the electrodes 14-14' will not sense 
any velocity. 
FIG. 2B is a similar cross-section of the transducer 10 also showing the 
electrodes, but now the flow has the direction indicated by the arrow 
labelled "Flow", which is not parallel to either of the diameters on which 
the electrodes exist. FIG. 2C is a side view illustrating the electrodes 
15 and 14 along with the flow direction. Because of the protrusion of 
electrode 14, the fluid velocity produces a disturbance, or wake, in which 
lines 20 represent streamlines thereof. The wake actually produced, of 
course, expands in three dimensions, although only two dimensions are 
illustrated in FIG. 2C. As is illustrated in FIG. 2C, the electrode 15 
lies within the wake. As a result, the velocity sensed by the electrode 15 
is disturbed by the wake. At the very least this degrades the performance 
of the transducer. 
With the transducer of the present invention, this difficulty is avoided by 
offsetting the electrodes. FIG. 2D shows a side view of a transducer in 
accordance with the present invention under the same conditions as that 
shown in FIG. 2C. However, as is illustrated in FIG. 2D, the electrode 15 
does not lie within the wake created by the electrode 14, since the 
electrodes 14 and 15 do not lie in the same plane perpendicular to the 
axis 11. 
If the transducer 10 is intended for an environment which limits the flow 
velocity to a direction perpendicular to its longitudinal axis, i.e., axis 
11, then the minimum offset, that is, minimum value of D, can be slightly 
larger than the electrode diameter. 
Most real transducers, however, are subjected to flows which are not 
limited to directions perpendicular to their axis 11. As will be 
understood by those skilled in the art, as the component of flow velocity 
parallel to the axis 11 increases, the minimum stagger, or offset, 
required to prevent an electrode from lying within the wake of another 
electrode, increases. For example, if a transducer is moored to a buoy, 
which is floating on the surface of a body of water, the action of water 
flow will cause the transducer to tilt. To prevent wake from one electrode 
from distrubing another electrode, the minimum offset has to be increased 
because of this tilt. The stagger distance cannot be increased to accept 
any arbitrarily large tilt angle. Preferably the stagger distance is 
arranged to allow a reasonable degree of tilt. Reasonable amounts of tilt 
may be angles on the order of 15.degree.-20.degree. with a 2-1 safety 
factor, the stagger distance will be arranged to accept tilt angles of 
40.degree.-45.degree., for example. 
Although the precise offset distance can best be determined experimentally, 
a reasonable offset for these circumstances is approximately 1 transducer 
diameter.