GB2322701A

GB2322701A - Apparatus for detecting the quantity of particles in a fluid medium

Info

Publication number: GB2322701A
Application number: GB9703980A
Authority: GB
Inventors: James Harvey Smith
Original assignee: Thales Underwater Systems Ltd
Current assignee: Thales Underwater Systems Ltd
Priority date: 1997-02-26
Filing date: 1997-02-26
Publication date: 1998-09-02
Also published as: GB9703980D0

Abstract

Apparatus for detecting the quantity of particles sinking through a fluid medium comprises a detector 11 positioned in the fluid medium at a predetermined depth below the surface 12 of the fluid. The detector comprises a diaphragm (21, figure 2) having beneath it a gas-filled cavity (22) containing a single microphone (23) or an array of microphones. The microphone detects the sound of particles (25) impacting on the diaphragm and passes signals indicative of these impacts to external circuitry 14 or to a remote station 16. The apparatus is intended to detect the quantity of feed pellets sinking through the water in a floating cage fish enclosure. Once the number of particles counted exceeds a predetermined threshold feed is deemed to be wasted.

Description

APPARATUS FOR DETECTING THE QUANTITY OF PARTICLES IN A FLUID MEDIUM This invention relates to apparatus for detecting the quantity of particles in a fluid medium, and in particular to the quantity of particles sinking through a fluid medium.

World aquaculture has grown at an average annual rate of 10% since 1984, reaching approximately 19.million tonnes of produce in 1992 and accounting for almost 20% of total fish production. A percentage of this production was fin fish, intensively cultured in floating cage structures at sea.

The stocks held in these cage usually has a long growing cycle, typically 12 to 24 months in the case of salmon. The largest single component of operating expenditure is considered to be that of feed, which may be as high as 50%. Feed wastage therefore presents a potential source of significant economic loss which may impact directly upon the viability of a fish-farming operation. In addition, feed wastage is an issue in terms of the impact on the marine environment in the vicinity of the farm cages. As well potentially disturbing the ecological balance below the cage, the build-up of uneaten food particles may increase the colonisation of marine bacteria which in turn may impact upon stock health and performance.

Current practice generally involves the use of hand feeding or automatic feeding systems which rely on little more than the intuitive judgement of the operator as to the level of satiation of the fish stock. These considerations alone make it desirable to find a mechanism for monitoring feed wastage in order to optimise the food conversion ratio and minimise any environmental impact.

It is an object of the present invention to provide apparatus for detecting the quantity of particles in a fluid medium and particularly for detecting the quantity of feed particles sinking through a fluid forming a fish farm environment.

According to the present invention there is provided apparatus for detecting the quantity of negatively-buoyant particles sinking through a fluid medium, which includes means for detecting the impact of such particles on a surface.

In this specification the term "particle" is used to denote fish feed of varying size and density. The term "satiation" is used to define the point at which the appetite of the fish stock at any particular time is satisfied and beyond which feed may be wasted. "Food conversion ratio" defines the ratio of the weight of food delivered to the weight of fish produced and is usually of the order of 1.1 to 1.8, with a value below 1.0 as the ultimate goal. The careful monitoring and subsequent control of the level of food wastage will contribute significantly in the attempt to achieve this goal.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of one form of apparatus suitable for applying the invention in a fish farm environment; Figure 2 is a schematic diagram of one form of detector suitable for use with the invention; and Figure 3 is a schematic diagram of an alternative form of detector.

Referring now to Figure 1, this shows an enclosure 10, usually made of nets, in which the fish stock is retained. A detector 11 is located in the water at a predetermined but variable depth below the surface 12. The detector 11 is connected via a cable 13 to drive and control circuitry 14 and, possibly, via a cable or radio link 15 to a remote control or alarm station 16.

The depth at which the detector 11 is located is dependent upon the feeding habits of the particular fish stock. It is necessary to locate the detector 11 at such a depth that any feed particles detected at this depth may be assumed to be waste. In the case of surface-feeding fish such as salmon a depth of, say, 5 metres below the surface will probably be suitable. In addition, the detector 11 should be located more or less directly under the feeding point 17 at which feed is introduced into the water, unless allowance is made for known currents in the enclosure.

The detector 11 may be mounted on the bottom of the enclosure, if this is at a suitable depth, or it may be supported from or tethered to the sea bed 18 or any suitable structure below the water, possibly in a cradle 19 as indicated in Figure 1.

Figure 2 shows one form of detector suitable for use with the invention. A watertight container 20 has its upper surface formed by a thin diaphragm 21, under which is a gas-filled cavity 22 containing a single omni-directional microphone 23. A cable 12 connects the microphone to the external circuitry 13 as shown in Figure 1. The container 20 may be weighted 24 in its lower part, as indicated at 24, to ensure that the container rests with the diaphragm in a particular desired orientation. The amount of weight will determine whether the container is positively or negatively buoyant. It may be convenient to arrange for the diaphragm to be substantially horizontal, so that any feed particles 25 will impinge upon it with maximum impact. If this introduces a problem due to the build-up of particles on the surface of the diaphragm, then the diaphragm may be formed into a non-flat shape, such as conical or inclined to a greater or lesser extent. Whilst this will reduce the problem of feed build-up it will also lessen the impact O of descending particles 25 upon the diaphragm. The gas filling the cavity below the diaphragm 21 may be air.

Figure 3 shows an alternative form of detector in which the single omni-directional microphone 21 of Figure 2 is replaced by an array of separate directional microphones 30. This arrangement may be more suitable if a shaped member is used in place of a flat member for the diaphragm 21.

By way of example only, fish-feed pellets typically descend at speeds between 10 and 20 cms/sec in non-turbulent water, depending upon their size and density. A standard 5mm diameter salmon feed pellet weighing 0.1 gms descends at a speed of the order of 15 cms/second.

The microphone or microphones detect the sound of the impact of feed particles 25 on the diaphragm 21. These individual detections may be processed in real time by the external circuitry 13 or may be transmitted to the remote station 15, or both. Particle detection may be compared with a satiation threshold above which feed is deemed to be wasted. The control or alarm station 15 may be arranged to provide an alarm in the event of detected wastage.

It will be apparent that the in-water detector may be movable at will within the enclosure or between separate enclosures.

The diaphragm 21, being immersed in water, possibly sea water, has to be made from or coated with a material which will resist corrosion under those conditions.

The apparatus described above is equally suitable for use in opensea fish farming or in fresh-water fish farming.

Claims

1. Apparatus for detecting the quantity of particles sinking through a fluid medium, which includes detection means for detecting the impact of such particles on a surface.

2. Apparatus as claimed in Claim 1 in which the detection means comprise a diaphragm on which the particles impinge and sound detection means operable to detect the sound produced by any such impacts.

3. Apparatus as claimed in either of Claims 1 or 2 in which the sound detection means comprise a single microphone located in a gas-filled cavity adjacent to the diaphragm.

4. Apparatus as claimed in either of Claims 1 or 2 in which the sound detection means comprise an array of microphones located in a gas-filled cavity adjacent to the diaphragm.

5. Apparatus as claimed in either of Claims 3 or 4 in which the gasfilled cavity contains air.

6. Apparatus as claimed in any one of Claims 1 to 5 in which the fluid medium is water containing fish constrained into an enclosure, the particles to be detected being fish feed introduced into the water at a feeding point.

7. Apparatus as claimed in Claim 6 in which the detector is located at a depth below the surface of the water below which the fish in the enclosure do not normally feed.

8. Apparatus as claimed in either of Claims 6 or 7 in which the detector is located substantially beneath the feeding point.

9. Apparatus for detecting the quantity of particles sinking through a fluid medium substantially as herein described with reference to the accompanying drawings.