This invention relates to electromagnetic flow meters which incorporate coils and have electrodes to measure the induced signal voltage as a result of flow through the conduit and the invention relates to the production of electromagnetic flow meters.

Flow meters of the type briefly mentioned above comprise generally of a conduit with an insulating liner through which a fluid flows with a flow rate to be measured. On the outside of the liner are located induction coils, usually two are arranged geometrically opposite each other on either side of the conduit. A pair of electrodes is also disposed onto the liner so they are in contact with the fluid and positioned perpendicular to the direction of flow through the conduit and perpendicular to the magnetic field created by the coils.

In operation current is passed through the coils in a series of alternating pulses. When the coils are energised the induced signal voltage as a result of the flow through the conduit is measured by the electrodes.

In designs where the flow conduit is narrow, it is difficult to attach coils to the side of a liner and often necessary to include a support onto which the coils are placed. In order to position the coils around the conduit, often a complex geometry of conduit and/or the coil arrangement is needed.

In order to suppress disturbances of the induced signal voltage from outside it is also known to wrap conductive material as an electrostatic screen around the liner. This screen has usually the form of a thin copper plate or foil. A known problem with such flow meters is that eddy currents are often caused in the screen and this leads to inaccuracy.

A further problem with the screen is that these flow meters are difficult to construct and manufacture. This is particularly difficult with the smaller flow meters which have complex geometry; this is obviously time consuming.

It is an object of the invention to overcome these problems and enhance the accuracy.

The invention comprises a flow meter comprising a conduit with an insulating liner onto which electrodes are attached, and onto which magnetic coils can be located, and said liner having a conductive coating surrounding the outer surface of the liner substantially in the area of the coils and electrodes .

The term "coating" and "coated" is used to distinguish between the prior art techniques of e.g. locating thin plate or wrapping in foil. The terms would be understood by the skilled person and includes painting, spraying, electro- depositing, vapour deposition and other such techniques.

Preferably the conductive coating is less than 50 microns and preferably less than 25 microns.

The conductive coating is preferably a paint such as carbon black filled, silver, copper, or nickel loaded paints. Such a coating may be brushed, dipped, electrodeposited, or sprayed. This means that components, in particular those that have complex geometries, can be easily coated.

When using such thin coating it has also been found that the undesirable eddy currents are better reduced compared with using thicker coatings such as metal foil or plate. This is an advantage and results in the possibility of making a flow

meter with higher excitation frequency. This property is important for batch applications for example.

Where the flow meter includes a coil support member (s) clamped generally around said liner the inner surface thereof is also coated.

Where the meter includes a coil bobbin, preferably the end- face surface of the bobbin proximal to the liner is also coated.

The invention will now be described in more detail and with reference to the following figures of which:

Figure 1 shows a view of a simple embodiment of the invention. Figure 2 shows a partially exploded view of another embodiment of the invention. Figure 3 shows the inner surface of a coil support in more detail.

Figure 4 shows a sectional side view of a coil bobbin.

Figure 1 shows a view of a simple embodiment of the invention. It shows a cylindrical shaped plastic or ceramic liner 1 of a conduit through which a fluid passes. On the outer diameter of the liner 1 is shown a pair of induction coils 2 which are opposite each other with respect to the linear longitudinal axis. These are energised by a series of pulses by appropriate means. A pair of electrodes 3 are also located on the liner (only one is shown) . These are also opposed to each other in similar fashion. Their purpose is to detect the induced signal voltage due to the magnetic field

created by the coils and due to flow of a fluid. The outer surface of the liner is painted with a copper-loaded paint 4.

Figure 2 shows a partially exploded view of another embodiment of the invention. It shows a narrow ceramic liner 1. Two sub-halves 5 of coil support member are shown exploded. In operation the two halves are fixed together around the conduit by appropriate means, e.g. adhesive, screw means. The two sub-halves also are configured and shaped to receive coil bobbins 6 around which coils are wound. Although the coil support halves are shown simplified in figure 2, figure 3 shows the inner surfaces in more detail showing the complex geometry.

Figure 4 shows a sectional side view of a coil bobbin. In one embodiment this is also coated with thin layer of a conductive material. All surfaces apart from those in contact with the coil are coated. The surfaces which are not coated are marked in the figure with a thick arrow.

The support member, i.e. both sub-halves, is coated with a copper or other conductive based paint. Because of the complex geometry of such supports dipping, painting or spraying are effective methods of coating. In addition the bobbin which holds the coil is also partially coated. The surfaces which may come into contact with the coil are not coated.