Measurement of particle concentration in a stream

A device for measuring, in a stream of fluid mixed with magnetic particles flowing through an elongate circumferential enclosure and having an axial flow direction, a concentration of the magnetic particles at an axial measurement location along the stream, wherein the circumferential enclosure, and therewith, the stream, has an axial center axis,

• wherein the device comprises:
• a coil with multiple electrically conducting windings, arranged to each tangentially surround the stream at the axial measurement location, and to together generate, when an electrical current flows through the windings of the coil, a magnetizing field having
• a radially inwards part of the magnetizing field within the radial extension of the stream, of which field lines extend inside the stream and axially through the coil, and,
• a radially outwards part of the magnetizing field outside a radial extension of the stream, of which field lines extend outside the stream and surround the axial extension of the coil, the coil having connection points for electrical connection to an external electrical current source,
• a sensor configured and arranged for producing a signal indicative of a magnetic flux density caused by the magnetizing field generated through and outside the coil and/or a change in this magnetic flux density, the sensor having connection points for connection to an external control unit,
• wherein the device further comprises, for increasing an electromagnetic permeability in the magnetizing field and therewith, the magnetic flux density,
• a permeability promotor, which comprises:
• a particle directing tool configured for directing the particles in the stream with respect to the center axis, in a radially outwards direction towards an inner circumference of the coil, such that within the radial and axial extension of the coil a particle concentration is higher in a radially outwards part of the stream relative to a radially inwards part of the stream, and/or
• in the radially outwards part of the magnetizing field, one or more magnetically conducting elements spaced from the coil by at most approximately half the diameter of the stream and extending so as to guide therethrough the field lines of the magnetizing field in their extension within the radially outwards part of the magnetizing field and concentrate these field lines in a plane transverse to the field lines.