We Claim:
1. Method for quantitively determining the number and size of particulate
components contained in a medium flowing along a flow channel, wherein
ultrasonic waves are coupled into the flowing medium, which are reflected at
least partially on the particulate components and the reflected ultrasonic wave
portions of which are detected in the form of ultrasonic time signals, on which
the quantitative determining is based, characterized by the following method
steps:
- coupling the ultrasonic waves into the flowing medium in such a way that at least one portion of the coupled-in ultrasonic waves is reflected on a wall region of the flow channel bordering the flow medium or a reflector introduced within the flow channel, via which an echo-ultrasonic time signal that can be associated with the wall region or the reflector is generated;
- determining at least one amplitude threshold value function which establishes an amplitude threshold value for each detected ultrasonic time signal, taking into consideration at least the echo-ultrasonic time signal;
- detecting amplitude values associated with the individual ultrasonic time signals, which are each greater than an amplitude threshold value established for each ultrasonic time signal; and
- assigning the detected amplitude values to values describing the size and number of the particulate components.
2. Method according to Claim 1,
characterized in that the ultrasonic waves are coupled into the medium flowing through the flow channel with a main direction of propagation directed at an angle to the direction of flow of the flowing medium, the ultrasonic waves travelling along the main direction of propagation are

orthogonal or oblique to the wall region delimiting the flow channel, or meet that reflector located inside the flow channel and are reflected, and the reflected ultrasonic wave portions reflected are detected in the area of or at the site of the coupling.
3. Method according to Claim 1 or 2,
characterized in that in order to calculate the amplitude threshold value function at least one of the following physical properties is considered:
- consideration of the ultrasonic field distribution in the flowing medium,
- consideration of the acoustic attenuation of the ultrasonic waves in the flowing medium,
- consideration of the coupling conditions of the ultrasonic waves in the flowing medium.
4. Method according to any one of Claims 1 to 3,
characterized in that the amplitude values which are individually larger than an amplitude threshold defined for the respective ultrasonic time signal are captured within a definable evaluation time range, which corresponds to a spatial measurement range within the flowing medium along the main direction of propagation and lies between the location of the coupling and the wall region delimiting the flow channel or the reflector.
5. Method according to any one of Claims 2 to 4,
characterized in that coupling of the ultrasonic waves into the flowing medium is focussed in such manner that the ultrasonic waves are focussed in a focal point located along the main direction of propagation which lies before or after the wall region that delimits the flow channel in the main direction of propagation.
6. Method according to any one of Claims 1 to 5,
characterized in that the assignment of the captured amplitude values to

values that describe the number of particulate components within the flowing medium is based on a number or statistical frequency with which the amplitude values captured per ultrasonic time signal lie above an amplitude threshold value defined for each ultrasonic time signal by the amplitude threshold value function.
7. Method according to any one of Claims 1 to 6, characterized in that the assignment of the captured amplitude values to values that describe the size of the particulate components is based on the respective numerical amount of the amplitude value of the ultrasonic time signal.
8. Method according to Claim 7,
characterized in that in order to obtain absolute size values a calibration value or calibration function is calculated by capturing an ultrasonic time signal, particularly the amplitude or/and signal shape thereof, which is obtained by reflection of the ultrasonic waves on a known ultrasonic reflector.
9. Method according to Claim 8,
characterized in that the at least one amplitude threshold value function is calculated using the calibration value or calibration function.
10. Device for quantitively determining the number and size of particulate
components contained in a medium flowing along a flow channel, wherein
ultrasonic waves are coupled into the flowing medium, which are reflected at
least partially on the particulate components and the reflected ultrasonic wave
portions of which are detected in the form of ultrasonic time signals, on which
the quantitively determining is based, characterized in that in order to couple
ultrasonic waves into the flowing medium at least a section of at least one
waveguide coupled acoustically to an ultrasound transducer is immersed in the
flowing medium,

the waveguide consisting of a waveguide material is surrounded by an outer layer at least in the region thereof which is immersed in the flowing medium, so that the outer layer is arranged between the other waveguide material and that the flowing medium, and the outer layer has a material composition which differs from that of the other waveguide material.
11. Device according to Claim 10,
characterized in that the waveguide has an end which is blunt, tapered or geometrically shaped on one side for coupling focussed ultrasonic waves into the flowing medium,
at least a portion of the end of the waveguide is surrounded by the outer layer, whose material composition is selected depending on the flowing medium in such manner that the material composition dissolves upon contact with the flowing medium.
12. Device according to Claim 10 or 11,
characterized in that the material composition of the outer layer contains at least one substance which initiates and/or supports the wetting of the flowing medium on the waveguide material.
13. Device according to Claim 12,
characterized in that the at least one substance is a smelting salt.
14. Device according to Claim 12 or 13,
characterized in that the at least one substance is surrounded by a cover or a matrix consisting of material that is meltable in the flowing medium.
15. Use der Device according to any one of Claims 10 to 14 for determining
the concentration of foreign bodies in an molten metal, particularly in an
aluminium melt as flowing medium.