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The contents of a liquid medium are determined using a light source and an optical detector, for example a spectrometer. A longitudinally displaceable piston sucks the medium into a cylindrical glass measuring chamber and evacuates the liquid from the chamber. At least one measuring beam is directed

The contents of a liquid medium are determined using a light source and an optical detector, for example a spectrometer. A longitudinally displaceable piston sucks the medium into a cylindrical glass measuring chamber and evacuates the liquid from the chamber. At least one measuring beam is directed through the medium, and at least one reference beam is directed outside the medium. The piston has cleaning/sealing rings mounted on its outer periphery which rub against an inner surface of the glass measuring chamber as the piston travels, to clean the surface.

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The invention claimed is: 1. A method for determining the content materials of a liquid medium using a light source and an optical detector arranged to define an optical beam path extending through a measuring chamber; wherein the light source directs at least one measuring beam along the optical b

The invention claimed is: 1. A method for determining the content materials of a liquid medium using a light source and an optical detector arranged to define an optical beam path extending through a measuring chamber; wherein the light source directs at least one measuring beam along the optical beam path through the medium, and at least one reference beam along the optical beam path outside of the medium; wherein the medium is sucked into, and evacuated from, the measuring chamber by the displacement of a piston device within the measuring chamber; wherein the piston device cleans at least a portion of an internal surface of the measuring chamber disposed in the optical beam path as the piston is displaced within the measuring chamber; and wherein the spectrometric measurement is carried out when a lower edge of the piston device is congruent with upper edges of respective optical lenses disposed in the optical beam path, and the results of the reference beam and of the measuring beam are calculated for each wavelength or for individual areas of the spectrum in accordance with the mathematical relations: description="In-line Formulae" end="lead"E1=L1* Espez+C description="In-line Formulae" end="tail" description="In-line Formulae" end="lead"E2=L2* Espez+C description="In-line Formulae" end="tail" wherein E1=measured extinction in measuring beam E2=measured extinction in reference beam L1=length of measurement path L2=length of reference path Espez=specific total extinction module of the medium C=extinction of the cylinder due to discoloration, opacity and contamination. 2. The method according to claim 1 wherein the measuring chamber comprises a light-transparent cylinder, the inside surface of which is cleaned by the piston device. 3. The method according to claim 1, wherein the cylinder comprises glass. 4. The method according to claim 1 wherein the spectrometer is configured as a probe; the measuring chamber is configured as a cylinder; and the piston device includes external cleaning/sealing lips; when the piston device is raised within the cylinder, medium is sucked into the cylinder through an opening of a base of the probe that is immersed in the medium, and an inside wall of the cylinder is cleaned by the cleaning/sealing lips; when the piston device is lowered within the cylinder, the medium is expelled from the cylinder through the opening, and the inside wall is cleaned by the cleaning/sealing lips; the piston device including a reference passage which during piston displacement becomes coincident with the optical beam path, whereupon the reference beam is directed along the optical beam path and through the reference passage to perform a reference measurement; and wherein the measurement beam is directed along the optical beam path and through the medium to perform a measurement of the medium when the piston device has passed the optical beam path. 5. The method according to claim 1 wherein the reference beam constitutes a first reference beam, and further including directing a second reference beam along the optical beam path, and selecting an optimal reference beam from among the first and second reference beams. 6. The method according to claim 1 wherein activated sludge is contained in the medium; wherein an upward displacement of the piston for sucking the medium upwardly into the measuring chamber is performed at a speed slow enough to prevent sludge solids from collecting in the optical beam path during a measurement of the medium. 7. The method according to claim 1 wherein activated sludge is contained in the medium; wherein an upward displacement of the piston device for sucking the medium upwardly into the measuring chamber is performed at a speed for preventing a de-blending of the medium and sludge solids; when a lower edge of the piston device coincides with respective upper edge of optical lenses arranged in the optical beam path, a level of settling of the sludge-medium blend is optically measured in second-intervals for determining: the sludge dry solid matter, the sludge volume, and the sludge index; wherein after the medium in the optical beam path is substantially free of sludge solids, the measurement of the medium is performed. 8. The method according to claim 1 wherein the light beam from the light source is fanned and brought into focus in an essentially parallel beam by at least one optical lens disposed in the optical beam path. 9. The method according to claim 1 wherein the optical detector comprises a spectrometer. 10. A probe for determining the content materials of a liquid medium, comprising: a measuring cylinder forming a measuring chamber and defining a vertical center axis; a lamp for producing an optical beam directed along an optical beam path passing through the cylinder perpendicularly to the center axis, the optical beam path including optical lenses disposed therein; a light guide for receiving the optical beam; a photo-detector for receiving the optical beam from the light guide; an evaluation unit operably connected to the lamp and the detector; a piston device displaceable vertically within the cylinder for sucking-in medium upon being raised, and for expelling the medium upon being lowered, a through-passage extending through the piston device and arranged to become coincident with the optical beam path during vertical travel of the piston device within the cylinder, the piston device including a lower edge positioned congruent with upper edges of respective optical lenses disposed in the optical beam path during a spectrometric measurement, and external sealing/cleaning lips arranged for cleaning at least a light-transparent portion of an internal surface of the cylinder disposed in the optical beam path as the piston is moved within the cylinder; and a motor operably connected to the piston device for effecting vertical movement thereof. 11. The probe according to claim 10 wherein the measuring cylinder includes a medium-conducting opening at a lower end thereof. 12. The probe according to claim 11 wherein the opening includes a micro-screen filter. 13. The probe according to claim 10 wherein the piston includes at least one additional through-passage for a reference beam. 14. The probe according to claim 10 wherein a diameter of the cylinder is between 5 and 30 mm. 15. The probe according to claim 10 wherein the measuring chamber comprises a glass cylinder. 16. The probe according to claim 10 wherein the photo-detector comprises a spectrometer. 17. The probe according to claim 10, wherein the piston device operates as a shutter by the positioning of the lower edge. 18. A probe for determining the content materials of a liquid medium, comprising: a measuring cylinder forming a measuring chamber and defining a vertical center axis; a lamp for producing an optical beam directed along an optical beam path passing through the cylinder perpendicularly to the center axis; a light guide for receiving the optical beam; a photo-detector for receiving the optical beam from the light guide; an evaluation unit operably connected to the lamp and the detector; a piston device displaceable vertically within the cylinder for sucking-in medium upon being raised, and for expelling the medium upon being lowered, a through-passage extending through the piston device and arranged to become coincident with the optical beam path during vertical travel of the piston device within the cylinder, the piston device including external sealing/cleaning lips arranged for cleaning at least a light-transparent portion of an internal surface of the cylinder disposed in the optical beam path as the piston is moved within the cylinder; and a motor operably connected to the piston device for effecting vertical movement thereof, wherein the piston includes at least one additional through-passage for a reference beam.