Abstract: A method for in situ self-calibrating of an electrochemical sensor for measuring the concentration of one or more species in an aqueous medium. The method includes: taking a first measurement of the current of a working electrode representative of the concentration of dissolved oxygen in the medium, applying an anode current of predetermined density and duration to a generating electrode to produce a defined increase in the local concentration of the dissolved oxygen, taking a second measurement of the current of the working electrode representative of the concentration of oxygen after applying the anode current to the generating electrode, and computing from the first and second measurements a calibration factor for one or more species that relates the oxygen concentration of the medium to be analyzed and the actually measured current between the working electrode and its counter electrode.

Abstract: In a method for measuring solid concentrations in liquids, the light from two differently modulated sources (Q1, Q2) directed on the liquid is measured at the same time and processed in a combined multi-beam measuring process consisting of a 90.degree. light scatter (Q1, P, D4; Q2, P, D3) and back-scatter process (Q1, P1, D1; Q2, P3, D2) and the solid concentration is found therefrom. The process is suitable for the simultaneous measurement of both extremely low and extremely high solid concentrations by using a 90.degree. light scatter process for low concentrations and a back scatter process for high concentrations. The multi-beam process which is based on two measurements through exactly the same window areas makes it possible effectively to compensate for undesired soiling of the window areas. Mechanical devices for cleaning the window areas and to eliminate interfering light scatter are proposed.

Abstract: In an apparatus for determining the oxygen content of a liquid by measuring the electric current between two electrodes (20, 21) of different materials which are immersed into the liquid, each electrode (20, 21) is embedded in its own body (18, 19) of insulating material in such a manner that only one end surface (20a, 21a) of each electrode is free and may be electrochemically active. The respective structural units (13, 14) consisting of an electrode and insulating body are pressed via associated springs (30, 31) at a common grinding device (28) displaceable by a driving apparatus, which grinding device serves for cleaning the free electrode end surfaces (20, 21a). The spring force by means of which the pressing to the grinding device (28) occurs is individually adjustable for each of the structural units (13, 14) by adjustment of a support ring (32,35) for the relevant spring (30, 31) to match the hardness, abrasion-resistance and corrosion properties of the material of the revelant electrode.