Acoustic cleaning of optical probe window

A system for removing deposits from a measuring instrument measuring chosen characteristics of a fluid medium contained in a pipe or container, wherein the measuring instrument comprises a first surface having contact with said medium, said first surface being positioned on one end defining a probe head of an elongated member adapted to extending extend through the wall of said pipe or container, on another end of said elongated member opposite the end being positioned outside said wall, the cleaning system comprises an acoustical transducer positioned on the other end of said elongated member, wherein said acoustic transducer is acoustically coupled to the elongated member so as to couple acoustic energy thereto, the acoustic energy propagating in said member to said first surface, wherein the probe head is provided with a gauge located in operational contact behind the first surface through which the characteristics are measured, wherein the gauge is acoustically insulated from the first surface, wherein a signal transmitter transmitting measurement signals from the gauge out of the system for further processing are arranged coaxially inside the elongated member and are partially acoustically insulated from said elongated member.