Patent ID: 7543508

Claim:
An ultrasonic flow sensor having at least one ultrasonic converter (A, B) for transmitting and receiving ultrasonic signals (A 0 , B 0 ) and a receiver unit ( 4 ) that is connected to the ultrasonic converter (A, B), monitors when the ultrasonic signal (A 0 , B 0 ) exceeds a predetermined threshold value (SW), and, depending on this event, determines a reception time (t 0 ) of the ultrasonic signal (A 0 , B 0 ), wherein the receiver unit ( 4 ) determines a piece of information about the amplitude (Amp) of the ultrasonic signal (A 0 , B 0 ) and adapts the threshold value (SW) based on the information determined; wherein the receiver unit ( 4 ) has a first S/H stage ( 12 ), whose input (US) is supplied with a converter output signal ( 5 ), and a subsequent second S/H stage ( 13 ), which adopts and stores the maximum value (Amp max ) of the first S/H stage ( 12 ).