Patent ID: 7694565

Claim:
A method of measuring a signal transit time in a medical liquid required by a signal to pass through a measurement zone from an ultrasonic transmitter ( 2 ) to an ultrasonic receiver ( 3 ), in which a line carrying the medical liquid is arranged in the measurement zone, comprising emitting a step-like signal ( 10 ) with the ultrasonic transmitter ( 2 ) such that the step-like signal ( 10 ) passes through the measurement zone, resulting in an oscillation-like received signal ( 12 ) oscillating about a resting level ( 11 ) on the ultrasonic receiver ( 3 ), the received signal being sampled at regular intervals Δt and detected, checking the oscillator-like received signal ( 12 ) on the basis of a selection criterion at least during a half-period ( 14 , 15 ) to determine whether the oscillator-like received signal ( 12 ) is the received signal produced by the step-like signal ( 10 ), and when the result of this check is positive, determining the signal transit time or the change in the signal transit time with an interpolated or extrapolated contact point ( 20 , 21 ) of the oscillator-like received signal ( 12 ) with the resting level ( 11 ) in a received signal-time diagram.