Method for operating an ultrasonic flowmeter and ultrasonic flowmeter

1 . A method for operating an ultrasonic flowmeter, wherein the ultrasonic flowmeter has at least one first ultrasonic transducer, at least one second ultrasonic transducer, at least one measuring tube with a measuring tube wall and at least one control and evaluation unit, wherein the first ultrasonic transducer is designed at least for emitting a measuring signal and wherein the second ultrasonic transducer is designed at least for receiving the measuring signal, wherein the first ultrasonic transducer and the second ultrasonic transducer are arranged on the measuring tube and wherein a medium to be measured flows through the measuring tube during operation, wherein, during a measurement, the method comprises the following steps:

emitting a measuring signal by the first ultrasonic transducer, wherein a first part of the measuring signal propagates as an interference signal along and/or within the measuring tube wall and wherein a second part of the measuring signal propagates as a useful signal through the medium;

receiving the interference signal and the useful signal by the second ultrasonic transducer and forwarding the interference signal and the useful signal to the control and evaluation unit;

determining an interference amplitude of the interference signal and determining a useful amplitude of the useful signal by the control and evaluation unit;

determining an amplitude ratio of the useful amplitude and the interference amplitude; and

providing a message if the amplitude ratio falls below a specified lower limit value or exceeds a specified upper limit value;

wherein the method further comprises one of:

averaging the amplitude of the interference signal over a plurality of peaks in order to determine the interference amplitude; and

averaging the received useful signal over a plurality of measurements so that a randomly present time offset between the useful signal and the interference signal is averaged out, and selecting the respective useful signals of a measurement as a time base, so that the interference signal has a randomly present time offset, and averaging the interference signal over a plurality of measurements, so that the interference amplitude is reduced.

2 . The method according to claim 1 , wherein the interference amplitude is determined from the interference signal lying temporally before the useful signal or from the interference signal lying temporally after the useful signal.

3 . The method according to claim 1 , wherein, in order to determine the amplitude ratio, the interference amplitude is corrected by a correction factor which takes into account that the determined interference amplitude does not correspond to the desired interference amplitude which impinges on the second ultrasonic transducer at the same time as the useful signal.

4 . The method according to claim 1 , wherein the amplitude of the useful signal is averaged over a plurality of peaks to determine the useful amplitude.

5 . The method according to claim 1 , wherein the useful amplitude and/or the interference amplitude corresponds to a peak-to-peak amplitude around a zero crossing of the interference signal or the useful signal.

6 . The method according to claim 1 , wherein, in order to determine the useful amplitude and/or the interference amplitude, an envelope of a time-limited interference signal or of a time-limited useful signal is determined; and

wherein the interference amplitude is a maximum of the envelope of the time-limited interference signal and/or wherein the useful amplitude is the maximum of the envelope of the time-limited useful signal.

7 . The method according to claim 1 , wherein the step of providing the message involves outputting and/or forwarding and/or displaying of a message.

8 . An ultrasonic flowmeter, comprising:

at least one first ultrasonic transducer

at least one second ultrasonic transducer;

at least one measuring tube with a measuring tube wall; and

at least one control and evaluation unit;

wherein the first ultrasonic transducer is designed at least for emitting a measuring signal and wherein the second ultrasonic transducer is designed at least for receiving the measuring signal;

wherein the first ultrasonic transducer and the second ultrasonic transducer are arranged on the measuring tube;

wherein a medium to be measured flows through the measuring tube during operation; and

wherein the control and evaluation unit is designed to:

determine an interference amplitude of a first part of the measuring signal that propagates as an interference signal along and/or within the measuring tube wall;

determine a useful amplitude of a second part of the measuring signal that propagates as a useful signal through the medium;

determine an amplitude ratio of the useful amplitude and the interference amplitude; and

provide a message if the amplitude ratio falls below a specified lower limit value or exceeds a specified upper limit value;

wherein the control and evaluation unit is further designed to one of:

average the amplitude of the interference signal over a plurality of peaks in order to determine the interference amplitude; and

average the received useful signal over a plurality of measurements so that a randomly present time offset between the useful signal and the interference signal is averaged out, and select the respective useful signals of a measurement as a time base, so that the interference signal has a randomly present time offset, and average the interference signal over a plurality of measurements, so that the interference amplitude is reduced.

9 . The ultrasonic flowmeter according to claim 8 , wherein the interference amplitude is determined from the interference signal lying temporally before the useful signal or from the interference signal lying temporally after the useful signal.

10 . The ultrasonic flowmeter according to claim 8 , wherein, in order to determine the amplitude ratio, the interference amplitude is corrected by a correction factor which takes into account that the determined interference amplitude does not correspond to the desired interference amplitude which impinges on the second ultrasonic transducer at the same time as the useful signal.

11 . The ultrasonic flowmeter according to claim 8 , wherein the amplitude of the useful signal is averaged over a plurality of peaks to determine the useful amplitude.

12 . The ultrasonic flowmeter according to claim 8 , wherein the useful amplitude and/or the interference amplitude corresponds to a peak-to-peak amplitude around a zero crossing of the interference signal or the useful signal.

13 . The ultrasonic flowmeter according to claim 8 , wherein, in order to determine the useful amplitude and/or the interference amplitude, an envelope of a time-limited interference signal or of a time-limited useful signal is determined; and

wherein the interference amplitude is a maximum of the envelope of the time-limited interference signal and/or wherein the useful amplitude is the maximum of the envelope of the time-limited useful signal.

14 . The ultrasonic flowmeter according to claim 8 , wherein the control and evaluation unit is designed to provide the message by outputting the message and/or forwarding the message and/or displaying the message.