Method for distinguishing between the presence of a foreign body or a gas bubble in a medium, and corresponding system

A system for distinguishing between a presence of a foreign body or a gas bubble in a flowable medium in a pipeline or a container, comprising:

• at least one microwave transmitter unit designed to transmit microwave signals into the flowable medium;
• at least one microwave receiver unit designed to receive microwave signals that have passed through the flowable medium;
• an ultrasonic transmitter unit is designed to introduce ultrasonic waves into the flowable medium;
• and
• a control/evaluation unit configured to distinguish between the presence of the gas bubble or the foreign body in the medium on the basis of the evaluation of the received microwave signals or a value derived therefrom,
• wherein the gas bubble present in the flowable medium is set into vibration by the ultrasonic waves, and a modulation of the received microwave signals or of the value derived therefrom is generated by the vibrating gas bubble,
• wherein the gas bubble is detected only when the ultrasonic waves are introduced and the received microwave signals or the value derived therefrom is/are modulated.