Method for measuring ultrasonically the fill level of a liquid

1. A method for measuring a fill level of a liquid additive in a tank using an ultrasonic sensor, the method comprising the following steps:

a) measuring a first propagation time of an ultrasonic signal from the ultrasonic sensor to a liquid level of the liquid additive;

b) cleaning at least one reference surface in the liquid additive by using at least one ultrasonic pulse;

c) measuring at least one second propagation time of an ultrasonic signal from the ultrasonic sensor to the at least one reference surface; and

d) calculating the fill level from the first propagation time and the second propagation time.

2. The method according to claim 1 , which further comprises providing the at least one ultrasonic pulse with a higher energy than the ultrasonic signals.

3. The method according to claim 1 , which further comprises emitting the at least one ultrasonic pulse from the same ultrasonic sensor as the ultrasonic signals.

4. The method according to claim 1 , which further comprises producing the at least one ultrasonic pulse in step b) by using a separate ultrasonic cleaning instrument.

5. The method according to claim 1 , which further comprises:

providing at least two reference surfaces; and

determining at least one of the following parameters in a step e) by using different second propagation times to different reference surfaces:

a quality of the liquid additive, and

a concentration of urea in the liquid additive.

6. The method according to claim 1 , which further comprises emitting at least two ultrasonic pulses in step b) for cleaning the at least one reference surface.

7. The method according to claim 1 , which further comprises providing the at least one ultrasonic pulse with a frequency between 20 kilohertz and 2 megahertz.

8. The method according to claim 1 , which further comprises providing the at least one ultrasonic pulse with a duration of between 10 milliseconds and 20 seconds.

9. The method according to claim 1 , which further comprises causing pressure peaks of at least 100 bar to occur at the at least one reference surface as a result of the at least one ultrasonic pulse.

10. A method for measuring a fill level of a liquid additive in a tank using an ultrasonic sensor, the method comprising the following steps:

a) measuring a first propagation time of an ultrasonic signal from the ultrasonic sensor to a liquid level of the liquid additive;

b) cleaning at least one reference surface in the liquid additive by using at least one ultrasonic pulse;

c) measuring at least one second propagation time of an ultrasonic signal from the ultrasonic sensor to the at least one reference surface; and

d) calculating the fill level from the first propagation time and the second propagation time;

after the at least one reference surface has been cleaned:

carrying out a test measurement in step b) using an ultrasonic signal from the ultrasonic sensor in order to check if the cleaning was successful, and

emitting another ultrasonic pulse for cleaning the at least one reference surface if the cleaning was unsuccessful.

11. A delivery unit for installation in a tank for a liquid additive, the delivery unit comprising:

at least one reference surface in the liquid additive;

at least one ultrasonic sensor; and

an electronics module for measuring a fill level of the liquid additive in the tank by using said at least one ultrasonic sensor, said electronics module being programmed to carry out the following steps:

a) measuring a first propagation time of an ultrasonic signal from said at least one ultrasonic sensor to a liquid level of the liquid additive;

b) cleaning said at least one reference surface by using at least one ultrasonic pulse;

c) measuring at least one second propagation time of an ultrasonic signal from said at least one ultrasonic sensor to said at least one reference surface; and

d) calculating the fill level from the first propagation time and the second propagation time.