Arrangement and method for testing a level gauge system

1. A test arrangement for a level gauge system comprising a measurement unit and a signal propagation device connected to the measurement unit, the test arrangement comprising:

a waveguide being connectable to the level gauge system at a connection to allow an electromagnetic transmit signal transmitted by the measurement unit to be propagated by said waveguide;

a first signal reflecting device arranged along said waveguide, at a first distance along said waveguide from said connection, said first signal reflecting device being configured to allow at least a portion of said electromagnetic transmit signal from the measurement unit to pass said first signal reflecting device and continue further along said waveguide; and

a second signal reflecting device arranged along said waveguide, at a second distance along said waveguide from said connection being greater than said first distance, for reflecting an electromagnetic signal traveling along said waveguide from said first signal reflecting device back towards said first signal reflecting device,

wherein said first signal reflecting device is configured to reflect an electromagnetic signal traveling along said waveguide from said second signal reflecting device back towards said second reflecting device and to allow, at least during a time period occurring at a predetermined time after transmission of said electromagnetic transmit signal from said measurement unit, an electromagnetic signal traveling along said waveguide from said second signal reflecting device towards said first signal reflecting device to pass said first signal reflecting device to allow said electromagnetic signal to be received by said measurement unit of the level gauge system.

2. The test arrangement according to claim 1 , wherein said first signal reflecting device comprises an impedance transition.

3. The test arrangement according to claim 2 , wherein said impedance transition is configured to reflect a first portion of a power of an electromagnetic signal incident on said impedance transition and to allow passage of a second portion of said power of said electromagnetic signal.

4. The test arrangement according to claim 3 , wherein said first portion is at least 20 percent of said power of the electromagnetic signal incident on said impedance transition.

5. The test arrangement according to claim 1 , wherein said first signal reflecting device comprises switching circuitry controllable between a signal conducting state and a signal reflecting state.

6. The test arrangement according to claim 5 , wherein said test arrangement further comprises timing control circuitry connected to said switching circuitry and to said waveguide between said connection and said switching circuitry,

said timing control circuitry being configured to sense said electromagnetic transmit signal transmitted by the measurement unit and to control a switching state of said switching circuitry based on a timing of said electromagnetic transmit signal.

7. The test arrangement according to claim 5 , wherein said switching circuitry is configured to have a switching time from said signal conducting state to said signal reflecting state being substantially longer than one nanosecond.

8. The test arrangement according to claim 5 , wherein said switching circuitry is configured to have a switching time from said signal conducting state to said signal reflecting state being shorter than a propagation time of an electromagnetic signal along said waveguide from said switching circuitry to said second signal reflecting device and back to said switching circuitry.

9. The test arrangement according to claim 1 , wherein said second signal reflecting device is arranged at an end of said waveguide.

10. The test arrangement according to claim 1 , wherein said waveguide is a coaxial cable.

11. A level gauge system for determining a filling level of a product contained in a tank, comprising:

a measurement unit;

a signal propagation device connected to said measurement unit for propagating an electromagnetic transmit signal from said measurement unit towards a surface of said product, and to return an electromagnetic reflection signal resulting from reflection of said electromagnetic transmit signal at said surface back to said measurement unit; and

a test arrangement according to claim 1 ,

wherein said level gauge system is controllable between:

a filling level determination state in which said measurement unit determines said filling level based on said electromagnetic reflection signal; and

a test state in which said waveguide of said test arrangement is connected to said measurement unit in such a way that said electromagnetic transmit signal transmitted by said measurement unit is propagated by said waveguide.

12. The level gauge system according to claim 11 , wherein said level gauge system, in said test state, is configured to determine a test result based on an electromagnetic signal received from said test arrangement only during a time period corresponding to the particular distance for which said level gauge system should be tested.

13. A method of testing a level gauge system comprising a measurement unit and a signal propagation device connected to the measurement unit, using a test arrangement comprising a waveguide, a first signal reflecting device arranged along said waveguide and a second signal reflecting device arranged along said waveguide spaced apart from said first signal reflection device, said method comprising the steps of:

transmitting an electromagnetic transmit signal from said measurement unit;

propagating said electromagnetic transmit signal along said waveguide from a connection at which said waveguide is connected to said measurement unit past said first signal reflecting device to said second signal reflecting device, where said electromagnetic transmit signal is reflected back along said waveguide towards said first signal reflecting device and then back and forth between said first reflecting device and said second reflecting device;

receiving, at said measurement unit, an electromagnetic reflection signal resulting from said electromagnetic transmit signal having traveled back and forth between said first signal reflecting device and said second signal reflecting device a preselected number of times; and

determining a distance corresponding to a time between transmission of said electromagnetic transmit signal and reception of said electromagnetic reflection signal.

14. The method according to claim 13 , further comprising the steps of:

detaching said measurement unit from said signal propagation device; and

connecting said waveguide of said test arrangement to said measurement unit.

15. The method according to claim 13 , wherein said method further comprises the steps of:

controlling switching circuitry comprised in said first reflecting device from a signal conducting state to a signal reflecting state after said electromagnetic transmit signal transmitted by said measurement unit has passed said switching circuitry; and

controlling said switching circuitry from said signal reflecting state to said signal conducting state after said electromagnetic transmit signal has traveled back and forth between said first signal reflecting device and said second signal reflecting device said preselected number of times.