Apparatus and method for measuring flowable substance and arrangement and method for controlling solid content of flowable substance

1. An apparatus for measuring a flowable substance which comprises:

at least three microwave sensor elements arranged at separate locations in a measurement chamber in contact with the flowable substance, wherein the at least three microwave sensor elements comprise only an odd number of sensor elements; and

said sensor elements being configured to operate as at least two transmitter-receiver pairs, distances between any two transmitter-receiver sensor element pairs deviating from each other, the at least two transmitter-receiver sensor element pairs being configured to transmit and receive microwave radiation via at least two paths of different lengths, through layers of dirt on the sensor elements and the flowable substance;

a switch matrix configured to temporarily form couplings between the sensor elements;

a source and a receiver for transmitting the microwave radiation through the flowable substance using different combinations of the sensor elements temporarily coupled by the switch matrix; and

a signal processing unit configured to receive signals via the at least two paths of different lengths between the at least two transmitter-receiver sensor element pairs, the signals carrying pieces of information about time of flights between the at least two transmitter-receiver pairs, and form a value of a desired property of the flowable substance on the basis of the pieces of information about the time of flights, and eliminate an effect of dirt layers on the sensor elements from the value of the desired property.

2. The apparatus as claimed in claim 1 , wherein the signal processing unit is configured to form the value of the desired property of the flowable substance on the basis of differences in pieces of information about the time of flights.

3. The apparatus as claimed in claim 1 , wherein at least one transmitting sensor element is configured to transmit a microwave signal to at least two receiving sensor elements.

4. The apparatus as claimed in claim 1 , wherein the apparatus comprises N microwave sensor elements configured to operate as N separate transmitter-receiver pairs, where N is at least two and different pairs have a physically separate receiving sensor, a physically separate transmitting sensor or both a physically separate receiving sensor and a physically separate transmitting sensor.

5. The apparatus as claimed in claim 1 , wherein in a measuring chamber, which has a circular circumference, a transmitter of the at least two transmitter-receiver sensor element pairs transmits in a direction of a chord towards a corresponding receiver.

6. The apparatus as claimed in claim 1 , wherein the chamber comprises electrically conducting material for causing an electric field polarized parallel to the wall direction of the chamber to be zero on the chamber wall.

7. A control arrangement for controlling solid content of a flowable substance, said control arrangement comprising:

at least three microwave sensor elements arranged at separate locations in a measurement chamber in contact with the flowable substance, wherein the at least three microwave sensor elements comprise only an odd number of sensor elements; said sensor elements being configured to operate as at least two transmitter-receiver sensor element pairs each pair of the transmitter-receiver sensor element pairs being configured to transmit a microwave radiation via a unique length of path through the flowable substance;

a switch matrix configured to temporarily form couplings between the sensor elements;

a source and a receiver for transmitting the microwave radiation through the flowable substance using different combinations of the sensor elements temporarily coupled by the switch matrix; and

a signal processing unit configured to receive signals via the unique paths of different lengths carrying information about time of flights from the at least two transmitter-receiver pairs form a value of a desired property of the flowable substance on the basis of the information about the time of flights and eliminate an effect of dirt layers on the sensor elements from the value of the desired property, and

the signal processing unit comprises a controller which is configured to control feed of at least one substance which has an effect on the measured value of the flowable substance to the chamber on the basis of the measured value of the desired property of the flowable substance.

8. The arrangement as claimed in claim 7 , wherein the controller is configured to control a feed of chemical which causes sedimentation in the flowable substance.

9. The arrangement as claimed in claim 7 , wherein the signal processing unit comprises at least one processor; and at least one memory including a computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the signal processing unit at least to perform

forming the value of the desired property of the flowable substance on the basis of the information about the time of flights, and

controlling the feed of the at least one substance which has an effect on the measured value of the flowable substance to the chamber on the basis of the measured value of the desired property of the flowable substance.

10. A method of measuring flowable substance, the method comprising:

transmitting and receiving with at least two transmitter-receiver pairs microwave radiation via at least two different paths through the flowable substance, the transmitter-receiver pairs being formed with at least three microwave sensor elements arranged at separate locations in a measurement chamber in contact with the flowable substance, the at least three microwave sensor elements comprising only an odd number of sensor elements, distances between any two transmitter-receiver sensor element pairs deviating from each other;

temporarily forming couplings between the sensor elements with a switch matrix;

transmitting the microwave radiation through the flowable substance using a source and a receiver with different combinations of the sensor elements temporarily coupled by the switch matrix; and

receiving, by a signal processing unit, signals via the at least two paths of different lengths between the at least two transmitter-receiver pairs, the signals carrying pieces of information about time of flights between the at least two transmitter-receiver pairs, and forming, by the signal processing unit, a value of a desired property of the flowable substance on the basis of the pieces of information about the time of flights and eliminating, by the signal processing unit, an effect of dirt layers on the sensor elements from the value of the desired property.

11. A method for controlling solid content of a flowable substance, the method comprising:

transmitting and receiving with at least two transmitter-receiver sensor element pairs microwave radiation via at least two paths of different lengths through the flowable substance, the transmitter-receiver sensor element pairs being formed with at least three microwave sensor elements arranged at separate locations in a measurement chamber in contact with the flowable substance, the at least three microwave sensor elements comprising only an odd number of sensor elements distances between any two transmitter-receiver sensor element pairs deviating from each other;

temporarily forming couplings between the sensor elements with a switch matrix;

transmitting the microwave radiation through the flowable substance using a source and a receiver with different combinations of the sensor elements temporarily coupled by the switch matrix; and

receiving, by a signal processing unit, signals via the at least two paths of different lengths carrying pieces of information about time of flights from the at least two transmitter-receiver sensor element pairs;

forming a value of a desired property of the flowable substance on the basis of pieces of information carried by the received microwave radiation;

eliminating an effect of dirt layers on the sensor elements from the value of the desired property; and

controlling the feed of the at least one substance which has an effect on the measured value of the flowable substance to the chamber on the basis of the measured value of the desired property of the flowable substance.