US Patent 7349814 Measurement method and arrangement

The invention relates to a method and an arrangement for measuring a liquid flow in connection with a pump system. The invention is preferably implemented in a pump system where the pump (240) is driven by an alternating-current motor (230), whose rotating speed is controlled by a control unit (220), such as a frequency converter, for instance. In accordance with the invention, the flow is determined without any direct flow measurement by utilizing characteristic data of the pump and parameters that can be easily and reliably measured. Such parameters comprise the rotation speed of the pump, the liquid pressure and/or the motor power. Both the motor power and the rotation speed can be measured i.a. at the frequency converter (220). In addition, the static liquid pressure can be measured by means of a straightforward and reliable pressure sensor (244, 245), which can be integrated in the pump system. In implementing the invention, two characteristic curves of the pump can be advantageously used; flow as a function of power and flow as a function of pressure. This achieves high accuracy of measurement both with low and high flow values.