Measurement method and arrangement

A method for measuring a flow in a pump system, in which a liquid flow is generated by means of a pump and the pump is actuated by an electric drive, in which the rotation speed of an alternating-current motor is controlled with a control unit, characterised in that the method comprises:

• measuring (402) a pump power (P) in the pump system,
• measuring a rotation speed (v) of the pump,
• measuring (504) a static pressure (Hs),
• setting an estimate of a dynamic pressure (Hd) to a pre-estimated constant value,
• determining (506) an estimate of a total pressure (H) on the basis of the static pressure and the estimate of the dynamic pressure,
• determining a first estimate (410, 416) of the liquid flow on the basis of the measured pump power (P) and rotation speed variables,
• determining a second estimate (514, 516) of the liquid flow on the basis of the estimate of a total pressure (H) and rotation speed variables,
• determining a flow measurement result by a logical selection or any other predetermined function on said first estimate of the liquid flow and said second estimate of the liquid flow,
• determining (520) a new estimate of a dynamic pressure (Hd) on the basis of the flow measurement result,
• re-determining the estimate of a total pressure (H) on the basis of the static pressure and the new estimate of the dynamic pressure,
• re-determining the second estimate of the liquid flow on the basis of the estimate of a total pressure (H) and rotation speed variables, and
• re-determining the flow measurement result by a logical selection or any other predetermined function on said first estimate of the liquid flow and said second estimate of the liquid flow.

An arrangement for measuring the flow in a pump system comprising a pump (240) for generating a liquid flow, an electric drive for actuating the pump, the electric drive comprising an alternating-current motor (230) and a control unit (220) for controlling the rotation speed of the alternating-current motor, characterised in that the arrangement comprises:

• means (221, 223) for measuring a pump power (P) in the pump system,
• means (221, 223, 228) for measuring a rotation speed (v) of the pump,
• means (244, 245) for measuring a static pressure (Hs),
• means (221, 222, 228) for setting an estimate of a dynamic pressure (Hd) to a pre-estimated constant value,
• means (221, 222, 228) for determining an estimate of a total pressure (H) on the basis of the static pressure and the estimate of the dynamic pressure,
• means (221, 222) for determining a first estimate of the liquid flow on the basis of the measured pump power (P) and rotation speed variables,
• means (221, 222) for determining a second estimate of the liquid flow on the basis of the estimate of a total pressure (H) and rotation speed variables,
• means (221, 222) for determining a flow measurement result by a logical selection or any other predetermined function on said first estimate of the liquid flow and said second estimate of the liquid flow,
• means (221, 222) for determining a new estimate of a dynamic pressure (Hd) on the basis of the flow measurement result,
• means (221, 222) for re-determining the estimate of a total pressure (H) on the basis of the static pressure and the new estimate of the dynamic pressure,
• means (221, 222) for re-determining the second estimate of the liquid flow on the basis of the estimate of a total pressure (H) and rotation speed variables, and
• means (221, 222) for re-determining the flow measurement result by a logical selection or any other predetermined function on said first estimate of the liquid flow and said second estimate of the liquid flow.