Fluid measuring device and method for determining a flow rate through a fluid measuring device

A method for determining a flow rate through a flow channel of a fluid measuring device having a measuring tube, in which a circumferentially closed flow channel for a fluid to be measured is formed, the flow channel having a polygonal cross section with several flat side faces arranged at an angle to each other, wherein at least one waveguide for surface acoustic waves is formed on one of the side faces of the measuring tube, and at least a first and a second signal converter element that are each fixedly mounted to the at least one waveguide at a predetermined distance from each other along a longitudinal direction of the flow channel,

• wherein the first and second signal converter elements each comprise at least two signal converters that are arranged beside each other in a transverse direction of the flow channel,
• each signal converter being controllable separately and each signal converter being designed to excite surface acoustic waves in the respective waveguide and/or to receive surface acoustic waves from the waveguide, wherein the signal converter is adapted to emit and/or receive surface acoustic waves and the respective waveguide is adapted to decouple surface acoustic waves from the waveguide as bulk acoustic waves that propagate through the fluid in the flow channel and/or to couple bulk acoustic waves into the waveguide as surface acoustic waves,
• the at least two signal converters of each signal converter element forming one center signal converter and at least one side signal converter, the center signal converters being arranged in the center of the respective side face with regard to the transverse direction and lying in a center plane parallel to the longitudinal direction and perpendicular to the respective side face, the side signal converters of the first and second signal converter element being arranged sidewardly of the respective center signal converter with regard to the transverse direction so that one side signal converter of the first signal converter element and one side signal converter of the second signal converter element lie in a side plane parallel to the longitudinal direction and parallel to the center plane, the method comprising the steps:
• measuring center signal data with the center signal converters and side signal data with the side signal converters of the first and second signal converter element,
• determining a correction coefficient from the measured center and side signal data, and
• determining a flow rate from the measured center and side signal data and the determined correction coefficient.