A signalling means for signalling to a control unit of a flow rate meter suitable for use in measuring a flow rate of a fuel injection system such as a diesel injection system; wherein the meter comprises a piston located within a main bore, a first chamber towards a first end of the main bore for receiving a substance, the substance being fuel, and a second chamber at a second end of the main bore remote from the first end for receiving air at a regulated air pressure via an inlet; the signalling means comprising a comparator means and at least a first pressure sensor configured to be provided in the first chamber, the signalling means further comprising a second pressure sensor configured to measure the air pressure within the second chamber; wherein a substance pressure, comprising pressure of the substance within the first chamber, applies a substance force to a first end of the piston; and wherein an opposing force resulting from the regulated air pressure is applied to a second end of the piston remote from the first end; wherein a difference between the substance force and the opposing force causes linear displacement of the piston, from an initial position, within and along an axial axis of the main bore; wherein measurements received by the control unit comprise measurements of linear displacement of the piston from an initial position received from a displacement measuring means, and measurements of time taken for a predetermined linear movement of the piston from the initial position received from a timing means; wherein, the comparator means is configured to receive measurements of substance pressure from the first pressure sensor, and calculates substance force using a predetermined area of the first end of the piston; and wherein the comparator means is configured to compare the substance force to the opposing force, wherein the opposing force is calculated by the comparator means from measurements of opposing pressure received from the second pressure sensor, and a predetermined area of the second end of the piston; and wherein based on the comparison of the substance force and the opposing force, the comparator means is configured to provide a signal to the control unit as to whether the control unit should record or disregard the measurements received from the measuring means and timing means, the timing means comprising a timing device, to measure the time taken for the predetermined linear movement of the piston to occur. the signalling means comprising a comparator means and at least a first pressure sensor configured to be provided in the first chamber, the signalling means further comprising a second pressure sensor configured to measure the air pressure within the second chamber; wherein a substance pressure, comprising pressure of the substance within the first chamber, applies a substance force to a first end of the piston; and wherein an opposing force resulting from the regulated air pressure is applied to a second end of the piston remote from the first end; wherein a difference between the substance force and the opposing force causes linear displacement of the piston, from an initial position, within and along an axial axis of the main bore; wherein measurements received by the control unit comprise measurements of linear displacement of the piston from an initial position received from a displacement measuring means, and measurements of time taken for a predetermined linear movement of the piston from the initial position received from a timing means; wherein, the comparator means is configured to receive measurements of substance pressure from the first pressure sensor, and calculates substance force using a predetermined area of the first end of the piston; and wherein the comparator means is configured to compare the substance force to the opposing force, wherein the opposing force is calculated by the comparator means from measurements of opposing pressure received from the second pressure sensor, and a predetermined area of the second end of the piston; and wherein based on the comparison of the substance force and the opposing force, the comparator means is configured to provide a signal to the control unit as to whether the control unit should record or disregard the measurements received from the measuring means and timing means, the timing means comprising a timing device, to measure the time taken for the predetermined linear movement of the piston to occur.