An arrangement of acoustic transducers for a flow meter suitable for water irrigation; the flow meter providing measurement of the rate at which water is flowing; the arrangement including a respective transducer set (7, 9, 11 13) for each edge of a notional regular polygon; the transducer sets (7, 9, 11 13) being mounted in a tubular cavity to carry the water; each of the transducer sets (7, 9, 11 13) respectively including two acoustic transducers (7a, 7b) oriented to provide only a single acoustic path with each other and lying in a measurement plane of the respective set; and another two acoustic transducers (7c, 7d) oriented to provide only another single acoustic path with each other and lying in the measurement plane of the respective set; and the transducer sets being positioned so that, in cross-section normal to the tubular cavity, the measurement plane of each respective transducer set is coincident with a respective edge of the notional regular polygon; characterized in that the arrangement including transducer units, each respective one of which carries two of the acoustic transducers (7a, 9a) and includes a mounting arrangement by which the respective one is mounted as a unit at least partially inside the wall of tubular cavity. the flow meter providing measurement of the rate at which water is flowing; the arrangement including a respective transducer set (7, 9, 11 13) for each edge of a notional regular polygon; the transducer sets (7, 9, 11 13) being mounted in a tubular cavity to carry the water; each of the transducer sets (7, 9, 11 13) respectively including two acoustic transducers (7a, 7b) oriented to provide only a single acoustic path with each other and lying in a measurement plane of the respective set; and another two acoustic transducers (7c, 7d) oriented to provide only another single acoustic path with each other and lying in the measurement plane of the respective set; and the transducer sets being positioned so that, in cross-section normal to the tubular cavity, the measurement plane of each respective transducer set is coincident with a respective edge of the notional regular polygon; characterized in that the arrangement including transducer units, each respective one of which carries two of the acoustic transducers (7a, 9a) and includes a mounting arrangement by which the respective one is mounted as a unit at least partially inside the wall of tubular cavity. two acoustic transducers (7a, 7b) oriented to provide only a single acoustic path with each other and lying in a measurement plane of the respective set; and another two acoustic transducers (7c, 7d) oriented to provide only another single acoustic path with each other and lying in the measurement plane of the respective set; and the transducer sets being positioned so that, in cross-section normal to the tubular cavity, the measurement plane of each respective transducer set is coincident with a respective edge of the notional regular polygon; characterized in that the arrangement including transducer units, each respective one of which carries two of the acoustic transducers (7a, 9a) and includes a mounting arrangement by which the respective one is mounted as a unit at least partially inside the wall of tubular cavity.