water the larger load 74 has this decrease in range at a higher water level the present invention makes use of this principle to estimate load size in fig5 l n denotes the minimum water level and ls denotes for each curve the water level at the switch point i e when the algorithm for the present invention is satisfied as will appear fig6 illustrates a simplified series of steps undertaken by the control circuit embodying the principles and the algorithm of the present invention control unit 76 determines the present average range of the pressure wave over a period of t seconds control is then passed to control unit 78 which stores the present average range as the largest range if it is larger than the previous largest stored pressure range control is then passed to control unit 82 which compares the present range to a predetermined fraction of the largest sensed pressure e g 3 4 of the largest sensed pressure plus a previously determined and experimentally derived constant if the present average range is not less than a predetermined fraction of the largest average range plus the predetermined constant then control unit 84 passes control back to control unit 76 where a new average range is found and the procedure is repeated if however the present average range is less than a predetermined fraction.