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The difference in effective velocity of sound pulses propagated alternately upstream and downstream through moving blood is a measure of the blood velocity. A blood flowmeter based on this principle has been developed and employed in routine experiments over a period of four years. The travel time of sound pulses passing diagonally through the blood vessel is measured by gating a ramp voltage generator during the transit of each sound pulse from the transmitter crystal to the receiver crystal. The peak voltage attained by the ramp is stored and compared with the peak voltage attained by the subsequent ramp when the direction of propagation is reversed. The cycle is repeated 400 times each second so that a 400 cps carrier is formed. The amplitude of the carrier is proportional to the magnitude of the flow, and the phase of the carrier indicates direction of flow. The total PP noise and long-term drift is less than 7(10)-11sec difference in transit time over a four-hour measurement period, under optimal conditions. Here a demodulator limited the rise time of the instrument to 20 msec. Each upstream-downstream measurement requires only 240 Â¿sec of the total available 2500 Â¿sec, so that a time sharing technique may be used to operate many flowmeters simultaneously without mutual interference. The transducers are light (about 5 grams) and economical to construct.