The measurement of low flow rates (0.1-30 ml/min) of conducting fluids.

The accurate measurement of flow rates in isolated-organ perfusion experiments remains a problem. This paper describes an instrument that is based upon the familiar drop counter, but permits greater accuracy, resolution and convenience of recording output than has hitherto been available with such a device. Long-term stability at low flow rates is excellent. The input transducer is a conventional drip chamber with platinum electrodes arranged so that each drop produces an electrical pulse. The time between pulses is measured (or groups of 10 pulses for higher flow rates) and a voltage output proportional to flow is derived by analogue computing elements. Calibration requires only the measurement of the volume of drops forming at very low flows, and a simple procedure to do this is described. Four switched ranges covering 0.1-30 ml/min are provided, and the agreement between flow recorded by the instrument and by timed collections of perfusate is generally better than 3% of full scale.