Patent GB-1361062-A (GB)
Title: Method and apparatus for drop flow control
IPC: A61M5/168
Filed: 1971-08-20
Abstract: 1361062 Drop-flow control IVAC CORP 20 Aug 1971 [30 Dec 1970] 39119/71 Heading A5R [Also in Division G3] Apparatus for controlling the rate of drop flow of liquid though a flexible tube 26, for example the feeding tube of a parenteral fluid administration set, compises a clamp 25, 27 for clamping the tube to stop liquid flow therethough and means for repeatedly releasing the clamp, the frequency of unclamping being such that a plurality of cycles of clamping and unclamping is required to produce each individual drop of flow. The actual drop flow rate through the flexible tube 26 is monitored by a drop sensor 11 which may be a photoelectric device positioned about a drop chamber. Output of the sensor is fed to a pulse generator 12, for example a monostable flipflop circuit, which produces a train of positive pulses at a frequency proportional to the detected drop flow rate and of a predetermined amplitude and width. This train of positive pulses is fed into a rate memory 14 through a current determining resistor R2. A train of negative pulses is generated by a variable frequency square wave generator 16 and is fed to the rate memory 14 via a current determining resistor R1. The signals arriving at junction 28 are added, and the resultant signal integrated by an operational amplifier 20 and a capacitor C1 to produce a d.c. control voltage on line 18. If the measured drop flow rate is lower than the desired rate the net input to the integrator will be negative and the d.c. control voltage will become more positive, whereas if the measured rate is too high the d.c. control signal will drift more negative. If the measured and desired rates are indentical, the net input to the integrator will be zero and the d.c. control voltage will remain constant. The output of the variable frequency pulse generator 16 is also fed over line 21 to act as the triggering signal of a variable width pulse generator 19. The variable width pulse generator may be a monostable flip-flop circuit the output pulse width of which is controlled by the d.c. control voltage on line 18. As the d.c. control voltage drifts more positive the output pulse width of the variable width pulse generator increases, and as the control voltage becomes more negative the pulse width decreases. Each output pulse from the generator 19 constitutes an energizing pulse and energizes an electromagnet 24 to retract the clamp member 25 and permit fluid flow through the tube 26, the duration of each period of fluid flow being equal to the energizing pulse width. By suitable choice of values of the resistors R1, R2 the energizing pulse frequency and thus the frequency at which the tube is unclamped can be made a high multiple of the desired drop flow rate, with the result that each drop is made up of many. For example 10 or 11, portions. Such make up results in consistent drop size, the effect being particularly noticeable if the pulse frequency to drop flow rate ratio is non-integral, for example 10¢ to 1. In use, the ratio of pulse frequency to drop flow rate is predetermined and the pulse frequency fixed. The actual drop flow rate is determined by the width of each energizing pulse, and this pulse width is varied by a feedback system to make the actual rae equal to the desired rate. In the event of a fault in the system, for example if the liquid source is exhausted, the d.c. control voltage may become excessively high or low. Such extremes of voltage may be detected by a monitoring system and used to sound an alarm.