Electronic Drip Timing Device

Abstract

Electronic timing device for timing drips in intravenous feeding and similar systems can be adjusted to provide an audible or visible signal in accordance with any desired drip rate between about 4 and 300 drips per minute. The device overcomes the inherent deficiency of conventional electronic metronomes which are unsuitable for adjusting drip rates since they have a first pulse period different from the remainder. The first pulse period is of the exact length of the remaining pulse periods due to the presence of a circuit which can be maintained in a standby condition with its capacitor partially charged. This permits the pulse producing circuit to be actuated simultaneously with the visual observance by an operator of a drip passing a predetermined point. If the actual drop rate is different from the desired rate, the operator adjusts it and again actuates the timing device. This operation is repeated until the timing rate and the drip rate are synchronized. The device is battery operated and sufficiently small as to be easily carried around the neck or in the pocket of the person using it.

Description

BACKGROUND OF THE INVENTION

This invention relates to timing devices and particularly to timing devices used to adjust the rate of flow of fluid to a patient such as in blood transfusions or intravenous feeding. Although there are fairly sophisticated devices known in the prior art for automatically regulating the flow of fluid or for indicating the rate of flow, such devices would be almost prohibitively expensive to use since a complete device would have to be provided at each location where fluids were being fed. It is the usual practice in the vast majority of hospitals for the drip rate to be adjusted by a doctor or a nurse who uses a stop watch to time the drip rate. Since different fluids being administered require different drip rates varying over a substantial range such as from 4 to 300 drips per minute, one can readily appreciate that it would be physically impossible to count very high drip rates and very time consuming to count and adjust low drip rates.

Since the drip rate range is generally within the range of timing pulses produced by electronic metronomes, one might think that such a device could be used to aid in the determination of drip rates. However, certain problems are presented depending on which type of metronome is used. For example, in a metronome or beat producing device such as shown in U.S. Pat. No. 3,271,670, a single rotatably adjustable control member is used to turn on the device and then set the frequency to a desired rate. Obviously, such a device could not be operated to produce a pulse period starting with a particular drip and thus, in order to time the drip rate one would have to set the metronome to the desired rate and then observe the exact position of a drip relative to the walls of a drip tube which contains it at the time of a pulse and then observe whether or not the next drip is above or below the point where the first one was observed at a time when the next pulse or beat is heard. Obviously, such a metronome could only be used with very high drip rates and/or long drip tubes to insure that a drip would be in motion in the tube at all times. For lower drip rates, the device would have to be continually set and reset until a drip was visible at the time of a pulse. The setting and resetting would have to be continually repeated until the actual drip rate was correctly adjusted.

Other electronic metronome devices, such as, for example, the device such as in U.S. Pat. No. 2,522,492, utilize separate controls for setting the beat rate and for turning the device on and off. Theoretically, it would seem that such a device could be used for timing drip rates since a user could set the metronome to beat at a rate equal to the required drip rate and then simply turn the device on when the drip is observed at a particular location such as the bottom of the drip tube. It has been found, however, that such a proposed method of operation of a metronome as a drip timer is not suitable since the very nature of electronic pulse producing circuits provides for a first pulse which is of longer duration than the remaining pulses. For the usual use of a metronome, this is of no particular importance since the device is merely used to provide a continuing beat of a desired frequency. In order, however, to utilize an electronic metronome or pulse producing device as a drip timer, it would appear to be desirable to be able to set the beat rate and then keep the device from pulsing or beating until a drop is observed hitting or passing a given point. Such operation would result in easier synchronization and determination of the correct rate. Obviously, if such a means of determining drip rates is to be useful, the first pulse period is very important and has to have the same duration as the remainder.

Electronic metronomes commonly include a resistance-capacitance network wherein the capacitor is alternately charged and discharged to provide a series of pulses. The time to charge the capacitor in such a circuit (the factor determining the duration of a pulse period) can be found from the following equation:

t = RC 1n V.sub.B - V.sub.o /V.sub.B - V.sub.c

wherein:

t = time

R = series resistance

C = capacitance

V.sub.B = battery voltage

V.sub.C = capacitor voltage just prior to discharge

V.sub.o = initial capacitor voltage

It can be readily seen from the equation that the first pulse period will be longer than the remaining ones when the circuit is turned on since the capacitor has to start charging from an initial voltage, V.sub.o = o rather than the higher initial voltage which it would have at the beginning of each following time period. It is thus apparent that trying to start synchronization with the capacitor at zero volts, such as would be the case if the capacitor were shorted out or the circuit closed by an on-off switch, would be unsatisfactory and would result in the first time interval being too long and thus providing a mismatch between drops and beats.

SUMMARY

Among the objects of this invention are to provide a simple, effective, economical, and compact device for aiding a nurse or other person in quickly and accurately adjusting the drip rate of a fluid feeding apparatus.

These and other objects are obtained by the present invention. In the preferred embodiment of the invention, the various elements are housed in a small case which can be easily supported by a cord placed around the neck of the nurse. The device includes a combination on-off and rate setting switch which connects an enclosed battery to a relaxation oscillator circuit, and also adjusts the circuit to change the beat rate of the device. The output of the device is preferably audible in which case the sound is heard from a speaker enclosed in the device, or by means of an ear plug. Alternatively, a visual signal such as a light could be used to provide a silent signal.

The basic operation of the circuit in the device is that when the on-off switch is turned on, a capacitor which is in parallel with a series connected transistor and speaker coil is subjected to a voltage across its terminals and initially resembles a short circuit. The voltage across the capacitor immediately starts to build up until it reaches the trigger voltage of the initially non-conducting transistor in parallel with it. At this point, the transistor becomes conductive and causes the capacitor to be discharged through the speaker. This cycle of alternate charging and discharging of the capacitor will then be repeated continually. The time period for charging and discharging the capacitor, and thus the time between pulses, can be varied by changing the resistance value in the R-C network. Obviously, when the R-C value is increased, it will take a longer time for the capacitor to charge and the beat rate will be slower. Conversely, decreasing the R-C value will cause the capacitor to discharge sooner and thus have a higher beat frequency. The voltage at which the capacitor will discharge, its trigger voltage, is determined by a plurality of resistors connected in series in a voltage divider network to a transistor. A compensating resistor and a blocking diode in the circuit provide a temperature compensation network.

A normally open switch adapted to be closed by a spring loaded reset button on the outside of the case or housing permits a fixed, low value resistance to be selectively placed in the circuit in parallel with the capacitor. The reset switch also electrically removes the relatively high value variable resistance used for changing the beat rate from the circuit. With the reset switch actuated, the reset resistance and the capacitor are each connected in series with a fixed resistance. The effect of the low value resistance which is inserted by the reset switch is to cause the capacitor to be charged to a voltage greater than zero and equal to the voltage which it assumes during normal operation of the device at the beginning of each charging cycle. A normal operating sequence would be to turn the device on, set the rate, (beats will be produced) and then hold the circuit in a partially "on" condition but one wherein no pulses are produced by means of the reset button. The reset button is released when the operator visually observes a drip hitting the bottom of a drip tube or some other point in a fluid dripping device such as an intravenous feeding apparatus. If the next drip observed comes either sooner or later than the first pulse or beat produced by the device, then the flow rate of the feeding apparatus is adjusted appropriately and the reset button is again held in until it is released when a drip is observed. This sequence of operations is then repeated until the actual drip rate corresponds to the desired drip rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic perspective view of a drip timing device in accordance with the invention;

FIG. 2 is a graph showing capacitor voltage versus time for a capacitor in a prior art pulse producing relaxation oscillator circuit; and

FIG. 3 is a circuit diagram of the electronic circuit of the device.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows the drip timing device of the invention indicated generally at 10 and including case 11 comprising a pair of case halves 12, 14 held together by a fastener member 16. The device can readily be stored in the pocket of an operator or may be hung around the operator's neck by means of a pair of integral lug members 18 about which a carrying cord 20 may be attached. At one end of the device 10, a dial 22 is provided which includes a plurality of indicia thereon corresponding to the varying beat rates from 4-300 beats per minute which may be produced. A rotary knob 24 having a pointer 26 overlies the dial 22 and is attached to an on-off switch S.sub.P and a variable resistance R.sub.2 (FIG. 3) mounted inside the case for turning the device on and off and setting the pulse or beat rate to any value throughout the range of 4-300 beats per minute. The audible signal produced by the device can be heard either through speaker openings 28 or by means of an ear phone (not shown) which may be connected to the device by means of jack 30. A reset button 32 on the side of the case 11 is adapted to be depressed against the force of a spring (not shown) to operate a reset switch S.sub.R and hold the circuit of the device in a partially "on" or standby condition as will be described more fully hereinafter.

In FIG. 2, a graph of capacitor voltage versus time is plotted for a capacitor in a known relaxation oscillator circuit. Referring to the graph, one can see that the capacitor in the circuit will alternately charge and discharge and that initially the capacitor will build up a charge from an initial capacitor voltage equal to zero to a final voltage V.sub.C which is the capacitor voltage at the instant prior to discharge. This increase in capacitor voltage with time as the circuit is first closed is illustrated by curve 40 while curve 48 represents the fall-off in voltage from value V.sub.C to a value V.sub.o, which is greater than zero, at the completion of discharge. The exact values of V.sub.o and V.sub.C depend upon the values of the various resistances in the circuit. After the first capacitor charging period represented by the curve 40, each remaining charging period can be presented by identical curves 42, 44 and 46 and each discharge period by identical curves 50, 52 and 54. Identical time periods T.sub.2, T.sub.3 and T.sub.4 are of a duration equal to the total time required for the capacitor to charge and discharge (the horizontal components of the charging and discharging curves 42, 50 respectively). It will be readily appreciated that the time period T.sub.1 corresponding to the first charging curve 40 and the discharge curve 48 will be longer than the remaining time periods T.sub.2, T.sub.3 and T.sub.4 by the amount of time necessary for the capacitor to charge when turned on from a voltage value equal to zero to a voltage value equal to V.sub.o.

Since it is necessary, in order for a pulse producing device to be useful in adjusting drip rates for the first time period T.sub.1 to be equal in length to the remaining periods T.sub.2, T.sub.3 ..... T.sub.N, the time period T.sub.1 must be shortened. This can be accomplished by the circuit shown in FIG. 3 which permits the length of charging curve 40 to be shortened by an amount equal to the length of curve 56 between voltage V.sub.o = o and the value of V.sub.o which is greater than zero which is indicated by the lower dotted line in FIG. 2.

Referring to the circuit diagram of FIG. 3, a battery indicated generally at 34 having a voltage of 4.2 volts is connected between ground and a primary on-off switch S.sub.P which opens and closes the battery circuit and connects the battery 34 by means of wires 60 to a common terminal point 62 connected to a resistance R.sub.1 having a value of 5.6K and a parallel connected resistance R.sub.3 having a value of 1K. Connected in series with resistance R.sub.3 are two more resistances, R.sub.4 having a value of 2K, and R.sub.5 having a value of 10K which form a voltage dividing network. Resistance R.sub.4 is a trimmer resistance which is tapped at a point 64 along its length by a blocking diode 66 which may be of the type sold by General Electric Co. under the number 1N 4154. The diode 66 is in series with a resistance R.sub.6 having a resistance of 1 meg and coacts with resistance R.sub.6 to form a temperature compensation network. The diode 66 is also connected to the gate G of a transistor 70 of a type sold by General Electric Co. under the number D 13T1 which has its cathode K connected to ground through the coil 72 of a 3.2 ohm speaker 74.

The 5.6K resistance R.sub.1 is connected in series with a variable resistance R.sub.2 having a maximum value of 420K which in turn is connected to the anode A of the transistor 70. The variable resistance R.sub.2 provides the variation in beat rate of the circuit and is adjustable by means of the knob 24. The basic relaxation oscillator circuit is completed by a capacitor C having a value of 22 mfd which is connected in parallel with transistor 70 and speaker coil 72 between the junction of resistors R.sub.1 and R.sub.2 and ground.

The circuit as described to this point is known and will produce pulses or beats having a time for the first pulse longer than the time for the remaining pulses as described in connection with the graph in FIG. 2.

The remainder of the circuit to be described is the portion which permits the time period of the first pulse to be equal to the time period of the remaining pulses and includes the resistance R.sub.7 having a value of 1.2K and the reset switch S.sub.R which has a first set of contacts 80 which can be closed to place the resistance R.sub.7 in series with the resistance R.sub.1 and in parallel with the capacitor C. A second set of contacts 82 on the reset switch S.sub.R electrically removes the large variable resistance R.sub.2 from the circuit and connects resistance R.sub.1 directly to the transistor 70 and capacitor C. Since the resistance R.sub.7 has a value 1.2K which is approximately 20 percent of the value of the 5.6K resistance R.sub.1, it is readily apparent that the voltage across the capacitance C will be approximately 20 percent of the 4.2 volt battery voltage or approximately 0.9 volts. Since this value is less than the trigger voltage of the transistor 70 which has a value of approximately 3.4 volts, it is obvious that the capacitor C cannot discharge through the transistor 70 and will retain its charge as long as the switch S.sub.P is closed and the reset switch S.sub.R is depressed by finger pressure on the reset push button 32. Since the values of the various resistances are chosen to cause the capacitor C to be charged to a voltage which is equal to the value V.sub.o on the lower dotted line in FIG. 2, it will be readily apparent that releasing the reset switch S.sub.R, such as when a drip is observed in an intravenous feeding apparatus, will result in a first pulse having a duration of time equal to that of the remaining pulses which will be produced after the reset switch S.sub.R is released.

The resistance R.sub.1 determines the shortest time period between pulses which the device can produce. In the present device, this shortest time period is 300 pulses per minute. The point 64 at which the blocking diode 66 is tapped into the trimming resistor R.sub.4 determines the trigger voltage of the transistor 70 which in the present embodiment is approximately 3.4 volts. Once the trimming resistor R.sub.4 is initially adjusted, to the remaining circuit components to provide an accurate beat rate, no further adjustment should be necessary in order to maintain the accuracy of the pulse rate produced by the device as long as the battery 34 is in good condition.

Although a speaker 74 has been shown as a means of rendering the discharge of capacitor C audible, it is obvious that an earphone could be substituted for the speaker coil 74 if the device were to be used in a relatively noisy room or if it were desired that patients not be awakened. For this purpose, an earphone can be inserted into the circuit by means of jack 30 shown in FIG. 1. Alternatively, a visual signal such as a flashing light could signal the beat rate if appropriate changes were made in the values of the various elements of the circuit.

Claims

I claim:

1. A device for producing a plurality of pulses having a period between pulses of a selected predetermined length comprising a relaxation oscillator circuit including a capacitor and a plurality of resistance members in series in a signalling means which have a time period of a predetermined length in accordance with the equation:

t = RC 1n V.sub.B - V.sub.o /V.sub.B - V.sub.c

where:

t = time

R = series resistance

C = capacitance

V.sub.B = battery voltage

V.sub.C = capacitor voltage

V.sub.o = initial capacitor voltage primary switch means for connecting the battery to close said oscillator circuit;

reset resistance means and reset switch means operable for selectively connecting said reset resistance means in said closed circuit in parallel connection with said capacitor and shunting one of said plurality of resistance members which has a greater value of resistance, said reset resistance means having a resistance value sufficiently low to prevent said circuit from oscillating to produce pulses while maintaining the voltage across the capacitor at a constant value equal to the value V.sub.o which it has when the reset switch is open and the circuit is oscillating to produce pulses.

2. The device as described in claim 1 wherein said reset switch means is operative when actuated in one mode to connect said reset resistance in said circuit in parallel connection with said capacitor and to remove said one of said plurality of resistance members from series connection with said capacitor, said switch being operative when actuated in another mode to remove said reset resistance from said circuit and to restore said one of said plurality of resistance members to series connection with said capacitor.

3. The device as described in claim 2 wherein said one of said plurality of resistance members comprises variable resistance means for varying the value of the series resistance R and thus the time t in said equation.

4. The device as described in claim 2 wherein said reset switch means comprises a momentary switch having a spring biased actuator witch is depressed to be actuated in said one mode and released to be actuated in said other mode.

5. The device of claim 3 wherein said primary switch means and said variable resistance means are operatively interconnected for actuation by a common control member.

6. The device of claim 1 wherein said signalling means includes a speaker for producing an audible sound.

7. The device of claim 2 wherein said oscillator circuit includes a transistor connected in fixed parallel relation to said capacitor and operative to trigger the discharge of said capacitor only when said reset switch is in said other mode and said transistor and capacitor are at a predetermined voltage in excess of said capacitor voltage V.

8. The device of claim 7 wherein a speaker is connected in series with said transistor, said capacitor being in parallel with said series connected transistor and speaker, said speaker producing an audible sound pulse when said capacitor discharges through said transistor.