Empty container detector with drop sensor.

This invention relates generally to fluid level detectors having an associated drop sensor. More particularly, this invention relates to a fluid level detector which can be associated with a fluid container to indicate when the container is empty of fluid. Additionally, the fluid level detector of the present invention incorporates an apparatus which senses the flow of fluid into the container. The present invention is particularly, but not exclusively, useful in the health care field for detecting the completion of fluid infusion by monitoring the fluid level of a drip chamber associated with the fluid source.

In the medical device field, the possibility of air entering an IV administration set is a major concern. Accordingly, attempts have been made to determine when air is present in the IV line between the fluid source and the patient and many devices have been proposed for this purpose. For example, U.S. Patent No. 4,ll4,l44 to Hyman, which is assigned to the same assignee as the present invention, discloses one of several types of air-­in-line detectors which employ optical circuits. In their operation, such air-in-line detectors typically rely on the physical properties of fluids present in the IV tube to refract a light beam. They make use of the refractive properties of fluids for directing a light beam onto or away from a photoelectric sensor and use the consequent sensor signal to indicate the presence or absence of a fluid. In a refinement of this technology, an air-in-line detector as disclosed in Application Serial No. 7l6,862 to Meijer and assigned to the same assignee as the present invention additionally negates the attenuating properties of varying fluid opacities by using different light intensity thresholds for the incorporated sensors.

Impliedly, air-to-line detectors are exclusively concerned with the detection of air bubbles in the IV line itself. It makes no difference how the air enters the line. In addition to the beneficial attributes of an air-in-line detector, there are situations involving medical devices, i.e., controllers and/or pumps, when it is desirable to cease the operation of the medical device upon the depletion of fluid from the fluid source. And this may be so even though there is not air in the IV line. Accordingly, the ability to determine when the fluid source is depleted has several advantages. First, if the depletion of the fluid source can be detected, the medical device can be stopped before air is brought into the operating system of the medical device and into the IV infusion tube. Second, the ability to stop the operation of the medical device before air enters the system allows for rapid replacement of the fluid source with no need to reprime the medical device. Additionally, the empty container detector envisioned by the present invention is placed in fluid communication with the fluid source and is not an integral part of the medical device. Thus, it has flexibility and can be associated with the fluid source of various medical devices regardless of the particular type or construction of the medical device.

Efficiency and cost effectiveness dictate that several functions be accomplished by a single IV administration set whenever practical. In IV infusion therapy it is important to know when fluid in the set has been depleted and to also know whether fluid is flowing in the set. Thus, a device which performs both functions is desirable. Typically, the determination of whether fluid is flowing in an IV set is accomplished by observing the flow of drops through a drip chamber. This task has been done visually for many years. Recently, electronic devices to perform this same function have also been proposed. For example, U.S. Patent No. 4,346,606 to Cannon et al. discloses a battery operable drop rate counter which senses successive drops of fluid entering a drip chamber. No single device, however, has been proposed which combines the drop sensing function with the fluid depletion detection function as proposed by the present invention.

In light of the foregoing, the present invention recognizes that there is a need for a detector which is capable of direct association with the fluid source and which is able to provide accurate information concerning the presence or absence of fluid in the container. Accordingly, it is an object of the present invention to provide an empty container detector which will provide accurate information on the fluid level in a fluid container. It is another object of the present invention to compare a control signal with a normalized reference signal which is independent of the material used for construction of the fluid container so long as the container is transparent to light. Still another object of the present invention is to provide a detector which is effective for determining fluid level in a fluid container regardless of whether the fluid is transparent, opalescent or opaque. It is still another object of the present invention to provide an empty container detector having an incorporated drop sensor for determining the rate at which fluid flows into the container. Yet another object of the invention is to provide an easily manufacturable, cost effective and easy to use empty container detector which is easily adaptable for use in a multitude of medical environments.

A preferred embodiment of the novel empty container detector includes a photoelectric transducer which is normally associated with a transparent fluid container below the fluid level in the container to generate a variable control signal. As envisioned by the present invention, the control signal is variable between a first state wherein fluid is detected and a second state wherein an absence of fluid is indicated. The empty container detector of the present invention further comprises another photoelectric transducer which is associated with the container above the fluid level in the container to establish a reference signal. This reference signal, which is the output of the upper photoelectric transducer, corresponds to an actual no-­fluid-in-container condition. Further, this reference signal is provided as input to the lower photoelectric transducer. Thus, electrical connecting means are incorporated to normalize and linearize the control signal relative to the reference signal. A comparator means is provided for the present invention to alarm and indicate an empty container condition whenever the normalized control signal is substantially equivalent to the reference signal. Stated differently, the detector of the present invention indicates an empty container whenever the control signal is in the second state and the second state is substantially equivalent to the reference signal.

The preferred embodiment of the novel empty container detector of the present invention incorporates a drop sensor which is able to sense successive drops of fluid entering the container. Specifically, the upper photoelectric transducer is positioned so its light beam is intersected and modulated by drops falling into the container. A first electrical means disregards the modulations caused by the falling drops and provides the normalized input for the lower photoelectric transducer. A second electrical means counts the interruptions in the circuitry of the upper photoelectric means to indicate the rate of flow of fluid into the container.

Reference is now made to the accompanying drawings taken in conjunction with the accompanying description in which similar reference characters refer to similar parts and in which: <ul id="ul0001" list-style="none"><li>Figure l is an elevational view of the empty container detector in association with a drip chamber;</li><li>Figure 2 is a cross-sectional view of the empty container detector and associated drip chamber as seen along the line 2-2 in Figure l with some parts removed for clarity;</li><li>Figure 3 is a cross-sectional view of the empty container detector and associated drip chamber as seen along the line 3-3 in Figure l with some parts removed for clarity;</li><li>Figure 4 is a schematic electrical circuit for use in the invention;</li><li>Figure 5A is a graphical plot of the variation in output voltage of a photoelectric sensor as indicated under certain operating conditions;</li><li>Figure 5B is a graphical plot of the electrical output of the circuitry shown in Figure 4 under certain operating conditions;</li><li>Figure 6 is a cross-sectional view of the empty container detector and associated drip chamber with some parts removed for clarity as seen along the line 2-2 in Figure l, when the detector performs a drop sensor function; and</li><li>Figure 7 is a schematic circuit diagram of the electronic componentry of the present invention incorporating a drop sensor.</li></ul>

Referring to Figure l, the empty container detector of the present invention is shown and is generally designated as l0. More particularly, it is seen in Figure l that the empty container detector l0 includes a casing l2 which is operatively associated in partial surrounding relationship with a drip chamber l4. As is well known to those skilled in the pertinent art, drip chamber l4 typically comprises a container l6 which is made from a clear resilient plastic material that allows pinching or squeezing of the container l6. Further, drip chamber l4 includes a standard type spike l8 which is associatable or connectable with a fluid source (not shown). Typically, drip chamber l4 is placed in fluid communication with a fluid source by the insertion of spike l8 into a stopple (not shown) or septum (not shown) which is associated with the fluid source. As also seen in Figure l, the drip chamber l4 has an associated tubing 20 which connects the drip chamber l4 into fluid communication with a medical device (not shown) or some other form of apparatus used to control the infusion to a patient of fluids which are passing through drip chamber l4.

Drip chamber l4 also includes an input orifice l06 through which fluid from the fluid source (not shown) flows into drip chamber l4. As is well known in the pertinent art, input orifice l06 is associated with a drop former l08 which causes fluid entering drip chamber l4 to be ordered into a stream of successive drops ll0.

In an operable state, drip chamber l4 is partially filled with fluid 22 to establish a fluid level 24 somewhere near the midpoint of container l6. As seen in Figure l, the empty container detector l0 is associated with drip chamber l4 in a manner to place a photoelectric transducer above fluid level 24 and another photoelectric transducer below fluid level 24. More specifically, as shown in phantom in Figure l, a photoelectric detector comprises an emitter (diode) 26 optically associated with a sensor (transistor) 28. As will be more apparent after further disclosure of the present invention, the emitter 26 and sensor 28 provide a photoelectric circuit which includes a light path 30 between emitter 26 and sensor 28.

As mentioned above, the circuitry created by emitter 26 and sensor 28 are disposed relative to drip chamber l4 so that light path 30 is above fluid level 24. As also seen in Figure l, empty container detector l0 further comprises an emitter (diode) 32 optically associated with a sensor (transistor) 34 to establish circuitry which includes a light path 36. Further, in accordance with the present invention, the circuitry created by emitter 32 and sensor 34 is positioned relative to drip chamber l4 so that when drip chamber l4 is normally operative, i.e., there is fluid in container l6, light patch 36 passes through fluid 22 at a point below fluid level 24. Further, as shown in Figure l, the light paths 30 and 36 are arranged so that they are substantially parallel to each other.

The actual operation of emitter 26 and sensor 28 circuitry will be best understood by reference to Figure 2 where it is seen that emitter 26 and sensor 28 are positioned relative to drip chamber l4 such that their alignment is offset from the longitudinal axis of drip chamber l4. Thus, it will be appreciated by the skilled artisan that light from emitter l6 when incident upon container l6 at the point 38 at any angle <u style="single">α</u> to the normal l00 will be refracted by the container l6. Further, upon emergence of the light from container l6 into the air above fluid level 24 in container l6 at the point 40, light path 30 will again be refracted. This time the light is refracted so that the portion of path 30 between points 40 and 42 is substantially parallel to the path 30 between emitter 26 and point 38. Subsequently, the incidence of light at point 42 and its emergence at point 44 causes a further refraction of light path 30 that brings the light into alignment as an extension of the origin of path 30 and directs the light onto sensor 28. Recognize that the above described light path 30 results only when light path 30 passes through air in container l6 above fluid level 24. When a medium in container l6, other than air, lies between emitter 26 and sensor 28, the resultant light path will be substantially altered.

A specific arrangement for the photoelectric detector comprising emitter 26 and sensor 28 occurs when the emitter 26 and sensor 28 are diametrically opposed to each other relative to the container l6, as shown in Figure 6. This arrangement positions light path 30 substantially perpendicular to the longitudinal axis of container l6 and accomplishes several objectives. Firstly, since light path 30 is normal to the surfaces of container l6 at its points of incidence and emergence, light path 30 will be unrefracted. This fact, however, does not in any way hinder the function of this upper photoelectric detector to provide a normalized input for a lower photoelectric detector. This function, and the associated electrical circuitry will be subsequently discussed in detail. Secondly, since light path 30 now passes generally through the centre of container l6, path 30 is properly positioned to be broken or interrupted by drops ll0 falling from drop former l08 into the container l6. The proper positioning of light path 30 with respect to drop former l08 is important to insure that a drop ll0 has in fact been formed before it interrupts light path 30. This can be easily accomplished by providing sufficient distance between drop former l08 and light path 30 to accomplish this purpose. The skilled artisan will recognize that an operational interaction of light path 30 with any container can be accomplished even where the container is not cylindrically shaped. Thus, in connection with electrical circuitry to be subsequently discussed in detail, sensor 28 can be used to detect the passage of drops ll0 through light path 30 as well as cooperate with other components to determine when container l6 is emptied of fluid.

Where a medium, such as a fluid, is held in container l6, the light path will be similar to that shown in Figure 3. In this case, although container l6 refracts light through the angle <u style="single">α</u>, light is refracted from the normal l02 by an angle <u style="single">ϑ</u> after entering the fluid in container l6. The light then continues along light path 36<u style="single">a</u> and exits container l6, as shown in Figure 3, without being incident upon sensor 34. This, of course, occurs so long as the fluid in container l6 allows for the passage of light. If the fluid in container l6 is opaque or sufficiently opalescent, the light will be attenuated instead of refracted. In either case, light will not be incident on sensor 34. Thus, regardless of the nature of the fluid, its mere presence will be sufficient to affect the incidence of light on sensor 34. According to the present invention, however, if the fluid level 24 in drip chamber l4 falls below the light path 36 as indicated in Figure l, the light is no longer either refracted or effectively attenuated and, instead, is subjected to the same physical properties during its passage between emitter 32 and sensor 34 as is the light in transit between emitter 26 and sensor 28. Accordingly, when the fluid level 24 in drip chamber l4 falls below light path 36, the passage of light between emitter 32 and sensor 34 will be as shown by the light path 36 in Figure 3. It will be understood by the skilled artisan that the present invention will be operable regardless whether visible or invisible light is used. In fact, it may be preferable to use infrared light rather than visible light.

It is recognized that a device incorporating a single photoelectric transducer circuit similar to the one shown in Figure 3 that comprises an emitter 32 and a sensor 34 is able to provide rough approximations of the presence or absence of fluid. Specifically, since the presence of fluid 22 within container l6 will cause light to be either attenuated or refracted along path 36<u style="single">a</u> at an angle <u style="single">ϑ</u> from the normal l02 and the absence of fluid 22 will cause light to be refracted along path 36 at an angle <u style="single">α</u> from the normal l02, the variation in light paths may be sufficient to establish the presence or absence of fluid in the container. However, in accordance with circuitry to be subsequently described in detail, light path 36 for the present invention is linearized and normalized with respect to light path 30 to account for physical variables which affect the functioning and reliability of the electrical circuitry between emitters 26 and 32 and their respective sensors 28 and 34. More specifically, in accordance with the present invention, variations due to the materials used for container l6 and changes in temperature are eliminated by referencing the lower photoelectric transducer with the upper photoelectric transducer. This is accomplished electronically by establishing the output of the upper photoelectric transducer 64, established by emitter 26 and sensor 28, as the input to both the upper and the lower photoelectric transducers 64, 84. The lower photoelectric transducer 84 is established by emitter 32 and sensor 34. As will be understood by those skilled in the pertinent art, the use of the output from upper photoelectric transducer 64 as its own input establishes a controlled feedback loop. Using this same output as input to the lower photoelectric transducer 84 establishes a normalized input for the lower photoelectric transducer. In effect, this establishes an environment for the lower photoelectric transducer which is dependent on conditions sensed by the upper photoelectric transducer 64.

Referring now to Figure 4, the electrical circuitry of the present invention is seen in a schematic form. Specifically, it is seen that emitter (diode) 26 and sensor (transistor) 28 are positioned to establish a light path 30. A resistor 50 is connected between the voltage source V_cc and transistor 28 to convert the current into the collector of transistor 28 into a voltage. The output of sensor (transistor) 28 is also connected with an amplifier stage 66 which comprises a resistor 52, and operational amplifier 56, a resistor 58 and a capacitor 54 connected as shown in Figure 4 in a manner well known in the pertinent art. The output from amplifier stage 66 is then connected with a current controller 68 which comprises a transistor 60 and a resistor 62. As will be appreciated by those skilled in the pertinent art, photoelectric transducer 64, amplifier stage 66 and current control 60 collectively establish a servo loop. For reasons well known in the pertinent art, current controller 68 is provided in the servo loop as a current amplifier. For this purpose, resistor 62 limits the maximum current available within the servo loop to prevent overdriving emiters 26 and 32. For instance, a blockage of light path 30 in photoelectric transducer 64 will cause excessive current flow. Further, the servo loop will operate at a set point voltage which is established by the system input voltage V_cc and the values used for resistor 70 and resistor 72. Since the container l6 is placed in light path 30, it is effectively made part of this circuitry. Thus, the materials used in the manufacture of container l6 affect the value of the current in the servo loop. Likewise, the individual component electrical and electro-optical parameters are affected by the ambient temperature. Consequently, these factors are no longer variables which must be accounted for and the current in the servo loop can be used as a reference signal.

Also shown in Figure 4 is a second photoelectric transducer 84 which is comprised of an emitter (diode) 32, a sensor (transistor) 34 and their associated light path 36. The output from photoelectric transducer 84 is shown in Figure 4 as an input to the comparator 80 which includes an operational amplifier 78 having positive feedback through a resistor 76. Also shown as part of the comparator 80 is a resistor 74. As will be appreciated by those skilled in the pertinent art, the value of resistor 76 can be made sufficiently high relative to the value of resistor 74 to provide hysteresis which, by virtue of positive feedback to operational amplifier 78, will sufficiently bias the output of comparator 80 so that variations in the input to comparator 80 which are less than the hysteresis will not affect the output of comparator 80.

A variable resistor 82 is shown in Figure 4 that is adjustable to establish a margin which is the effective difference between the input to amplifier stage 66 and the input to comparator 80. In accordance with well known servo loop techniques, the input voltage to amplifier stage 66 through resistor 52 is held equal to the reference voltage determined by the values set for resistors 70 and 72. On the other hand, variable resistor 82 will establish the input voltage to comparator 80. And, for reasons to be discussed, a difference in these inputs is generally necessary. Of course, when container l6 is emptied of a clear fluid, it can be expected that current flow through transistor 34 will substantially equal the current flow through transistor 28 since emitters 26 and 32 are in series. However, when an opaque or opalescent fluid is emptied from container l6, a residue may remain on the sides of container l6 which will attenuate, but not refract light path 36. In this latter case, the current flow through transistor 34 will remain less than the current flow through transistor 28 despite the fact that container l6 is empty. Thus, it is necessary that variable resistor 82 be set at a level which allows a lower output voltage from transistor 34 to be handled by detector l0 as substantially equivalent to the higher output voltage of transistor 28. In other words, variable resistor 82 establishes a minimum level above which the output voltage of comparator 80 will alarm the detector l0. As envisioned by the present invention, the output voltage of comparator 80 is a control signal which is continuously compared with the reference signal established in the servo loop. When the control signal output of comparator 80 rises to a predetermined fraction of the reference signal set by the servo loop, the detector 20 alarms to indicate an empty condition for container l6. In other words, when light patch 30 and light path 36 are substantially similar and photoelectric transducer 84 is providing an output voltage that is greater than a predetermined fraction of the output voltage of photoelectric transducer 64, an alarm is triggered in detector l0 to indicate an empty container l6.

A further appreciation of the relationship between voltage output of the photoelectric transducer 84 and control signal output from comparator 80 can be had by reference to Figures 5A and 5B. Both Figures 5A and 5B are graphical plots of voltage with respect to time. More specifically, Figure 5A is a time representation of the signal voltage change at the collector of transistor 34 as fluid level 24 drops below light path 36. As seen in Figure 5A, the voltage of sensor (transistor) 34 will be in the region 90 when the fluid level 24 is above the light path 36. As the fluid level 24 recedes across light path 36, the voltage output of sensor (transistor) 34 will vary according to time as shown in the region 92 of Figure 5A until a voltage corresponding to region 94 is attained when the fluid level 24 is below the light path 36. It will be appreciated that the transition through region 92 will depend in large part on the opalescence of the fluid. Thus, it can be seen in Figure 5A that for normal operation with fluid level 24 above the light path 36, corresponding to fluid present in the container, a positive interval 86 is established between the operating voltage and a reference voltage indicated by the dashed line l04 in Figure 5A.

The comparison of Figure 5A with Figure 5B shows that the voltage output of comparator 80 represented in Figure 5B is dependent upon the voltage output of sensor (transistor) 34 shown as the voltage level 96 in Figure 5A. The control voltage 98 output from comparator 80 is seen to appear as a step-function which changes state according to the voltage level 96 in Figure 5A. Through appropriate circuitry, the voltage level 98 representing the output from comparator 80 can be used by electronic circuitry well known in the art to trigger an alarm which indicates the absence of fluid in the light path 36. From the foregoing it will be recognized that the absence of fluid in light path 36 corresponds to a condition where fluid 22 has been depleted from drip chamber l4 and which further indicates that the fluid source which is connected in fluid communication with drip chamber l4 has likewise been depleted of fluid.

When detector l0 is used as a combination empty container detector and drop sensor, the output from sensor 28 must be used for two separate signals. On the one hand, as an empty container detector, the output of sensor 28 must be normalized and linearized as previously discussed. For this function, the output of sensor 28 needs to be a relatively steady signal. On the other hand, for the use of detector l0 as a drop sensor the output of sensor 28 must provide a variable and intermittent signal which will correspond to, and permit detection of, fluid drops flowing into container l6.

Figure 7 shows the electronic circuitry of the detector l0 when it is used both as an empty container detector and a drop sensor. It is to be understood that as drops ll0 fall into container l6, in the manner shown in Figure l, they will interrupt light path 30. It happens that as the drops ll0 interrupt light path 30, the incident light on sensor 28 is modulated and a positive voltage pulse occurs at the collector of sensor 28. Thus, in accordance with drops falling into container l6, the actual output of sensor 28 is variable and intermittent. For purposes to be subsequently discussed this signal is well suited for use by detector l0 in its drop sensing function. Such a signal, however, is inappropriate for use by the detector l0 as an empty container detector. An explanation as to how the output is modified to accomplish both functions requires further reference to Figure 7.

For the purpose of using detector l0 as an empty container detector, despite the fact the output of sensor 28 is modulated by drops ll0 falling through light path 30, the circuitry previously disclosed for amplifier stage 66 is adequate. Specifically, the pulse generated in the output of sensor 28 by drop ll0 interrupting light path 30 is integrated by amplifier stage 66 to effect a negligible change in the current flowing from amplifier stage 66 through photoelectric detectors 64 and 84. As is well known by the skilled artisan, this is accomplished by placing capacitor 54 in a feedback loop around operational amplifier 56.

For the purpose of using detector l0 as a drop sensor an additional comparator circuit l22 must be added to the circuitry previously disclosed for detector l0 in Figure 4. Specifically, comparator l22 requires electrically connecting operational amplifier ll2 to the output of sensor 28. Also, electrically connected to operational amplifier ll2, in a manner as shown in Figure 7, are resistors ll4 and ll6 which provide a hysteresis feedback to amplifier ll2. Further, resistors ll8 and l20 are connected between the voltage source V_cc, resistor ll6 and ground as shown in Figure 7 to create a voltage divider which establishes the threshold for comparator l22. For the purposes of the present invention, an appropriate threshold equals the sum of the DC operating point for sensor 28 and approximately fifty percent (50%) of the pulse amplitude created when a drop ll0 interrupts light path 30. Thus, when a drop ll0 creates a pulse at sensor 28 the threshold for comparator l22 will be exceeded. This, in turn, creates a negative going logic level signal as the output l24 of comparator l22. As will be appreciated by one skilled in the art, output l24 from comparator l22 can be used as input to electronic components (not shown) which will detect the drops ll0, count them, and calculate the rate at which they fall into container l6.

OPERATION

In its operation the empty container detector l0 of the present invention is operatively associated with a drip chamber l4 which has been connected in fluid communication with a fluid source. Upon connection of drip chamber l4 with a fluid source by means of spike l8, the container l6 of drip chamber l4 is squeezed to allow a partial filling of the drip chamber l4 to a fluid level 24 as shown in Figure l. It will be appreciated that with drip chamber l4 in fluid communication with a fluid source, the air trapped in drip chamber l4 above fluid 22 will maintain the fluid level 24 substantially as shown in Figure l so long as there is fluid in the fluid source. Thus, it is a purpose of the empty container detector l0 of the present invention to determine when the fluid level 24 falls below a predetermined level in drip chamber l4 and then alarm to indicate the depletion of fluid from the fluid source. Accordingly, the empty container detector l0 comprises a first photoelectric transducer 64 which is operatively associated with the drop chamber l4 and disposed relative to drip chamber l4 above fluid level 24. Also, the empty container detector l0 includes a phototelectric transducer 84 which is associated with drip chamber l4 at a point below fluid level 24 in drip chamber l4.

A reference signal is established by photoelectric transducer 64 and a control signal is established by photoelectric transducer 84. Since photoelectric transducer 64 is disposed above fluid level 24, the reference signal corresponds to air in container l6. On the other hand, transducer 84 is disposed below fluid level 24. By virtue of the refraction of light path 36 in photoelectric transducer 84 in the presence of fluid 22 in drip chamber l4, the photoelectric transducer 84 will have a minimal output. Upon depletion of fluid from the fluid source, the partial vacuum above fluid 22 in drip chamber l4 is relieved and the fluid level 24 recedes through the drip chamber l4. Upon passage of fluid level 24 past the light path 36 of photoelectric transducer 84, the output of photoelectric transducer 84 is changed since light path 36 is no longer refracted or blocked by the presence of fluid in drip chamber l4. Accordingly, the control signal output of comparator 80 is altered. An alarm is triggered to indicate depletion of fluid from the fluid source, i.e., an empty fluid container, when the control signal exceeds a predetermined fraction of the reference signal.

When upper photoelectric transducer 64 is positioned so that drops ll0 falling into container l6 interrupt and modulate light path 30, the detector l0 can be used as a drop sensor. In its operation as a drop sensor, the output from photoelectric transducer 64 of detector l0 is modulated by drops ll0 interrupting light path 30. This output is integrated by amplifier stage 66 to smooth the modulations before it is used by associated circuitry to accomplish the empty bottle detector function. Additionally, the output from transducer 64 is connected directly to comparator l22. As an input to comparator l22, this output is not integrated by amplifier stage 66. Instead, the modulated signal is connected directly to other electrical componentry to indicate the passage of drops 220 into container l6. The output from comparator l22 can be then connected to appropriate drop sensing components.