EP0042400A4

EP0042400A4 - Bubble detector in a flow metering apparatus.

Info

Publication number: EP0042400A4
Application number: EP19810900165
Authority: EP
Inventors: John E Gupton; Norm Shim; Nicholas Zissimopoulos; John Baron
Original assignee: Baxter Travenol Laboratories Inc
Current assignee: Baxter International Inc
Priority date: 1979-12-13
Filing date: 1980-11-24
Publication date: 1982-04-22
Also published as: WO1981001655A1; EP0042400A1

Abstract

In a flow metering apparatus (10) a bubble detector for detecting bubble formation in tubing (16) subject to deformation from internal fluid pressure includes a light source (60) and a first light detector (61). The light detector is positioned on the opposite side of the tubing from the light source such that the light transmitted through the tubing to the detector is dependent on the presence of fluid in the tubing and on the shape of the lumen of the tubing. A control circuit (100) responsive to the output of the detector interrupts operation of the metering apparatus (10) when the light transmitted through the tubing falls below a predetermined minimum level. False interruptions resulting from deformation of the tubing by pressure changes in the fluid are prevented by forming members (90, 91) which engage the wall of the tubing (16) adjacent the light source (60) and light detector (61). For improved sensitivity a second light detector (100) is positioned at an angle to the light path between the light source (60) and the first detector (61) so as to receive increased light from the light source in the absence of the Focusing effect of fluid in the tubing. Intensity control means (102) responsive to the output of the light source decreases the intensity of the light source (60) with increased incident light at the second detector (100) to increase the responsiveness of the bubble detector to the absence of fluid in the tubing.

Description

BUBBLE DETECTOR

Background of the Invention

The present invention relates generally to fluid infusion apparatus, and more particularly to an improved system for detecting the formation of bubbles in such systems.

The infusing of fluids such as parenteral fluids and blood into the human body is usually accom¬ plished by means of an administration set and metering apparatus which controls the rate of flow of fluid through the set. Peristaltic-type pumps, which function by re¬ petitively compressing and expanding a section of tubing, have proven particularly attractive for use in such metering apparatus since they do not introduce the pos¬ sibility of leakage or contamination into the system, while providing positive control of fluid flow through the system. One form of metering apparatus employing a peristaltic-type pump is described in U.S.. Patent 4,155,362, which issued to Thurman S. Jess on May 21, 1979, and is assigned to the present assignee. A suc- cessful commercial embodiment of this apparatus is marketed as the Travenol Model 2M8014 infusion pump by Baxter Travenol Laboratories, Inc., of Deerfield, Illinois.

One problem which arises with the use of liquid infusion sets is that dissolved gases in the liquid being infused may be released as bubbles as the liquid is subjected to pressure and/or temperature changes as it passes through the pump of the metering apparatus. These bubbles may coalesce and form larger bubbles or pockets of gas which may be infused along with the liquid into the body, an occurrence which may be harm¬ ful or even fatal to the patient under certain circum¬ stances.

To prevent gas from being infused it has be- come common practice to locate a bubble detector down¬ line of the metering apparatus pump to automatically stop the apparatus should gas bubbles be detected. Such detectors typically employ a light source and a light de¬ tector positioned on opposite sides of the administration set tubing to monitor the level of light transmitted through the tubing. Operation of the metering apparatus is interrupted and an alarm is sounded when the trans¬ mitted light level falls below a predetermined level. To this end, the lens effect of the fluid in the lumen of the tubing may be employed to enhance the difference in transmission levels between fluid and no fluid con- ditions.

One problem encountered with such bubble de¬ tectors is that fluid pressure changes in the tubing of the administration set such as may result from the use of a downline flow restriction, as in the Travenol Model 2M3014 infusion pump, may deform the wall of the tubing from its unstressed shape and diminish the lens effect. This has the potential of diminishing the re¬ liability and sensitivity of the bubble detector. The present invention is directed to a bubble detector wherein forming members are provided in association with the light source and light detector to prevent variations in effectiveness as a result of changes in fluid pres¬ sure.

In bubble detectors which utilize the focusing or lens effect of the fluid in the lumen of the tubing to provide a difference in light transmission levels through the tubing between fluid and no fluid conditions, the change in transmission levels under certain circum¬ stances, such as where the fluid is cloudy, may not be adequate. The present invention is further directed to the use of a second light detector and feedback circuit in such systems whereby improved responsiveness to the absence of fluid in the tubing is achieved.

Accordingly, it is a general object of the present invention to provide a new and improved bubble detector.

It is another object of the present invention to provide a new and improved bubble detector wherein means are provided for preventing deformation of the tubing wall as a result of internal fluid pressure.

It is a further object of the present inven- tion to provide a new and improved bubble detector suit¬ able for use with vinyl tubing or the like subject to deformation from internal fluid pressure.

It is another object of the present invention to provide a new and improved bubble detector having improved responsiveness to the^' absence of fluid in the tubing being monitored.

It is a further object of the present inven¬ tion to provide a bubble detector wherein the lens ef¬ fect of fluid within the lumen of the tubing being mon- itored is utilized to detect with improved responsive¬ ness the absence of fluid within the tubing.

Summary of the Invention

The invention is directed to a flow metering apparatus for controlling the flow of fluids through an administration set of the type having transparent tubing subject to deformation from internal fluid pressure. The apparatus includes a bubble detector comprising a light source arranged at one side of the tubing, and a light detector generally arranged at the opposite side of the tubing opposite the light source and defining a light path through the tubing. The detector generates an output signal in response to the intensity of light from the light source transmitted through the tubing, the intensity of the transmitted light being dependent on the presence of fluid within the lumen of-the tubing and on the shape of the lumen between the source and the de¬ tector. Control circuit means responsive to the output signal are provided for interrupting operation of the flow metering apparatus upon the intensity of the trans¬ mitted light falling below a predetermined minimum level. Platen means including at least one forming member en¬ gaging the tubing about a substantial portion of its circumference adjacent the light path are provided for preventing deformation of the tubing and consequent changes in the intensity of the transmitted light with fluid pressure changes in the lumen of the tubing.

The invention is further directed, in a flow metering apparatus for controlling the flow of fluid through an administration set of the type having trans¬ parent tubing, to a bubble detector comprising a light source arranged at one side of the tubing, and a first light detector generally arranged at the opposite side of the tubing and defining in conjunction with the light source a light path through the tubing. The first light detector generates a first output signal in response to the intensity of light from the light source incident thereon, the intensity of the incident light increasing in the presence of fluid within the lumen of the tubing as a result of the focusing effect thereof. A second light detector generally arranged at an angle to the light path is provided to generate a second output signal in response to the intensity of the light incident there¬ on, the intensity of the incident light decreasing in the presence of fluid within the lumen of the tubing as a result of the focusing effect thereof. Detector circuit means responsive to the first output signal are provided for interrupting operation of the flow metering apparatus upon the intensity of the light incident of the first light detector falling below a predetermined minimum level. Intensity control means responsive to the second detector output signal decreases the light output of the • light source upon the light incident on the second de¬ tector increasing to increase the effectiveness of the control circuit means in responding to the absence of fluid in the lumen of the tubing.

Brief Description of the Drawings

The features of the present invention which are believed to be novel are set forth with particularity in the amended claims. The invention, together with the further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, and the several figures of which like reference numerals identify like elements, and in which: Figure 1 is a perspective view of a metering apparatus incorporating a bubble detector constructed in accordance with the invention.

Figure 2 is an enlarged front elevational view of the metering station of the flow metering apparatus partially in section and partially broken away to illus¬ trate the operation thereof.

Figure 3 is a cross-sectional view of the metering station taken along line 3-3 of Figure 2.

Figure 4 is an elongated front elevational view of the bubble detector head of the metering station. Figure 5 is a cross-sectional view of the bubble detector head taken along line 5-5 of Figure 4. Figure 6 is an exploded perspective view of the bubble detector head showing the principal compon- ents thereof.

Figure 7 (A) is a diagrammatic depiction of the bubble detector head useful in depicting operation of the bubble detector in the absence of fluid.

Figure 7 (B) is a diagrammatic depiction similar to Figure 7 (A) showing operation of the bubble detector with fluid present.

Figure 8 (A) is a simplified diagrammatic de¬ piction of a prior art bubble detector head illustrating the effect of deformation of the tubing wall on the operation of the bubble detector. Figure 8(B) is a simplified diagrammatic de¬ piction of the flow detector head of the invention use¬ ful in understanding the operation thereof.

Figure 9 is a simplified functional block diagram of the control system of the metering apparatus of Figure 1.

Figure 10 is a simplified schematic diagram of a preferred detector circuit for use in conjunction with the bubble detector of the invention. Figure 11 is an enlarged cross-sectional view of an alternate embodiment of the bubble detector head similar to Figure 5.

Figure 12 (A) is a diagrammatic depiction of the bubble detector system utilizing the bubble detector head of Figure 11 useful in depicting operation of the system in the absence of fluid.

Figure 12 (B) is a diagrammatic depiction sim¬ ilar to Figure 12 (A) showing operation of the bubble detector system with fluid present.

__oiapι_: Description of the Preferred Embodiment

Referring to the Figures, and particularly to Figure 1, a peristaltic-type flow metering apparatus 10 for use in conjunction with an administration set for controlling the flow of fluid into a vein or artery in¬ cludes a generally rectangular housing 12 having a handle 13 at one end thereof for convenient carrying. The front surface of the housing includes a control panel 14 which allows the operator to control and monitor the operation of the metering apparatus, and a peristaltic-type flow metering head 15 for compressing a section of tubing 16 of the administration set to effect control of fluid flow therein. A channel 17 is provided above the meter¬ ing head 15 for maintaining a portion of the tubing seg- ment in convenient view of the operator whereby flow irregularities can be more readily observed. .

The administration set, of which tubing seg¬ ment 16 is a part, and which may be conventional in design and construction, is preferably formed of a plastic material such as vinyl and packaged in a sterile and non- pyrogenic condition. To avoid the danger of contamin¬ ation, the administration set is normally utilized for one application only, and is disposed of after a single use. The operating mode of metering apparatus 10 is controlled by means of a push button STOP switch 20, a bush button START switch 21, and a push button power ON-OFF switch 22. Each of these push button switches includes an internal indicator lamp which provides a positive indication of the operation of the operating mode of the apparatus. Various abnormal operating con¬ ditions are annunciated by means of indicator lights 23 contained on the control panel to the left (as viewed in Figure 1) of the mode control push buttons. Control panel 14 further includes a digital display 30 of volume infused, a digital display 31 of volume to be infused, and a digital display 32 of the fluid flow rate. The volume displayed by display 30 is the volume of fluid actually infused, and can be reset to zero by the operator by means of a push button RESET switch 33. The volume to be infused by display 31 is preset by the operator by means of a set of push button switches 34 to indicate a desired volume of fluid to be infused. Similarly, the infusion rate display 32 is preset by the operator by means of a second set of push button switches 35 to indicate the rate at which in¬ fusion is to take place.

The operation of the various indicators, control switches and other features of metering appar- atus 10 is described in detail in the copending United States patent application of Thurman S. Jess and Norm Shim, Serial Number 856,863; Norm Shim, Serial Number 857,018; Norm Shim and Vincent L. niggee, Serial Number 856,927; and Thurman S. Jess, Serial Number 356,926; all filed December 2, 1977.

Referring to Figures 2 and 3 , the peristaltic metering head 15 includes a rotor 40 having four pressure rollers 41 disposed in equi-spaced relation about its circumference. The rollers are each mounted on a shaft 42 for free rotation, and the shafts are carried on carriages 43 and constrained to radial movement by re¬ spective radial slots 44. Each carriage is mounted for reciprocation within a radial recess 45 and spring loaded radially outward by a helical spring 46 disposed within the recess.

The pump also includes a pressure plate 50 having an arcuate working surface 51 which substantially corresponds in shape to the circumference of rotor 40. The working surface brings tubing 16 into compressive engagement with rollers 41 around at least a portion of the rotor circumference corresponding to the spacing between adjacent rollers. The pressure plate may be reciprocated toward and away from rotor 40 to facilitate installation and removal of tubing 16 by rotation of an eccentric cam 52, which is constrained to operate within a vertical slot 53 provided on the pressure plate. Rotation of the cam is accomplished by a shaft 54 and a user-actuable lever 55 operatively connected to the cam. When the lever 55 is in its vertical position, as shown in Figure 3, the pressure plate is moved sufficiently close to the rotor circumference to cause tubing 16 to be completely occluded by one of the pressure rollers 41.

After passing through metering station 15, tubing 16 extends between a light source 60 and a photo- detector 61, which together comprise a bubble detector head 62. This head, combined with associated control circuitry forms a bubble detector system which discon¬ tinues operation of the metering apparatus and alerts the operator upon formation of a bubble in the tubing. The tubing next passes through a flow restric¬ tion station 63. This station includes a pressure block 66 and a slidably mounted plunger 67 biased against the sidewall of tubing segment 16. The end of plunger 67 which engages the tubing segment includes a generally L-shaped head portion 68 having a wedge-shaped working surface 70 which occludes the tubing and a generally flat control surface 71 which responds to fluid pressure changes. Plunger 67 is slidably received within a mount¬ ing block 73, and extends through the center of a helical compression spring 74 which biases head 68 into engage¬ ment with the tubing. The occlusion of the tubing by the flow restriction station increases the pressure of the fluid in the tubing at the point of engagement of the rollers 41 of rotor 40 to assist in restoration of the tubing following compression by the pressure rollers for improved metering accuracy.

Plunger 67 can be opened to facilitate loading or unloading of tubing 16 by means of a lever 76. The plunger is locked open by means of a latch member 77 which is pivotally mounted at 78 to pressure plate 50 and biased by a helical spring 79 for operation within a plane perpendicular to the plunger. Latch member 77 is received in a slot 80 on the plunger when the plunger is moved to its full open position. To insure that plunger 67 will be released when pressure plate 50 is subsequently closed, an actu¬ ator pin 82 having a tapered end surface displaces latch member 77 from slot 80 when the pressure plate is re¬ turned to its closed position by rotation of knob 55. This prevents inadvertent operation of the system with¬ out the back pressure and gravity flow protection pro¬ vided by the plunger. Also, when the pressure plate is opened, the displacement of latching member 77 pre¬ vents the plunger from being latched open. In accordance with the invention, metering apparatus 10 includes a bubble detector head 62 which renders the bubble detector system of the apparatus immune to variations in fluid pressure in tubing 16. Referring to Figure 4, the bubble detector head is seen to comprise first and second forming members 90 and 91 disposed on opposite sides of tubing 16. The first form member 90 is secured to housing 12 by a bolt 93, extend¬ ing through a flange portion 92. Similarly, the second form member 91 is secured to the slidable pressure plate 50 by means of a bolt 94 extending through a flange por¬ tion 95 of the base member.

To provide mounting means for light source 60, the first form member 90 is provided with a bore 96 per¬ pendicularly aligned to the axis of tubing 16. To pro- vide a receptacle for photodetector 61, form member 91 is similarly provided with a perpendicularly aligned bore 97. Bores 96 and 97 are each dimensioned with an inside diameter just slightly larger than the outside of light source 96 and photodetector 97, respectively, to provide a fit for these elements sufficiently tight to maintain the elements in alignment.

It will be noted that form members 90 and 91 define inwardly concave mandrel surfaces 98 and 99, respectively, between which tubing 16 is held when the form elements 90 and 91 are in their closed position, as shown in Figures 4 and 5. The curvature of these mandrel surfaces is dimensioned to correspond closely to the natural or unstressed curvature of the outside surface of tubing 16 so that when the tubing is engaged to the form members the tubing lumen is maintained in its unstressed cross-sectional shape notwithstanding pressure changes in the fluid contained therein.

To remove the tubing, it is merely necessary to separate form members 90 and 91, as shown in Figure 6. In metering apparatus 10, this is accomplished auto¬ matically upon the operator actuating knob 55 to open metering station 15, since form member 90 is mounted to a stationary housing member, and form member 91 is mounted to the movable pressure plate 50. To illustrate the benefit of maintaining the cross-section of tubing 16 constant, reference is made to Figures 7 (A) and 7 (B) which illustrate the lens effect upon which the detector depends. In Figure 7 (A) no fluid is present in the lumen of tubing 16, and light source 60 diverges as it passes through the transparent walls of the tubing. As a result, only a small portion of the light transmitted through the tubing actually falls upon light detector 61, and the resulting signal produced by that device is small. In contrast, when liquid is present in the lumen of the tubing as shown in Figure 7 (B) , the circular cross-section of the fluid mass, as defined by the inner surface of the wall of tubing 16, forms a lens which focuses the light on de¬ tector 61. As a result, a greater portion of the trans- itted light is actually incident on the detector and the resulting detector output signal is stronger. By comparing the light detector output signals for the con¬ ditions shown in Figures 7 (A) and 7 (B) , appropriate bubble detector circuitry within metering apparatus 10 determines the presence or absence of fluid in the tubing, Referring to Figure 8 (A) , in prior art bubble detectors no provision was made for maintaining the tubing in constant cross-section. As a result, as the pressure of the fluid in the tubing lumen increased the walls of the tubing became deformed. This caused the light from light source 60 to be only partially focused on ight detector 61, so that the output signal developed by that device was weaker than the signal would have been had the distortion not taken place. As a result, the capability of the bubble detector to distinguish between liquid and no liquid conditions was diminished. As shown in Figure 8 (B) , the invention over¬ comes this deficiency by maintaining tubing 16 is con¬ stant cross-section regardless of pressure variations in fluid in the tubing lumen. The light from light source 60 continues to be focused on light detector 61 and an optimum output signal is developed for maximum capability in distinguishing between fluid present and fluid absent conditions. The output of light detector 61 is applied to appropriate bubble detector circuitry wherein it is utilized to develop a control signal suitable for con¬ trolling the operation of the metering apparatus. Re¬ ferring to Figure 9, in the present embodiment the output of light detector 61 is applied to a bubble detector circuit 100 wherein a control signal is developed indi¬ cative of the presence or absence of fluid in tubing segment 16. This control signal is applied to one input of an AND gate 101, wherein it serves to control the application of control pulses to the motor drive circuit 102 of a stepper motor 103, which is utilized to drive the peristaltic rotor 40 of the apparatus.

Control pulses for drive circuit 102 are ob¬ tained from a pulse source in the form of a clock 104. The clock pulses are divided to a lower frequency by a variable-rate divider 105, and applied through AND gate 101 to the motor drive circuit. The division factor of rate divider 105 is selected by the operator to obtain a desired rate. The pulses derived from divider 105 are also applied to a* volume register 106 wherein they are counted for use by volume display. 30. The divided pulses are also applied to a bi-directional register 107 which supplies an inhibit signal to AND gate 101 upon the desired volume having been infused. The count- ing state of this register is displayed by display 31. Referring to Figure 10, a preferred bubble detector circuit 61 may comprise a multi-vibrator 111 consisting of three NAND gates 112, 113 and 114. A cap¬ acitor 115 connected to the output of gate 113 and a potentiometer 116 connected to the output of gate 114 provide an RC time constant which determines the fre¬ quency of the ulti- ibrator output signal in a manner well known to the art. A diode 117 is connected between the arm of potentiometer 116 and the output of gate 114 to vary the duty cycle of the oscillator output signal. A fixed resistance 118 connected in series with the body of potentiometer 116 provides a desired adjustment range.

The AC signal generated by multi-vibrator 111 is applied through a resistance 119 and transistor 120 to light source 60. The AC signal developed by multi¬ vibrator 111 is amplified by transistor 120 and utilized to drive the LED, causing the LED to produce a light output which varies at a rate dependent on the output frequency of the multi-vibrator.

The alternating light developed by the LED is converted by phototransistor detector 122 to an output signal indicative of the strength of the transmitted light. The emitter of transistor 123 is connected to ground through a resistor 124, and is connected through respective diodes 125-127 to respective inputs of a threshold trigger device in the form of a dual Schmitt trigger 128. The cathodes of diodes 125-127 are con¬ nected to ground by respective parallel combinations of capacitors 130-132 and resistors^* 133-135. These elements serve in conjunction with the diodes as alternating current detectors, generating a DC signal at the inputs of trigger 128 dependent on the amplitude of the AC sig¬ nal produced by detector 61. The dual Schmitt trigger 128, which may be a commercially available component such as the type NC14583B Schmitt trigger marketed by Motorola, Inc., of Schaumburg, Illinois, produces an output upon reduction of either of its input signals falling below a predetermined threshold level. The input associated with diode 125 functions as an enabling input for both triggers. The outputs of Schmitt trig¬ gers 128, which comprise a first control signal, are applied to one input of a logic OR gate 12 .

The emitter of transistor 123 is also con- nected to ground through series-connected resistors 136 and 137. The signal developed at the junction of these two resistors is filtered by a series-connected resis¬ tor 138 and a short-connected capacitor 139 and resis¬ tor 140 connected to ground. This forms a second control signal, which is applied to the remaining input of OR gate 129. In this way, OR gate 129 is provided with the output signal developed by the dual Schmitt trigger 128 , and with the DC control signal developed across capacitor 139, either of which can result in an output from the gate in the event of the occurrence of a bubble in tubing 16. The output of Schmitt triggers 128 and the output of OR gate 129 are also connected to the positive uni¬ directional current source of the system by respective resistors 143 and 144. Since the output of OR gate 129 is dependent on both the amplitude of the AC signal as rectified and applied to the parallel-connected Schmitt triggers 128, and on the DC signal developed across capacitor 139, the bubble detector utilized in the metering apparatus pro- vides two control channels. The first channel, ^"which utilizes Schmitt triggers 128, establishes a highly pre¬ cise threshold below which an alarm output is produced. The second channel, which depends only on the input characteristic of gate 129, serves to provide an alarm output in event of failure of resistor 124 in the photo- detector bias circuit.

In order for bubble detector 62 to not provide an output, it is necessary that the DC signals applied to the Schmitt triggers as a result of rectification by diodes 126 and 127 be above a predetermined minimum level, which is possible only when there is fluid within tubing segment to provide a lens to direct light from light source 60 to light detector 61.

Referring to Figures 12 (A) and 12 (B) , operation of the bubble detector is again seen to be dependent on a focusing or lens effect produced by fluid in the lumen of the tubing. As shown in Figure 12(A) , in the absence of fluid light from light source 60 diverges as it passes through the transparent walls of the tubing. As a result, only a small portion Of the light transmitted through the tubing actually falls upon light detector 61, and the resulting signal produced by that device is small. In contrast, when liquid is present in the lumen of the tubing as shown in Figure 12 (B) , the circular cross- section of the fluid mass, as defined by the inner sur¬ face of the wall of the tubing, forms a lens which focuses the light on detector 61. As a result, a greater portion of the transmitted light is actually incident on the detector and the resulting detector output signal is stronger. By comparing the light detector output signals for the conditions shown in Figures 12 (A) and 12 (B) , appropriate bubble detector circuitry within metering apparatus 10 determines the presence or absence of fluid in the tubing. In accordance with another aspect of the in¬ vention, the difference in incident light levels on light detector 91 between fluid and no fluid conditions is accentuated to obtain a more positive response upon de¬ tection of a bubble. This is accomplished by providing a second light detector 160 (Figure 11) arranged gen¬ erally at the opposite side of the tubing from the light source and aligned at an angle to the light path between light source 60 and detector 61 by the lens effect of tubing in the lumen. Thus, a greater amount of light is incident on detector 100 when the lumen is empty than when the lumen is filled with fluid, as shown in Figure 12(B) .

The output signal produced by light detector 160 is applied to an amplifier 161, wherein it is am- plified. The resulting amplified signal is applied to an intensity control circuit 162, wherein it is utilized to vary the output of a light source to control the light output of light source 60.

In operation, intensity control circuit 162 decreases the light output of light source 60 as the

O PI 4 . IPO _. output of the second light detector 160 increases. This has the effect of reducing the light incident on light detector 61 below the level it would be at if the in¬ tensity control circuit was inoperative, thereby increas- ing the net change in signal level available for con¬ trolling operation of the metering apparatus. Although the signal incident on the second light detector 160 also falls with reduction in light output from light source 60, the gain provided to the output of detector 160 by amplifier 161 and intensity control circuit 162 is sufficient to overcome this reduction and achieve a net increase at the output of detector 61.

The output of light detector 61 is applied to detector circuit 100 wherein it is utilized as previously outlined to develop a control signal suitable for con¬ trolling the operation of the metering apparatus.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made therein without departing from the invention in it^'s broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifi¬ cations as fall within the true spirit and scope of the invention.

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AΛ, I

Claims

- ^' l9 -

WE CLAIM:

1. In a flow metering apparatus for controlling the flow of fluid through an administration set of the type having transparent tubing subject to deformation from internal fluid pressure, a bubble detector cσm- prising, in combination; a light source arranged at one side of said tubing; a light detector generally arranged at the opposite side of said tubing opposite said light source and defining a light path through said tubing, said de¬ tector generating an output signal in response to the intensity of light from said light source transmitted through said tubing, the intensity of said transmitted light being dependent on the presence of fluid within the lumen of said tubing and on the shape of the lumen between said source and said detector; control circuit means responsive to said out¬ put signal for interrupting operation of said flow metering apparatus upon the intensity of said trans- itted light falling below a predetermined minimum level; and means including at least one forming member engaging said tubing about a substantial portion of the circumference of said tubing adjacent said light path for preventing deformation of said tubing and consequent changes in the intensity of said transmitted light with fluid pressure changes in said lumen of said tubing.

2. A bubble detector as defined in claim 1 wherein the engaging surface of said forming member con¬ forms generally to the non-expended shape of said tubing.

3. A bubble detector as defined in claim 2 wherein said non-expanded shape of said tubing is cylindrical.

4. A bubble detector as defined in claim 1 wherein said platen means comprise a pair of forming members engaging said tubing from opposite sides thereof.

5. A bubble detector as defined in claim 4 wherein at least one of said forming members is slid¬ ably mounted in a direction generally perpendicular to the axis of said tubing so as to be disengageable from said tubing to facilitate insertion or removal of said tubing from said platen means.

6. A bubble detector as defined in claim 1 wherein the tubing of the flow metering apparatus is formed of vinyl.

7. In a flow metering apparatus for controlling the flow of fluid through an administration set of the type having transparent tubing, a bubble detector com¬ prising, in combination: a light source arranged at one side of said tubing: a first light detector generally arranged at the opposide side of said tubing opposite said light source and forming in conjunction with said light source a light path through said tubing, said first detector generating a first output signal in response to the intensity of light from said light source incident thereon, the intensity of said incident light increasing in the presence of fluid within the lumen of said tubing as a result of the focusing effect thereof; a second light detector generally arranged at an angle to said light path, said second detector gener- ating a second output signal in response to the inten¬ sity of light from said light source incident thereon, the intensity of said incident light decreasing in the presence of fluid within the lumen of the tubing as a result of the focusing effect thereof; detector circuit means responsive to said first detector output signal for interrupting operation of the flow metering apparatus upon the intensity of the light incident on said first light detector falling below a predetermined minimum level; and intensity control means responsive to said second detector output signal for decreasing the light output of said light source upon the light incident on said second detector increasing to increase the effec¬ tiveness of said control circuit means in responding to the absence of fluid in the lumen of said tubing.

8. A bubble detector as defined in claim 7 wherein said tubing is cylindrical in form.

9. A bubble detector as defined in claim 7 wherein said second light detector is disposed generally on the opposite side of said tubing to receive light at an angle to said light path.

10. A bubble detector as defined in claim 9 wherein said angle is approximately from 35° to 45° to said light path at the axis of said tubing.

11. A bubble detector as defined in claim 7 wherein said light path between said light source and said first light detector extends substantially through the axis of the tubing.

12. A bubble detector as defined in claim 7 including a power source for said light source, said intensity control means acting to modulate the output of said power source to control the light output of said light source.