Abstract:
An optical sensor includes a sample cell through which a fluid flows, an optical sensor formed by an optical emitter and an optical detector. The sample cell and the optical sensor use light refraction to determine the presence and size of a bubble passing through the sample cell. A housing may also be included to provide better control over light refraction and to protect the optical sensor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to systems for administering solutions to patients in such a manner as to decrease the risk that air bubbles will be inadvertently provided to the patient. More specifically, the present invention relates to a system for optically detecting bubbles in solution being provided to a patient. 
     2. State of the Art 
     Parenteral and Enteral feeding systems are used to provide solutions to patients. In parenteral systems, solutions such as balanced saline solutions are administered to the patient&#39;s circulatory system. Enteral feeding systems are provided for patients who, for one reason or another, are unable to eat for themselves. 
     One common concern with both parenteral and enteral feeding systems is that it is undesirable for large quantities of air to be provided with the solution. In enteral systems, excessive air may irritate the digestive system of the patient and complicate other medical conditions. Additionally, the air can render the volumetric calculations of the enteral feeding pump inaccurate. 
     In parenteral applications, the risk can be much greater. While air in a parentral infusion line is undesirable, large quantities of air can cause serious problems in the vascular system. In extreme cases, excessive air can even cause death of the patient. Thus, it is critical in parenteral application that air not be delivered to the patient&#39;s vascular system. 
     In addition to the health concerns posed by the air being released into the patient&#39;s body, the presence of air in the parenteral or enteral feeding tube also means that the desired solution is not being delivered to the patient. Each cubic centimeter of air is a cubic centimeter of enteral feeding solution, medication, etc. which is not delivered to the patient. Without being able to detect the quantity of air passing through the system, the system is unable to accurately determine the actual amount of solution which has been delivered to the patient. Over a prolonged period of time, even modest amounts of air passing through the system can cause significant disparities in the amount of solution the system indicates to be delivered and the actual amount delivered. 
     There are numerous mechanisms available for detecting air in liquid passing through a tube. Many of these mechanisms provide marginal accuracy or are complex to use. Others, while relatively accurate, require considerably more power draw than is necessary. Yet other air detectors do not provide an inherent integrity check to prevent failure of the sensor from giving erroneous information regarding air in the conduit. 
     While all of the above are disadvantageous, a principle disadvantage of most air detectors which are used in enteral feeding pumps and the like is the cost. Most enteral feeding pumps utilize ultrasonic sensors to check for bubbles. Such sensors, however, can cost fifty times or more the cost of an optical sensor. 
     Thus, there is a need for an improved sensor for determining the presence of bubbles, which is less expensive, and which is easy to operate. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved sensor for detecting bubbles in a conduit; 
     It is another object of the present invention to provide such a sensor which is easy to make and use. 
     It is yet another object of the present invention to provide such a sensor which utilizes refraction of light to determine the presence of air within the conduit. 
     It is still another object of the present invention to provide such an optical sensor which is relatively inexpensive. 
     The above and other objects of the invention are realized in specific illustrated embodiments of an optical air/liquid sensing system which utilizes a sample cell. The sample cell has a pair of sidewalls which refract light into the liquid in the conduit at such an angle that the light passes out of the sample cell in a first direction when no air is present in the conduit, and in a second direction when air is present in the conduit. 
     The sample cell is disposed between an optical emitter and an optical detector. Light is emitted from the optical emitter and is refracted as it passes through the sample cell. The presence of air within the sample cell changes the refraction of light, thereby changing the light received by the optical detector. It should be appreciated that, as used herein, the term light is not limited to electromagnetic radiation in the spectrum visible to humans. To the contrary, a preferred embodiment of the invention utilizes infrared radiation. 
     In accordance with one aspect of the present invention, the amount of air in the conduit affects the amount of light refracted in the second direction. The amount of light refracted into the first direction versus the amount refracted in the second direction indicates the relative amount of air in the conduit. 
     In accordance with another aspect of the invention, the sample cell is configured and disposed to always allow some light to pass through the sample cell and to be received by the optical detector. If no light is detected by the optical detector, the system is readily able to determine that the system has failed. 
     In accordance with another aspect of the invention, the sample cell is situated so that all of the light emitted from the optical emitter does not reach the optical detector. Thus, if the optical detector indicates that nearly all of the light emitted from the optical detector has been received, the optical sensor system can readily determine that the sample cell is not properly loaded between the optical emitter and optical detector of the sensor. 
     In accordance with another aspect of the invention, the exterior of the sample cell is triangular. The shape of the sample cell regulates the flow of light through the sample cell and thereby directs light to the optical detector depending on whether air is present in the conduit. 
     In accordance with still yet another aspect of the present invention, a housing is provided and spaced apart from the sample cell by an air channel. As with the shape of the sample cell, the housing helps to direct light through the sample cell at a desired angle to facilitate the determination of whether air is present in the conduit in the sample cell. 
     Preferably, the housing is formed of a similar material to the sample cell and is disposed at an angle parallel to the sidewalls of the sample cell. This provides for the refraction of light at desired angles through the sample cell. 
     In accordance with another aspect of the present invention, a sample cell container is provided with a channel disposed therein. Properly mounting a flexible tube in the channel causes the tube to form first and second sidewalls which are disposed at desired angles to refract light in one direction when a solution is present, and to refract light in second direction, i.e. toward an optical signal detector, when the conduit is filled with air. 
     In a preferred embodiment of the invention, the sample cell has sidewalls which are disposed at an angle of between about 45 and 100 degrees relative to one another are disposed at the same acute angle relative to a plane extending from the optical signal emitter and the optical signal detector. More preferably, the two sidewalls are disposed at an angle of 60 degrees from one another and are disposed at the same relative acute angle from a horizontal or other plane passing through the sample cell from the optical signal emitter and the optical signal detector. By same angle it is meant that each sidewall has a similar acute angle from the plane, although they are in opposite directions. 
     In accordance with another aspect of the invention, it has been found that the sample cell can be used to determine the presence of air so long as the sample cell wall is less than normal from the plane along which the light is omitted. The closer the sample cell wall is to normal, however, the further away the optical sensor components must be from the sample cell wall. Additionally, the positions of the optical signal emitter and the optical signal detector can be adjusted to ensure light is refracted to the optical sensor when air is present, but not when liquid is present, or vice versa. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will become apparent from a consideration of the following detailed description presented in connection with the accompanying drawings in which: 
     FIG. 1 shows a perspective view of an optical sensing system made in accordance with the principles of the present invention; 
     FIG. 2A shows a cross-sectional view of the optical sensing system shown in FIG. 1; 
     FIG. 2B shows the cross-section view of the optical sensing system shown in FIG. 2A with a conduit of the sample cell filled with liquid; 
     FIG. 2C shows the cross-sectional view of the optical sensing system shown in FIG. 2A, wherein the conduit has an air bubble disposed therein; 
     FIG. 2D shows the cross-sectional view of the optical sensing system of FIG. 2 with an opaque solution therein; 
     FIG. 3 shows a bottom perspective view of an adaptor for use in enteral feeding pumps having the sample cell of the optical sensor system disposed therein; 
     FIG. 3A shows a top perspective view, exploded view of the adaptor and an enteral feeding pump; 
     FIG. 4 shows a cross-sectional view of an alternate configuration of a sample cell made in accordance with the principles of the present invention; 
     FIG. 5 shows a cross-sectional view of an yet another configuration of a sample cell made in accordance with the principles of the present invention; 
     FIG. 6 shows a perspective view of a sample cell container made in accordance with the principles of the present invention in an open position; 
     FIG. 6A shows a perspective view of the sample cell container of FIG. 6 in a closed position; 
     FIG. 6B shows a cross-sectional view of the sample cell container of FIGS. 6 and 6B, forming a sample cell therein; 
     FIG. 7 shows a perspective view of yet another embodiment of a sample cell in accordance with the principles of the present invention; 
     FIG. 7A shows a cross-sectional view of the sample cell of FIG. 7; 
     FIG. 8 shows a cross-sectional view of yet another embodiment of the present invention; and 
     FIG. 9 shows still another embodiment of a sample cell and optical sensor system made in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to the drawings in which the various elements of the present invention will be given numeral designations and in which the invention will be discussed so as to enable one skilled in the art to make and use the invention. It is to be understood that the following description is only exemplary of the principles of the present invention, and should not be viewed as narrowing the pending claims. 
     Referring to FIG. 1, there is shown a perspective view of an optical sensor system, generally indicated at  100 , made in accordance with the principles of the present invention. The optical sensor system  100  includes an optical sensor, generally indicated at  104 . The optical sensor  104  has an optical emitter portion  108  which emits light, and an optical detector portion  112  which detects light emitted by the optical detector and generates a voltage that is proportional to the amount of light received. The optical emitter portion  108  and the optical detector portion  112  define a cavity  116  through which the light travels between the two portions. (Those of skill in the art will appreciate that the optical sensor  104  can be made of a paired emitter and detector, or could be formed from two discrete parts.) 
     The optical sensor  104  further includes a plurality of leads  120  which are used to send and receive electrical signals from the enteral feeding pump with which the optical sensor is associated. Those skilled in the art will appreciate that optical sensors which function in such a manner are inexpensive and readily available. 
     Positioned to extend through the cavity  116  in the optical sensor  104  is a sample cell  130 . The sample cell  130  includes a sample cell wall  134  which defines a conduit  138 . In a presently preferred embodiment, the sample cell wall  134  defines a triangular conduit. Most preferably the sample cell wall  134  forms a conduit which, in cross-section, is an inverted equilateral triangle. The sample cell wall  134  also preferably forms a base  142  disposed at the bottom tip of the triangle. 
     Disposed adjacent to and extending along the sample cell  130  is a housing  150 . The housing  150  has a base portion  154  which preferably extends horizontally, a pair of sidewalls  158  which extend upwardly and outwardly, and an upper flange portion  162 . The respective sidewalls  158  of the housing  150  preferably are disposed in parallel to the respective side walls  134   a  and  134   b  of the sample cell wall  134 . Depending on the plastics used, the angle of the sidewalls  134   a,    134   b  and  158  will preferably be between 45 and 100 degrees. Most preferably, they are disposed at an angle of 60 degrees from one another and from a plane along which light would flow uninterrupted between the optical signal emitter  108  and the optical signal detector  112 . 
     The housing  150  and the sample cell  130  are spaced apart from one another and define an air chamber  170  therebetween. The housing  150  serves several purposes. First, the housing  150  prevents water or other liquids from getting into the cavity  116  where it could damage the optical sensor  104 . Second, the housing  150  is designed so that even if water were to fill the air chamber  170  between the housing and the sample cell  130 , the flow-path of light indicating the presence of liquid or air would be consistent. 
     Turning now to FIG. 2A, there is shown a cross-sectional view of the sample cell  130  disposed in cavity  116  between the optical emitter  108  and the optical detector  112  of the optical sensor  104 . The housing  150  is disposed in parallel and spaced away from the sample chamber  130  to form the air chamber  170 . 
     As shown, the sample cell  130  is preferably formed by a three part sample cell wall  134  defining a triangular conduit  138 . Most preferably, the conduit has a cross-section which is an inverted equilateral triangle. In such a configuration, the two sidewalls  134   a  and  134   b  of the sample cell wall  134  are offset from one another sixty degrees. The two sidewalls are also offset from the horizontal plane  2 A— 2 A at an angle of sixty degrees. The horizontal plane  2 A— 2 A represents the plane along which light passing directly from the optical signal emitter  108  to the optical signal detector  112  would pass. 
     It has been found that such an angular arrangement provides a light flow pattern which readily facilitates the determination if there is air in the sample cell  130  and the relative amount of air present. As will be explained in additional detail below, having the sidewalls disposed at corresponding angles between 47-70 degrees and preferably 60 degrees allows light to be conveniently refracted in one direction if the conduit has liquid therein, and in a second direction, back toward the plane  2 A— 2 A when the conduit is filled with air. 
     Disposed at the bottom of the sample cell  130  is a base portion  142  which extends horizontally (i.e. parallel with the plane  2 A— 2 A) a short distance, rather than forming a point similar to an inverted equilateral triangle. The base portion  142  allows a certain amount of light to pass through the sample cell  130  and to be received by the optical detector  112  regardless of whether there is air or water in the conduit  138  of the sample cell. This forms an inherent integrity check, as the lack of any detected signal will mean the optical emitter  108  is not working and the optical sensor  104  must be replaced. 
     The housing  150  is positioned to both protect the optical sensor  104  from being damaged by water, etc., and to assist in the flow of light through the sample cell  130 . As shown in FIG. 2B, the light from the optical emitter  108  refracts as it passes into and out of the housing  150 . It refracts again as it passes into the sample cell wall  134   a.    
     If water or some other translucent solution is present in the conduit  138 , the water refracts to a very small degree as it passes from the sample cell wall  134   a  into the liquid. This is because the refraction index of the plastic which forms the sample cell wall  134  and the refraction index of the liquid is very similar. 
     Due to the similarities in the indices of refraction, the majority of the light will follow a generally straight path through the sample cell  130  and will not return to the optical detector  112 . Of course, a small amount of the light will pass through the base portion  142  and will be detected by the optical detector  112 . 
     Turning now to FIG. 2C, there is shown a similar view to that of FIG. 2A, but with the conduit  138  having a large air bubble disposed therein. The path of the light passing through the base  142  is the same i.e. along plane  2 A— 2 A. The path of the remaining light is the same as FIG. 2A until it reaches the conduit  138 . Because air and plastic have significantly different indices of refraction, the light is bent so that it travels generally horizontally across the conduit  138 . When the light impacts the opposing portion of the sample cell wall  134   b,  the light is refracted downwardly. The light is again refracted as it enters and leaves the air chamber  170  and the housing  150 , and is directed to the optical detector  112 . The amount of light which is received by the optical detector  112  is roughly proportional to the size of the bubble in the conduit  138 . Thus, a small bubble will create a small voltage increase, while a large bubble will create a substantial voltage increase. By monitoring the voltage produced by the optical sensor  104 , the approximate size of the bubble can be determined. In practical application in an enteral feeding context however, the number of air present signals generated over a given time will generally be more germane. Thus, for example, ten air present signals will indicate that the solution has run dry or has a very high number of air bubbles. 
     Utilizing the configuration shown, a voltage reading of 0 indicates the sensor is malfunctioning, as some light should be passing through the base portion  142  of the sample cell  130 . A reading of 1 volt indicates that the sample cell is full of liquid and that the pump is delivering the indicated volume. 
     In contrast, a reading of 3-4 volts indicates that a much larger amount of light is being returned than expected, thereby indicating the presence of a bubble. Where the voltage falls within this range indicates the size of the bubble. A reading of 5 volts indicates that the sample cell  130  and housing  150  are not properly mounted in the optical sensor  104  and an alarm is sounded. 
     By monitoring the number of air bubble signals within a given amount of time, the pump with which the sensor is used can adjust to ensure that the volume of solution delivered to the patient is accurate. If the number of air bubble signals is too high, the pump may shut down and generate an alarm indicating that the solution has run dry or is not properly loaded. 
     Turning now to FIG. 2D, there is shown a cross-sectional view of the sample cell  130  having an opaque solution in the conduit  138 . Because the solution is opaque, the light from the optical signal emitter  108  is blocked by the solution in the conduit  138 , preventing it from being directed to the optical signal detector  112 . Substantially the only light which does reach the optical signal detector  112  is that passing through the base portion  142 . Thus, the optical signal detector  112  receives the same signal regardless of whether the sample cell  130  is filled with a transparent solution or an opaque solution. If a large bubble replaces the solution, whether transparent or opaque, the bubble will cause the optical detector  112  to detect more light and indicate the presence of the bubble. 
     Turning now to FIGS. 3 and 3A, there is shown, respectively, a bottom perspective view of an adaptor, generally indicated at  180 , and the adaptor  180  in conjunction with a feeding pump  190 . The adaptor  180  includes a sample cell  130  of the optical sensor system  104 . As shown in FIG. 3, the base portion  142  of the sample cell  130  is disposed on the bottom of the sample cell to allow some light to pass to the optical detector regardless of the contents of the sample cell. 
     The adaptor  180  enables the sample cell  130  to be conveniently mounted to the enteral feeding pump  190 . As explained in additional detail in U.S. patent application Ser. No. 09/836,851 (which is expressly incorporated herein), the pump generally includes a pair of channels  192  and  194  which receive two sides  180   a  and  180   b  of the adaptor along with a working portion  196  of an infusion set which is attached at opposing ends to a first connector  184  and a second connector  188 . 
     The sample cell  130  is formed in the first connector and is configured to rest in one channel  192  in the enteral feeding pump  190 . The housing  150  (FIGS. 1 through 2D) is typically formed as a wall of the channel  192  associated with the sample cell  130 . The optical emitter  108  and the optical detector  112  (not visible in FIG. 3A) are typically disposed on opposing sides of the channel  192  to function in the manner discussed above. 
     The adaptor  180  also includes an anti-freeflow device  212  configured for positioning in the other channel  194  of the pump  190 . Typically, the sample cell  130  and the housing  150  will be disposed upstream, while the anti-freeflow device  212  is typically disposed downstream from a rotor  204  which engages the working portion  196  of the infusion set and moves liquid through the sample cell  130  and past the anti-freeflow device  212  by a plurality of rollers  206  which compress the working portion. 
     The adaptor  180  enables an infusion set to be quickly loaded into an enteral feeding pump. As the adaptor  180  is pushed into place, the sample cell  130  is automatically positioned between the optical signal emitter  108  and optical signal detector  112  housed in the pump  190 . If the pump  190  forms the housing  150 , the sample cell  130  will also preferably be positioned away from the channel wall sufficiently to define the air chamber. 
     If the adaptor  180  is not properly loaded in the pump  190 , the amount of light received by the optical signal detector  112  will fall outside a predetermined range. Typically, the optical signal detector  112  will receive much more light than normal, thereby indicating the sample cell  130  is not in place. 
     Turning now to FIG. 4, there is shown, a cross-sectional view of a sample cell  230  made accordance with the principles of the present invention. While the sample cell  130  forming a conduit with a cross-section which is an inverted equilateral triangle is preferred, such a configuration is not required for the present invention to function properly. Thus, as shown in FIG. 4, the conduit  238  may have a cross-section which is diamond shaped. Because of the angled sidewalls  234   a  and  234   b,  are angled between 45 and 100 degrees, and preferably 60 degrees, from one another and are generally the same acute angle from the plane of the light emission, the light follows the same path as discussed above in FIGS. 2B and 2C, thereby enabling the sample cell  230  to be used with the same housing  150  and sensor  104  configuration discussed above. Thus, as shown in FIG. 4, the air bubble in the conduit  238  causes light to be refracted to the optical signal detector  112 . 
     One advantage of the configuration shown in FIG. 4 is that the sample cell  230  will allow a greater amount of solution to flow through the conduit due to its increase in size. Such a configuration, however, may increase the likelihood of a partially filled conduit passing some air which is undetected. 
     FIG. 5 shows a cross-sectional view of another embodiment of a sample cell  250  in accordance with the principles of the present invention. The sample cell  250  has a rounded top wall  254  to allow a greater amount of solution to flow through the conduit  258 , while keeping the sidewalls  254   a  and  254   b  disposed at an angle between about 45 and 100 degrees, and most preferably about 60 degrees, to provide the desired light refraction when air is present. 
     Because the most important aspects of the sample cell are the tangential configuration of the sidewalls and the acute angle from the plane of light transmission, those skilled in the art will appreciate that numerous other configurations could be used for forming the conduit. For example, the conduit could have a cross-sectional shape which forms an isosceles triangle, or could form a pentagon or some other shape. In order to properly refract light back to the optical emitter, however, the sidewalls should be separated by an angle between about 45 and 100 degrees, and should have a similar acute angle relative to the plane of light transmission. 
     Turning now to FIG. 6, there is shown a perspective view of yet another embodiment of a sample cell container, generally indicated at  300 , made in accordance with the principles of the present invention. The sample cell container  300  has an upper portion  304  and a lower portion  308  which are engageable one with another. The lower portion  308  has a channel  312  formed therein for receiving a flexible tube. The channel  312  preferably has a base portion  316  for receiving the bottom of the tube and a center portion  320  which is formed by two opposing sloped sidewalls which are preferably sloped downwardly and inwardly toward the base portion at an angle of between about 45 and 100 degrees from each other and with a similar acute angle, i.e. each has the same relative angle with respect to a horizontal plane so that the two would eventually intersect and have an angle between 45 and 100 degrees, and most preferably about 60 degrees. 
     The channel  312  also has a top portion  322  which is configured to receive a protrusion  330  disposed on the upper portion  304 . When the upper portion  304  is closed, as shown in FIG. 6A, the protrusion  330  extends down into the channel  312  to force the flexible tube disposed therein into contact with the central portion  320  and base portion  316 . This, in turn, causes the flexible tubing to conform to the shape of the channel and form a tube which has a shape similar to that of the sample cell  130  shown in FIGS. 1-2D. 
     Turning now to FIG. 6B, there is shown a cross-sectional view of the sample cell container  300  with a flexible tube  340  disposed therein. The protrusion  330  has forced the flexible tube  340  downwardly, wherein the tube substantially conforms to the shape of the channel  312  and has a base portion  342 , a central portion having two sidewalls  346   a  and  346   b  which are both disposed at the same angle from the horizontal plane, although in opposing directions, and a generally horizontal top portion  346   c.  In such a manner, the flexible tube  340  forms a sample cell which defines a conduit which is generally triangular. When disposed between an optical emitter and an optical detector, the sample cell formed by the flexible tube  340  will function in substantially the same manner as the sample cell  130  discussed above with respect to FIGS. 1 through 2D. 
     Those skilled in the art will appreciate that such a configuration is desirable because it allows a conventional infusion set of an enteral feeding pump to be adapted to provide a sample cell in accordance with the principles of the present invention without having to cut the tubing or add an adaptor having a sample cell. Additionally, the sample cell container  300  could be used repeatedly as infusion sets are replaced, thereby keeping cost to a minimum. 
     Turning now to FIGS. 7 and 7A, there is shown a perspective view and a cross-sectional view of a yet another embodiment of a sample cell of the present invention. The sample cell  350  forms a housing  354  having a rectangular cross-section, and a triangular conduit therethrough. As with the previous embodiments, the conduit  358  has a cross-section which is preferably an equilateral triangle, although other configurations, such as an isosceles triangle, diamond, pie, or other shape will work as well provided that the light refraction is disposed to facilitate different directions of light refraction depending on the contents of the conduit. 
     Those skilled in the art will appreciate that the configuration shown in FIGS. 7 and 7A are advantageous in that they can be used as a connector for conventional infusion sets for enteral feeding pumps to provide optical bubble detection at minimal cost. 
     While the embodiments discussed above having two sloped sidewalls disposed between 45 and 100 degrees, an in particular about 60 degrees, from one another are a preferred configuration for carrying out the present invention, it has been found that the angles can be much broader while still obtaining some of the benefits of the present invention. As shown in FIG. 8, there is shown a cross-sectional view of a sample cell, generally indicated at  400 . The sample cell  400  is: formed by a cell wall  404  which has a first sidewall  404   a  and the second sidewall  404   b.    
     An optical sensor system, generally indicated at  408  includes an optical emitter  412  and an optical detector  416 . The optical emitter  412  emits light (i.e. electromagnetic radiation) along a plane  8 A. The optical signal emitter  412  and the sample cell  400  are arranged so that the sidewall  404   a  Is disposed at an angle less than normal to the plane  8 A. As such, the sidewall  404   a  causes the light to refract as it contacts the outside of the sidewall. 
     If the sample cell  400  has clear liquid disposed therein, the light undergoes minimal refraction as it passes out of the sidewall  404   a,  through the liquid  424 , and through the opposing sidewall  404   b  as indicated by plane  8 A′. Thus, the light does not reach the optical signal detector  416 . Of course, if the liquid disposed in the sample cell  404  is opaque, the liquid will stop the light and prevent it from being received by the optical signal detector  416 —giving the same result as a clear liquid. 
     If an air bubble  424 ′ is disposed in the sample cell  400 , the light traveling along plane  8 A will be refracted both as it enters and exits the first sidewall  404   a,  and typically as it enters and exits the second sidewall  404   b,  so that the light follows plane  8 A″ and reaches the optical signal detector  416 . Thus, even using a relatively small angle in the sample cell wall  404 , the light can be directed to the optical signal sensor when air is present, and not when the sample cell is filled with liquid. 
     While the sample call  400  shown in FIG. 8 has a wall  404   a  which is less than normal and a wall which is disposed substantially normal to the plane  8 A, it will be appreciated that by moving the optical signal emitter  412  and/or the optical signal detector, a variety of different wall configurations, including two less than normal, could be used. 
     Turning now to FIG. 9, there is shown yet another embodiment of a sample cell, generally indicated at  450 . The sample cell  450  has a pair of sidewalls  450   a  and  450   b  which are disposed slightly off  90  degrees from the plane  9 A. The sidewalls  450   a  and  450   b  refract light in a manner similar to that discussed above. However, because the angle less than normal (i.e. less than 90 degrees) is so small, the optical signal emitter  462  and the optical signal detector  466  are spaced further from the sample cell  450 . 
     When the light traveling along plane  9 A impacts the sidewall  450   a  it is refracted. If relatively clear liquid is disposed in the sample cell  450 , the light follows a relatively straight line, plane  9 A′ and does not reach the optical signal detector  466 . If, however, a predetermined amount of air is present in the sample cell  450 , the light is refracted back toward the optical signal detector  466  as it passes from the first sidewall  450  into the conduit  458  in the sample cell, from the conduit into the second sidewall  454   b  and from the second sidewall back into the air between the sample cell and the optical signal detector. The distance of the optical signal emitter  462  and optical signal detector  466  from the sample cell  450  exaggerates the amount of refraction by providing more distance for the light to travel along the refracted plane. 
     Thus there is disclosed an improved optical bubble detector. Those skilled in the art will appreciate numerous modifications which can be made to the embodiments and methods discussed herein without departing from the scope and spirit of the present invention. The appended claims are intended to cover such modifications.