Method and Apparatus for Detecting Bubbles in a Liquid

This invention relates to sensing the presence of bubbles in a liquid, and a particular application thereof is the detection of air bubbles in serum or blood during infusion thereof into a patient. In such procedures it is important to insure that gas or air be prevented from being infused with the liquid, since air emboli can be extremely dangerous in a patient, particularly if the patient is very sick or is a premature or sick newborn.

Various instruments have been developed for preventing air bubbles from being infused into a patient along with a liquid, typical of which are those disclosed is U.S. Pat. Nos. 2,835,252, 3,812,842, 3,935,876 and 4,114,144. Such instruments typically provide an alarm and/or shut off mechanism which is activated when air bubbles are sensed in the infusion liquid being monitored. They may also provide readout means indicating the volume infused up to the cut-off point. Such prior instrumentation commonly suffers from unreliable operation because of low signal level, electronic drift, and/or difficult mechanical alignment.

The present invention avoids problems inherent in certain of the prior instruments primarily by virtue of the markedly increased signal response generated by a sensor when the presence of a bubble in the liquid being monitored is sensed thereby.

More particularly, the invention exploits the radiation absorptivity characteristics of the liquid being monitored to produce a markedly increased differential between the output of the sensor when a bubble is not sensed and the output thereof when a bubble is sensed, such increased differential being effected by directing radiation through the liquid and toward the sensor, and by limiting the radiation which can reach the sensor to wavelengths which are strongly absorbed by the liquid.

The preferred embodiment of the invention is particularly adapted for sensing the presence of air bubbles in an aqueous liquid such as serum or blood. Markedly increased signal response is produced as a result of limitation of the radiation which can reach the sensor to wavelengths above one micrometer, and preferably about 1.92 micrometers, water being strongly absorptive of radiation of such wavelengths.

Other objects, features and advantages of the invention will become apparent as the description proceeds, reference being had to the accompanying drawing.

In the drawing,

FIG. 1 is a diagrammatic illustration of the preferred form of the invention; and

FIG. 2 is a graphic logarithmic illustration of the absorption coefficient of water with respect to radiation having wavelengths of from 0.70 to 2.50 micrometers.

FIG. 1 diagrammatically illustrates apparatus for detecting bubbles in a liquid in accordance with the present invention. In FIG. 1 the numeral 5 indicates a tube shown in transverse cross-section, which tube is formed of radiation-transmissive material such as transparent glass or plastic. Tube 5 may form part of a system (not shown) for infusion of a liquid such as blood or serum into a patient and is adapted to contain the liquid 6 to be monitored for bubble content as it flows from a source to the patient.

Means is provided for directing radiation from a radiation source 7 through the liquid 6 in tube 5 and toward a radiation-responsive sensor 8. The radiation source 7 preferably takes the form of an incandescent lamp connected to a suitable source of electric power (not shown) and operated at a low color temperature, i.e., at about 1500.degree. K. As shown by the arrows, radiation from lamp 7 is focused on the bore of tube 5 by a lens 9 which may be carried by a tube 10.

Wide aperture radiation collection means 11, which may take the form of a fibre optic cyclinder or other suitable wide aperture optical system, is positioned on the opposite side of tube 5 from lamp 7 in alignment with the tube 10. Collection means 11 collects radiation from source 7 which has passed through the tube 5 and transmits such radiation longitudinally thereof toward the sensor 8.

Means is provided for limiting the radiation from the source 7 which can reach the sensor 8 to a predetermined wavelength which is strongly absorbed by the liquid 6. In the illustrated embodiment such means takes the form of a filter 12 which is interposed between the collection means 11 and the sensor 8.

The apparatus of the present invention may form a part of a system (not shown) having an electrical control circuit to which the sensor 8 is connected. Such system may have means responsive to an output signal from sensor 8 for cutting off the flow through tube 5 and for indicating the amount of liquid infused up to the cut-off point and/or for sounding an alarm when a bubble is present in the tube 5 and intersects the radiation path therethrough.

The inventive concept underlying the method and apparatus disclosed herein will be best understood with reference to FIG. 2 of the drawing. In FIG. 2 the absorption coefficient of water (.alpha., CM.sup.-1) with respect to radiation of wavelengths between 0.70 and 2.50 micrometers (.mu.) is plotted on a logarithmic scale. It will be observed that the curve developed is characterized by a series of peaks of dramatically increasing absorptivity for radiation of linearly increasing wavelengths. The following table sets forth the approximate wavelength/absorptivity relationships in the areas of the aforenoted peaks.

Prior art photodetector instruments in general utilize radiation of visible wavelengths and into the infra red range up to about 0.98.mu.. It will be observed that the absorption coefficient of water for radiation of a wavelength of 0.98.mu. is 0.45.alpha., CM-.sup.1. However, the absorption coefficient of water for radiation of longer wavelengths of 1.19, 1.45 and 1.92.mu. is respectively 2.22, 55.55 and 235.55 times the 0.45.alpha., CM.sup.-1 absorption coefficient of water for radiation of 0.98.mu. wavelength.

The present invention exploits the illustrated absorptivity characteristics to provide a dramatically increased differential between the response of the sensor 8 when a bubble is not present in the radiation path and the response thereof when a bubble is present in the radiation path. This is accomplished by the use of the filter 12 to limit the radiation which can reach the sensor 8 to that which is strongly absorbed by the liquid 6, i.e. radiation having a wavelength greater than one micrometer. The dramatically increased differential mentioned is reflected in a corresponding marked increase in signal response to sensor 8 when a bubble is present in the radiation path.

While the invention will produce substantially increased signal response by sensor 8 if filter 12 limits the radiation which can reach the sensor to any wavelength above 1.mu., including the peaks at 1.19, 1.45 and 1.92.mu., it is presently preferred to use as filter 12 a narrow bandpass interference filter which limits such radiation to that having a wavelength of 1.92.mu., i.e. the wavelength at which the strongest absorption occurs on FIG. 2. Wavelengths longer than 2.5.mu. can advantageously be used, but they are not preferred because of the requirement for special circuitry or other provision for physically cooling the sensor 8 when such wavelengths are used.

The sensor 8 selected preferably has peak responsivity at or near the wavelength of the radiation to be sensed thereby. More particularly, when a filter 12 is used which limits the radiation which can reach the sensor 8 to a wavelength of 1.92.mu., the preferred sensor 8 for use therewith is a lead sulfide thermal detector which has a response range of from about 1.2.mu. to about 2.5.mu. and a peak responsivity at about 2.0.mu..

Referring again to FIG. 1, the operation of the illustrated apparatus will now be described. With an aqueous liquid 6 to be monitored, such as serum or blood, flowing through the tube 5, radiation from the lamp 7 is focused on said liquid by the lens 9 and is partially absorbed by said liquid. The portion of such radiation which is not absorbed enters the radiation collector 11 and is transmitted to the filter 12 which permits only that portion of such radiation having a wavelength of 1.92.mu. to reach the sensor 8. As shown on FIG. 2, water absorbs such radiation very strongly, hence little such radiation reaches the sensor 8 and the output thereof is substantially zero.

If an air bubble passes through the tube 5, displacing the liquid 6 in the path of radiation from lamp 7 to sensor 8, only a negligible amount of such radiation is absorbed by such bubble. Such radiation passes through the tube 5 substantially undiminished, is collected by collector 11 and transmitted thereby to the filter 12. Since filter 12 permits only that portion of such radiation having a wavelength of 1.92.mu. to reach the sensor 8, the difference in absorptivity of radiation of that wavelength by air as compared to absorptivity thereof by water results in a very large differential in the signal response of sensor 8 as compared to that produced when a 0.98.mu. filter commonly used in prior art photodetectors is used.

The increased differential in signal response produced by the present invention can be illustrated by the following procedure. The sensor 8 of the apparatus illustrated in FIG. 1 is connected to the input of an amplifier (not shown) whose output is adjustable and is read in millivolts. The tube 5 is removed from the radiation path so that the sensor 8 is exposed to the maximum amount of radiation from lamp 7. The output of the amplifier is then adjusted arbitrarily to 100 millivolts, after which the tube 5 is repositioned in the radiation path as shown in FIG. 1. Because of reflection, refraction and/or absorption of radiation resulting from the repositioning of the tube 5, the radiation reaching the sensor 8 is reduced, and the sensor output is correspondingly reduced, so that the output of the amplifier drops to 90 millivolts. Since no liquid is present in the tube 5, this represents the response of the sensor 8 when an air bubble is present in the radiation path. Moreover, the output of the sensor is likely to be substantially the same in this situation if the filter 12 is one which limits the radiation which can reach the sensor 8 to 0.98.mu. wavelength or is one which limits such radiation to 1.92.mu. wavelength.

The tube 5 is then filled with water, and for the filter 12, a filter is used which limits the radiation which can reach the sensor 8 to that having a wavelength of 0.98.mu.. Since the absorptivity of water is only about 0.45.alpha., CM.sup.-1 at this wavelength (see FIG. 2), i.e. it is very low, the amount of radiation of 0.98.mu. wavelength reaching the sensor 8 is only very slightly reduced by water absorption so that the output of the amplifier drops to 88 millivolts.

A 1.92.mu. filter is then used as the filter 12 instead of the 0.98.mu. filter. Since the absorptivity of water at this wavelength is 106. .alpha., CM.sup.-1 (see FIG. 2), i.e. very strong, substantially all of the radiation of 1.92.mu. wavelength is absorbed by the water so that the output of the sensor 8 and of the amplifier drop to substantially zero.

From the foregoing illustration it is apparent that by limiting the radiation which can reach the sensor to that which is strongly absorbed by the liquid being monitored, the signal response of a photodetector to the presence of a bubble in the liquid can be dramatically increased. More specifically, it will be observed in the foregoing illustration that when a 0.98.mu. filter is used, the amplifier output when a bubble is present is only two millivolts higher than when liquid is present in the radiation path. In marked contrast to the minimal response produced when a 0.98.mu. filter is used, when a 1.92.mu. filter is used, the amplifier output when a bubble is present is 90 millivolts higher than when liquid is present in the radiation path. The signal response resulting from the practice of the present invention is thus about 45 times greater than that which is attained in prior art bubble sensing apparatus.

Having described one embodiment of the invention, it will be understood that various changes and modifications may be made in the disclosed embodiment without departing from the spirit of the invention, and all of such changes are contemplated as may come within the scope of the appended claims.