WO1989001796A1

WO1989001796A1 - Bubble detector

Info

Publication number: WO1989001796A1
Application number: PCT/GB1988/000702
Authority: WO
Inventors: Geoffrey Cross
Original assignee: Bellhouse Technology Ltd
Current assignee: Bellhouse Technology Ltd
Priority date: 1987-08-28
Filing date: 1988-08-26
Publication date: 1989-03-09
Anticipated expiration: 1990-02-28
Also published as: GB8720454D0

Abstract

In order to detect bubbles in a fluid flowing along a passageway (2), a portion (C) of the passageway (2) is formed with an elongate cross-section having parallel longer side walls (7). A first light path passes across the passageway portion (C) and a second light path not passing across the passageway is provided as a reference. When a bubble bigger than the gap between the side walls (7) of the passageway portion (C) passes into the passageway portion, the amount of light passing along the first light path increases and, if the ratio of light passing along the first light path to light passing along the second, reference light path exceeds a predetermined value, a bubble is deemed to have been detected.

Description

DESCRIPTION

BUBBLE DETECTOR

This invention relates to a detector for detecting bubbles in liquids, particularly blood.

Blood which is going to be introduced into a patient's body should not contain air bubbles big enough to be clinically significant because such bubbles are potentially lethal. Devices are known for detecting bubbles in liquids such as blood. For example, in US-3921622 and ϋS-4418565, a tube through which blood flows is sandwiched between a pair of ultrasonic transducers which detect bubbles in the blood from the change in acoustic impedence. Small bubbles in the middle of the blood flow and larger bubbles which span the flat faces of the squashed tube can be detected but it is difficult to arrange the device to detect only bubbles that are clinically significant and not to generate repeatedly alarms in response to insignificant bubbles.

In fact, mass production of bubble detectors using the ultrasonic technique is made difficult by the fact that there are variations of up to a factor of 10 in the characteristics of supposedly identical ultrasonic elements, owing to differences in resonant frequencies and differences between resonant and operating frequencies. Also, it is difficult to produce repeatedly the same coupling between the ultrasonic transducers and the tubing containing the blood, thereby leading to variations in radiated power, radiation pattern and resonant frequency. It is also difficult to produce an evenly radiated ultrasonic field throughout the cross-section of the tubing and to ensure uniform receiver sensitivity to the bubble irrespective of its location. O-86/04409 discloses the use of a light beam to detect the absence of liquid in a tube but, if a bubble big enough to displace the liquid level down beneath the level of the light beam should occur, then the device will also serve the purpose of being a bubble detector. In this particular instance, the bubble will have to occupy almost the entire cross-section of the tube, which represents the cross-section of the flowing liquid. However, bubbles considerably smaller than this may be clinically significant in blood.

It is an object to the present invention to provide a bubble detector which detects reliably all bubbles of a size which is significant but appreciably smaller than the cross-section of the flow of liquid and which does not involve time-consuming manufacturing techniques to make allowance for the variable characteristics of radiation emitters and detectors.

According to the present invention, a detector for detecting bubbles in a liquid comprises a passageway along which the liquid, in use, passes, the passageway including a portion having an elongate cross-section with parallel longer side walls; a first light path passing transversely across the portion of the passageway, between the parallel side walls; a second, reference light path not passing through the passageway; a light source for directing radiation along the first and second light paths; and detector means for detecting the radiation after it has been passed along the first and second light paths, whereby, when, in use, a bubble less absorbent of the radiation than the liquid and of diameter sufficient to fill the gap between the parallel side walls of the passageway portion passes along the passageway portion, there is an increase in the amount of radiation falling upon the detector means.

With this arrangment, the detector detects bubbles that are bigger than a predetermined size. In relation to blood, the size can be set to be the threshold value for clinically significant bubbles. Because the passageway portion has an elongate cross-section, with the bubbles only needing to straddle the small gap between the pair of parallel longer side walls, the bubbles do not have to be of the same size as the passageway itself.

The reference light path provides a datum amount of light falling on the detector means, against which transient increases of light passing along the first light path, through bubbles in the passageway portion, can be detected. When detecting bubbles by looking for ratio increases in the amount of light falling on the detector means, variations in the characteristics of the light source and the detector means due to the effects of temperature changes and component ageing will have no effect on the detection operation. For use with blood, the light source will preferably produce light in the green (550nm) to near infra-red (lOOOnm) region of the spectrum, so as to make use of the absorption characteristics of haemoglobin. As is known, an LED may conveniently be used to produce the radiation.

Preferably, the first light path is wide enough to span the gap between the shorter side walls of the passageway portion so as to ensure that the entire flow through the portion is monitored for bubbles. In this case, the detector means may also span the entire width between the shorter side walls.

Conveniently, the light source is positioned on one side of the passageway portion and the detector means on the other side. The detector means may be arranged to produce a single signal representing the total light intensity that has passed along the first and second paths. An initial signal value {b_ ) corresponding to light passing along only the second (b) path may be stored, and a current signal value (a_c+b_c) corresponding to light passing along the first (a) and second (b) light paths taken and the ratio (a_c+b_c) :bj calculated and compared with a predetermined value (e.g. 2) to determine whether a bubble is present in the passageway portion. If the spacing between the longer side walls of the passageway portion is set to that of the critical bubble diameter, then the predetermined value will need to be not much greater than unity. If, however, the gap size is set to be less than the critical bubble diameter, the value of a_c produced by a bubble of the critical size may be of the same order of magnitude as b_c (and b^, assuming the quantity of light passing along the second path has not changed substantially) and hence the predetermined value may be set at some value considerably larger than unity (e.g. 2, or any greater integral value) so as to avoid increases in the value of b_c and general system noise triggering false indications of the presence of a critically sized bubble. With regard to minimizing false triggering due to system noise, the amount of light passing along the second light path, and hence the resulting value of b_c, should be made sufficiently large as to swamp the intrinsic electrical background noise of the detector means.

If the light source is unfocused, then a cylindrical rod may be used as a.lens for focusing the radiation to bathe some or all of the gap between the shorter side walls of the passageway portion. The light source may be pulsed at a high frequency (e.g. at kHz frequencies, such as 125kHz and higher) so that the effect of natural and ambient light on the detector means may be filtered out electronically. The detector means may be one large collector, such as a solar cell, or a row of collectors. The detector may comprise a cell within which the passageway is located. The cell will therefore in use be plumbed into the piping through which the blood passes. The ratio of the longer side walls to the shorter side walls of the passageway portion will normally be no smaller than 3:1, although it is envisaged that lower limits of 5;1 or even 10:1 may be set. The shape of the passageway through the detector, and in particular the aspect ratio of the passageway portion, may be arranged so that the hydraulic resistance through the cell is matched to that of the piping with which it is to be used. An infusion needle is usually used to return blood to a donor, and hence the cell may conveniently be arranged to have approximately the same hydraulic resistance as this needle. In the direction of flow, the longer side walls of the passageway portion may be curved convexly, although, for ease of manufacture, they may be parallel to one another. Also for ease of manufacture, the shorter side walls joining the longer side walls may be curved concavely in the direction transverse to that of the blood flow. The second, reference light path may pass through a transparent portion of the cell itself. If it passes immediately adjacent to the passageway, the size of the detector means needed to detect the radiation from both light paths may be minimised.

The passageway may comprise a piece of piping. The passageway portion may comprise a portion of the piping squeezed to give the elongate cross-section with the parallel longer side walls. The ratio of the longer side walls to the shorter side walls of the passageway portion will normally be no smaller than 2:1, although limits such as 3:1, 5:1 or even 10:1 may be set.

Examples of detectors in accordance with the present invention will now be described with reference to the following drawings, in which:-

Figure 1 is a perspective view of a cell of a first example of detector;

Figure 2 is a horizontal section through the cell of Figure 1;

Figure 3 is a section on the line III-III of Figure 2;

Figure 4 is a diagramatic side view of the detector of Figure 1; Figure 5 is a diagramatic plan view of the detector of Figure 1;

Figure 6 is an enlarged view of part of Figure 2 showing a bubble of less than the critical size;

Figure 7 is a view similar to that of Figure 6 but showing a bubble of greater than the critical size;

Figures 8, 9 and 10 are respectively a sectional elevation, sectional plan and end view of a modified cell for the detector of Figure 1; Figures 11 and 12 are respectively a plan and side view of piping forming part of a second example of detector; and.

Figure 13 is a plan of a modified clamping arrangement for the piping of Figures 11 and 12. As shown in Figures 1 to 3, the first example of detector includes a cell 1 having a passageway 2 formed between a transparent body portion 3 and a transparent plate 4 sealed onto one face of the body portion. Blood enters the passageway 2 through an inlet adapter 5 and exits through an exit adapter 6.

Viewed in the direction of flow, side walls 7 of the passageway 2 initially taper towards one another, then run parallel to one another in a central portion C and finally diverge apart before reaching the exit adapter 6. Top and bottom walls 8 of the passageway 2 run parallel to one another in the direction of flow for the entire length of the passageway, but they may alternatively be arranged to diverge apart, run parallel in the central portion C and then taper together. This would help to give the passageway 2 a constant cross-section along its direction of flow and a faired inlet and outlet.

The central portion C of the passageway 2, as may been seen from Figure 3, has a very elongate cross-section and the side walls 7 are parallel - to one another both transversely and parallel to the direction of flow. The former ensures that the size of the gap through which the blood flows is constant (as viewed in Figure 3) at any height of the central portion C. This is essential for accurate sizing of bubbles in the blood. Opaque material 9 (shown in Figure 3, but of exaggerated thickness, for clarity) is provided between the body portion 3 and plate 4 in such a way as to leave, above and below the central portion C only a small window 10 of transparency. The window 10 may alternatively be provided by using an opaque layer outside or immediately adjacent to the cell 1 and having a slot therein corresponding in shape to the central portion C but also extending above or below, or both above and below, the portion C so as to define a transparent window or windows through the cell.

In Figures 4 and 5, the only parts of the cell 1 shown are the central portion C and window 10. The other components have been omitted for clarity. A non-lensed LED 11 produces a wide beam of radiation having wavelengths between 550 and 600 nm. In order to permit the detection of fast moving bubbles, the LED 11 is driven by circuitry (not shown) so as to flash at a rate of 125kHz, with detection circuitry filtering out the DC effects of ambient natural light and the mains frequency (50Hz) effects of artificial lighting. Light from the LED 11 is focused by a perspex cylindrical lens 12 onto the central portion C and window 10. The arrangement of components is such that the intensity of light varies by no more than ± 10% from the top of the central portion C to the bottom of the window 10. Light passing through the central portion C follows a first light path and light passing through the window 10 follows a second light path, both light paths beginning at the LED 11 and terminating on a detector 13 in the form of a large area solar cell. A screen 14 with a slot 15 therein is provided between the cell 1 and detector 13 so as to minimise the amount of light falling on the solar cell 13, other than that which has passed along the two light paths. By using one LED 11 and one detector 13, changes in their characteristics affect both light path channels equally. Thus, the provision of the second light path through the window 10 allows detection circuitry (not shown) to compensate for changes in characteristics affecting the first light path (e.g. temperature and ageing effects).

The solar cell 13 produces a single signal proportional to the total amount of light falling on it. This signal is processed by the external detection circuitry to determine when a critically sized bubble is present in the central portion C of the cell 1.

The central portion C has dimensions H, W and L (indicated in Figures 2 and 3) of respectively 8mm 0.3mm and 1mm. Spacing W of 0.3mm has been chosen to permit the detection of critically sized bubbles of diameter 0.5mm or greater. When a bubble of considerably less than this size passes through the central portion C, see Figure 6, the bubble does not bridge the gap between the side walls 7 and hence there is no increase in the amount of light passing along the first light path through the central portion. A bubble of 0.3mm or greater in diameter will bridge the gap and the amount of light passing along the first light path and falling on the detector 13 will increase. A bubble of 0.5mm diameter, illustrated in Figure 7, produces a circular detection window of diameter D^, even though the maximum diameter of the squashed bubble is D . The quantity of light that will pass through a window of diameter D_j can be calculated and the detector 13 set to use this as a threshold level above which the presence of a critically sized bubble is indicated.

The electronics (not shown) associated with the detector 13 can be arranged to store an initial signal value (b^) resulting from light passing along only the second light path. In use, when there are no bubbles larger in diameter than the gap w between the side walls 7, the current signal value outputed by the detector 13 will still substantially correspond to light passing only along the second light path, giving a current output signal value of b_c. If a bubble big enough to bridge the gap passes through the central portion, some light will reach the detector 13 from along the first light path to contribute a component a_c to the total current signal value of (a_c+b_c) . The detector electronics is arranged to calculate the ratio (a_c+b_c):bi. Assuming that no changes in the output characteristics of LED 11 or detector 13 have occured, b_c=bi. Thus, the ratio in question is (a_c+bj):bi. By calculating the value that a_c will reach when a bubble equal to the critical bubble size is present in the gap between the side walls 7 of the central portion C, a suitable value of the ratio of (a_c+bi) :bi can be determined and used as a predetermined threshold value which, if exceeded, indicates the presence of a bubble equal to or bigger than the critical size. In this case, an alarm may be triggered or other appropriate action taken.

The detector will treat a passageway passing just air as a bubble bigger than the critical size and hence sound the alarm. The modified cell shown in Figures 8, 9 and 10 is similar to the cell of Figures 1 to 3 and equivalent components have the same reference numerals. It differs by having the passageway 2 formed between two mirror-image transparent body parts 20 and inlet 21 and outlet 22 piping received directly in inlet 23 and outlet 24 bores respectively. The cell 1 is to be used with an independent slotted opaque layer of the type referred to previously so as to define two transparent windows 10. The dimensions W and L are 0.3mm and 1.5mm respectively.

Figures 11 to 13 illustrate how the dedicated cell 1 of Figures 1 to 10 may be dispensed with. Instead, piping 30 having a passageway 2 therein is squeezed by a clip 31 to produce a portion C in the passageway that is of elongate cross-section.

In Figures 11 and 12, the clip 31 includes a pair of identical side plates 32 having elongate slots 33. In Figure 13, the clip 31 is slightly different in that it includes a back plate 34 having an elongate slot 33 therein and a movable plate 35 having an elongate slot 33 therein.

A first light path 36 passes through a first one of the slots 33, through the portion C of the passageway 2 and then through the other of the slots 33. A second, reference light path (not shown)is separate from the piping 30 and clip 31. The piping 30 and clip 31 replaces the cell 1 of Figures 1 to 10 but otherwise operates in the same manner as described in relation to Figures 1 to 10 to detect bubbles B.

Claims

1. A detector for detecting bubbles comprising a passageway (2) along which the liquid, in use, passes, the passageway including a portion (C) having an elongate cross-section with parallel longer side walls (7); a first light path passing transversely across the portion (C) of the passageway, between the parallel side walls (7); a second, reference light path not passing through the passageway; a light source (11) for directing radiation along the first and second light paths; and detector means (13) for detecting the radiation after it has been passed along the first and second light paths, whereby, when, in use, a bubble less absorbent of the radiation than the liquid and of diameter sufficient to fill the gap between the parallel side walls (7) of the passageway portion (C) passes along the passageway portion (C) , there is an increase in the amount of radiation falling upon the detector means.

2. A detector according to claim 1, wherein the first light path and detector means (13) are wide enough to span the gap between the shorter side walls (8) of the passageway portion (C) , thereby to ensure that the entire flow through the portion is monitored for bubbles.

3. A detector according to claim 1 or claim 2, wherein the light source (11) is positioned on one side of the passageway portion (C) and the detector means (13) on the other side.

4. A detector according to any one of claims 1 to 3, wherein the detector means (13) is arranged to produce a single signal representing the total light intensity that has passed along the first and second paths.

5. A detector according to claim 4, wherein the detector means (13) is arranged to store an initial signal value ( i) corresponding to light passing along the second path, to produce a current signal value (a_c+b_c) corresponding to light passing along the first and second light paths, to calculate the ratio (a_c+b_c):bi and to compare the calculated ratio (a_c+b_c) :b£ with a predetermined value to determine whether a bubble is present in the passageway portion (C).

6. A detector according to any one of the preceding claims, wherein the light source (11) is unfocused and the detector futher comprises a cylindrical rod (12) arranged to focus the radiation on the passageway portion (C) .

7. A detector according to any one of the preceding claims, wherein the light source (11) is arranged to be pulsed at a high frequency.

8. A detector according to any one of the preceding claims, wherein the passageway (2) is located within a cell (1) .

9. A detector according to claim 8, wherein the second light path passes through a transparent portion (10) of the cell (1) .

10. A detector according to any one of claims 1 to 7, wherein the passageway (2) is located within a piece of piping (30) having a portion squeezed to produce the passageway portion (C) .