GB2065311A - Particle study device aperture cleaning systems and methods - Google Patents

Abstract

Residue left within the internal surfaces of the apertures of a particle study device (10) in which particles in suspension are passed through one or more microscopic apertures (14) to generate particle related signals for study of the particles in the suspension is cleared by applying energy through the apertures sufficient to boil the fluid therein. The energy is applied to the electrodes (28) (30) (32) of the device (10) in the form of a D.C. current at a constant magnitude level for a continuous predetermined length of time, generally between passages of the particle suspensions being monitored with the fluid suspension at rest. <IMAGE>

Description

SPECIFICATION Aperture cleaning systems and methods The invention relates generally to particle study devices having one or more apertures through which particles suspended in a fluid are passed for study and more particularly to cleaning the residue left by the particles and fluids on the internal walls of the aperture to eliminate a build up of foreign matter in the apertures which would effect the accuracy of the signals caused by the particles passing through the apertures.

A particular device for studying particles of microscopic size suspended in a fluid of electrolyte whose electrical impedance or resistivity is substantially different from that of the particles shown and described in U.S. Pat. No. 3,259,842.

The fluid is passed through a microscopic aperture formed in an insulating wall. Simultaneously, an electrical current is established in the aperture providing a sensing zone whose impedance is changed in proportion to the size of the particle passed through the zone. The change in impedance is detected and a signal is generated whose amplitude is proportional to the particle size and whose duration is equal to the time that it required for the particle to pass through the sensing zone.

The signals can be counted for any given volume of suspension passed through the zone to determine the particle concentration or the signals can be segregated according to size and number to determine the particle size distribution of the suspension. The particle information derived from the signals is utilized in hospitals and laboratories and it is extremely critical that the information be accurate. The devices are typically utilized in large complex apparatus whose operators do not have the time and/or may not have the expertise constantly to monitor and correct every phase of operation of the apparatus.

Various types of apparatus have been developed to overcome such problems as an aperture being blocked by particles or other problems which cause erroneous information to be developed. One device developed to overcome the problems presented in employing a single aperture device is shown in U.S. Pat. No.

3,444,463.

In this system the sample fluid is passed through three apertures simultaneously and separate respective detecting signals are developed in response to the particles passing through each of the three apertures. The signals developed by each aperture are compared and then by a process commonly known as "voting", should one of the signals developed be beyond a predetermined limit from the average of the other two signals, that aperture is considered to be malfunctioning and the signal from that aperture is ignored in the processing of information using such signals.

The probability of more than one aperture becoming blocked or otherwise significantly malfunctioning at the same time is remote. The reliability of the information received from the particle study is thus enhanced as a blocked or otherwise faulty aperture is ignored in the study.

A system utilizing the multiple apertures and voting as described above is disclosed in the U.S.

Pat. No. 3,549,994. This system was developed especially for use in the field of medicine and biology for studying body fluids. As is well known, body fluids such as blood are studied to obtain information to be used in the diagnosis and treatment of patients. The need for accuracy of this information is thus very critical.

Blood is composed of microscopic cells or particles suspended in a serum and various of these cells are important in the study of the blood.

Three types of blood cells may be of particular interest including red and white blood cells which are on the order of seven or more microns in size and platelets which may range from one to four microns in size.

In the systems utilizing multiple apertures and voting, the need for accuracy of the detecting signals developed by the passage of the particles through each aperture is extremely important. If the signals which are "voted" on the voting apparatus are not equal for the same sized particles for any reason, then the circuitry may vote out an aperture which is functioning normally. Even if the signal differences are not sufficient for one aperture to be voted out, the resultant data and information developed in the particular study will not be accurate.

The apertures in these and other particle study devices including those for industrial use, may range from 30 microns to 500 microns in diameter with a typical range of 50 to 100 microns for studying body fluids. The apertures generally are formed in wafers of sapphire or ruby and have typical manufacturing tolerances which may cause the residue buildup to be greater in one aperture than another of the same or different size.

Systems are available for detecting and clearing the complete or substantially complete blockage of an aperture in particle study devices.

One such circuit is shown and described in U.S.

Pat. No. 3,259,891. This patent shows several debris clearing devices which require either complex mechanical linkages in order to mechanically remove the aperture debris or the actual removal of the aperture and/or aperture tube to manually remove the debris. The mechanical linkages are somewhat difficult to utilize and are cumbersome in operation. In the case of actually removing, cleaning and replacing an aperture tube, time is consumed which is to be avoided in operating the study devices, especially in a structure with more than one aperture and furthermore this severely limits the throughput of the devices.Another debris clearing device shown in the patent employs a capacitor charged to a high potential which is discharged via the electrodes creating a very high initial current flow through the aperture, thereby literally heating the contents of the aperture to explode and driving the obstruction out of the aperture. The rate of application of energy from the capacitor is not optimum or uniform and when sufficient energy is utilized to clear a blockage it creates a serious threat of damage to the aperture material or aperture holding structure.

A second type of aperture clearing circuitry is shown in U.S. Pat. No. 3,963,984 which includes a pulse generator coupled to the electrode inside the aperture tube and to the electrode outside the particle tube in the fluid suspension. A pulse generator is coupled to the first and second electrodes and develops a combination of pulses having predetermined characteristics which are coupled to the electrodes and hence are coupled through the liquid contents of the aperture where they cause the liquid to vaporize and cause a microscopic explosion. Again, the force of the explosion is intended to be controlled to dislodge the debris without causing damage to the aperture or aperture structure; however, even at the optimum such an RF burst is highly energy wasteful. Furthermore, RF frequency applied to the aperture may cause a noise problem in the particle device itself.Further, it has been found that such a high frequency combination of pulses does not clean the internal surface of the aperture as completely as desired.

The particle signals may be significantly effected even though the apertures are not blocked. It therefore would be useful to maintain each aperture as clean as possible without wasting energy, deteriorating the aperture structure and so that the aperture does not become increasingly smaller as a number of particle fluid samples is passed therethrough Accordingly, the invention provides a method of aperture cleaning for a particle study device having at least one aperture through which particles in fluid suspension are passed for study: comprising the step of applying energy continuously at a substantially constant level for a predetermined period of time to the fluid in the aperture sufficient to boil the fluid therein.

Further, apparatus also is provided for the practice of the above method including a particle study device having at least one aperture for passage of fluid therethrough and through which particles in suspension are passed for study, comprising a power supply connected to apply energy continuously at a substantially constant level for a predetermined duration to the fluid in the aperture sufficient to boil the fluid therein.

The preferred embodiments of this invention now will be described, by way of example, with reference to the drawings accompanying this specification in which: Figure 1 is a partial block and schematic diagram of a particle study device including an aperture cleaning system embodying the invention; Figures 2A and 2B comprise a schematic diagram of one example of a cleaning energy generation circuit; and Figure 3 is a schematic diagram of a modified cleaning energy generation circuit.

Referring now to Figure 1, a particle study device is indicated generally at 10. The type of particle study device is not critical; however, each device of interest generally will include a particle analyzer 12 which is coupled to at least one aperture 14 through which the particles in suspension are passed. The structure containing the apertures also is not critical and can be a glass vessel or bath 1 6 into which a suspension of particles to be studied is moved.

For the proper operation of the device 10 and to enable accurate data or information to be obtained therefrom, it is essential that the amplitude of a signal produced for a given size particle be proportional to the size of that particle and, in addition, that the actual amplitude of that same signal be identical for all apertures in a device having a plurality of apertures if an identical particle had passed through each of the apertures.

Three aperture tubes 1 8, 20 and 22 are immersed in a main body 24 of the vessel 16. The aperture tubes are preferably mounted on a plate 26 which engages an upper entrance of the vessel 1 6 as a type of closure. Each of the aperture tubes 1 8, 20 and 22 has the microscopic aperture 14 in the bottom end thereof and individual electrodes 28, 30 and 32 internally of the respective aperture tubes. The individual electrodes enable individual detecting circuits each to be coupled to the body of the suspension contained in the respective aperture tubes. The vessel 10 includes an electrode 34 common to all of the electrodes in the respective aperture tubes which is coupled by a lead 36 to ground.Each of the tubes is coupled to individual leads 38, 40 and 42 extending from respective electrodes 28, 30 and 32 to individual electronic detecting circuits in the particle analyzer 12 through a switch 44.

Each suspension of particles to be studied can be admitted into the vessel 1 6 by means of a sample conduit 46. Each of the aperture tubes 18, 20 and 22 is coupled to a suitable pressure differential (not shown) so that the suspension is sucked into or pushed through all the aperture tubes simultaneously, through the respective apertures 14, when the particle studying device 10 is in operation.

A power supply 48 is coupled to each of the leads 38, 40 and 42 through the switch 44 and hence to the respective electrodes 28, 30 and 32.

In operation, electric current from the power supply passes through the respective apertures to the common electrode 34 and then to ground. The current passing through the suspension in each aperture will produce a volume of relatively high current density compared to the current density elsewhere in the suspension, thereby establishing the abovementioned sensing zones in the respective apertures 14 and their immediate vicinities.

Since the suspension is chosen to be of a substantially different impedance than the particles suspended therein, as the particles pass through each of the apertures they displace a finite amount of the suspension and hence change the impedance by a finite amount in each sensing zone. As previously mentioned, the signals thus produced are dependent not only upon the size of the particles passing through the apertures but upon the internal dimensions of the apertures 14 themselves.

The change in impedance by the particles passing therethrough can be detected in individual circuits in the particle analyzer 1 2 which is coupled to the respective apertures over respective leads 38, 40 and 42 through the switch 44. The particle analyzer 12 can be of the type described in U.S. Pat. No. 3,549,994. The variations in impedance in each sensing zone will cause an independent detecting signal to be developed indicative of the particle passing through the respective aperture or sensing zone.

The signals from the three sensing zones then can be compared by the voting circuitry as previously described if each is the same size. The resulting data or information derived from the signals from properly operating apertures can be displayed on a readout device (not shown) or otherwise processed and used.

In operation of the particle study device 10, a suspension or sample fluid containing particles is introduced into the body of the vessel 1 6 through the sample conduit 46. The suspension is then sucked or pushed through the apertures 14 of all three aperture tubes while simultaneously electric current is passing through the apertures from the power supply 48. As each particle from the main body of suspension passes through one of the apertures, the impedance in the respecting sensing zone will vary. This is detected by its individual detecting circuit in the particle analyzer 12 by means of the respective leads 38, 40 and 42. The amplitudes of the individual detecting signals caused by the particles passing through each of the sensing zones are then compared. The sensing signal amplitude generated should be substantially equal.Variations in the initial physical constructions of the apertures 14 can be balanced out in the particle analyzer 12 as described in U.S. Pat. No. 4,078,211.

Once a sufficient amount of suspension has been passed through each of the apertures, the solution is drained through a drain conduit 50 and is followed by a rinse solution introduced through a rinse conduit 52. Once the vessel has been thoroughly rinsed, the rinse solution is drained through the conduit 50 and the next sample is introduced through the conduit 46.

As previously mentioned, the particular type of structure in which the apertures 14 are mounted and the particle analyzer 1 2 is not critical; however each of the devices will include at least one aperture and will pass the particles in suspension therethrough for measuring as described above. In examining white cells, the red cells are lysed and in doing so their structure is destroyed releasing their internal chemicals and protein into the suspension which then is passed through each of the apertures. The fluid of the suspension or electrolyte itself also contains chemicals and the protein and chemicals may build up on the internal surfaces of the apertures 14. Further, as previously mentioned, the buildup may not be uniform in each of the apertures 14 which becomes a problem when there is more than a single aperture.As the buildup increases, the size of the aperture decreases and hence the signal will vary although the same size particle has been passed therethrough each time. The rinse solution does not eliminate this problem. As mentioned above the apertures may be cleaned physically or may be cleared by high bursts of energy applied to the apertures, but these are not without resulting or potential problems including loss of time and/or energy.

The apertures 14 generally being formed in members of sapphire or ruby which are then cemented or otherwise affixed to the aperture tubes or other aperture mounting structure are somewhat fragile. Applying too high a burst of energy such as a high frequency burst or a capacitor discharge may rupture the aperture itself or the bond between the aperture and the mounting structure which will result in erroneous readings until replacement of the aperture and/or aperture structure. It has been found that a constant energy level applied to the fluid contents in the aperture for a predetermined time will boil the fluid therein and clean the aperture without affecting the aperture material or structure. The constant energy will be a D.C. current applied through each aperture 14.

The switch 44 is coupled to a control 54 which switches an additional power supply in the supply 48 across each of the leads 38, 40 and 42 to pass the required D.C. current through the apertures to cause the aperture contents to boil between each of the cycles of passing the sample suspension through the apertures 14. Preferably, the switch 44 also switches the particle analyzer 12 off so that the higher energy supplied for the cleaning function will not damage the analyzer but if, as is generally the case, the analyzer 1 2 contains circuitry to protect against transients in the system, the analyzer may just remain coupled to the leads 38, 40 and 42 without regard to the cleaning power supplied. The cleaning power is preferably supplied each cycle during the rinse portion of the cycle between passing the sample suspensions through the apertures. The cleaning energy also can be supplied from a separate power supply.

One example of the power supply 48 and control 54 utilizes a separate power supply from the normal aperture excitation power supply and is illustrated in Figures 2A and 2B.

The power supply 48 has a pair of input leads 56 and 58 which are coupled between a source of AC power such as a standard (in U.S.A.) 110 volt 60 cycle supply and a full wave bridge rectifier 60.

The D.C. rectified voltage is coupled on a line 62 across a current limiting resistor 64 to a filter formed by a capacitor 66 and a resistor 68. The filtered voltage is coupled on a line 70 to a line voltage regulator including a resistor 72 and three zener diodes 74, 76 and 78. The regulated voltage output of the resistor 72 and diodes 74, 76 and 78 is preferably approximately one volt from the desired power supply output voltage. The approximate voltage is coupled to a resistor and capacitor noise filter consisting of a resistor 80 and a capacitor 82 and then is applied to the base of a gate regulator 84 and the collector of a control transistor 86.

The emitter of the control transistor 86 is coupled to a voltage dividing resistor 88 and an output line 90 on which is supplied the regulated power supply voltage coupled to the lines 38, 40 and 42 hence to the apertures, as previously described. The output on the line 90 is maintained very precisely by the control transistor 86 sensing at its base the voltage on a resistor 92 which changes the impedance seen by the gate regulator 84 as the load varies to maintain a constant output on the line 90. The output voltage is coupled across a last filter capacitor 94.

The control 54 includes a manual and an automatic trigger to generate the cleaning cycle control timing pulse on an output line 96 which switches the output voltage on the line 90 to the leads 38, 40 and 42. A manual switch 98 can be depressed, on operator demand, to provide the control timing pulse from a D.C. voltage supplied on a line 100 over a filter formed by a pair of capacitors 102 and 104. The D.C. voltage is coupled through a current limiting resistor 106 when the manual switch 98 is engaged. The control 54 is protected from back EMF by a diode 108. The automatic pulse control section of the control 54 receives a trigger pulse on a line 110 which can either be from the cleaning cycle control circuit or from the particle analyzer 12.The trigger pulse is applied on the line 110 following the passage of the sample suspension through the aperture or apertures 14 and can be applied between each cycle or at a predetermined frequency of cycles, again during the rinse portion of the operation.

The trigger pulse is applied through a line filter formed by a resistor 112 and a capacitor 114 to a timer 116. The timer 116 includes a resistor 118 and a capacitor 120 coupled to a D.C. power source to provide the R.C. time constant for the timer 11 6. The timer 11 6 generates a time pulse 122 of the proper duration on a line 124. A load resistor 126 is also coupled to the line 124 and the pulse 122 is passed through a current limiting resistor 128 to the base of a relay control transistor 130 which switches a relay in the switch 44 to couple the voltage output on the line 90 to the lines 38, 40 and 42.

It is preferable that the fluid flow be stopped when the cleaning power is applied to the apertures. The rinse cycle then is continued to flush any bubbles remaining from the boiling of the apertures as well as any debris boiled away from the surfaces thereof prior to running the next sample through the apertures 14. It is not necessary to stop the fluid flow, but would require an increased amount of energy to boil the fluid passing through the apertures. Further, although the cleaning pulse can be provided between each passage of sample through the aperture or apertures 14, this is not necessary and as stated before, can be provided at a predetermined frequency of cycles of sample passage. Again, it is preferable that cleaning be effected after each sample passage.The protein does not necessarily build up during each sample passage through each of the apertures 14, but the buildup varies and it essentially is impossible to predict how fast or on which passage of the sample a significant buildup will occur and in which aperture. Further, allowing minute buildups to occur each sample run makes it more difficult to boil away the residue on the walls. Therefore, it is preferable to utilize the cleaning pulse after each passage of sample fluid through the apertures 14.

An advantage in applying energy at a low continuous rate through the apertures 14 has been the heating not only of the aperture material, but of the surrounding structure at a slow rate decreasing the thermal gradient between the aperture material and the aperture mounting structure. This causes a virtually minimum amount of thermal shock to the aperture while still supplying sufficient energy to the volume of fluid or electrolyte in the aperture to cause the fluid to boil and cleanse the aperture.

In a specific example, the voltage applied on the line 90 is 65 volts which is applied with a timing pulse 122 of 2.5 seconds which applies approximately .813 watt seconds to each of the apertures 14 having a diameter of 50 to 100 microns. The power necessary to boil the fluid in the aperture most depends upon the volume of electrolyte or fluid which the aperture contains since this is the fluid which must be heated to a sufficient temperature to boil. The determination of the voltage on the line 90 and the duration of the pulse 122 are determined by the watt seconds necessary to boil the fluid in the apertures 14. The volume of fluid is the major factor, but the resistance and temperature of the fluid also should be taken into account.The example of 65 volts for 2.5 seconds is illustrative of one particular example and essentially the same power could be supplied by a pulse of 130 volts applied for 1.25 seconds.

The size range of the apertures of interest is 30 to 500 microns and although it is preferable to apply the power at a low rate, it generally is necessary to apply no less than .2 watt seconds.

The preferable range for a 50 to 100 micron aperture is .7 to 1.1 watt seconds, but again depends not only upon the aperture size but also upon the time interval between the sample passages through the aperture 14. Thus, in an automated instrument the time impulse 122 can be dependent upon the time allowed, between the passage of each sample through the apertures 14 by the study device 10 which is done in a repetitive cycle for numerous different sample suspensions.

A second embodiment of the invention employs a combined control and power supply circuit 1 56 and is illustrated in Figure 3. The separate power supply 48 and separate control 54 as well as the switch 44, are eliminated. In this embodiment the circuit 1 56 is not separately switched from the particle analyzer 12 and the increased voltage level cleaner pulse supplied to the lines 38, 40 and 42 and the apertures 14 is blocked by the internal protection circuit in the particle analyzer 1 2. This protection circuit can include various fiiters and neon protection circuits which will prevent the power cleaning pulse from damaging the circuitry in the particle analyzer 12.

The circuit 1 56 includes an input line 1 58 which receives a negative trigger pulse 1 60 whenever the cleaning cycle is initiated. The pulse 1 60 can be generated automatically following the end of each sample suspension passed through the apertures 14, during the rinse cycle of the particle study device 10, while the flow is stopped in the apertures 14 to conserve energy. The pulse 1 60 also can be manually generated. The rinse cycle then is continued to flush any bubbles remaining from the boiling of the apertures as well as any debris boiled away from the surfaces thereof prior to running the next sample through the apertures 14.As with the earlier described embodiment of Figure 2, the cleaning pulse 1 60 can be applied each time the rinse cycle occurs or once every predetermined number of sample cycles. Therefore, in order to maintain the cleanliness and most accurate aperture surfaces in the apertures 14 and application of the pulse 1 60 for each rinse cycle is preferable.

The trigger pulse 1 60 is coupled through a current and power limiting resistor 1 62 to the base of an inverting transistor 1 64. A line 1 66 couples the operating power for the control 1 56, such as a D.C. voltage, through a load resistor 1 68 to the collector of the transistor 1 64.The negative going trigger pulse 60 produces a positive pulse 1 70 at the collector of the transistor 1 64. The pulse 170 is coupled on a line 172 over an isolation capacitor 1 74 to a timer 1 76. The isolation capacitor 174, a diode 1 78 which clips the positive portion of the pulse and a load resistor 180 generate a negative trigger pulse 1 82 from the trailing edge of the pulse 170. The pulse 182 is coupled to the timer 1 76 to trigger the operation of the timer 176.

The timer 1 76 generates a specific duration pulse 1 84 which operates both as a control and a switching pulse. The timing of the timer 1 76 is controlled by a fixed resistor 186, a capacitor 1 88 and a trimmer resistor or a potentiometer 1 90.

The timer also includes a bypass capacitor 1 92.

The duration pulse 184 is coupled on a line 1 94 through a current limiting resistor 1 96 to the base of a transistor 198 which inverts the duration pulse 1 84 to switch or toggle a pair of transistors 200 and 202. The collector of the transistor 1 98 also is coupled to the power line 66 through a load resistor 204. The negative pulse from the transistor 1 98 is coupled on a line 206 to the base of the transistor 200. The collector of the transistor 200 is coupled to a fixed D.C. supply voltage of, for example 300 volts, supplied on a line 208 through a load resistor 210. The collector also is coupled by a line 212 to the base of the transistor 202. The collector of the transistor 202 is coupled to the line 208 through a load resistor 214.

The leading edge of the pulse from the transistor 1 98 turns on the transistor 200, which then turns on the transistor 202 to couple the voltage on line 208 to a voltage dividing circuit on a line 21 6. The voltage dividing circuit includes a pair of resistors 218 and 220 connected to an output line 222. The collector of the transistor 200 is protected by a blocking diode 224. The transistor 200 remains on for the duration of the pulse from the transistor 1 98 which maintains the transistor 202 conducting which produces the cleaning pulse on the output line 222 for the duration set by the timer 1 76. In this case utilizing a 300 volt source on the line 208, a 40 millisecond pulse 1 84 is utilized.This generates a cleaning pulse of 300 volts having a duration of 40 milliseconds on the output line 222 which is coupled to the leads 38, 40 and 42 and hence the apertures 14.

The determination of the voltage on the line 208 and the duration of the pulse 180 are determined by the watt seconds necessary to boil the fluid in the apertures 14. This is in the main part determined by the size of the aperture in the range from 30 to 500 microns since the major determining factor is the volume and resistance of fluid in the aperture. A minor factor to be taken into consideration is the temperature of the fluid in the aperture which also will affect the amount of energy necessary to boil the fluid. Generating 300 volts for 40 milliseconds results in a power or energy applied to the apertures of approximately .27 watt seconds, since some of the power is dissipated in the protection circuits in the particle analyzer 12.This figure is chosen for the particular device 10 involved and could be any other combination to provide the necessary watt seconds to boil the fluid in the apertures 14. For instance, instead of 300 volts for 40 milliseconds, a 400 volt pulse could be applied for 30 milliseconds to apply essentially the same power to the apertures 1 4.

Thus we note that there are two effective ways to effect clearing boiling of the debris which may be in the aperture; first, to apply a low magnitude constant current for a longer duration than the suggested duration of application where the other embodiment consists of employing a short duration higher level (voltage) D.C. current. The basis requirement is to provide enough continuous energy to bring the liquid within the aperture to a boil, that is not instantaneously like vaporization but with the production of bubbles.

Other modifications and variations of the present invention within the scope of the appended claims. For example, one can provide a resistor in parallel with the normal excitation load current resistance which can be switched into the load circuit to increase the voltage for the cleaning cycle.

Claims (27)

1. A method of aperture cleaning for a particle study device having at least one aperture through which particles in fluid suspension are passed for study, said method comprising: applying energy continuously at a substantially constant level for a predetermined period of time to the fluid in the aperture, the energy being sufficient to boil the fluid therein.

2. A method according to claim 1 in which the energy applied is greater than two tenths watt seconds.

3. A method according to claim 1 in which the energy applied is less than two tenths watt seconds.

4. A method according to any one of claims 1, 2 or 3 in which the movement of the fluid through the aperture is materially reduced while applying said energy to the fluid to conserve cleaning energy applied to the aperture.

5. A method according to claim 1 in which the energy is applied by passing a substantially constant D.C. current through said aperture.

6. A method according to claim 5 in which the D.C. current is supplied at a low constant rate.

7. A method according to claim 5 in which the D.C. current is applied at less than three hundred volts for greater than one second.

8. A method according to claims 5 or 6 in which the D.C. current is applied at greater than 300V for less than one second.

9. A method according to any one of claims 1 to 8 in which different samples of particles in suspension cyclically are passed for study through the aperture, and the energy is applied to boil the fluid therein between cycles of passing said particle suspensions through said aperture.

10. A method according to any one of claims 1 to 9 in which the movement of said fluid through said aperture is stopped while applying said energy.

11. A method according to claim 9 in which the energy is applied in the form of a substantially constant D.C. current through each of said apertures.

12. An aperture cleaning system including a particle study device having at least one aperture for passage of fluid therethrough and through which particles in suspension are passed for study, characterized by a power supply connected to apply energy continuously at a substantially constant level for a predetermined duration to the fluid in the aperture, the energy being sufficient to boil the fluid therein.

13. An apparatus according to claim 12 in which the applied energy is in the range of seven tenths to one and one tenth watt seconds.

14. An apparatus according to claim 12 in which said power supply energy is greater than two tenths watt seconds.

15. An apparatus according to any one of claims 12 to 14 further including means for materially reducing the movement of said fluid through said aperture while applying said energy to said fluid.

16. An apparatus according to any one of claims 12 to 15 wherein said power supply includes circuitry capable of directing a substantially constant magnitude D.C. current through said aperture.

17. An apparatus according to claim 12 in which the energy is applied in the form of D.C.

current supplied at a low constant rate.

1 8. An apparatus according to claim 12 in which the energy is applied in the form of D.C.

current less than three hundred volts for greater than one second.

19. An apparatus according to claim 12 in which the energy is applied in the form of D.C.

current at least at three hundred volts for less than one second.

20. An apparatus according to any one of claims 12 to 1 9 including means for cyclically passing different samples of particles in suspension for study through the aperture and means for applying the energy to boil the fluid therein between cycles of passing said particle suspensions through said aperture.

21. The apparatus according to claim 20 wherein flow of fluid through said aperture is stopped during application of said energy.

22. A method of aperture cleaning for a particle study device having at least one aperture through which particles in fluid suspension are passed for study, the method being substantially as herein described with reference to and as illustrated by the accompanying drawings.

23. A method according to claim 22 substantially as herein described with reference to and as illustrated by Figures 1, 2A and 2B of the accompanying drawings.

24. A method according to claim 22 substantially as herein described with reference to and as illustrated by Figures 1 and 3 of the accompanying drawings.

25. An aperture cleaning system including a particle study device having at least one aperture for passage of fluid therethrough and through which particles in suspension are passed for study, the system being substantially as herein described with reference to and as illustrated by the accompanying drawings.

26. An apparatus according to claim 25 substantially as herein described with reference to and as illustrated by Figures 1, 2A and 28 of the accompanying drawings.

27. An apparatus according to claim 25 substantially as herein described with reference to and as illustrated by Figures 1 and 3 of the accompanying drawings.