CA1163295A - Aperture cleaning system - Google Patents

Abstract

ABSTRACT

A cleaning system fox clearing residue left within the internal surfaces of the apertures of a particle study device in which particles in suspension are passed through one or more microscopic apertures to generate particle related signals for study of the particles in the suspension by applying thereto energy through the apertures sufficient to boil the fluid therein. The energy is applied in the form of a low magnitude D.C.
current at a constant level for a continuous predetermined length of time, the energy applied being less than two tenths watt seconds.

Description

l 1~329$
The ~nyentio~ xelates ~enexally to p~rticle study devices having one or more ~pertures through which particles suspended in a fluid are passed for study and mo~e particularl~ to cleanin~ the residue left b~v the pa~t~cles and fluids on th.e internal ~alls of the apexture to eli~inate a build up of foreign m3tter in the apertures which would ef~ect the accuracy o~ the signals caused by the PArticles passin~ throu~h the ape~tures~
A particular device for studyin~ particles of microscopic size suspended in a fluid of electrolyte whose electrical impedance or resistivity is substanti~ different from that of the paricles. shown and described in U.,~s Pat, No~ 3,259~842. The flui~d is passed through.'a miCxoscopic aperture formed in an ~nsulating wall, ~ulta,neousl~ ~n electrical cu~rent is eStablished in the a,pertu.re providing a sensing zone whose ~mpedance is changed ~n pro~ortion to the size of the particle ~assed through the æone~ 'The change in impedance ~s detected ~nd a si~nal ~s genexated ~hose ~mplitude i~ px4po~tional to the particle size and whose' durat~on is e~ual to th.e t~ç that it ~equ~red ~o~ thç
p~rticle to ~ss throu~h the ,sensing zo~e~
The s~nals can ~e counted ~or any ~iYe~/volume o~ suspension passed th~ough the zone to dete~,ne the particle concentration or the sign~ls c~n be segxe~ated accordin~ to size and number to determine the ~article size distribution of the suspension.. Thç p~xticle ~nfo~m~t~on derived ~rom the 5ignals is utllized in hospit~
laboratorie$ and it is extremely cri,tic~l t~t the information be accurate. The deyices are t~ypicall~
utilized in large compl~x apparatus whose opera~tors do ~ 2 ~ 1~329S
not have the time and~or ma~ not hAve the expext~se constantly to monitor and coxrect every ph~se o~ ope~tion of the apparatus.
V~rious types o~ appaxatus ha~e been deYeloped to overcome such problem,S a~ an apertu~e ~e~ng blocked by particles oX other problems which cause e~roneous in~ormation to be developed. One dev~ce developed to overcome the problems presented in emplo~ing a single apexture device is shown in UeS, Pat~ No, 3~444,463, In this system the sam~le ~luid is. passed throu~h three apertures simultaneously and sep~ate reSpective detecting si~n~ls are develo~ed ~n response to the p~rticles passin~ through each o~ the three a~ertures, The s~nals developed b~ each aperture ~e co~p~rea and then bY a p~ocess commonly known as ~Vo~.~n~ should one o.~ the signAls deyeloped be beyond a predete~ined limit ~rom the aVerage o~ the othex tWo Signals~ that apertuxe is conside~ed to be ~alfunction'in~ and the signAl ~om th~t aperture'is ignoxed in the proces~s~ng ~ in~oxmation using such'~s~gnals.
The pxobab~l~t~ o,f, ~oxe than one apertuxe becoming blocked or othe~ise ,signiflc~ntl~ ~a,l~unctionin~
~t the s~e ti~e I~S xe~ote~ The rel~Ability o~ the in~or~ation ~eceiyed ~xo~ the p~rticle study is thus enh~nced as a blocked o~ othe~wise faulty apexture ~s ignored in the stud~r, ~ s~stem utilizing the multiple ~pertu~es And ~oting AS desc~ibed ~bVe is disclosed in the U, ~r P~t. No, 3,549,994. This s~stem w~s developed es~ec~
~ox use in th.e field o~ medicine and biol~y fo~ stud~ng body ,~lu~ds, As is well known, bod~ fluids such ~s blood l 163295 are studied to obtain information to be used in the diagnosis and treatment o~ patients. The need for accuracy of this ln~ormation is thus ~ery critical Blood is composed of microscopic cells or .5 particles suspended in a serum and ~arious o,~ these cells are important in the stud~ of the blood, Th~ee t~pes of blood cells may be o~ part~cular interest lnclud2n~ ~ed and white blood cells which are on the order o~ seven or more mic~ons in size and platelets wh~ch:~y ran~e ~o~
one to ~oux microns in size5 In the s~stems utilizing ~ult~ple ,a~extures and votin~, the need for accur~cy o~ the detect~ng s~gn~l~
developed by the passage of the particles' through e~ch apeXtu~e ~s ext~emely ~mpoxtant. If the s~gnals ~h~ch ~xe "voted" on the V~t~ng ap~aratus a~e not e~u~l ~o~ the same s~zed particles ~ox any Fe~son~ then the cixcu~tr~ ~ay yQte ~ut an ~exture which i5 functionin~ nvxmall~, ~ven ~f the s~nal d~fexence~ a~e not suf~c~ent fox'one ~perture to be voted out~ the ~e~ult~nt d~ta and in~ormation developed ~n the pa~ticula~ study w~ o~ ~e accu~ate.
The ~pertu~e~ in these ~nd othex p~tl,c~e S~ud~
devices includin~ those $oF industxial u~er ~a~ x~n~e ~xom 30 ~icrons to 500 ~cXon~ i~ di~eter ~th'~ typ~c~l ~an~e of 50 to 100 ~ic~ons ~ox stud~ing bod~ fluids, The ~extues generally are'~o~ed ~n w~fe~s of sa~phire QX xub~ ~nd h~e t~p2ca1 manu~actuXin~ tole~ances which ~y c~use'the residue buildup to be ~eater in one apextuxe than ~,nothe~
of the same ox di~exent s~ze~

~ 163295 S~stems aXe ~yail,a,ble ~OX deteçt~ng ~nd cle~rin~
the complete or subs~antially complete blockage of an aperture in particle stud~ devices, ~ne such circuit is shown and described in U.$, Pat. No. 3,259~891. This pa,tent shows sevexal debxis clearing devices ~hich re~ui~e either complex mechanical linkages in order to mechanically remove the aperture debris QX the actual xemoval of the ~erture and/or aperture tube to manually remoye the debris, The ~echanical l~nka~es a~re so~ewhat d~fi,cult to utilize and are cumberso,me in operation, ~n the case of actually removing, cleaning and replacing an aperture tube~
t~me is consum,ed whlch ~s to be avoided ~n o~era,ting the study devices~ es~ecially ~n a ~tructure w~th moxe than one apertuXe and ~urthe~o~e th~s seYe~el~ limits the throuyh~
put o~ the deviçes, Another debris clear~n~ device shown in the p~tent employs a ca~citor chaFged to ~ high potentia wh~ch is discha,rged Yi~ the electrQdes creatin~ a ye~ h~gh lnitial current flow throu~h the ~perture, the~eb~ l~texally heating the contents o~ the ape~ture'to explode and driv~ng the obstructlQn out o~ the apertuxe, The rate of appli,cation o~ ener~y ~ro~ the capqc~toX i~ not Q~tim~m X un~for~ and ~hen suf~c~ent enexgy ~S utilized to clear a ~lockage Xt cre~tes a se~ious thre~t of dam~e to the ~perture material or aperture holding structure.
A second type o~ aperture clearl,n~ c~rcuitry ~s shown in U.S. Pat. No. 3,963~984 which includes a, pulse generator coupled to the elect~ode ~nside the apertu~e tube and to the electrode outside the particle tube ~,~ the ~luid suspension. A pulse gener~tor is coupled to the ~irst ~nd second electrodes and develops a com~in~tion of pulses J 16329~

havin~ predetermined char~cteri~tics which are coupled to the electrodes and hence are coupled through the l~quid contents of the aperture where they c~use the li~uid to yapoFize and cause a microscopic explosion. ~gain, the fo~ce of the explosion ~s intended to be controlled to dislodge the debris without ca,u~ing dama~e to the apertuxe OX aperture structure~ howe~ex~ eVen at the opt~mum such an P~ burst ~9 highIy enex~y wasteful.
Furthermore~ RF fre~uenc~v a~plied to the apertu~e may cause a noise problem ~n the'partlcle device ~tself. Further~ ~t h~s been found th~t such'a high frequenc~ combin~tion of pulses does not clean the ~,n~e~nal souxfa,ce of the ~pe~tuXe ~s completeI~v as desixed.
The particle si~n~ls ~ay be s~gn~ficantl~v effected even thou~h'the'~Rextuxe~s ~e ~ot'blocked~
It therefoxe would be sueful to m~intain e~ch apertuxe ~s clean ~s pQ~sible w~thout w~st~ng enexg~ dete~orating the ~pe~tuXe structure and so that the ~pertu~e does not become incxeasin s~aller as a number of p~xt~cle fluld s~mples ~S
passed therethrou~h.

~ 6 1 1632g~

~ ccordingly, the invention provides a method of aperture cleaning for a particl~ study device havlng at least one apexture through which particles in fluid suspension are passed'for study, the step of applying energy continuously at a substantially constant level for a pxedetermined period of time to the fluid in the aperture sufficient to boil the ~luid therein, the ap~lied energy bein~
less than two tenths ~att seconds~
Further, apparatus also is provided for the practice o~ the above method.
The preferred embodiments of this inVention now will be described by'way of example, with reference to the drawings accompanying this speciflcation in which:
Figure 1 is a partial block and schematic diagram of a pa,rticle study device embodying the aperture cleaning system o~ the in~ention, and ~ igures ~.~ and 2B comxise t~gethe~ ~
sche~atic diagram of a cleaning energy generation circuit according to the invention.

1lfi32g5 Referxing noW to Fi~ure 1, the particle study device is indicated ~enerall~ at 10. The t~pe o~
particle study device is not critical; however~ each device of interest generally will include a particle analyzer 12 which is cou~led to ~,t least one aperture 14 through which the paxt~cles in suSpension are passed. The structure containing the apertures also is not cr1,tic~1 and may be a glass vessel or b~th 16 into Which a suspension of particles to be studied is moved, Fo~ the propeX opex~tion of the deyice 10 and to enable accurate data OX infoxmation to be o~ta~ned '. therefxom, it is essent~al that the emplitude o~ ~
si~nal p~oduced ~OX a given s~ze particle be ~ropoxt~on~l to the s~ze o~ that p~xticle And, 1~ addit~on, that the ~ctu~l ~m~litude o~ th.at same sign~l be identlcal ~or . all apertuXes in a deyice h~ying ~ plux~lity o~ ape~ture~s.
if an identical particle had passed thxou~h each'of the a~ertures, Three ape~tu~e tubes 18, 20 a~d 22 are immex~ed in a ~ain body 24 o~ the vessel 16, The ape~tu~e tubes may be mounted on ~ pl~te 26 Which. engages an uppe~ ent~nce o~ the.~essel 16 as a, type o~ closuxe~ Each o~ the aperture tubes 18~ 20 and 22 has the m~crosco~ic ape,rture 14 in the bottom end thexeof and individu~l electrodes 28, 30 and 32 inte~nall~ o~ the respective apertu~e tubes, The individu~l eIectrodes en~ble indiv~du~l detect~ng cirCuits each to be coupled to the bod~ o~ the "suspension contained in the respectiYe ~perture tube~ The yessel 10 includes an electrode 34 common to all of the elect~ode~
.tn the respective ape~tuxe tubes which is cou~led b~

~ 8 --~ 1632~5 lead 36 to g~ound ~ach of the tube~ is coupled to individual leads 38, 40 ~nd 42 extending from respectiVe electrodes 28, 30 and 32 to individual electronic detectin~ clrcuits ln the pa~ticle analyzer 12 throu~h a switch 44~
Each sUspension o~ particles to be studi.ed ~ay be admitted into the vessel 16 by means of a sa~ple conduit 46~ ~ach of the ape~ture tubes 18~ 20 and 22 is coupled to a suitable Pressure di~ferent~al (not sh.ownl so that the suspension is sucked into o~ pushed th~o.-u~h all the apertu~e tubes si~ultaneously~ throu~h the respecti~e'apertures 14, when the particle study~n~
dev~ce 10 ~s in o~e~t~on~
A power supoly 48 is coupled to each of the lea,ds 38, 40 and 42 thxou~h the ~s~itch'44 and hence to the ~es~ectiye electrodes 28~ 30 ~nd 32, In operat~on~
eIectric current fxom the power suppl~ passes throu'~h the xespectlve apextures to the common eIectrode 34 and then to ground. The cU~rent p~ssin~ thr!ou~h'the suspension in each apextuxe will produce a volume o~ relat~velnY hi~h curXent densit*Y comp~xed to the current densit~v else~here in the suspension, thereb~ establishin~ the'aboYe.-mentioned sensin~ zones in the respectiVe apertures 14 and their immediate v~c~nitl.es Sin~e the suspens~ion is cho~en to be o~ a substantiall~ different ~mpedance than the p~t~cles suspended the~eln, as the p~ticle~ pass th.~ou~h each o~
the apertures they displace a ~inite a,~ount of ~h.e suspension and hence chan~e'the impedance by a ~x~te amount in each sensin~ zone~ As previousl~ menti.oned, ~ 16329~
the signals thus P~oduçed a,xe dependent not onl~ upon the - size of the particles passing thxou~h the'apertures hut upon the internal dimentions o~ the apextures 14 themselves.
The change i,n impedance by the'particles passin~ therethrou~h ~a~ be detected in ind~Y~dual circuits in the particle analyzer 12 wh~ch'~.s coupled to the respective apertures oyex respec~Ive le~ds 38, 4Q and 42 through the switch 44, The particle analyzex 12 ma~ be of the type described in U,S~ Pat. No. 3,549~994, The variations in impedance in e~ch sensing zone ~11 cause an independent detecting signal to ~e deyeloped indicative o~ the particle passin~ through.the respectiYe apexture or sensin~ æone, The s~nals ~rom the three sensing zones then may be co~pared by the ~otin~ circuit as previously described if each is the same s'.~ze, The resulting data or in~ormation derived from the $i~nals ~rom propexlv opeXa,ting a,~ertures may be d~splayed on a readout device (not shown) or otherwise ~rocessed ~d used.
ln ope~ati,on o$ the particle stud~ dev~çe lQ, a suspension or sample :Elu~d containin~ pa~t~cles ~S
introduced into the body o~ the vessel 16 thxough ~he sample conduit 46~ The suspension is then sucked or pushed through the apertures 14 of all ~h~ee a~erture tubes while simultaneousl~ electric current ~s pass~ng through the apertures from the powex suppl~ 48, As each particle fro~ the ~ain body o$ suspenslon pas~ses throu~h one of the apertures~ the impedance in the respect~ng sensing zone will va,ry~ Th~s is detected hy its individua l 163295 detectin~ circuit ~n the Raxticle analyze~ 12 b~ means o~
the respective leads 3B, 40 and 42~ The amplitudes of the individual detecting s~gnals caused by the paxticles pass~ng through each o~ the sensin~ zones are then compared The sensiny signal amplitude ~enerated should be substantially equal. Variations in the initial physical constructions of the apertures 14 may be balanced out in the particle analyzer 12 as described ln U.S. Pat. No. 4,078,211.
Once a sufficient amount of suspension has been passed through each of the ape~tures, the solution ma~ be drained through a drain conduit 50 and may be followed by a rinse solution introduced throu~h a rinse conduit 52.
Once the vessel has been thorvu~hly rinsed~ the rinse solution may be drained through the conduit 50 and the next sample may be ~nt~oduced throu~h-the conduit 46.
As previousl~ ment~oned~ the p~rticular type of structure in which the ape~tures 14 are mounted and the p~rticle analyzer 12 is n~t cr~tical; ho~ever, each o~ the deyices will include at least one aperture and will pas~
the particles in suspension therethrough ~or measurin~ as described above~ In examining white cells, the red cells are lysed and in do~ng so their structure is destroyed releasing their internal chemicals and protein into the suspension whlch then ls passed through each of the apertures. The fluid o~ the suspenslon 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 l 16329~

increases, the size of the ape~tu~e decreases ~nd hence the signal will vary although ~he s~e size part~cle has been passed therethrou~h each tlme~ The rlnse s~lution does not eliminate this problem~ As ~entioned abo~e the apertures may be cleaned physically or ~ay be cleared b~ high ~ursts of energy applied to the apertures, but these are not without resulting or potential p~oblems including loss o~ time and/or energy.
The apertures 14 genexally being foxmed in ~embers o~ sapphire or rub~ Wh~Ch are then cemented or ~therwise affixed to the aperture tubes or other apexture mounting structure are somewhat ~ragile~ Applying too high a burst of energy such as a high fxequency burst or a capacitor discharge may rupture the aperture itself or the bond between the aperture and the mountin~ structure which w~ll result in erroneous readings until replacement o~ the ape~ture and~or ape~ture structure. It h~s been found that a constant energy level applied to the fluid contents in the a~erture for a predetermined time w~ll bo~l the fluid thexein and clean the aperture without a~fecting the aperture matex~al or structure.
The constant enexgy will be a D.C. cuxrent applied through each aperture 14.
The switch 44 ~ay be coupled to ~ control 54 which may switch an addit1onal poWer suppl~ in the supply 48 across each of the leads 38, 40 and 42 to pass the re~uixed D.C.
current through the apertures to cause the ~pertu~e contents to boil between each o$ the cycles o~ passing the ,sample - suspension through the ape~tu~es 14~ The switch 44 also ~ay switch the partlcle analyze~ 12 o~ so that the hl~her energ~
supplied for the clean~ng ~unctxon will not damage the analyzer or ~s is ~ener~lly the case, the analyzer 12 may ~ 163295 contain circuitry to protect against transients in the syste~n.
In that case the analyzer may just remain coupled to the leads 38, 40 and 42 without regard to the cleaning power supplied.
The cleaning power ~ay be supplled each cycle during the rinse portion of the cycle between pasSing the sample suspensions through the apertures~ The cleaning ener~y also can be supplied from a separate power sup.ly.
One embodiment of the power supply 48 and control 54 utilizes a separate poweX suppl~ fr~m the normal aperture eXcitation power suppl~ and is illustrated in Figure 2A amd 2B.
The power supply 48 has a pair of input leads 56 and 58 which are coupled bet~een a source of AC power such as a standard 1].0 volt 60 c~cle supply and a full wave bridge recti~ier 60. The P,C~ rectified voltage is coupled on a line 62 across a cuxrent limttin~ resistox 64 to a ~ilter formed by a cap~citor 66 ~nd a resistor 68, The filtered voltage is coupled on a line 70 to a line voltage regulator includin~ a resiStor 72 and three zener diodes 74 t 76 and 78. The ~egulated voltage out~ut of the resiStor 72 and diodes 74, 76 and 78 may be appxoximatel~ one volt from the desired poWer suppl~ output.voltage~ The approxi~ate volta~e ls coupled to a xesisto~ and capac~tor noise filter consisting o~ a resistor 80. and a capacitor 82 and then is applled to the base o~ ~ gate re~ulator 84 a.nd the collec~or o~ a control trans~sto~ 86.
The emitter of the control transistor 86 is coupled to a voltage di~idin~ ~esistor 88 and an output line 90 on which is supplied the re~ulated poWer supply ~olta~e coupled to the lines 38, 40 ~nd 42 hence to the apertures, as ~eviousl~ descxibed. The output on the line 90 is ~ 1~3295 maintained ver~ precisel~ by the control transistor 86 sensing at its base the voltage on a resistor 92 which changes the impedance seen b~ the gate regulator ~4 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 geneXate the cleanin~ cycle control timin~ pulse on an output line 96 which switches the output voltage on the line 90 to the leads 38, 4Q and 42, A manual switch 98 may be depressed, on operator demand, to provide the control timing pulse from a D,C. volta~e supplied on a line 100 over a filter formed ~y a pair of capacitors 102 and 104. The D.C. voltage is coupled through a cuxrent limiting resistor 106 when the m~nu~l switch 98 is engaged~ The control 54 is protected ~rom back E~F by A diode 108~ The automatic pulse control section o~ the control 54 receiVeS a trigger pulse on a line 110 which ma~ elthex be ~rom the cleaning cycle control circuit OX fro~ the particle analyzer 12.
The trigger pulse is applied on the line 110 ~ollowin~ the passage o~ the sa~ple ~uspension throu~h the aperture or apertures 14 and ma~ be a~plied between each cycle or at a predetexmlned frequenc~ of cycles, again durin~ the rinse portion of the operation~
The trlg~er pulse is applied throu~h a line filter ! formed by a resistor 112 and a capacitor 114 to a timer 116.
¦ The timer 116 includes a ~esistoX 118 and a capacitor 120 coupled to a D.C. powex souXce to provide the R.C. time constant fox the timeX 116, The timer 116 ~enerates a time pulse 122 of the proper durat~on on a line 124~ A load 1, l 16329S
resistor 126 ~s also couPle~ to the l~ne 124 ~nd the ~ul~e 122 is passed throu~h a current limitin~ 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~
- lt is preferable that the fluid flow ~e stopped ~hen the cleaning power is~applied to the apertures. The rinse cycle then is continued to flush any bubbles remaining from the voilln~ of the aperture$ as well as any debris boiled away from the sur~aces the~eof prior to running the next sample through the ape~tures 14, It is not necessary to stop the ~luid flow, but would re~uire an lncreased amount of energy to boil the ~luid pAssing through the apertures.
Further, although the cleaning pulse can be provided between each passage o~ sample throuyh the aperture or apertures 14, this is not necessar~ and as stated before, can be provided at a predetermined ~requency of cycles of sample passage.
~gain, it is p~e~erable that cleaning be e~ected after each sample pas~a~e. The prote~n does not necessaril~ bulld up durin~ each sample passa~e through each o~ the ~pertures 14, but the buildup varies and it essentially is impossible to predict how fast or on wh~ch passage o~ the sample a significant buildup w~ll occu~ and in which apexture. Further, allow~ng minute buildups to occur each s~mple run makes it more difficult to ~o~l ~WR~ the residue on the ~alls~
Therefore, it is prefe~able to utilize the cleaniny pulse a~ter each passaye of sample ~luid through the apertures 14.
An advanta~e in a~plying energy at a low continuous ~ate throu~h the apertures 14 has been the heatln~ not only of the aperture matexial, but of the surrounding structuxe ~ 1632~

at a slow rate decreasing the ther~al ~radient between the aperture material and the aperture mountln~ structure~ This causes a minimum amount o~ thermal shoc~ to the aperture while still supplying suf~c~ent energy to the volume of fluid or electrolyte in the aperture to cause the fluid to boil and cleanse the aperture.
In a specific e~ample~ the voltage applied on the line 90 is 65 volts which is applied with a timing pulse 122 of

2,5 seconds which applAes approximately .813 watt seconds 1~ to each'o~ the apertu~es 14 having a diameter of 50 to 100 microns. The powex necessary to boil the flu~d in the aperture ~ost depends upon the ~olume of electrolyte or fluid which the aperture conta~ns since this is the fluid which must be heated to a sufficient temperature to boil.
The determination o~ the voltage on the line 90 and the duration o~ the pulse 122 are determined b~v the watt seconds necessary to boil the ~uid in the apertures 14. The volume of fluid ls the ma~ox ~actor~ but the resistance and temperature o~ the ~luid ~lso ~hould ~e taken into account.
The example of 65 vQlts ~or 2,5 seconds is illustrative o one particular example and essentiall~v the same power could be supplied by a pulse o 130 volts applied for 1~25 s'econds, The size ran~e of the apertures of interest ~s 30 to 500 microns and alth~ugh'it is preferable to apply the power at a low rate, it ~enerally is necessary to apply no less than ~2 w~tt,,seconds. The preferable range ~or a 50 to 100 micron aperture is ~7 to 1.1 watt seconds, but a~ain depends not only upon the aperture size but also upon the time interval between the sample passages through the apertures 14~ Thus, in an automated instrument the time ~ 163295 i~pulse 122 ~ be de~endent u~on the ti~e a.llo~ed~ bet~?een the pass~ge`of e~ch ~a~ple through the ~pertures 14 by the study device 10 wh~ch is done in ~ repetitive cycle for - ~
numerous differen~ sample suspensionsr , 17 ~

Claims (13)

The embodiment of the invention in which an exclusive property or privilege is claimed is defined as follows:

1. A method of removing deposit from the inner wall of the scanning aperture of an electronic particle study device of the type wherein particles of microscopic size are suspended in a liquid having an electrical characteristic substantially different from that of the particles, the liquid being passed through a microscopic aperture of predetermined diameter, volume and configuration formed in an electrically insulating wall, a sensing zone being provided within the aperture by establishing an electrical current therein, passage of each particle through the sensing zone changing an electrical characteristic and the change in said electrical characteristic being detected and monitored, said method comprising the steps of: noncyclically applying sufficient low level energy in the form of a d.c. current of substantially constant magnitude at less than three hundred volts for a duration greater than one second to the liquid within the aperture whereby the applied energy is less than two tenths watt seconds to boil the liquid and the energy being applied continuously for the period of time indicated to bring the liquid within the aperture rapidly to a boil without vaporizing the liquid substantially instantaneously into its gaseous state thereby producing a plurality of scrubbing gas bubbles and directing said scrubbing gas bubbles in a stream against the inner wall of the aperture removing any accumulated deposit thereon.

2. A method as defined in claim 1 wherein the energy is applied in the form of a D.C. current at less than three hundred volts applied for greater than one second.

3. A method as defined in claim 1 wherein different samples of particles in suspension cuclically are passed for study through the aperture, and the energy is applied to boil the liquid therein between cycles of passing said particle suspensions through said aperture.

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

5. A method as defined in any one of claims 1, 2 or 3 in which the applied energy is in the form of a substantially constant D.C. current passed through said aperture.

6. A method as defined in any one of claims 1, 2 or 3 wherein the movement of said liquid through said aperture being stopped during application of said energy.

7. A method as defined in claim 3 wherein said energy is applied in the form of a substantially constant D.C. current through each of said apertures.

8. A wall cleaning system for removing deposit from the inner wall of the scanning aperture of an electronic particle study device of the type wherein particles of microscopic size suspended in a liquid having an electrical characteristic substantially different from that of the particles are passed with the suspending liquid through a microscopic precisely dimensioned aperture formed in an electrically insulating wall and constituting a sensing zone when an electrical current is established in the aperture, an electrical characteristic within the sensing zone being changed upon passage of each particle through the sensing zone, the change in the electrical characteristic being detected and monitored, the microscopic aperture having a predetermined diameter, volume and configuration; means for passing the liquid suspension in a path through said aperture and liquid always remaining with the aperture, the improvement comprising: power supply means for applying low level energy of magnitude less than 0.2 watt seconds in the form of a d.c. current applied at less than 300V to the liquid in the aperture for greater than one second, said energy being sufficient rapidly to generate a plurality of gaseous scrubbing bubbles against the inner wall which defines the interior of said aperture, said bubbles effecting scrubbing of the inner wall to remove therefrom any deposit which may have accumulated on the surface thereof and control means for applying said energy in a continuous substantially constant level for the predetermined duration, said scrubbing bubbles being generated by effecting rapidly gentle boiling of the liquid within the aperture at a rate sufficient to generate distinct bubbles and insufficient to effect conversion of all said liquid substantially instantaneously into a gaseous state by vaporization.

9. An apparatus as defined in claim 8 further including means for materially reducing the movement of said liquid through said aperture during application of said energy to said liquid.

10. An apparatus as defined in claims 8 or 9 wherein said energy is applied in the form of a substantially constant magnitude D.C. current.

11. An apparatus as defined in claims 8 or 9 in which said power supply supplies substantially constant D.C. current applied at least at three hundred volts for less than one second.

12. An apparatus as defined in claims 8 or 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 liquid therein between cycles of passing said particle suspensions through said aperture.

13. An apparatus as defined in claim 8 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 liquid therein between cycles of passing said particle suspensions through said aperture, flow of liquid through said aperture being stopped during application of said energy to said liquid.