Scanning Element And Aperture Wafer For Electronic Particle Counting And Sizing Apparatus

Abstract

An aperture wafer which is adapted to be mounted to an aperture tube scanning element for use with a Coulter-type electronic particle counting and sizing apparatus. The wafer is formed of the usual materials such as ruby, sapphire and the like. The aperture is located substantially in the center of the wafer and dimensioned as conventionally formed apertures in known wafers, but differs from the prior art by having the body of the wafer thickened as much as feasible contiguously to the aperture. The configuration of the resulting structure provides a conical chamber leading to the aperture, which consequently is located at the apex of the chamber. The chamber and aperture are symmetrical about the aperture axis. The aperture wafer provides decreased capacitive losses and also focuses the field of the aperture giving increased resolution without substantial loss in sensitivity. The scanning element has the wafer mounted thereto with the chamber opening to the inside of the aperture tube.

Description

BACKGROUND OF THE INVENTION

The field of the invention is the art of studying particulate matter suspended in diluents. U.S. Pat. No. 2,656,508 discloses an apparatus which operates on the so-called Coulter principle in accordance with which a particle flowing in a liquid suspension through an electric field established in a path of very small dimensions will change the impedance characteristics of that field in a detectable manner. Apparatus constructed in accordance with that principle has generally used a microscopic aperture in an insulating wall to provide the path, placing an electrode in each of the respective bodies of liquid located on opposite sides of the wall and connecting these electrodes to a source of current so that the electric current will flow through the aperture. The current is of low current density while passing through the bodies of liquid to the aperture, but will be concentrated in a relatively high current density when said electric current flows through the aperture. A detector also connected across the same electrodes will respond to the changes in the impedance of the aperture each time that a particle passes through the same, the theory involved being well known. Briefly, the normal particle may be considered an insulating body passing through the aperture and displacing its own volume of the conductive diluent which is in the aperture. The decrease in the volume of conductive liquid caused by the presence of a particle will increase the impedance of the aperture by a measurable amount which is proportional to the amount of liquid displaced.

The aperture of such apparatus is desirably quite small, having an internal diameter of the order of several times the diameter of the particles to be studied, for example. In practice, apertures even several hundred times the volume of the particles being studied will give satisfactory results. Thus, for red blood cells having an equivalent spherical diameter of about five and one-half (51/2) microns, apertures of the order of seventy (70) microns in diameter to several times that have been successfully used. The problem of providing an aperture with stable dimensions of this size has been solved by utilizing wafers of corundum, such as ruby or sapphire, having microscopic holes drilled in them by watch-jewel-making techniques. The resulting wafer is mounted in an orifice that is formed in the side wall of a glass tube, by any suitable method. One effective method is by fusing.

The construction of aperture tubes using this type of wafer and the techniques involved are disclosed in U.S. Pat. No. 2,985,830. In this latter patent, the aperture tube with its aperture is referred to as a scanner element because in effect the stream of particles in suspension is scanned for obtaining signals when it passes through the aperture that is provided in the sidewall of said tube.

The current distribution through the aperture is not perfectly uniform in its geometry, because the liquid of the suspension spreads immediately outside of the ends and hence the current density fans out also. This bulges the strong field within the aperture and makes the aperture in effect slightly longer than its actual axial dimension. It is desirable to have as short an aperture as possible to obtain maximum sensitivity of the signals produced by the passage of particles therethrough, but as long an aperture as possible to obtain maximum resolution. It can also be seen readily that the statistical advantage of passing high-concentration suspensions through an aperture can be decreased by the presence of more than one particle in the aperture, because the latter occurrence will produce only one signal, albeit of greater amplitude. Shortening the aperture will decrease the probability of coincidence and may enable multiple particles to be distinguished from large particles by the character of their signals but will lower resolution also.

In the case of apparatus which utilizes alternating current in the aperture, another problem arises. The wafer itself will pass capacitive current depending upon its dielectric constant, this being referred to as displacement current. If the displacement current can be decreased, thereby decreasing the losses in the electrolyte in the immediate vicinity of the aperture, the signal-to-noise ratio and hence the sensitivity and response of the scanner element will be increased.

In any aperture, the forces of liquid flow and electric current flow are extremely difficult to predict and ascertain. Several factors and the interactions between them are at least the partial cause for this difficulty, including the formation of the vena contracta, the response of liquid flow to the presence of sharp corners, the distribution of current caused by sharp corners, the presence of stagnant sections within the aperture because of vena contracta flow, the effects of heating due to high current concentration, etc. Accordingly, the solution to the problems described above is not obvious or simple.

The solution of the problem which arises in connection with apertures that are required to pass electric current at high frequency obtained by the invention herein has additionally provided an improved resolution of signals, even where the aperture current is DC or of very low frequency. A theory for this benefit will be set forth as a suggestion, but the exact reason is not certain and no limitations are intended on account of such explanation.

SUMMARY OF THE INVENTION

According to the invention, a wafer of corundum or other hard material is constructed in which there is an aperture of axial dimension not substantially different from the axial dimension of similar diameter apertures formed in known wafers, but in which the wafer itself thickens quite substantially contiguous to the entrance and/or exit of the aperture, preferably as a conical chamber, the aperture and conical chamber or chambers being substantially symmetrical about the axis of the aperture. The wafer is secured to the wall of an aperture tube by any known technique in order to form a scanning element. In the case of a wafer which has only one end of the aperture provided with a conical chamber, the wafer is mounted in such a manner that the conical chamber opens on the downstream side considering the flow of the liquid suspension through the aperture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a median sectional view on an exaggerated scale of an aperture wafer constructed in accordance with the invention, portions being broken away to enable proper proportions to be illustrated;

FIG. 2 is a view similar to that of FIG. 1 but showing a slightly modified form of the invention;

FIGS. 3 and 4 are views similar to those of FIGS. 1 and 2, but illustrating wafers constructed in accordance with the prior art;

FIG. 5 is a view similar to that of FIGS. 1 and 2, but illustrating another form of the invention;

FIG. 6 is a fragmentary sectional view of the aperture of the structure of FIG. 5;

FIG. 7 is a diagrammatic sectional view showing an aperture tube in the environment of normal use, having a wafer of the preferred construction mounted in the aperture tube; and

FIG. 8 is a fragmentary vertical median sectional view taken through the aperture wafer of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The prior art structures which are referred to in said U.S. Pat. No. 2,985,830 have generally taken the form of those wafers which are illustrated in FIGS. 3 and 4. The wafers 10 and 12 are cut, ground and drilled from a highly heat resistant material, such as ruby or sapphire, with their surfaces lapped and finished. The wafer 10 is in the form of a circular disc having a central aperture 14 which is drilled as cylindrical as possible. The surface 16 and 18 are ground, polished, and/or lapped flat and parallel one to the other. The aperture 14 is drilled in the center of a spherical depression 20 that is formed below the surface 18. This arrangement is quite similar to the construction of watch jewels. The dimensions of the aperture are fairly square, that is, the length C along the axis perpendicular to the surfaces 16 and 18 is substantially the same as the diameter B. In many wafers this ratio may be 2/3 or 3/2 or thereabouts. This is generally accepted as standard in aperture wafers. The thickness E of the wafer 10 for this type of structure is of the order of 0.5 to 1 millimeter. Since this is several times thicker than the wafer 12 of FIG. 4, it is relatively easier to manufacture. The diameter A of the wafer 10 is of the order of 3 or 4 millimeters, since this provides an easily manipulated article and also provides sufficient overlap to enable the wafer to be mounted to a glass tube. Practically all wafers have approximately this diameter.

The wafer 12 is much thinner than the wafer 10, since it has no spherical depression similar to 20. All of the dimensions of this wafer are about the same as those of the wafer 10 except for the thickness. The square dimensions are usual for the aperture 22, and normally the thickness E is the same as the axial dimension of the aperture 22. The surfaces 24 and 26 are flat and without depression. The short axial length of the aperture 22 is a compromise of sensitivity and resolution. The corners defined by the ends of the aperture 22 are fairly sharp. It has been found that it is easier to clear debris from an aperture that opens to a flat surface, and when both ends of an aperture open to flat surfaces, there is no need to choose one of a pair of different opposite faces to be secured to the glass tube. For example, in the case of the wafer 10, debris will be found more readily to clog the aperture 14 if the flow is into the depression 20 than out of the depression 20. Furthermore, even if the aperture clogs, it is easier to clear from the flat side 16 if this is the upstream end of the aperture.

The most simple forms of the invention are illustrated in FIGS. 1 and 2, and in each case the same reference characters will be used to designate the same or equivalent parts.

The two wafers are designated generally 40 and 40'. The wafers 40 and 40' are substantially thicker than the wafer 12 but may be as thick as the wafer 10. The aperture 42 is drilled at the apex of a conical chamber 44 that opens to the surface 46. The chamber 44 is preferably at the downstream end of the aperture 42, so that the surface 48 which is shown to be flat, has the upstream end of the aperture 42 opening directly to it. In each case the angle of the conical chamber is about 90.degree. as indicated, but this is a noncritical dimension. The chamber 44 of the wafer 40 terminates in a slightly spherical surface 50 which has a radius R which is about three times the diameter B of the aperture 42. This structure provides a relatively wider mouth than the straight conical chamber 44 of wafer 40' where the formation 50 is omitted.

Insofar as the decrease of displacement current is concerned where high-frequency current is used in the aperture, it will be seen that the thickness of the wafer at 52 immediately contiguous to the aperture has in each case been substantially increased over those instances where the aperture opens directly to a flat surface. This latter is true for the wafers 10 and 12.

Insofar as improved resolution is concerned, this is believed due to current focusing effect. In some manner, the sharp corner current density concentration which is believed to occur in prior wafers seems to have been shifted toward the axis of the aperture, or is decreased so that the amplitudes of signals derived from the aperture have dependence upon the portion of the aperture traversed by the respective particles producing the same. For example, a particle coming through an area of high current density at a sharp corner of prior apertures will produce a different signal than if it went through the center of the aperture. In the case of the aperture wafers 40 and 40' this disparity seems to have been reduced. In any event, such aperture wafers have better resolution without substantial loss of sensitivity.

The effect of such structures is to render the data obtained from the sizing of particles to be more accurate since many of the larger and apparently spurious signals due to nonuniform current density in the effective portion of the aperture will not be generated.

In FIGS. 5 and 6 there is illustrated a wafer 60 in which the aperture 62 opens at both ends to conical chambers 64 and 66. In this structure the benefits of the increased resolution are provided at both sides of the aperture 62, and it is less fragile and thus easier to handle during mounting to a glass tube. The face which is secured to the surface of the tube may be either 68 or 70 which obviates the need for making a proper choice of face during assembly. The thickness of the wafer at 72 contiguous to the aperture is much greater than if only one conical chamber were provided. Accordingly, the signal-to-noise ratio for apparatus which uses aperture currents of high frequency would be improved over that using a one-sided conical chamber in its wafer.

FIG. 7 illustrates the environment in which the invention is used and shows a scanning element 78 comprising a glass aperture tube having a wafer such as 40 secured in its wall 80 as shown in FIG. 8, being mounted to the outer face 84 of the wall 80 adjacent the bottom thereof. The scanning element 78 is here shown having a configuration which is well known, having an upper flared mouth 77 by means of which it is connected into the closed fluid system to provide for aspiration in the manner described in U.S. Pat. No. 2,869,078. Only a portion of the glassware normally associated with such a system is shown at 79. In the use of this type of scanning element 78, it is immersed in a body of fluid 81 carried in a vessel 83, such vessel being a small beaker or cuvette containing a suspension to be tested, and being brought into engagement with the bottom end of the scanning element 78. The interior of the scanning element 78 and the remainder of the fluid system will contain the second fluid body 85 whose composition is normally of little consequence, providing it is conductive. Generally, it may comprise saline solution previously introduced, or may contain the remains of several prior-tested samples.

The electrodes 87 and 89 in the respective bodies of fluid enable the coupling of the apparatus to the electrical portions of the apparatus by way of suitable conductors, such as the leads 91 and 93. The block 95 generally represents the electrical components, including a source of electric current, detectors, counters, controls, etc. The benefits of the invention are particularly advantageous where the source of current includes at least one alternating current power source, but, as previously mentioned, were also found to improve the operation of the apparatus even where the current source is DC or low frequency.

The view of FIG. 8 is taken as a vertical median section through the left side of the aperture tube 78 of FIG. 7 but on a greatly enlarged scale. As noted, the flat surface 48 of the wafer 40 faces outwardly and normally will be slightly raised from the glass face 84 so that the operator may easily rub his finger over such surface. In the event that debris is captured by the aperture, it will generally block across the opening to the aperture 42 or a portion will hang in the aperture and protrude slightly. The rubbing of the finger or other implement over such surface will clear the debris in practically all cases. This is not as readily accomplished with apertures whose entrances are below the surface of the wafer. The situation can be envisioned by considering that the flow of suspension is from right to left in FIG. 8, instead of from left to right as indicated by the arrow. The setup of FIG. 7 contemplates that the normal usage of the apparatus will require the movement of the test suspension from the body of fluid 81 through the aperture 42 and to the body of fluid 85.

The wafer 40 of FIGS. 7 and 8 has been fusedly secured to the wall 80 of the tube 78 over a generally conical orifice 86 formed in the tube by glassblowing or other glass-handling techniques. The orifice is relatively large but somewhat smaller than the diameter A of the aperture wafer 40. The surface 46 faces inwardly and hence the chamber 44 also faces inwardly, this being the downstream end of the liquid flow through the aperture 42.

In the construction of the aperture wafers, the angle of the conical chamber should be chosen so that the current density in the cone will not be high enough to produce any substantial signal after the particle has passed through the aperture. The angle should likewise not be so flattened that the effect will be not much different from a wafer of the type shown in FIGS. 3 and 4. The 90.degree. angle chosen has given excellent results for the conditions under which it was used. Variations of this also produced advantageous results. What it is desired to secure by Letters Patent of the United States is the following.

Claims

We claim:

1. An aperture wafer used in combination with electronic particle counting and sizing apparatus of the type which includes structure establishing an electric field in a path of very small dimensions whereby a particle flowing in a liquid suspension through said field changes the impedance characteristics of said field in a detectable manner and wherein said path is defined by said aperture wafer which comprises a relatively thin disc of highly heat resistant material having an aperture therethrough and which is mounted to the wall of an aperture vessel and in which apparatus electrical means are provided to detect the passage of said particles in terms of change of the impedance characteristics of said field, said aperture wafer disc has opposite substantially parallel faces, said aperture being formed through the disc substantially at the center of said disc and having a generally cylindrical configuration, with the axis of the cylinder perpendicular to the faces of the disc and the diameter and axial length of said aperture being of the same order of dimension, a chamber of generally conical configuration formed in said wafer coaxial with the aperture and having its apex end connecting with one axial end of the aperture and its base end opening at one face of the disc, the second axial end of said aperture being connected with the second face of said disc, and the configuration of the chamber being such as to provide a portion of the disc contiguous to the one axial end of said aperture which is substantially thicker in an axial direction than the axial length of said aperture.

2. The aperture tube as claimed in claim 1 in which the second axial end of the aperture opens directly to the second face of the disc.

3. The aperture tube as claimed in claim 1 in which there is a second substantially similar conical chamber formed in said wafer in mirror position relative to said first-mentioned chamber, and the second axial end of the aperture connects with the apex end of said second chamber.

4. The aperture wafer as claimed in claim 1 in which the conical angle of said chamber considered on a diametral plane is approximately 90.degree..

5. The aperture wafer as claimed in claim 1 in which the apex of the chamber is slightly spherical with the radius of such apex being no more than several times the diameter of the aperture.

6. The aperture wafer as claimed in claim 1 in which the juncture between the chamber and the aperture is substantially sharp.

7. A scanner element used in combination with electronic particle counting and sizing apparatus of the type which includes a vessel having an aperture therein through which fluids carrying suspensions of particles are adapted to pass and in which there is established an electric field and electrical means are provided to detect the passage of particles in terms of a change in the impedance characteristics of said electric field, said scanner element comprising:

a. an aperture tube adapted to be connected into a closed fluid system so that a suspension may be drawn into the interior of the tube through a wall thereof and

b. means providing a passageway through the wall of the tube, said means comprising

i. a flat-faced corundum disc secured to said wall,

ii. a cylindrical aperture in said disc having a diameter and length of the same dimensional order,

iii. the aperture axis being substantially perpendicular to the faces of said disc,

iv. a chamber of generally conical configuration formed in said wafer coaxial with said aperture and having its apex connecting with one axial end of said aperture and its base end opening to one face of said disc, and

v. the second axial end of said aperture being connected with the second face of said disc.

8. The scanner element as claimed in claim 7 in which there is one conical chamber only and the one face of said disc to which the base end of the conical chamber opens is on the interior of the aperture tube the second axial end of the aperture opens directly to the second face of the disc.

9. The scanner element as claimed in claim 8 in which the disc is fusedly secured to the wall of the aperture tube with its flat surface disposed in position to be rubbed from the outside of the aperture tube.