Electricity: measuring and testing – Determining nonelectric properties by measuring electric... – Particle counting
Reexamination Certificate
2000-04-10
2002-07-30
Le, N. (Department: 2858)
Electricity: measuring and testing
Determining nonelectric properties by measuring electric...
Particle counting
C324S071100, C702S100000
Reexamination Certificate
active
06426615
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an apparatus and method for analyzing particles suspended in a fluid. More particularly, this invention relates to an apparatus for counting, measuring, differentiating, manipulating, and controlling the movement of particles suspended in a fluid having electrical properties different from that of the particles by determining electro-physical properties, e.g., electrical impedance, of the particles.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 2,656,508 to Coulter discloses what is commonly referred to as the “aperture impedance” or the “Coulter” principle for counting and sizing particles. An exemplary arrangement utilizing this principle is shown in
FIGS. 1
,
3
,
5
, and
7
. Through a small aperture
1
, the fluid
4
containing the particles in dilute suspension, is aspirated from one electrically insulating vessel
3
into another similar vessel
5
. This aperture
1
provides the only path for fluid or electrical communication between the two vessels
3
and
5
. One electrode
7
is immersed in the fluid in the first vessel
3
, and a second electrode
9
is immersed in the fluid in the other vessel
5
. The passage of a particle through the aperture
1
causes a brief change in electrical impedance measured between the two electrodes
7
and
9
. The magnitude of the transient resistance change, called a “resistive pulse”, is a measure proportional to the size of the particle. Several thousand particles may be measured in a few seconds, and the data may be sorted into classes to provide a distribution histogram showing the number of particles falling into each size range. However, this basic arrangement has suffered drawbacks, and drawbacks in accuracy can be significant. For example, measurement of particle size range is critical for the production of a wide range of products including ceramics; toners; dyes; powders; cement; sugar; pharmaceutical products and photographic materials. Variations in particle size can critically influence both the manufacturing processes and the characteristics of the final product.
There have been many attempts to address the drawbacks associated with this basic design. However, none of these attempts have been entirely successful. These drawbacks have resulted in limitations to the smallest particle that can be measured with a given aperture size, orientation errors, coincidence errors, trajectory errors, and extended sensing zone errors.
For small particles, the electrical and acoustic noise compete with the small resistive pulse signal generated by the particles resulting in low S/N ratio. Therefore, the smallest particle measurable by the aperture impedance principle is typically 2% of the aperture diameter. With very small apertures, such as a sub-micrometer aperture, the lower limit is higher than 2% because the noise floor rises substantially due to the increased resistance. The noise goes as the square root of the aperture resistance and the aperture resistance is inversely proportional to the square of the aperture cross-sectional area. Therefore, as the aperture becomes smaller, the resistance increases and so does the associated noise. Additionally, for the instruments based on this aperture impedance or electrical sensing zone method, in the measurement of small particles, thermal aperture noise continues to exceed all other noise contributions by more than an order of magnitude. Further improvements in the circuitry cannot lead to better resolution.
The prior art embodiment of
FIG. 1
does not take into account the shape of the particle and this leads to an inability to obtain important information about the particles and significant particle orientation errors. Thee electrical response for cylindrical shaped particles measured by this aperture impedance method can be proportional to the size deduced from a calibration using spherical particles. This may be errors as high as 25%. There is a complex relationship between hydrodynamic forces, deformation of particles, aperture dimensions and pressure and therefore it is not possible to relate the characteristics of the pulse to the shape of the particle.
In an attempt to get more information on the particles, prior art designs have simultaneously passed high and low-frequency currents through the aperture. While the use of appropriate filtering techniques can permit detection of both the low frequency resistance and high frequency reactance of the particle traversing the aperture, the interference created between the two separate current sources employed to create the high frequency and the low frequency current within the aperture cannot be eliminated. Any slight change in conditions can cause either, or both of the two frequencies to become de-tuned.
Further, it is known that generally, due to the hydrodynamic focusing in most instruments, elongated particles will be aligned with their elongated axis substantially parallel to the center axis of the orifice. With two particles of equal volume, one being spherical and one being elongated, the spherical particle while passing thorough the orifice, will have a greater cross section perpendicular to the current flow than the elongated particle. Hence, the spherical particle will distort the field in such a manner that it will give a greater measured size than the elongated particle, despite their equal volumes.
FIGS. 1 and 2
illustrate the error in the prior art due to the difference in orientation of the particles. Aperture
1
in the insulator
2
establishes the constricted electrical path of external electrodes. Consider a non-spherical particle
8
with its main axis along the aperture axis, and another non-spherical particle
6
with its main axis perpendicular to the aperture axis. The particle
6
with its main axis perpendicular the aperture axis would obstruct the electric field in the aperture
1
significantly more, and would result in a higher peak
10
as compared to the peak
12
of other particle
8
with its main axis along or parallel to the aperture axis. Thus, it is evident that particle size measurements for non-spherical particles can be fairly erroneous.
Another limitation with prior art devices results in certain instruments, counting losses of up to 20% due to random coincidences of particles in the orifice. Simultaneous presence of more than one particle in the aperture can occur without detection. The prior art neglects the co-incident pulses most of the time or provides imprecise corrections. Statistical methods are used to compensate for neglecting these pulses. This inherently limits the accuracy of the instrument.
FIGS. 3 and 4
illustrate the error in the prior art due to the coincident presence of particles in the sensing zone. Assume that a second particle
15
enters the sensing zone before a first particle
17
has left the sensing zone. The result is that the pulse
16
due to the first particle
17
is superimposed with the pulse
18
due to the second particle
15
resulting in a much larger pulse
14
.
An additional problem in the prior art is due to trajectory errors. This may arise due to non-uniform current density at different cross-sectional locations within the aperture of the instrument. Because of the non-uniform current density, the pulse height of the related shape depends on the path an individual particle takes through the aperture. The current density is significantly higher at the edges of the entrance and exit of the aperture. Also,the electrolyte stream velocity is higher in the center of the aperture than in the periphery due to boundary development. Some particles approaching the aperture obliquely travel close to the wall. These particles move slower than those that pass through the center of the aperture. The particles enter and leave the aperture boundaries through the zones of higher current density and may suffer shape distortions as a result of higher shear force near the wall resulting from the higher stream rate associated with the boundary layer. Errors may therefore result because pulse width measurements of la
Kerveros James
Le N.
Nirmel Chittaranjan N.
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