Apparatus For Particle Size Distribution Display With Means To Selectively Alter The Display By Adding Predetermined D.c. Signals To The Data

Abstract

The system includes a plurality of potentiometers and summing networks and a switching mechanism for alternately connecting the potentiometers to a neutral position, to a negative voltage source or to a positive voltage source. Each potentiometer is connected through a switch and one of the summing networks to the output of an accumulator in a particle size analyzing device and is adapted to add or subtract a signal to or from the accumulator output signal or, when the switching mechanism is in the neutral position, to have no effect upon the accumulator output signal. Each accumulator accumulates pulses generated by particles within a particular size range and the accumulator output signal is indicative of the quantity of particles in that size range. The particle analyzing device includes an electronic switching device for sequentially and cyclically connecting the outputs of the accumulators to a visual display device for presenting a visual display of a particle size distribution curve. The system of the invention permits the size distribution curve to be altered to compensate for erroneous signals (noise). It also permits the showing of a graph of the deviation between a standard particle size distribution curve and the sample particle size distribution curve. It further permits the replacement of the output signal from a selected accumulator with a desired D.C. signal level.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my copending U.S. application Ser. No. 195,730 filed Nov. 4, 1971, and now abandoned.

Description

BACKGROUND OF THE INVENTION

This invention relates to a system for altering the visual display of data distribution. More specifically, it relates to a system for altering a visual display of a curve of size distribution of particles in a sample.

In the field of particle analysis it is often important to determine the size distribution of particles in a given amount of sample of particulate matter or of particles suspended in a fluid. To obtain an indication of the size distribution of particles in the given amount of sample, the given amount of sample is passed through a particle analyzing device (e.g., a Coulter particle analyzing device). The particle analyzing device produces signals, i.e., particle pulses. The amplitude of each pulse is related to the size of a particle sensed. To determine how many particles are in a given size range, the pulses are applied to a plurality of so-called "window comparators" each of which is adapted to sense pulses between predetermined lower and upper threshold levels which are directly related to a particular particle size range. Upon sensing a pulse having its maximum amplitude falling between the predetermined threshold levels, a window comparator will produce an output pulse having a predetermined amplitude. The output pulses from the window comparators are applied to individual accumulators. The output signal level of each accumulator changes-increases-with time as pulses are accumulated therein and is indicative of the number and/or total volume of particles sensed within a given particle size range. The accumulators are sequentially connected in a predetermined order to a visual display device such as an oscilloscope to provide a visual display of a particle size distribution curve on cartesian coordinates where the horizontal or x axis represents particle size and the vertical or y axis represents the accumulated quantity of particles. Typically an electronic switching device is provided for cyclically connecting the accumulators to the visual display device in the preselected order such that successive curves are traced or displayed on the visual display device. As more and more particles are sensed, the maximum amplitude of each curve traced increases until the given amount of sample has been passed through the particle analyzing device, at which time the curve being traced is the particle size distribution curve for the given amount of sample.

In the presently available electronic particle size analysis systems for providing a visual display of a particle size distribution curve, the beginning or leading edge of the curve is irregular as a result of noise. Such noise may be caused by extraneous signals picked up by circuit elements of the system or may be caused by undesirable debris -- extraneous particles -- which have become entrained in the sample. As a result, the beginning of the distribution curve is not a true picture of the quantity of the smaller size particles in the sample and, it is desirable that some means be provided for eliminating or canceling out the noise appearing at the beginning of the distribution curve to obtain a more accurate picture of the beginning of the particle size distribution curve.

Also, it is desirable to provide a more accurate representation of particle size distribution at the "very small-particle end" and the "very large-particle end" of the particle size spectrum by replacing the signals defining the ends of the particle distribution curve with extrapolated, predetermined DC signal levels.

From time to time it is desirable to compare the distribution curve so produced with a standard distribution curve to find out if the concentration of particles of a given size in the sample has changed. It is desirable, therefore, to provide means for comparing the distribution curve with a standard and for displaying the deviation, if any, from the standard.

Also, it is often desirable to formulate a mixture of particles having a preselected size distribution of particles. When preparing such a mixture, it is desirable to have some means for indicating when the desired size distribution of particles has been obtained.

Heretofore, various electrical circuits have been proposed for altering signals in diverse systems and examples of such electrical circuits may be found in the following patents:

United States Patent Nos. ______________________________________ 2,171,216 2,812,494 2,412,350 2,858,475 2,480,636 3,532,977 ______________________________________

However, these systems were not concerned with particle analysis and the electrical circuits did not provide all the desirable features noted above.

It is therefore a primary object of the invention to provide a system for altering or replacing signals in a predetermined manner to obtain a desired visual display of the signals as altered or replaced.

SUMMARY OF THE INVENTION

According to the invention, there is provided a system for altering the visual display of a particle size distribution curve obtained by the sequential and cyclical application of the outputs of a plurality of accumulators in a particle size analyzing device to a visual display device, the system including apparatus for adding or subtracting predetermined signals to the outputs of selected accumulators and for replacing the outputs of selected accumulators with predetermined signals.

Preferably, the apparatus includes a plurality of potentiometers each connected to the output of one accumulator, and a switching mechanism for connecting the potentiometers to a positive voltage source, to a negative voltage source, or to a neutral position where the potentiometers have no effect on the output signals from the accumulators. Also preferably, each potentiometer is connected to the output of an accumulator through a summing network and a disconnect switch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the system of the invention connected to the outputs of a plurality of accumulators in a particle size analyzing device.

FIG. 2 is a series of graphs showing the timing pulses for actuating the switches between the outputs of the accumulators shown in FIG. 1 and a visual display device.

FIG. 3 is a schematic diagram of one of the electronic switches shown in FIG. 1.

FIG. 4 is a graph of the particle size distribution shown on the visual display device.

FIG. 5 is a graph of the deviation between a standard particle size distribution curve and a sample particle size distribution curve.

FIG. 6 is a graph of a integral curve of the particle size distribution curve shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, the signal altering system of the invention is generally indicated at 10 and is connected into and forms part of a particle size analyzing apparatus 12. The apparatus 12 includes a particle sensing or scanning device 14 (such as a Coulter type particle analyzing device) which senses particles as they pass through a sensing zone and which produces a signal in the form of a particle pulse for each particle sensed. Typically, the amplitude of each particle pulse is directly related to the size or volume of the particle sensed. The particle pulses are applied to a pulse amplitude discriminating apparatus 16 which includes a plurality of so-called "window comparators" (not shown). This amplitude discriminating apparatus may incorporate means for disabling the output to any of the subsequent accumulators, at the discretion of the operator. Alternatively, and as will be described hereinafter, the system 10 can include disconnect switches for disconnecting the accumulator outputs from the circuits of the apparatus 12. Each of the window comparators senses particle pulses having amplitudes falling within predetermined lower and upper threshold levels and for each particle pulse sensed produces an output pulse having a predetermined amplitude and duration. The lower and upper threshold levels are related to a particular particle size range. The number of window comparators will depend upon the number of size ranges it is desired to analyze. In one pulse amplitude discriminating apparatus 16 used in the apparatus 12, sixteen window comparators are provided.

The output pulses from each window comparator are applied to an individual accumulator or integrator connected thereto. In FIG. 1, four such accumulators, the first, second, third, and nth accumulator, are shown at 21-24.

Each window comparator upon sensing a particle pulse having an amplitude falling within the predetermined upper and lower threshold levels, produces an output pulse which is accumulated by one of the accumulators 21-24, and the output signal level from each accumulator 21-24 will increase as output pulses from the window comparators are accumulated therein. The output signal level from each accumulator or integrator 21-24 is sequentially and cyclically applied via an output channel 31, 32, 33, 34 to a visual display device 40 which is typically an oscilloscope. The output signal levels can also be applied alternately or concurrently to a plotter shown in phantom lines at 42.

It will be understood that the output signal level on each one of the channels 31-34 must be applied to the oscilloscope in a predetermined sequence for predetermined times. For this purpose, the apparatus 12 includes an electronic switching device 44 which is connected to an electronic switch in each one of the channels 31-34. The electronic switches are identified by reference numerals 51-54.

The output signal level on each channel 31-34 when applied to the oscilloscope 40, is first passed through a buffer amplifier 56 which is preferably an operational amplifier having a low input impedance.

It will be understood that the output signal level on the first channel 31 is indicative of the quantity of particles in the sample having a size falling within a first size range. This size range can be from some small amount above the noise level to 1 x cubic microns. The second channel will have an output signal level indicative of the number or concentration of particles in the sample having a size falling within a second size range such as from 1x to 2x cubic microns. The output signal level on the succeeding channels will likewise be indicative of the number or concentration of particles having a size falling within a particular-larger-size range. By plotting the output signal levels indicative of the quantity of particles accumulated with respect to particle size, one can obtain a particle size distribution curve. In order to obtain a visual display of such a curve of particle size distribution, the output level signals on the channels 31-34 are sequentially applied in a predetermined order to the vertical sweep of the oscilloscope 40. It will be appreciated that to provide the desired visual display of a particle size distribution curve on the scope 40 in the operation of the switches 51-54 by the electronic switching device 44 must by synchronized with the period of the horizontal sweep of the oscilloscope 40. Such timed operation of the switches 51-54 is obtained with the electronic switching device 44 which can include a crystal oscillator for providing a clock signal which is applied to an n counter. The n counter then operates a one of n decoder to provide output pulses for operating-closing-the switches 51-54. The switch operating pulses are shown at 61-64 in graphs A-D of FIG. 2. As shown, the pulse 61, which operates or closes the switch 51, is applied to switch 51 during the interval t1-t2. Then during the interval t2-t3 the pulse 62 is applied to the switch 52, and so on. In this way, the scope 40 traces a curve of particle size distribution on cartesian coordinates where the horizontal or x axis represents particle size and the vertical or y axis represents the quantity of particles of a particular size. Such a particle size distribution curve is indicated at 70 in FIG. 1. Although the curve 70 is shown as being continuous it is to be understood that it is actually a plurality of dots each dot representing an output signal level from one of the accumulators 21-24. Three of these dots are shown at 72-74 in FIG. 4 which is an enlarged view of the particle size distribution curve 70 shown in FIG. 1.

The electronic switches 51-54 are all the same and a schematic diagram of the switch 51 is shown in FIG. 3. As shown, the electronic switch 51 includes a shunt-connected field-effect transistor 76 and a series-connected field-effect transistor 78. The control pulse 61 is applied by the electronic switching device 44 via a conductor 81 at the gate 82 of the series-connected field effect transistor 78 to trigger the same into a low impedance or conductive state where the signal on the channel 31 is passed to the buffer amplifier 56. At the same time the shunt connected field effect transistor 76, which is connected between the channel 31 and a common conductor 83 connected to the common or ground potential of apparatus 12, is triggered to a high impedance or nonconducting state. When the control pulse or signal 61 from the electronic switching mechanism 44 is terminated, the series-connected field effect transistor 78 goes from a low impedance conducting state to a high impedance nonconducting state to block the passage of the output signal level on the channel 31 to the buffer amplifier 56. At the same time the shunt connected field effect transistor 76 is rendered conductive to shunt the output level signal on the channel 31 to ground, and thereby protect the series connected field-effect transistor 78, and improve switching.

The apparatus 12 is also adapted to present on the oscilloscope 40 an integral curve which is the integral of the particle size distribution curve 70. This is achieved by latching each of the switches to an ON condition. When such an integral curve (such as curve 160 or curve 165 in FIG. 6) is desired, the electronic switching mechanism 44 is operated to turn on each of the electronic switches 51-54 in a predetermined order and to hold each switch on for one cycle or sweep of the scope 40. This is accomplished by maintaining the control signal 61 for the duration of the cycle or sweep such as indicated by the signal 61a in graph A of FIG. 2. Likewise the succeeding control signals or pulses are maintained for a longer duration as indicated at 62a, 63a and 64a in graphs B,C, and D, respectively.

As best shown in FIG. 4, the leading edge 84 of the particle size distribution curve 70 does not start at 0 as one would expect it to. This is caused by noise picked up in the apparatus 12. For example, the noise may be the result of extraneous signals being picked up by circuit elements of the apparatus 12. On the other hand, the noise may be the result of foreign or extraneous particles in the given amount of sample.

The system 10 forming part of the apparatus 12 provides a means for eliminating or compensating for this noise.

As best shown in FIG. 1, the signal altering system 10 includes a plurality of disconnect switches 85-88 in each of the channels 31-34 respectively and a plurality of summing networks 91-94 connected into each one of the channels 31-34. Each of the networks 91-94 includes a first resistor 91a, 92a, 93a, 94a which is series-connected in the channel 31, 32, 33, or 34 and each of the switches 85-88 is connected between the output of one of the accumulators 21-24 and one of the resistors 91a-94a. As shown each switch 85-88 is a single pole double throw switch which can be closed as shown, open, or connected to the common conductor 83. Each network 91-94 also includes a second resistor 91b-94b which is connected as shown between the channel 31, 32, 33 or 34, and an adjustable contact 101, 102, 103 or 104 of a potentiometer 111, 112, 113 or 114. As shown, the resistance element of each of the potentiometers 111-114 is connected between the common conductor 83 of the apparatus 12, and a movable contact 115 of a potentiometer 116. The resistance element of the potentiometer 116 is connected between the common conductor 83 and a movable contact 118 of a switching mechanism 120. As shown, the switching mechanism 120 can be operated to connect the movable contact 118 either to a source of positive voltage, to a source of negative voltage, or to a neutral position where the contact 118 is connected to the common conductor 83.

It will be understood that the value of the resistance elements of the potentiometers 111-114 and 116 is much smaller than the resistance value of the resistors 91a-94a and 91b-94b so that when the movable contact 118 is in the neutral position connecting the potentiometer 116 to the common conductor 83, the effect is the same as if the resistors 91b.94b were connected directly to the common conductor 83. In this neutral position the resistors (such as the resistors 91a and 91b) of each of the summing networks 91-94 function to divide or attenuate the output signal level on the respective channel 31-34. However, when the potentiometer 116 is connected by the switching mechanism 120 either to the source of positive voltage or the source of negative voltage, the networks 91-94 function as summing networks from summing a signal with the output signal level on the respective channel 31-34. In this way a plus or minus signal, e.g., a plus or minus current is added to the output signal level on the channel 31,32,33 or 34.

Although not shown, it will be understood that the movable contact 101-104 and 115 of the potentiometers 111-114 and 116, as well as the movable contact 118 of the switch mechanism 120 are connected to knobs mounted on a console containing the circuit elements of the apparatus 12. In this way, an operator can add or subtract a signal to the output signal level of each channel merely by rotating the knobs connected to respective movable contacts. Thus, by means of a knob connected to the movable contact 118, the plurality of fixed (current) signals added to one or more of the channels 31-34, can be an additive signal or subtractive signal depending upon the needs of the operator or the process being monitored or observed on the oscilloscope 40. As a result of the buffer amplifier 56 being an operational amplifier which presents a low input impedance when it is connected via one of the switches 51-54 to one or more of the channels 31-34, the inputs to the amplifier 56 are currents and equal to the voltage drops across the resistors (such as the resistors 91a and 91b) in one summing network (such as the network 91) which is connected to the amplifier 56 divided by the resistances (such as the resistances of resistors 91a and 91b). In one embodiment of the invention all the resistors 91a-94a and 91b-94b have the same resistance value. Of course, the resistance value of each one of these resistors need not be equal and can be varied as desired.

The signal altering system 10 of the invention can be utilized in various ways. For example, and with reference to FIG. 4, the system 10 can be utilized for canceling out the noise which occurs at the beginning of the particle size distribution curve 70. In other words, it can be used for eliminating the leading edge 84. When the system 10 is to be used in this manner, the apparatus 12 can be operated without a given amount of sample such as by passing a fluid or diluent through the particle analyzing device. The visual display on the oscilloscope will essentially be only the leading edge 84 of the curve which, however, tapers to 0 as indicated by the phantom line 122. This fictitious curve 122 or particle size distribution for the smaller sized particles can then be eliminated from the visual display by operating the mechanism 120 to connect the potentiometer 116 to the negative voltage source, and then by operating the first few potentiometers 111 and 112 . . . associated with the first several channels 31, 32 . . . until the visual display on the oscilloscope 40 is 0. In effect, a plurality of negative signals equal to the positive signals from the accumulators is added to the positive signals from the accumulators to obtain a 0 net output signal. The negative signals added to the fictitious or erroneous positive signals represented by the curve 122, is indicated by the phantom line curve 124 in FIG. 4. As a result, the particle size distribution curve for the next sample analyzed will have a more correct leading edge as indicated by the broken line 126 in FIG. 4.

Of course, it is to be understood that as particles are being sensed and output pulses are being accumulated in the accumulators, the switching device 44 is cyclically and sequentially operated to close the switches 51-54 such that the developing particle size distribution curve is continually being traced on the oscilloscope 40 as indicated by the curves 131-133 shown in broken lines on FIG. 4. After the given amount of sample has been passed through the particle analyzing device, the output signal level on each one of the accumulators 21-24 will have reached a steady state so that successive traces of the particle size distribution curve on the oscilloscope 40 will be the curve 70.

Another way in which the signal altering system 10 of the invention can be utilized in a particle size analyzing apparatus 12 is by passing a known sample having a desired particle size distribution through the particle analyzing apparatus 12 to obtain a desired particle size distribution curve on the oscilloscope 40. Then the switch 120 is operated to connect the potentiometer 116 to the negative voltage source and each one of the potentiometers 111-114 is rotated until the visual display on the oscilloscope 40 is 0. In other words, until the visual display is a line lying on the horizontal axis. Then, of course, when the accumulators are cleared of the stored output signal levels therein, the visual display on the oscilloscope 40 will be a negative representation of the desired sample distribution curve. Stated otherwise, a negative mirror image of the desired particle size distribution curve will now appear on the scope as shown in phantom line at 140 in FIG. 5. Next, a given amount of sample which should have the desired particle size distribution is passed through the particle analyzing device. The particle size distribution curve which would normally be generated by the given amount of sample, is indicated by phantom lines at 142 in FIG. 5. However, since the signal altering system 10 is adding in negative signal levels, as represented by the curve 140, to the output signal levels from the accumulators 21-24, the resulting signal appearing on the oscilloscope 40 is a curve 144 which represents the deviation in particle size distribution in the given amount of sample tested with the desired or standard particle size distribution curve, 140.

Of course, instead of generating a negative sample distribution curve such as the curve 140 in the manner just described above, an operator can overlay the screen of the oscilloscope 40 with a mask containing a desired negative particle size distribution curve and then adjust the potentiometers until the curve traced on the screen of the oscilloscope 40 is coincident with the desired curve on the mask. Once the potentiometers are properly adjusted in this manner, a given amount of sample of a mixture being prepared can be passed through the particle analyzing device and the deviation of the particle size distribution in the mixture can be noted and then the addition of particles of known size can be added to the mixture or the pressure on grinding wheels can be adjusted until the deviation is essentially zero.

When the signal altering system 10 of the invention is being utilized to produce a curve of the deviation between a standard particle size distribution curve and the particle size distribution curve of a given sample, the leading edge of the curve can be eliminated. In this respect, the first few potentiometers associated with the first few channels can be set at 0 and the accumulators connected to the first few channels can be disabled such that no signal appears before the time T as shown in FIG. 5. In the alternative, or where the accumulators do not have disabling means, the switches 85, 86, etc., can be opened or connected to the common conductor 83. Then, the potentiometers can be used to substitute for disabled or disconnected accumulators, thus providing extrapolated points for inclusion in a subsequently generated integral curve. This ability of the system 10 to extrapolate a curve into a region where for one reason or another it is impossible to take good data is important where an integral curve of the particle size distribution curve is desired. In this respect, at the "very small-particle" end of the particle size spectrum, it is known that not all of the particles are sensed by the scanning device 14 and at the "very large-particle" end it is known that large particles may "settle out" and not be sensed. As a result the particle size distribution curve does not include 100 percent of the data, i.e., particles. Now, where the integral curve, presumptively based on 100 percent of the data, is utilized to obtain information regarding the percent of particles above a given size it is necessary that the integral curve be based on 100 percent data. Accordingly the missing data at the ends of the particle size spectrum must be obtained by further measurement or by extrapolation and included in the data utilized in generating the integral curve. The system 10 provides a simple means for introducing, by extrapolation, the missing data into the data utilized for generating the integral curve. This is illustrated graphically in FIG. 6 where there is shown an integral curve 160 of the particle distribution curve 70, as altered by extrapolation. As shown, the erroneous leading edge 84 below a first particle size P, is replaced by an extrapolated leading edge 161 and an erroneous trailing edge 162 above a particle size P.sub.2 is replaced by an extrapolated trailing edge 163. Typically, the resulting integral curve 160 is inverted to the curve 165 so that the percent of particles having a size above a given size P can be read directly from the visual display device 40. For this application, it would be possible to reduce the number of potentiometers to those channels which measure only the ends of the size distribution.

It will be appreciated that by only showing the deviation of a particle size distribution from a standard as shown at 144 in FIG. 5, the apparatus can be very effectively used for quality control.

From the foregoing description it will be apparent to those skilled in the art of data distribution analysis that obvious modifications in variations can be made to the system of the invention, and an apparatus incorporating same. For example, although the system 10 has been particularly described with reference to its use in a particle size analyzing apparatus, it is to be understood that it can be utilized in other apparatus where the distribution of data is to be analyzed, particularly, where it is desirable to compare, eliminate, or replace the deviations between a known data distribution and a data sample. Accordingly, the scope of the invention is only to be limited as necessitated by the accompanying claim.

Claims

What it is desired to be secured by Letters Patent of the United States is:

1. In a method for presenting a visual display of particle size distribution including the steps of passing a given amount of sample containing a plurality of particles through a sensing zone, generating a particle pulse for each particle sensed, each particle pulse having an amplitude related to the size of the particle sensed, segregating the particle pulses into a predetermined number of pulse amplitude ranges for classifying the particles according to size, generating in each range an output pulse for each particle pulse in a given range, each output pulse having an amplitude related to the size of the particle classified, separately accumulating the output pulses in each range to obtain a plurality of D.C. output levels each being indicative of the number and/or total volume of particles in a given size range and cyclically applying the D.C. output levels in a timed sequence to a visual display device to present thereon a particle size distribution curve, the improvement comprising the steps of adding predetermined D.C. signal levels to selected D.C. output levels and/or replacing selected D.C. output levels with a chosen set of D.C. signal levels to alter the particle size distribution curve in a predetermined manner.

2. The method according to claim 1 wherein said predetermined D.C. signal levels constitute the negative equivalent of a standard particle size distribution curve and are added to or compared with the D.C. output levels representing the sample particle size distribution curve whereby the curve appearing on the visual display device represents the deviation in particle size distribution between the sample particle size distribution and the standard.

3. The method according to claim 1 including the steps of sequentially, and continuously for one cycle, applying the unaltered, altered and/or substitute D.C. levels to the visual display device to present thereon an integral curve which is the integral of the particle size distribution curve.

4. In a particle size analyzing apparatus including a particle sensor which produces a particle pulse upon sensing a particle in a given amount of sample when the particle is in a sensing zone, the amplitude of each particle pulse being related to the size of the particle sensed, a plurality of particle pulse amplitude discriminators for sensing particle pulses within predetermined pulse amplitude ranges related to given particle size ranges and for producing an output pulse having a predetermined amplitude for each particle pulse sensed within one of said amplitude ranges, accumulators for separately accumulating the output pulses from each discriminator, an output channel connected to each accumulator, a visual display device, and an electronic switching device for sequentially, alternately and cyclically connecting each of said output channels to said visual display device for presenting on the device the accumulator output signal levels on said channels in spaced relation thereby tracing a particle size distribution curve for the given amount of sample on said visual display device, the improvement comprising means for adding predetermined D.C. signal levels to selected ones of the channels and/or for replacing selected output signal levels applied to the channels from the accumulators with a chosen set of D.C. signal levels whereby the curve traced on said visual display device can be altered at the discretion of the operator such as for eliminating noise from the particle distribution curve thus formed and for comparing the curve traced with a standard so as to present a curve of the deviation of the particle size distribution from the standard.

5. The combination according to claim 4 wherein said signal adding and/or replacing means includes a plurality of disconnect switches, each switch being located in one of the selected channels for disconnecting the associated accumulator from that channel.

6. The combination according to claim 4 wherein said signal adding and/or replacing means includes a plurality of potentiometers equal in number to the number of selected channels, each potentiometer being connected to one of the selected channels and being adapted to be connected to a source of negative or positive voltage potential.

7. The combination according to claim 6 wherein said signal adding and/or replacing means includes a plurality of summing networks equal in number to the number of potentiometers, each summing network including an impedance element in one of the selected channels and an impedance element connected between that selected channel and one of said potentiometers.

8. The combination according to claim 7 wherein said signal adding and/or replacing means includes a disconnect switch in each of the selected channels between the output of one of the accumulators and the said impedance element in the channel, each said switch being operable to disconnect the associated accumulator from the channel so that the accumulator output signal level can be replaced with one of the chosen D.C. signal levels.

9. The combination as claimed in claim 6 including a switching means for alternately connecting said potentiometer to a negative voltage source for adding a negative signal to each selected channel, to a positive voltage source for adding a positive signal to each selected channel and to a neutral position where no signal is added to each channel.

10. The system according to claim 4 wherein said signal adding and/or signal replacing means is operable to replace the signal levels in the channels which contain signal levels forming the leading edge of the particle size distribution curve.