Method and Apparatus for Correcting Total Particle Volume Error Due to Particle Coincidence

FIG. 1 is a block diagram of the error correction circuit of this invention shown incorporated into a portion of FIG. 1 of the patent which is incorporated by reference.

FIG. 2 shows the waveforms at various points of the block diagram shown in FIG. 1.

When a circuit corresponds to a circuit in the incorporated patent the circuit number in this application will be followed by an a subscript in order to simplify crossreferencing to the incorporated patent.

For a complete understanding of the purpose and the operation of the Particle Size Analyzing Apparatus and Method Using Threshold Level Control, reference is to be made to the patent incorporated by reference. However, for purposes of a more clear understanding of this application, some explanation of the circuitry shown and described in the incorporated patent, and operatively associated with the error correction circuitry, is in order. The explanation will be given with respect to FIG. 1 of this application.

In FIG. 1, particles in a particulate system are passed through a Coulter type aperture tube 10a. An electric current also is passed through the aperture tube 10a. Particle pulses are produced as a result of the modulation of this current by passage of the particles through the aperture tube 10a. Each pulse amplitude is proportional to the size or volume of the pulse producing particle. These particle pulses are amplified by amplifier 14a and coupled to a comparator 20a, via conductor 18a. One such amplified pulse is shown in FIG. 2, waveform A. If the particle pulse exceeds a preset level or threshold, as determined by voltage source 28a, comparator 20a will develop a comparator signal such as shown in FIG. 2, waveform C which is coupled to an input of AND gate 36a by conductor 38a. This pulse is present only during the period that the amplified pulse exceeds the preset threshold level. The pulse shown in FIG. 2, waveform A exceeds this threshold during the period exceeding from time T.sub.1 to T.sub.2. AND gate 36a will develop an output signal in response to the comparator signal which is coupled via conductor 40a to electronic switch 22a for operating switch 22a to allow the particle pulses from amplifier 14a to pass through electronic switch 22a to a pump circuit 44a. Pump circuit 44a converts each pulse to a charge which is coupled via conductor 46a to integrator 48a which accumulates the charges. Integrator 48a in effect adds all of the signals passed by the electronic switch 22a, and the voltage level at its output terminal 50a at any time represents the accumulated signals. If the signals are accumulated for a predetermined number of particles, the voltage level will also represent the mean particle volume. It is assumed that the count of the number of particles accumulated also has been corrected for coincidence loss by some arrangement such as is described in copending patent application Ser. No. 441,752 filed Feb. 12, 1974 so that the mean particle volume can be obtained based upon a correct particle count. Because of particle coincidence the voltage or output signal data appearing at output terminal 50a, will be slightly in error if samples having high particle concentrations are employed.

In order to eliminate the coincidence error in the output signal data, a coincidence error correction circuit, shown in dotted lines and identified generally by the number 100 is employed. The amplified particle pulse such as shown in FIG. 2, waveform A, also is coupled by conductor 18a to a pulse stretcher 104. Pulse stretcher 104 will develop an output signal voltage that increases in amplitude to a voltage which is equal to or is related to the amplitude of the particle pulse developed at the output of amplifier 14a. Accordingly the output voltage is equal or related to the particle size. Once the maximum amplitude of the pulse has been reached, pulse stretcher 104 will maintain that maximum amplitude. The stretched signal output developed by pulse stretcher 104 is coupled by a conductor 106 to the first input of a second comparator 108. The stretched signal output developed on conductor 106, and produced by the pulse shown in FIG. 2, waveform A, is shown in FIG. 2, waveform B.

In addition to coupling the output of comparator 20a to AND gate 36a as previously noted, the output of comparator 20a is also coupled to a trailing edge detector 110 and a rate meter 112, both in coincidence correction circuit 100. Rate meter 112 is of the type commonly known in the art which will develop an output voltage at conductor 114 that varies in accordance with the repetition rate of the pulses coupled thereto. Accordingly, the voltage at conductor 114 will be at some amplitude which is determined by the repetition rate over a preceding predetermined time period of pulses such as shown in FIG. 2, waveform C.

Trailing edge detector 110 responds to the trailing edge of each pulse developed by comparator 20a and develops a pulse which is coupled to the set input 116 of a bistable multivibrator 118. The pulse developed by trailing edge detector 110 in response to the signal shown in FIG. 2, waveform C is shown in FIG. 2, waveform D. Bistable multivibrator 118, better known as a "flip-flop", responds to the set pulse at input 116 and develops a control signal pulse of fixed amplitude at its output. The control signal produced by flip-flop 118 in response to waveform D is shown in FIG. 2, waveform E. This pulse is coupled by a conductor 120 to the control input 122 of analog switch 124 and to the input of an integrator 126. Analog switch 124 operates in response to the control signal coupled to input 122 to couple the voltage developed by rate meter 112 at conductor 114 to summing resistor 128. Summing resistor 128 converts the voltage to a charging current whose amplitude is proportional to the amplitude of the voltage developed by rate meter 112. As the rate meter voltage amplitude varies in accordance with the repetition rate of particle pulses, the charging current amplitude will vary with the particle pulse repetition rate. The charging current is coupled from summing resistor 128 to integrator 48a in the same mnner as the charging current developed by pump circuit 44a, and is accumulated in integrator 48a along with the currents from pump circuit 44a.

The pulses developed by flip-flop 118 such as for example the pulse of FIG. 2, waveform E, are also coupled to integrator 126. Integrator 126 will integrate each pulse coupled thereto and develop an integration voltage such as shown in FIG. 2, waveform F. Waveform F is superimposed upon waveform B in order to simplify certain portions of this explanation. The integration voltage developed by the integrator 126 is coupled via conductor 130 to the second input of comparator 108. As previously noted the voltage amplitude at the first input of comparator 108 is equal to or related to the particle size. When the voltage developed by integrator 126 reaches and exceeds the voltage developed at the output of pulse stretcher 106, comparator 108 will change states and develop a comparison signal at its output which is coupled by the conductor 132 to a monostable multivibrator 134, more commonly known as "one shot". One shot 134 will change states in response to the comparison signal and develop a pulse for a very short period of time as shown in FIG. 2, waveform G. This pulse is coupled via conductor 136 to the reset inputs of integrator 126, flip-flop 118 and pulse stretcher 104 resetting all three devices. With flip-flop 118 reset, the pulse developed at conductor 120 and coupled to control input 122 of analog switch 124 terminates as shown in FIG. 2, waveform E, thus terminating the voltage coupled from rate meter 112 to summing resistor 128. Accordingly, the charging current coupled to integrator 48a for providing the volume coincidence correction to the voltage developed at conductor 50a is terminated. As flip-flop 118 is set, or on for a time period which is related to the size of particles the charging current duration is related to particle size.

In the preferred embodiment, the component values employed in rate meter 112, integrator 126 and current limiting resistor 128 are determined via trial and error method. Specifically, a known volume of particulate matter in a quantity of liquid is passed through the apparatus and the voltage representing the volume of particulate matter is noted. The particulate concentration is then changed, changing the total volume of particulate matter in the sample. Preferably, the dilution is doubled, thus halving the concentration. The sample then is again passed through the apparatus and the total voltage representing the total volume of particulate matter is noted. If the component values selected for rate meter 112, integrator 126 and charging resistor 128 are correct, the voltage for the diluted sample should be one-half the voltage developed due to the original sample .