Title of Invention

AN APPARATUS USEFUL FOR MEASURING PARTICLE SIZE OF A POWDER SAMPLE

Abstract

Apparatus useful for measuring particle size of a powder sample comprises a light source(3), having a collimating lens (4) in front of it in such a manner so as to effect parallelism of the light beam, an infra red filter (5) being fixed in front of the said lens (4) in such a manner so as to enable absorption of the infra red and impinge the said light beam on to a sample (2) being contained in a transparent settling column (1), an optical detector (6) being fixed in a manner so as to detect and feed the light signal so received through the said sample to an amplifier circuit (7) to record the output, the said entire assembly being housed in a light proof box (8).

Full Text

The present invention relates to an apparatus useful for measuring particle size of a powder sample. Determination of particle size distribution is very important in applications where the particle size influences process parameters, such as in cement, paints, powder coating, food processing industries and the high-technology areas of thick-film and hybrid electronic circuit manufacturing and in TV picture tube phospors. Of special importance of estimating particle size in a powder sample is in the drugs industries who make medicinal tablets and capsules involving many ingredients. The estimation of particle size in all the above mentioned applicatins can be considered as an idispensible quality control instrument. There are several methods for particle size analysis including Wagner's method which is the one which had been in use in the cement industry for a long time. This and similar methods employing the principle of sedimentation and turbidimetry have become outmoded with the advent of modern methods based on lasers and high resolution imaging type photodetectors and computerized data processing. The instruments based on these modern techniques are very expensive and apply to a wide range of particle sizes from submicron to several hundreds of microns. Turbidimetric principle is well established as a means for measurement of particle size distribution of powder samples. It uses Stokes1 law of settling of small particles through a measured height in a liquid. When the terminal velocity of a particle is reached in a liquid, the size (weight, diameter) is a function of settling time for equal height and is expressed by d/t=K, where d is the Stoke's diameter of the particle and . K is the constant of proportionality containing terms such as viscosity of the liquid etc. The principle used in Wagner's method is a well established empirical relationship that when a light is shone through a settling column at a fixed height, the transmitted light intensity It which changes with time due to particles gradually settling out of the suspension, is related to turbidity T by the following relationship, T = - log I, Turbidity in turn is defined by, T = k w/d The turbidity at any instant during settling is a function of w, the weight of particles in suspension and inversely as their Stokes1 diameter, k is light extinction coefficient, which is unity for particles above about 1 micron for which the laws of geometric optics holds. Normally all known techniques (such as Wagner's) of turbidimetric analysis is relative, and a change in turbidity is measured to obtain the fractional weight in a small settling duration. This eliminates all unnecessary constants in calculations. Therefore, in a range t1 and t2, T1 and T2 yield the weight w12, and log (l2/Ii)= k (wi2/d12) or, W12 = log (I2/I1) dn/k where I1 and I2 are the intensities Finally, the weight fraction in the diameter range d12 is given by, Wi2 / (Wl2+W23+W34 + ) This is plotted against diameter to obtain the size distribution. The two main drawbacks of the conventional turbidimetry method as above are, (i) the large concentrations of suspension which leads to inaccurate results due to hindered settling (i.e., Stokes' law is not strictly obeyed) of particles in liquid, and (ii) the necessity to convert the transmitted light signal readings to the corresponding logarithms. Using small concentrations, hindered settling is avoided. However, the turbidity signal -(T1 -T2) being small requires amplification. Amplification by conventional means is not possible due to the transmitted light signals being quite large. The main object of the present invention is to provide an apparatus useful for measuring particle size of a powder sample which obviates the above noted drawbacks. Another object of the present invention is to facilitate measurement of particle size by the principle of turbidimetry. Yet another object of the present invention is the measurement of particle size by using small concentrations of the particles in any solvent. In the device of the present invention the clear liquid transmission signal( I0) is eliminated in the amplifier circuit, and it is no longer necessary to obtain logarithms of light intensity readings, since dT = d(logl) = dI/I0, and the signal is linearly proportional to turbidity. In the drawing accompanying the specifications fig 1. shows the schematic diagram of the apparatus of the present invention wherein (1) is a transparent settling column, (2) is the sample of which the particle size is to be determined, (3) is an incandecent bulb used as a light source, (4) is a lens for effecting collimation of the light beam from the light source (3), (5) is an infra red filter for absorbing IR , (6) is a photodetector, (7) is a low noise high gain signal amplifier and (8) is an enclosure for housing all the components and for preventing any external light to enter the system. Accordingly the present invention provides an apparatus useful for measureing particle size of a powder sample, which comprises a light source (3), having a collimating lens (4) in front of it in such a manner so as to effect parallelism of the light beam , an infra red filter (5) being fixed in front of the said lens (4) in such a manner so as to enable adsorption of the infra red and impinge the said light beam on to a sample ( 2) the particle size of which is to be determined, the said sample (2) being contained in a transparent settling column (1), an optical detector (6) being fixed in such a manner so as to detect and feed the light signal so received through the said sample to a signal amplifier circuit (7) recording unit for recording the output, the said entire assembly being housed in a light proof box (8). In an embodiment of the present invention the light source (3) used may be such as any ordinary incandacent bulb. In still another embodiment of the present invention the lens (4) used may be of such a focal length so as to make the beam collimated. In yet another embodiment of the present invention the infra red (IR) filter (5) used may be any conventional IR filter. In another embodimewnt of the present invention the optical detector (6) used may be any device working on the principle of photoelectricity such as a photovoltaic cell. In yet another embodiment of the present invention the signal amplifier circuit (7) used may be a low noise and high gain amplifier. The operational detail of the device of the present invention used to determine the particle size of a powder is described as follows: Shining a light beam on a liquid 2 in a column 1, from a suitable source 3 collimated rthrough a lens 4 and trapped infra red radiation by a fiilter 5 , and setting the initial conditrion oh the amplifier circuit by a zero offset, again shining the light form the same light source through the same arrangement to a liquid having the experimental sample and finally recording the signal on a suitable detector with time. The present invention for particle size measurement combines the use of low concentration of suspension of a powder sample in a liquid, and electronic amplification of the small changes in the transmitted light signal or turbidity due to particles settling out of suspension, to readily estimate the size distribution of the powder in the range of 1 to 50 microns. The range below (to 0.5 microns) and above (to 100 microns) can be measured with a somewhat lesser accuracy by varying the height of settling column (suspension) and duration of measurement. In the device of the present invention the particle size of a powder sample is measured using the principle of turbidimetry. Small suspension concentration is used for unhindered settling of particles, and the small difference in turbidity (the signal) is amplified by an operational amplifier by simultaneously eliminating the clear liquid signal I0. Additionally, by using small concentrations, it is no longer necessary to obtain logarithms of light intensity readings, since dT = d(log I) = dl/10, and the signal is linear. In fig 2 of the drawing is shown the intensity variation in clear and turbid liquid due to different settling time dependent on the particle size . In fig 3. of the drawing an embodiment of the output signal detection circuit has been shown. The present invention is herein described with reference to accompanying figures 1, 2 & 3. A settling column (1), initially contains clear water (2). The light from a stable light source (3) is passed through a lens (4) and an optical filter arrangement (5) to absorb the infra-red of the light and then to convert it to a parallel beam before traversing the column width. At the other side it is detected by a photodetector (6) connected to an amplifier circuit (7) in which an op amp is used as adifferential amplifier. The whole optical arrangement is housed in a box to absorb stray light. Figure 2 shows schematically typical variations of light intensity (transmitted), It with time t. Curve (9) corresponds to normal turbidity (high concentration), the transmitted light intensity increases with time due to particles settling out of suspension and liquid becoming clearer. (The intensity levels out but does not quite reach the clear liquid value lo, due to very small particles with large settling time remaining in suspension.) Curve (10) corresponds to the case of small turbidity (small concentration), the case being dealt with in the present invention. As can be seen, the signal (the curve) is superimposed on a large optical signal I0. In fig. 3. which shows an embodiment of output signal detection circuit the op amp is used as a differential amplifier which first cancels out the signal ( corresponding to the impinged light) at input (13) with the help of an opposing voltage at the other input (14). That is, the input at (14) is adjusted so that it just balances the light signal input at (13), and there is no amplified signal at the output. The description of the op-amp circuit is given here with reference to Fig. 3. Light signal (18) (It) is converted to electrical signal by photodetector with supply voltage (17), and the signal across load (21) is conveyed to input (13) of op-amp (19) through an impedence (22). Power supply (15) provides the necessary dual polarity supply voltage of 9V to the op-amp. A variable bias I0 is provided by a battery (21)and potentiometer (24)at the second input of the op-amp. The amplified signal is obtained across load (20) at the output. The output may be connected by a conventional bnc connector and coaxial cable to an ADC card fitted to a personal computer. After the above procedure of zero adjustment, the column of clear suspension is replaced with an identical settling column with uniformly turbid suspension, and the reading of turbidity or light intensity is monitored with time as shown in curve (11) in fig.2 This signal can be easily amplified since it is clamped at zero base as shown in curve (12) in fig. 2. This directly yields particle size distribution. This method of analysis is also amenable to computerization, whereby the data are fed to the computer via an Analogue toDigital Circuit (ADC) interface. The following examples are given by way of illustration only and should not be construed to limit the scope of the present invention. EXAMPLE-1 In the present examle particle size of a lead oxide phosphor is determined.. The settling column chosen is made of glass . The experiment is started by first putting acetone in the column and recording the intensity variation . Next the differential mode of the operatinal amplifier is used to offset the signal obtained from the clear liquid. Then after this the powder whose particle size is to be measured is put into the settling column containing the liquid, acetone in this case. The concentration of the sample liquid being kept at 7.97 wt.%. As the particles start settling down and the amplifier already adjusted by zero offset to nullify the initial signal from the clear liquid, the light intensity variation due to different settling times is recorded with time and the diameter of the particles measured were 150.07, 97.2,82.1, 75.4, 63.7, 53.8, 49.4, 45.4 and 41.0 microns. EXAMPLE -2 In the present examplere size of a lead oxide phosphor is determined.. The settling column chosen is made of glass . The experiment is started by first putting acetone inthecolumnand recording the intensity variation . Next the differential mode of the operatinal amplifier is used to offset the signal obtained from the clear liquid. Then after this the powder whose particle size is to be measured is put into the settling column containing the liquid, acetone in this case, the concentration of the sample liquid being kept at 2.09wt%. As the particles start settling down and the amplifier alresdy adjusted by zero offset to nullify the initial signal from the clear liquid, the light intensity variation due to different settling times is recorded with time and the diameter of the particles measured were 30.4, 32.4,29.0 and 27.4 microns. EXAMPLE -3 In the present example particle size of a lead oxide phosphor is determined.. The settling column chosen is made of glass .The experiment is started by first puttingacetone in the column and recording the intensity variation . Next the differential mode of the operatinal amplifier is used to offset the signal obtained from the clear liquid. Then after this the powder whose particle size is to be measured is put into the settling column containing the liquid, acetone in this case, the concentration of the sample liquid being kept at 1.35 wt % .As the particles and the amplifier alresdy adjusted by zero offset to nullify the initial signal from the clear liquid, the light intensity variation due to different settling times is recorded with time and the diameter of the particles measured were 17.9, 15.2, 13.9, 11.8, 10.8, 9.1, 8.4, 7.7, 7.1 and 6.5 microns. The main advantages of the present invention are: 1. The apparatus is amenable to computerisation. 2. Determination of particle sizes in an economic and cost effective manner. 3. Small concentrations of the solvent with the powder sample is required, 4. Gives direct readings . We Claim; 1. An apparatus useful for measuring particle size of a powder sample , which comprises a light source (3), having a collimating lens (4) in front of it in such a manner so as to effect parallelism of the light beam, an infra red filter (5) being fixed in front of the said lens (4) in such a manner so as to enable adsorption of the infra red and impinge the said light beam on to a sample ( 2) the particle size of which is to be determined, the said sample (2) being contained in a transparent settling column (1), an optical detector (6) being fixed in such a manner so as to detect and feed the light signal so received through the said sample to a signal amplifier circuit (7) recording unit for recording the output, the said entire assembly being housed in a light proof box (8). 2. An apparatus as claimed in claim 1, wherein the light source used is any ordinary incandescent bulb. 3. An apparatus as claimed in claim 1, wherein the infra red filter used is any conventional IR filter. 4. An apparatus as claimed in claim 1, wherein the optical detector used is any device working on the principle of photoelectricity such as photovoltaic cell. 5. An apparatus as claimed in claim 1, wherein the signal amplifier circuit used is a low noise and high gain amplifier. 6. An apparatus useful for measuring particle size of a powder sample substantially as herein described with reference to the example4s and drawings accompanying this specification.

Documents:

1684-del-1998-abstract.pdf

1684-del-1998-claims.pdf

1684-DEL-1998-Correspondence-Others.pdf

1684-del-1998-correspondence-po.pdf

1684-del-1998-description (complete).pdf

1684-del-1998-drawings.pdf

1684-del-1998-form-1.pdf

1684-del-1998-form-19.pdf

1684-del-1998-form-2.pdf