Presently, instruments employed in the measurement of particle size have attained a degree of accuracy that was unimaginable years ago. Tolerances are constantly being minimized, and the newest commercial versions work faster and more accurately than their predecessors.
The launch of software graphics together with multitasking operating systems coupled with ever more robust computers has resulted in higher reliability and reproducibility than in the past. The results acquired in this way are reliable.
That's the theory. However, in practice, the situation is occasionally a little more complex. Although the analysis assures precision and accuracy, eccentric results mysteriously occur, raising uncertainties about the highly praised instruments which promise a lot. Yet, the true reason for the anomalous results is not in the testing process with a floating decimal point or operator error; it is just because of the procedure of the preparation of samples before they are presented to the measurement system.
It is an incredible fact that the market for high-precision completely automatic analyzers is continually growing, while no one seems to be apprehensive about the preparation and collection of the samples, factors which are equally, if not more significant, in obtaining a reliable result. Insight on the aspect of particle size has improved as higher levels of accuracy have been realized.1 The importance has currently shifted toward the sample-preparation phase of the measurement process.
The Task at Hand
The practical application of costly, superior-quality analyzers is directly linked to the sample presented for analysis. The analysis error always comprises both measurement and sampling errors. Based on the law of error propagation, the determined total error in the value of the results is2
Stot = √s2 measurement + s2 sample
The standard deviation of the value of the results in relation to the analyzed material is established by the analyzers to a great extent only if the variance for the sample preparation is evidently less than the variance of the analyzers. Thus, the results can be reproduced only if the sample to be tested is entirely representative of the material to be analyzed.
In this situation, “representative” is usually understood to mean that the samples taken can be equated with the whole batch a priori. If the mixture state of the original sample is illustrated by a property function, the sampling is illustrative if the measured values distributed over the location sufficiently approximate the property function.3
If several individual samples are obtained from a well-mixed whole, its composition is subject to a statistic variation insofar as a particular feature is concerned. An error can creep in because a sample is obtained from a segregated standard quantity and always occurs in this case, since mixtures are never suitable. To reduce the errors, it is best to obtain as many small samples as possible from random locations in the basic whole and then mix them.
Typically, the sample available to the laboratory may be 2000 g, while the quantity used for the analysis is less than 200 mg. Therefore, an accurate technique for sub-dividing the lab sample is required, so that the trial 200 mg used for the analysis is completely illustrative of the original. This is the perfect situation and can only be accomplished with the help of an accurate sample divider. Whether division is achieved in one or more steps, it has no relation to error preparation only on the costs of the extra cleaning processes.
The FRITSCH Rotary Cone Sample Divider LABORETTE 27 is thus available with a division ratio of 1:30 among others. This makes it possible to split total samples of up to 300 ml into 30 individual samples in a single step. Moreover, dividing cones with ratios of 1:8 or 1:10 are available in many materials so that they can be matched to various types of samples.
The patented design is based on the combination of three division techniques within one instrument. The sample is fed onto a dividing cone through a hopper. The dividing cone is built to emulate the process of “Coning & Quartering” which is accepted as being the most accurate method for sample division. The whole system rotates and the sample material is accelerated outward such that it is moved into the guide channels, making sure that nearly 30 individual samples are gathered. The rotation of the dividing cone boosts the number of divisions to nearly 2600 per minute; hence, the final sample is made up of a very large number of individual samples.
A mixture comprising of about 800 g of quartz sand was produced and divided. Figure 1 compares the FRITSCH Rotary Cone Sample Divider with a rotary sample divider with local feeding directly over the sample containers. To realize the same ratios, the dividing cone for eight 500 ml sample containers was used. An examination of equal division of samples to the sample containers showed that the LABORETTE 27 is obviously superior to traditional sample dividers.
Figure 1. Sample division with eight sample containers.
Figure 2 illustrates no substantial differences in the particle size distribution for each receiver. The deviations per sample are very minute. The analysis was done with the help of the FRITSCH Particle Sizer ANALYSETTE 22.
Figure 2. Particle size distribution.
The presentation of the d50 values as seen in Figure 3 illustrates that the median value of distribution in all sample containers is properly denoted.
Figure 3. Median values per container.
Figure 4 illustrates the median values of particle size distribution in the individual sample containers in series 1, while eight reproducibility measurements of the same material are plotted in series 2 without any change in the instrument parameters.
Figure 4. Comparison of multiple measurements with sample division.
The median average of the eight reproducibility measurements is x = 63.1925 ± 0.012 (µm), while the median average of the independently measured samples in the eight sample containers is x = 63.1975 ± 0.027 (µm). The comparison shows that the accuracy of the analyzer is naturally substantially higher than the accuracy after sample division. However, it is obviously evident that the systematic error caused by the LABORETTE 27 is in fact very minute.
The dividing accuracy has a critical impact on the precision of a feature analysis. It can be reduced through design measures. In this case, FRITSCH succeeded with the help of the LABORETTE 27 — the rotary cone sample divider enables advanced analyzers to completely exploit their capabilities. The LABORETTE 27 should be on par with every advanced analyzer.
References and Further Reading
- Dipl.-Phys. Götz von Bernuth, “Probenvorbereitung,” Kontrolle, September 1984
- Dr.-Ing. H. Müller, Dr.-Ing. D. Espig, Ing. S. Kauter, “Probenvorbereitung auf wissenschaftlicherGrundlage,” Aufbereitungstechnik, 8/1986
- Prof. Dr.-Ing. Karl Sommer, “Seminar Fritsch Probennahme,” Mannheim, Germany, June 28, 1995
This information has been sourced, reviewed and adapted from materials provided by FRITSCH GMBH - Milling and Sizing.
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