Correlating ‘laser diffraction’ and ‘conventional’ Blaine measurements

Blaine number measurement is based on a packed-bed pressure-drop method and gives a single averaged result for each sample. Laser diffraction particle size measurement is very different, an ensemble sizing method that determines the size distribution of particles present, from an understanding of the way in which they scatter light. Using particle size distribution data it is relatively easy to calculate an averaged specific surface area figure that equates to Blaine number - a ‘laser diffraction’ Blaine. This can be valuable for building confidence when switching from one technique to another, but in the long term it is not the optimum way to use laser diffraction particle size distribution data.

A recent analysis of measurement sensitivity has verified the ability of a Malvern laser diffraction instrument to resolve the 7% variations in Blaine numbers of the samples tested. In further work to develop a correlation between “laser diffraction” and “pressure drop” Blaine, samples of a particular type of cement representing the high-acceptable, low-acceptable, and average-acceptable Blaine characteristics (i.e. the control range of interest) were measured. Conventional Blaine measurements were performed on these samples by the product manufacturer, and compared with results obtained using the Insitec system. This comparison indicates that the Blaine number, or specific surface area (SSA), yielded by the optical instrument can be used as a surrogate for a pressure-drop Blaine analysis.

Correlation between Blaine measurement and Insitec

Allen (Allen, Terence, "Particle Size Measurement," Vols. I & II, 5th Edition, Chapman and Hall, 1997) has analyzed the reliability of the Blaine concept and states: "The assumptions made in deriving the Carman-Kozeny equation (on which the Blaine number is based) are so sweeping that it cannot be argued that the determined parameter is a surface," and "The determined surface areas are usually lower than those obtained by other measuring techniques." This last comment is obviously consistent with the current optical results, which give higher values than the Blaine number.

These initial successful comparisons of Blaine and SSA measurements encouraged more comprehensive test comparisons. The figure below shows the results of a two-week test series comparing independent Blaine results and the optical SSA. The SSA values were normalized to the Blaine number results by a constant, determined in a previous test series. On this graph, the Blaine number error bars indicate one standard deviation (about 1.5% as estimated by ASTM for an individual technician). For multiple technicians, it is known that different techniques may be used and the standard deviation would be of the order of 2.5%.

Correlation between off-line Blaine measurement and on-line Insitec SSA measurement

Note that the two different measurement techniques agree quite well, with a few data points at the beginning and end of the test series showing deviations greater than 1.5%. An error bar has not been applied to the optical instrument measurements, but it can be assumed that it is of the same order, namely 1% to 1.5%. The error bars of the two sets of measurements would overlap in all cases, indicating that the two techniques are statistically in agreement. That is, there is no statistical discrimination for choosing one method as being more accurate or precise than the other.