## 1 Introduction  
The mean particle size, particle size distribution, and specific surface areas of powders are evaluated by a variety of analytical methods. Some of them, which use measuring principles based on light scattering, electric field perturbation, particle settling in a liquid, gas permeability, and gas absorption, are well-suited for testing fine and superfine powders, i.e., powders composed of particles smaller than 45 and 10 \(\upmu\)m, respectively. In recent years, laser light diffraction has become the most widely accepted method of determining the particle size distribution of metal powders [1; 2]. The commercially available instruments operate on the principle of coherent light scattering on particles most often suspended in water or an organic liquid. The liquid dispersant is re-circulated continuously through the measurement cell. From the angle and intensity of the diffracted laser beam, the particle size distribution is calculated according to the Fraunhofer-Mie theories [3; 4]. A typical instrument control, data acquisition and data analysis software, has also the capability to calculate the specific surface area and two mean particle sizes defined in the ASTM E799-03 standard [5], namely, the De Broukere mean diameter and the Sauter mean diameter. The simplest equipment has an operating range of around 1 to 300 \(\upmu\)m. More sophisticated designs, incorporating a dual light source (red laser light and blue laser light), offer significant improvements in the measurement resolution below 1 \(\upmu\)m and can effectively measure particles falling into the whole range of particle sizes used in powder metallurgy (P/M) applications (0.1-1000 \(\upmu\)m). When analyzing fine powders, especially the superfines, care must be taken to properly disperse the powder in the slurry by choosing appropriate suspension liquid and, if necessary, adding a surfactant or dispersant to the suspension. It is common practice to use ultrasonic sound waves to break up weakly agglomerated particles. Some newly developed instruments are additionally fitted with dry dispersion units. They allow a dry powder sample (4-10 g) to be suspended in a flowing air stream, pass through the measuring zone, and be sucked into a vacuum cleaner. This usually results in poorer powder dispersion, but in conjunction with an analysis of liquid suspension, it can be useful in assessing the amount of agglomerated material in the dry state. Laser light diffraction offers certain advantages over other testing methods. The ease of operation, rapid analysis, and relatively simple specimen preparation are primary reasons for its wide usage in research and quality control routines for comparing different lots of metal powders [1; 4]. Several limitations must be addressed, however, in using coherent light scattering for particle size analysis. First, the principle behind light scattering assumes the spherical shape of particles. Therefore, deviations from sphericity contribute to a great extent to errors in the particle size distribution results. Second, the equipment design precludes analysis of concentrated suspensions. Hence, the tested sample is hardly representative of the powder lot. Third, the instrument cannot distinguish between dispersed/primary particles and agglomerates. This leads to an overestimation of coarser particles. A comprehensive determination of physical particle properties by laser light diffraction alone is impossible. Therefore, complementary techniques such as scanning electron microscopy (SEM) and testing the gas permeability of a packed bed of powder are also used extensively throughout the P/M research centers and industry [1]. SEM serves as a means to measure individual particles and affords the opportunity for particle morphology and shape analysis. Gas permeability testing is most often carried out with two commercial test devices, i.e., the Fisher Sub-Sieve Sizer (FSSS) and the Sub-Sieve AutoSizer (SAS).