DmP Pof15 500

where dm is diameter, /'m. and /-and P0 are densities of particle and liquid, respectively. Typical values are:

Density, g/cm3 2 5 10 17 dm (max), ■■m 500 62 19 8

Below a value of 100 for this function, sieving becomes increasingly effective. Thus, some samples may require both sieve and electrozone analyses, with blending of the two data sets in the overlap region of 100 to 500 for this size-density function.

Metal Conductivity/Magnetism. The oxide surface layer present on most metal particles causes them to appear to be nonconductive, in effect creating a barrier impedance. Electrozone currents high enough to produce 1 to 3 V across the surface layer overcome this barrier impedance, thus causing conduction for only part of the pulse cycle (prior to the occurrence of sulfur polarization and ion depletion). Consequently, the particle pulse will erroneously appear to be smaller.

This effect can be detected by test observation of the total relative volume of particles per unit volume of suspension with increased current value. Samples may be treated with coating reagents or surfactants to increase surface layer impedance. Orifice current settings then must be kept below the level where this effect appears. Similarly, porous particles appear to be too small, due to the conductance of the pores aligned with the electric field, unless they are plugged with nonconductive material.

Magnetic powders may be dispersed for a sufficient time to permit electrozone measurement if they are degaussed for 20 min at temperatures as high as 750 °C (1380 °F) (but below the sintering point, however) and/or spatulated with viscous honey to inhibit reagglomeration. A microscope slide covered with the honey suspension is then dipped into the electrolyte in the stirred sample beaker. In this manner, data acquisition is initiated instantly, while the suspension is being formed from the dissolving honey.

Advantages. The electrozone method rapidly measures particle size distribution with excellent low-end sensitivity and with good resolution (volume response) and precision (minimal side effects).

By contrast, photozone (photicsensing zone) methods, which include light-beam scattering, blockage, and diffraction, have much lower resolution and precision. This is due to their area response and several significant side effects, including photic properties of particles, high coincidence levels, and beam/sensor instabilities. Sieving and sedimentation methods have similar limitations to response and side effects and are much slower in producing data.

0 0

Post a comment