Photosedimentometers

Latex particle sizes and polydispersity were measured by use of an ICI Joyce Loebl disc centrifuge (JLDC) photosedimentometer (11,12). 

An Improved Disc Centrifuge Photosedimentometer and Data System for Particle Size Distribution Analysis... 

An Improved disc centrifuge photosedimentometer (DCP) was developed for use in the determination of the particle size and size distribution of latices, pigments and other particulates. Separation is based on Stokes Law for the sedimentation of particles in a centrifugal force field and does not rely on the use of particle size calibrants or standards. The DCP Instrument provides accurate stable particle size analyses over a wide range of conditions while at the same time is rugged enough for heavy use in both a research and quality control environment. A stand-alone data collection, analysis and management system was developed both for routine quality control operation and for research use of the instrument. 

Koehler, M. E., Zander, R. A., Gill, T.T., "An Improved Disc Centrifuge Photosedimentometer and Data System", This Volume. 

The photosedimentometer combines gravitational settling with photoelectric measurement. The principle of the technique is that a narrow... 

A photosedimentometer has also been described for measuring free-falling diameters up to 20 cm in size with an application using coke. The system has been used to accurately measure a range of materials [31 ]. 

Fig. 3.29 Sedimentation kinetics of a 10 w/w suspension of LiChrospher Si 100, 7 im in three different solvent mixtures using a Laser photosedimentometer (Hallmann, 1992). Solvents (1) n-heptane, (2) dioxane-cyclohexane (50/50 v/v), (3) dioxane-cyclohexane-n-octanol (45 45 10 v/v/v).

If the stream of moving material can not be diverted, several samples should be taken and uniformly mixed prior to splitting. For that purpose a conveniently located access door must be provided large enough to accommodate a container which is as wide as the stream of material and can be inserted for a short period of time. If only the installation of small openings is feasible, samples should be taken at different locations across the material flow. The same is true if suspended particles are removed from the process with probes. If the size of a representative sample of suspended solids is compatible with the requirements of the test method, analyses can be carried out directly, e.g. in a photosedimentometer. Dry samples from various points must be mixed and split prior to analyzing. If correctly prepared, the results of such tests can be very accurate. 

The photosedimentometer (Figure 35) uses a stabilized, often monochromatic, light source and a simple lens and aperture system. The light beam thus defined penetrates the suspension in a thermostatically controlled cuvette its intensity is reduced according to the law of Lambert-Beer. The photo element produces a current that is proportionate to light intensity / and is recorded as a function of time. The percentage passing D is calculated with... 

The advantage of the photosedimentometer is that sedimentation is not disturbed by taking samples for the measurement of concentration. Furthermore, of all methods of analysis, this apparatus requires the smallest amount of the powder to be analyzed. In some instruments the duration of the test can be shortened by reducing the height of sedimentation during measurement of the finer particle fraction another possibility to carry out sedimentation analyses more rapidly is the use of a scanning (X-ray) beam. 

The rate of coalescence of the droplets in a macroemulsion is stated to be the only quantitative measure of its stability (Boyd, 1972). It can be measured by counting the number of droplets per unit volume of the emulsion as a function of time in a haemocytometer cell under a microscope (Sherman, 1968) or by means of a Coulter centrifugal photosedimentometer (Groves, 1964 Freshwater, 1966). 

Particle sizes are sometimes misrepresented by the particle size analyzers due to shape. In most cases the instruments determine or calculate the diameter of the circular outline of particles passing an opening (e.g., sieving) or of circles with the same area as projections of real particles dispersed in space (e.g., photosedimentometer [B.12, B.48]. Discrepancies between the measured particle sizes and reality are of particular concern if the shape of the particulate solid is elongated (e.g., rods or needles, Fig. 10.9). 

Figure 3. A typical variation of settling velocity distribution of a natural suspension underlying aggregating phenomena in a flowing system. Data are observed with a photosedimentometer in an aggregating kaolinite suspension, where the suspending medium was a water sample from a river, upstream (A) and downstream (O) from a waste discharge.

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