PDA3000 Particle and Flocculation Monitor

The flowing suspension is illuminated by a narrow beam of light perpendicular in the direction of flow. For the PDA3000, a novel flow cell has been developed; transparent flexible tubing fits in a detachable block that houses a light source (miniature light-emitting diode) and a sensitive photodetector. The LED has wavelength in the in the near infrared and the detector response is closely matched to this. The sample flows through transparent plastic tubing located in a slot between the light source and detector. A narrow light passes through the flowing sample by means of two aligned pinholes about 1mm in diameter. The standard cell designed for tubing of 3mm bore, but other cells, for 1mm bore and 5mm bore tubing can be supplied.

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The output from the photodiode is converted to a voltage, which consists of a large average (DC) component, together with a small, fluctuating (AC) component. The DC component is simply a measure of the average transmitted light intensity and is dependent on the turbidity of the suspension. The AC component arises from random variations in the number and size of particle in the sample volume (i.e. the volume illuminated by the light beam, typically of the order of 1mm(3) with a 3mm bore tube). Because the suspension flows through the cell, the actual sample in the light beam is continually being renewed and local variations in particle number and size give fluctuations in the transmitted light intensity. These fluctuations cease when the flow is stopped.

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The root mean square (RMS) value of the fluctuating signal is related to the average number concentration and size of the suspended particles. For fairly uniform suspensions, estimates of particle size and number concentration can be made, but the main use of the PDA3000 is in the monitoring of flocculation and dispersion processes.

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The RMS value of the fluctuating signal increases when aggregation of particles occurs. Measurable changes in the RMS value occur long before any visible signs of aggregation are apparent. Conversely, when aggregate are broken, the RMS value decreases, reaching a minimum when disaggregation (or dispersion) is complete. The DC value (related to the turbidity) is much less sensitive to changes in the state of aggregation, but can still give useful information. In many cases, the Ratio value (RMS/DC) is the chosen parameter, since this is not affected by drift in the opto-electronic components for by fouling of the tube wall by adhering particles. The Ratio is often referred to as the Flocculation Index.