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PDA3000 Particle and Flocculation Monitor

The PDA3000 Particle and Flocculation Monitor is a simple, rugged, but very sensitive monitor for flowing suspensions and emulsions, especially for assessing their state of aggregation. The method is based on the Turbidity Fluctuation principle and was originally developed at University College London. The unit is very compact and lightweight.

Applications include:

  • Monitoring coagulation/flocculation processes and selection of optimum dosages of additives. Particles from sub-micron to over 1 mm can potentially be monitored.
  • Comparison of strength of aggregates (flocs) under different conditions.
  • Control of dispersion and emulsification processes.

Some advantages of the PDA3000 over other techniques are:

  • Simple to operate—little attention needed.
  • Powered from computer—no separate power supply needed.
  • A wide range of suspension concentrations can be directly monitored.
  • No orifices to clog.
  • Minimal problems due to contamination of optical surfaces.
  • Novel flow cell using inexpensive, disposable plastic tubing—no connection problems.
  • Cell can be mounted remotely up to 5 m from the controller.

Principle of Operation

The flowing suspension is illuminated by a narrow beam of light perpendicular to 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 a wavelength 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 beam passes through the flowing sample by means of two aligned pinholes about 1 mm in diameter. The standard cell is designed for tubing of 3 mm bore, but other cells, for 1 mm bore and 5 mm bore tubing can be supplied.

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 particles in the sample volume (i.e. the volume illuminated by the light beam, typically of the order of 1 mm3 with a 3 mm 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.

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.

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 aggregates 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 or by fouling of the tube wall by adhering particles. The Ratio is often referred to as the Flocculation Index.


The PDA3000 is operated using our free Rank Brothers Logger software. This allows the light intensity and sampling interval to be set and any two of the outputs (DC, RMS, and RMS/DC) to be displayed graphically. Logged data can be saved to a file or exported to a spreadsheet.


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