PDA3000
Particle and Flocculation Monitor
The PDA3000 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:
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Monitoring coagulation/flocculation processes and selection of optimum dosages of additives. Particles from sub-micron to over 1mm can potentially be monitored.
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Comparison of strength of aggregates (flocs) under different conditions.
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Control of dispersion and emulsification processes.
Key Features
Simple to operate – little attention needed.
Powered from your 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.
Flow-through operation, ideal for on-line applications.
Novel flow cell using inexpensive, disposable plastic tubing – no connection problems.
Cell can be mounted remotely up to 5m from control unit.
Specifications
Available with 3 different cell sizes to allow for a variety of flow through tubing bores to suit your application.
Plug and play operation with self detecting cell size.
Width
80mm
Height
45mm
Length
200mm including cell
Weight
375g including cell
1mm Cell
for 1mm bore tubing
3mm Cell
for 3mm bore tubing
5mm Cell
for 5mm bore tubing
Selecting the cell size
The appropriate tube size depends on the specific application requirements.
Generally, the size of the flocs is as critical as the turbidity, due to the minimum flow rates associated with each tube size. When using a 1mm tube, the shear rates are quite high, which can potentially break up weaker flocs, causing operational issues. Additionally, if the flocs become large relative to the tube size, it is advisable to use the next larger tube size.
The 3mm tube is considered standard and is the most popular choice.
Software
The supplied software provides simple operation of the PDA3000, and only a few settings need to be adjusted.
The light intensity and sampling interval can be set, and two of the parameters, DC, RMS and RMS/DC can be chosen to be displayed graphically. The software allows numerical values to be saved to a file or exported to a spreadsheet.
Typical results using the PDA3000, with the DC and Ratio (RMS/DC) values shown as red and blue curves respectively.
A suspension of silica particles in London tap water was used with aluminium sulphate as coagulant. The silica was added at 120s (A) (note the small drop in DC value) and the coagulant at 300s (B), with stirring at 50 rpm. At 620s (C) the stirring speed was increased to 400 rpm for 20s and then returned to 50 rpm. The Ratio value clearly shows the growth of flocs up to a maximum size and their rapid break-up as a result of the increased stirring speed. There is then a partial re-growth of flocs at the lower stirring rate. During the entire process there is very little change in the DC value.
Requirements
Operating System
Any supported Windows based OS.
Processor
No specific requirements.
Memory (RAM)
4GB recommended.
Storage
At least 1GB of free space.
Graphics Card
No specific requirements.
Display
No specific requirements.
Principles 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 infra-red 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 1mm in diameter. The standard cell is designed for tubing of 3mm bore, but other cells, for 1mm bore and 5mm 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 cubic mm 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.
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’.
