I have since found quite a good method that specialises in fertiliser granules which can be found below for others that may be interested:

http://ferret.com.au/articles/EF/0C030AEF.asp

This however is quite expensive so I am still searching... Ultrasonics seems to be the best value.. if they can do the job.

Tane ■

HelloTane Wallace, Malvern Instruments carry a product which is designed for applications just like yours. Using a particle counting technique with a static probe, the device monitors the PSD of your production (your application would best require it installed after your 4mm screen)

The device is typically used on granulation processes as a product monitor and can offer Particle Size feedback to control stations

The advantages it offers are its static atributes. Any change in the product can be accurately reported with measurement intervals in seconds rather than minutes and it monitors continuously.

You can view details of this product at ouir web site www.malps.com and look under the PARSUM tab. We are using it for measurement of particles up to 5mm at this time.

You can contact me directly for more information. ■

Dear Tane Wallace, The response from Nordell is both informative and interesting. Image techniques are favoured but have their own set of limitations, eg; how much is the information is skewed by natural size segregation when measuring from a static bulk - as on a conveyor line? Also particle field density will have an effect on representativity.

Similarly particle counting techniques typically will be questioned on their represetnativity due to the number/volume issue - how much of the product is actually measured? The Parsum probe promoted by Malvern has advanced both number and volume capabilities for a counting device but all of the same challenges apply to both techniques (Image and counting).

I am, however, a great supporter of Image based techniques because the advantages of non-contact measurements speak for themselves.

One of the major operational considerations would be on the usability of any measurement system. Traditionaly particle size measurement systems using light scattering techniques, and have been employed for at-line or on-line solutions are both fragile in use and prone to varying uncertainties based on the operator contribution to the analysis performed and the repeatability of the measurement in terms of both sample representivity and the physical measurement reproducability. The best solution is Malvern's Insitec analyser but this runs out of range for you at approx 1mm !!

The laser Probe method (particle counter) I referred to from Malvern Instruments uses a dedicated optical cleaning system (air jet) that has proven to achieve very long run-times between maintenance (MTBM), Months in many cases. This kind of performance attribute should be investigated as a key decision parameter for a process/on-line/real-time application.

The optimum measurement zone in the process will typically be as soon after the particles have been produced and a flowing stream of particles with particle-particle separation would probably benefit the measurement technique for both Laser-based and optical measurement platforms. Ideally in such a location there will be minimal size segregation caused by any bulk effects.

I do not believe light extinction, ultrasound, x-ray, gamma ray devices have the resolution to perform as a product (particle size) monitor so your best choice will be somewhere between a (streaming) image type of system or a laser/counter type of system. True laser diffraction is not relaible at such large particle sizes but this would otherwise be the best solution as it would allow for full cross stream sampling and complete measurement of the particle size distribution.

This discussion about the best measurement technique has been going on for years and will continue to do so. Establishing your requirement criteria and trading compromises against that is the best advice I can offer .... I wish you well in your search and selection! ■