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Comparing Laser Diffraction and Electrozone Counter for Particle Size Analysis

Table 1 gives the measured variability of the off-line electrozone counting PSA compared with that of Malvern Insitec on-line laser diffraction PSA in a toner milling line.

PSA instrument Measurement technique On-line? Sample throughput Update period / timelag Standard Deviation for Dv(50) measurement
Coulter
Multisizer 		Electrozone
counting 		No 5g every 2
hours 		1 measurement
every 2h with at
least 20mins time
lag before result

>=0.1
Malvern
Insitec 		Ensemble laser
diffraction 		Yes 20kg/h 1 measurement
every 10s (average
of 20,000
measurements), real
time updates. 		0.01 - 0.1 (product dependent)

Table 1: On-line laser diffraction PSA versus off-line electrozone counting PSA

The higher variability of the off-line electrozone method is due in part to the manual sampling and sample preparation, which is performed by several different operators. Malvern Insitec laser diffraction method has proved to be more sensitive to real process changes than off-line measurement, allowing even subtle variations to be corrected.

In Table 2, an example is given of the measured variablity of each PSA technique for one product type (measurements carried out in same conditions).

PSA technique Points measured Standard Deviation for Dv(50) measurement
Coulter Multisizer (Off-line electrozone counting)

 108 0.19
Malvern Insitec (on-line laser diffraction) 8000 0.07

Table 2: Measurement variability of laser diffraction PSA versus off-line electrozone counting PSA