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
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