Micron Powder Inspection Methods
Basis of the Van Moppes Quality Inspection System
The Van Moppes micron diamond powder inspection system is probably the most demanding and most complete system used amongst diamond powder manufacturers. For powders in the "visible" range, i.e. containing no particles under 0.5µ, it is based on the following measurements, which are made on all micron powders before they are allowed to go to inventory:
- Statistical size distribution
- Particle shape measurement
- Oversize particle scan
The size distribution is obtained through the direct optical measurement of the greatest lengths of the particles contained in a microscopic slide. Submicron powder inspection follows different rules, as described below. Inspection data is recorded on a Quality Inspection Record, which is kept for at least 10 years.
Inspection Instruments
a) Image Analysis System
This proprietary system measures the longest particle dimension on all powders over approx. 0.5µ. It is calibrated against the projection microscope. The microscopic slide has to be of perfect quality, with a single particle layer and few touching particles. Measurements are made on approx. 500 to 1000 particles, which ensures excellent consistency.
b) Projection Microscope
This was used as the calibration instrument for the whole system. It can be used to measure all powders above approx. 0.4µ, a limitation due to the wavelength of light and it is also used for the oversize scan. Particles are measured by the circumscribed circle method, virtually identical to the longest dimension, except for very rare triangular particles. By definition, the projection microscope requires no calibration, but the process is time-consuming and requires excellent operator skills and constant training to ensure consistent measurements over time.
c) Disk Centrifuge
This special instrument is used for the measurement of submicron powders only, between 0.01 and 1µ. At the 1µ interface, it produces a distribution similar to an optically measured numbers/lengths distribution. Disk centrifuge distributions are however based on a volume measurement of particle volumes, as no numbers distribution can be obtained in such small sizes, nor could it be converted mathematically from a volume distribution without significant errors. Measurement accuracy and repeatability are remarkably good.
Basis of Particle Measurement
The image analysis measures particles by their longest dimension (L), which constitutes an additional quality guarantee for micron powders. Particle shape is measured simultaneously, by the aspect ratio (A/R, or L/W), as shown here.