While instruments that measure the size and distribution of individual particles in a powder form a significant market, there is also an interesting but smaller market for instruments that measure the bulk properties of powders. These measurements are largely motivated by problems in industrial processing, where the behavior of powders passing through hoppers and bins, or settling in containers, is of critical importance. Caking, cohesion and other problems with powder flow can lead to final products being out of specification or costly production-line shutdowns. These issues can also depend on seemingly minor changes in the powder due to the length of storage time or environmental factors such as humidity.

For these and other reasons, methods and instrumentation have been developed to measure the properties of bulk powder samples. Many of the instruments in this category work on principles first developed for manual methods. For instance, one basic measurement is the angle of repose. A powder poured onto a horizontal surface tends to form a cone with a fixed angle of inclination. Manual systems with a chute and a protractor-like scale are read by eye, but fully automated instruments use cameras or other sensors to measure the angle. The angle of fall is measured by tapping the horizontal surface as the sample is being poured, which makes the powder settle into a shallower cone. Use of automation can simplify the measurement and make the method far more repeatable. These measurements are useful since they help to specify the geometry of bins and hoppers most suitable for use with these powders in a production setting.

Another common powder measurement is shear, which is also related to the flow properties of powders. In a typical powder shear measurement, the powder is packed into a circular or annular trough. The lid of the shear cell has a textured surface that is brought in contact with the sample under a load. The instrument then measures the torque required to rotate the lid with respect to the sample. A common variation of this technique involves bringing the powder sample in contact with a sheet of the material intended to be used in the process equipment. This provides a direct measurement of wall friction, which again is a useful parameter in designing industrial machinery for a particular purpose.

Other powder measurements include aerated density, packed density, compressibility and other flow-related properties, such as the total time required for a sample to empty out through a passage of a given diameter and shape. Instruments exist for the automated measurement of all of these properties.

Bulk powder testing is a relatively uncommon technique. Many of the instruments on the market have been developed by independent testing labs or process consulting groups that have designed their own equipment for testing and later produced them for sale.

Bulk Powder Testing at a Glance:

Leading Suppliers

• Hosokawa Micron

• Mercury Scientific

• Jenike & Johanson

Largest Markets

• Food

• Chemicals

• Pharmaceuticals

Instrument Cost

• $2,000–$50,000