A brief guide to particle counters

A particle counter is used to count and size particles directly, rather than other methods where numbers, volumes and sizes are estimated. Many counters use optical techniques such as direct imaging, light scattering or light obscuration, and variations can be used to count particles in a liquid, aerosol or dry powder.

One method of counting particles involves shining a high-energy light source like a laser beam or halogen light through the chamber, which allows only single particles to pass through. The particles can then be measured via many techniques. Direct imaging uses a high-resolution camera to produce an image that is analysed by a computer. An obscuration technique will use the particles’ light-scattering properties to determine their size and number via photo detector.

In condensation particle counting the particles are detected in an aerosol by increasing them in size to produce droplets in a super-saturated gas. Tunable resistive pulse sensing (TRPS) uses a nanopore through which the individual particles are propelled and picked up as a small change in ionic current flow.

The need for particle counting and sizing is very important in many different areas, such as in medicine for the injection of liquids and in microelectronics where pure water is needed. Particle counters are also important where clean air is required.

Coulter counters are often used when there is a need to count biological particles like bacteria, cells or viruses suspended in electrolytes. This equipment detects electrical resistance changes caused by the particles as they pass two different chambers, which is related to the volume of the particle.