Optical particle counting methods – light blockage and light scattering

Optical particle counting methods – light blockage and light scattering

Single particle counting is an optical laser measurement method. A laser beam illuminates the entire profile area of a flow-through measuring cell and is captured by a photodetector. Each particle in the liquid absorbs and reflects the light from the laser beam. This allows the number and size of particles in the liquid to be determined by a detector. The two main methods for optical particle counting are the principle of light blocking, which is used by the PAMAS HCB-LD and HX sensor series, and the principle of light scattering, on which the PAMAS SLS-25/25 sensor is based.

A. The principle of light blockage

As for the principle of light blocking, the laser light illuminating the liquid reaches a photodiode directly. Particles present in the liquid interrupt the laser beam when passing through the measuring cell and cast a shadow on the photodiode, which is detected as a change in current. The number and strength of the signals are translated into particle numbers in channels of different sizes using a calibration curve. With this measuring principle, single particles with a minimum size of 1 µm can be detected.

B. The principle of light scattering

The smaller the particles to be measured, the lower the light blocking signals produced. Below 1 µm, the signals are so low that a reliable measurement with a light blocking sensor is no longer possible. For this reason, the light scattering principle is used. In contrast to the light blocking principle, a photodiode detects the light scattered by the particle as a current change. This signal is proportional to the size of the measured particle. The detector only captures the scattered light. The intensive laser beam is absorbed by a light trap.

Depending on the size of the particles to be measured, the sensors can be determined. In order to obtain reliable and accurate measurement results, the particles must pass through the measuring cell separately, thus avoiding overlapping, which would result in two or more small particles being counted as one large particle. This is known as a random error, which occurs more quickly when it comes to larger particle sizes. Therefore, the maximum measurable particle concentration is much higher for sensors with small profiles.

Particle counters for liquids: www.pamas.de (WoTS2022 Hall 7, Stand No. F080)