Focused Beam Reflectance Measurement (FBRM®) provides the unique ability to measure particles and droplets in concentrated suspensions and emulsions – at full process concentration without the need for sample extraction and sample preparation.
FBRM® technology is designed to provide in-process particle system characterization in real time. METTLER TOLEDO continually researches and improves FBRM® technology - pushing the boundaries of in-process measurement of high concentration slurries and particle systems.
The unique design features of FBRM® ensure a repeatable and reproducible measurement of particle dimension and relative particle count – even in highly concentrated and opaque solutions.
The basic measurement principle of the FBRM® technique is a relatively simple concept. This cutaway schematic in Figure 1 illustrates the key internal components of each probe-based instrument.
A solid-state laser light source provides a continuous beam of monochromatic light that is launched down FBRM® probe. An intricate set of lenses focuses the laser light to a small spot. This focal spot is carefully calibrated to be positioned at the interface between the probe window and the actual process. Tightly controlling the position of the focal spot is necessary for a sensitive and repeatable measurement.
A precision motor - pneumatic or electric - is used to rotate the precision optics at a constant speed. The speed is carefully monitored and controlled throughout the measurement to ensure maximum precision in the data. (Standard probes operate to provide a fixed 2 m/s scan speed. Some models are capable of faster scan speeds and may be calibrated to allow operation at different speeds to improve performance in particular applications.)
As viewed from the probe window in Figure 2, the focused beam scans a circular path at the interface between the probe window and the particle system. As the scanning focused beam sweeps across the face of the probe window, individual particles or particle structures will backscatter the laser light back to the probe.
Particles and droplets closest to the probe window will be located in the scanning focused spot and backscatter distinct pulses of reflected light (Figure 3). These pulses of backscattered light are detected by the probe and translated into Chord Lengths based on the simple calculation of the scan speed (velocity) multiplied by the pulse width (time); a chord length is simply defined as the straight-line distance from one edge of a particle or particle structure to another edge.Thousands of individual chord lengths are typically measured each second to produce the Chord Length Distribution (Figure 4) which is the fundamental measurement provided by FBRM®. The Chord Length Distribution, as a “fingerprint” of the particle system, provides the ability to detect and monitor changes in particle dimension and particle count in real time.
Note that unlike other particle size analysis techniques, with FBRM® measurement there is no assumption of particle shape. This allows the fundamental measurement to be used to directly track changes in the particle system without unnecessary complex mathematical assumptions that could introduce significant errors to the measurement.