The Lasentec focused beam reflectance measurement (FBRM) probe provides in situ particle characterisation over a wide range of suspension concentrations. This is a significant advantage over conventional instruments that require sampling and dilution. However, FBRM gives a chord distribution, rather than a conventional diameter distribution. Both theoretical and empirical methods for converting from chord to diameter data are available, but the empirical method was found to be more successful. The FBRM response has been compared to conventional particle sizing techniques (laser diffraction and electrical sensing zone) for a range of sieved aluminium or calcite suspensions. The mode average of the square-weighted chord length was found to be comparable to other sizing techniques over the range from 50 to 400 pm. The square-weighted FBRM results were essentially unaffected by changes to the instrument focal position, the suspension fluid flow velocity, or the solid fraction in the range 0.1-20% w/v.