The design, construction and commissioning of a low-cost optical particle size analyser specifically for measurement of settling velocities and size of flocs

This paper presents the problems encountered and methods undertaken in the design and construction of equipment for measuring the particle size and settling velocity of flocs. The paper reviews existing settling columns and the development of an experimental system suitable for such measurements. The equipment consisted of a rectangular glass column (1.5 m high, 1.5 cm deep and 3 cm wide) filled with water, into which flocs were introduced at the top and their size and settling velocity measured. This column was housed in a cylindrical aluminium vessel temperature controlled by air, using a fan-forced heat exchanger which was in turn temperature controlled by a water bath. Both the air column and all air lines were insulated. Air was fan forced into the aluminium vessel at the top and bottom to achieve an internal temperature of 25 degreesC, thus eliminating any convection currents caused by temperature variations. The image-capturing equipment consisted of a CCD camera, with illumination provided by a stroboscope fitted with a polarizing and a red filter to achieve optimum contrast. All the equipment was mounted on a heavy aluminium-skirted base fitted with 50 squash balls to eliminate any vibration causing disturbance to the settling column and associated optics. The signal from the CCD camera was fed into a video recorder and analysed by image analysis software (WiT) on a standard computer. The magnification range for the optical system was, 1300 times, capable of measuring in the range of 2-240 mum. The maximum settling velocity that can be measured is 0.225 m s(-1). The field of view was 240 mum by 180 mum.