On a new sensing strategy using a combination of ultrasonic and photoacoustic techniques

The process industry is today aiming for more advanced process control strategies. These strategies rely on quick and accurate sensing of process variables. Considering processes where particles are suspended in a fluid, e.g. paper and pulp industry, there is a need for development of a new or improved in-line sensor technique determining different properties of the particles. In this study we present a sensing strategy where a combination of acoustical and optical measurement techniques is used. For this purpose have we designed and built a measurement cell which utilises the ultrasonic signal generated from an ultrasonic transducer as well as the ultrasonic, or photoacoustic, signal generated using a pulsed laser. The photoacoustic method is using the same transducer as in the transducer based method above. In the transducer based method, the well known pulse-echo technique is used. The pulse is generated by an ultrasonic transducer, travels through the suspension and reflects at a steel reflector. In the photoacoustic method, the ultrasonic transducer receives the ultrasound that is generated when a pulse of laser light is scattered and absorbed in the suspension. The laser beam enters the cell through fused silica windows. The laser beam is crossing the cell orthogonally to the ultrasonic transducer. The photoacoustic signal contains two echoes. The first echo is the acoustic signal which is generated when photons are absorbed by the medium. The second echo is the signal that has been travelling through the suspension and reflected at the steel reflector. The two measurement techniques were tested experimentally in the cell using aqueous suspensions of Nylon 66 fibres. The samples were mixed from fibres of three different diameters 17, 51 and 55 µ m with the length of 1.2mm for the 17µm and 1.5 mm for the 51 and 55µm. The fibres were suspended in distilled and degassed water to mass fractions of 0.12% and 0.25%. Distilled and degassed water was also used as a reference medium when determining the calibration constants for the cell. The photoacoustic signal is generated using a double-pulsed laser having pulse energy of 10 mJ and the laser beam is focused at the centre of the cell. The suspension was pumped around in a closed loop using a peristaltic pump and the temperature of the suspension was monitored during the measurement cycle. The result shows that the signal generated using the transducer based method scales linearly with mass fraction for each type of fibre. The results also show that the photoacoustic signal is sensitive to the number density of scatters in the suspension, i.e. the amplitude of the signal decreases with decreasing fibre diameters. These preliminary results indicate that a combination of these two techniques might be used in resolving the proportion of different particles with different diameter in a suspension.Validerad; 2006; 20061229 (ysko)