Measuring fluid and slurry density and solids concentration non-invasively

Staff at Pacific Northwest National Laboratory have developed a highly sensitive, non-invasive, self-calibrating, on-line sensor to measure the density, speed of sound, and attenuation of ultrasound for a liquid or slurry flowing through a pipeline; the approach can also be applied for measurements made in vessels. The sensor transducers are mounted directly upon the stainless steel wall and the pipeline wall becomes part of the measurement system. Multiple reflections within the stainless steel wall are used to determine the acoustic impedance of the liquid, where the acoustic impedance is defined as the product of the density and the speed of sound. The probe becomes self-calibrating because variations in the pulser voltage do not affect the measurements. This feature leads to the stability of the measurements and the instrument requires much less time and effort to calibrate. Further, the calibration remains constant in time, because it does not depend upon the pulser voltage remaining at a given value. By basing the measurement upon multiple reflections, the sensitivity of the measurement is significantly increased. For slurries with wt% solids concentration of 1% or less, high sensitivity is gained by analyzing attenuation measurements obtained from multiple paths through the slurry. For slurries with higher concentrations of solids, sufficient sensitivity is obtained by analyzing data from a simple transmission. Data are presented that show probe performance for each of these cases: very dilute and highly concentrated kaolin clay slurries.