Self-Calibrating Sensor for Measuring Density Through Stainless Steel Pipeline Wall

Abstract

An ultrasonic instrument to measure the density of a liquid or slurry through a stainless steel pipeline wall is described. By using multiple reflections of the ultrasound within the stainless steel wall, the acoustic impedance (defined as the product of the density of the liquid and the velocity of sound in the liquid) is determined. Thus, the wall is part of the measurement system. The density is obtained by coupling the acoustic impedance measurement with a velocity of sound measurement. By basing the measurement on multiple reflections, instrument sensitivity is increased by the power of the reflection coefficient. The measurement method is self-calibrating because the measurement of the acoustic impedance is independent of changes in the pulser voltage. Data are presented over a range of pulser voltages for two wall thicknesses. These results can be applied to develop an ultrasonic sensor that (1) can be attached permanently to a pipeline wall, possibly as a spool piece inserted into the line or (2) can clamp onto an existing pipeline wall and be movable to another location. The self-calibrating feature is very important because the signal strength is sensitive to the pressure on the clamp-on sensor. A sensor for immersion into a tank could also be developed.