Articles | Volume 5, issue 1
Regular research article
17 Jun 2016
Regular research article |  | 17 Jun 2016

Instrumented flow-following sensor particles with magnetic position detection and buoyancy control

Sebastian Felix Reinecke and Uwe Hampel

Abstract. A concept for buoyancy control and magnetic position detection has been developed for the improvement of instrumented flow-following sensor particles. The sensor particles are used for investigation of hydrodynamic and biochemical processes in large-scale vessels such as biogas fermenters, bioreactors and aerated sludge basins. Neutral buoyancy of the sensor particles is required for tracing of the fluid flows. Buoyancy control is performed by adjustment of the sensor particles' volume, which is altered by an integrated piston. A miniaturized linear actuator, namely a stepper motor with linear transmission, is operated by a microcontroller to drive the piston. The buoyancy control unit enables accurate automated taring of the sensor particles in the stagnant process fluid to achieve neutral buoyancy. Therefore, the measured vertical position of the sensor particle as a function of the hydrostatic pressure is used as feedback. It has an incremental density change of 0.0136 % as compared to water and a residual drift velocity of approximately 3.6  ×  10−3 m s−1. Furthermore, a minimum density of 926 kg m−3 can be set by full extension of the piston, which allows floating of the sensor particles after a defined event, namely critical charge of battery, full data storage or the end of a fixed time cycle. Thus, recovery of the sensor particles can proceed easily from the fluid level. The sensor particles with a buoyancy control unit are tested for depths up to 15 m. Also, detection of a local magnetic position marker by the sensor particles has been implemented to enhance movement tracking. It was tested in a lab-scale biogas digester and was used for estimation of the liquid circulation time distribution and Peclét number to describe the macro-flow.

Short summary
Sensor particles with buoyancy control and position detection are presented, which are used for flow tracking in large vessels, such as biogas digesters and waste water tanks. They were tested in the realistic flows of a biogas digester. The buoyancy control allows taring for good flow tracing by the sensor particles, and it lets them float to the surface after data acquisition for easy recovery. The fluid mixing was estimated from detected passages of sensor particles at a magnetic coil.