Abstract
Certain applications require a contactless measurement to eliminate the risk of sensor-induced sample contamination. Examples can be found in chemical process control, biotechnology or medical technology. For instance, in critically ill patients requiring renal replacement therapy, continuous in-line monitoring of blood conductivity as a measure for sodium should be considered. A differential inductive sensing system based on a differential transformer using a specific flow chamber has already proven suitable for this application. However, since the blood in renal replacement therapy is carried in plastic tubing, a direct measurement through the tubing offers a contactless method. Therefore, in this work we present a differential transformer for measuring directly through electrically non-conductive tubing by winding the tube around the ferrite core of the transformer. Here, the dependence of the winding type and the number of turns of the tubing on the sensitivity has been analyzed by using a mathematical model, simulations and experimental validation. A maximum sensitivity of 364.9 mV/mol/L is measured for radial winding around the core. A longitudinal winding turns out to be less effective with 92.8 mV/mol/L. However, the findings prove the ability to use the differential transformer as a truly contactless sensing system.
Highlights
To avoid possible contamination of a sample induced by a sensing system, in some applications it is essential to perform contactless measurements
We use a differential transformer consisting of three printed circuit board (PCB) coils located on a ferrite core for the experimental characterization of a sample
The tubing can be wrapped longitudinally to the ferrite core, i.e., a multi-layer winding in z-direction with nL turns, or a planar multi-layer winding with nR turns in radial direction lateral to the ferrite core is possible, or both
Summary
To avoid possible contamination of a sample induced by a sensing system, in some applications it is essential to perform contactless measurements An example of such an application is the continuous in-line monitoring of blood conductivity as a measure of the sodium concentration of blood serum [1,2,3,4,5,6]. Such monitoring is useful for critically ill patients in intensive care units with severe dysnatremia [7,8,9]. A patient-individualized therapy should be used requiring continuous monitoring of the patient0 s sodium concentration [13,14,15]
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