Abstract
The electrical and dielectric properties of liquids can be used for sensing. Specific applications, e.g., the continuous in-line monitoring of blood conductivity as a measure of the sodium concentration during dialysis treatment, require contactless measuring methods to avoid any contamination of the medium. The differential transformer is one promising approach for such applications, since its principle is based on a contactless, magnetically induced conductivity measurement. The objective of this work is to investigate the impact of the geometric parameters of the sample or medium under test on the sensitivity and the noise of the differential transformer to derive design rules for an optimized setup. By fundamental investigations, an equation for the field penetration depth of a differential transformer is derived. Furthermore, it is found that increasing height and radius of the medium is accompanied by an enhancement in sensitivity and precision.
Highlights
The height hM and the volume of the sample is of particular interest, since often, the sample volume is defined by certain specifications coming from the application. Such restrictions can be found in the continuous in-line monitoring of the sodium concentration of the blood in extracorporeal circuits during dialysis treatment, where, in addition to flow requirements, the sample volume should be kept as low as possible in order to draw as less blood as possible from the patient
We have investigated the influence of the sample compartment geometry on the sensitivities Sκ and Sc and the precision of a differential transformer
The experimental investigations were conducted by using a printed circuit boards (PCB) differential transformer
Summary
It is of great importance to determine the electrical and dielectric properties of a sample contactless in order to avoid sample contamination by the sensing system. Examples of these applications are the contactless blood conductivity measurement in medical technology as a measure of the sodium concentration or the contactless determination of the polarizability of a medium as a measure of the biomass in biotechnology. Since the sample or medium is often in materials with low permittivity such as polymers or glasses in these applications, contactless capacitive sensors usually suffer from poor penetration of the electric field through these materials. There are systems like capacitively coupled contactless conductivity detection (C4 D) [2], allowing for capacitive measure through a usually special fused-silica capillary with a small cross-section [3,4]
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