How Geometry Affects Sensitivity of a Differential Transformer for Contactless Characterization of Liquids

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.

Differential Inductive Sensing System for Truly Contactless Measuring of Liquids′ Electromagnetic Properties in Tubing

“…However, after a maximum at 4 and 5 windings, the sensitivity declines again. This behavior of decreasing sensitivity after a certain maximum was already predicted in a mathematical model for a continually distributed sample with increasing outer sample radius in [ 45 ]. The reason for the decreasing sensitivity is an increasing penetration of both secondary coils with a uniform magnitude of the secondary magnetic flux density due to the rising mean radius of the sample.…”

“…The primary magnetic flux density B P of the primary coil, orientated in the z -direction, induces circular eddy and displacement currents I S into the sample located above L Sec1 [ 45 ]. These circular currents rotate around the z -axis in the x-y -plane or around the ferrite core, respectively.…”

“…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. This real differential transformer was designed in previous works [ 9 , 45 ]. The PCBs contain 35 µm thick copper tracks forming the coils.…”

“…Since we have already investigated the general effects of the sample geometry on the sensitivity of a differential transformer in [ 45 ], this paper is a direct continuation of the previous work. The objective is to develop and investigate a new approach of a truly contactless method to measure the electromagnetic properties of samples in electrically non-conductive tubing without the need for a special designed flow chamber.…”

“…In addition, the chamber must meet the requirements of hemo and biocompatible materials, as it is in direct contact with the blood. All the aforementioned specifications make the design process highly complex and sometimes contradicts the requirements for a highly sensitive sensing system since, e.g., the radius and the height of the flow chamber cannot be changed arbitrarily [ 45 ]. Furthermore, disposables are often used in medical technology to prevent elaborate sterilization.…”