A Technical Evaluation of Capacitive Level Sensors for use in Hemodialysis Machines on Chambers Filled with Dielectric Liquids

Abstract: A series of tests were performed to explore the limits of two capacitive level sensors using the principle of parallel finger layout (sometimes called comb fingers) for sensing. The goal was to present a decision basis for Baxter to use when choosing a sensor to implement in their next generation hemodialysis machine. The devices under tests were 1) sensor type A: developed in-house and 2) sensor type B: designed by a third party. A survey of the system and requirements on the sensor, followed by a brief overview of capacitance, permittivity, thermal expansion, and the Wien bridge oscillator converged in a set of tests. These tests determined 1) that both sensor types are able to measure the liquid level with fair accuracy in a controlled environment; 2) type A is more sensitive to temperature change and induced noise; 3) type A drifts over a 4 hour time period; 4) type A changes its resolution in different liquids; 5) type B cannot successfully measure liquid levels at a temperature of 96oC. The recommendation is to move forward with the type B level sensor and to further test the sensors’ durability over time and in different climates. The method used to measure in type B is unknown, making it hard to anticipate and to find solutions to problems that may occur in the future.