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

Stevander, Viktor LU and Petersson, Viktor LU (2016) EEM820 20161
Department of Biomedical Engineering
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... (More)
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. (Less)
Popular Abstract
Uncovering flaws in capacitive level sensors.

To improve their dialysis machine, Baxter wanted to implement capacitive level sensors. Two sensor models were suggested and a deep dive revealed some major flaws in one of them.

Whenever it comes to medical engineering, the equipment needs to be reliable and work when the condition changes without compromising the patient. When Baxter in the next generation dialysis machine wanted to implement capacitive level sensors, an extensive evaluation of two sensor models began. The sensors function was to measure the liquid level in chambers where dialysis fluid is mixed and where air bubbles removed.

The two sensor models tested are type A, an in-house developed sensor, and type B, a sensor... (More)
Uncovering flaws in capacitive level sensors.

To improve their dialysis machine, Baxter wanted to implement capacitive level sensors. Two sensor models were suggested and a deep dive revealed some major flaws in one of them.

Whenever it comes to medical engineering, the equipment needs to be reliable and work when the condition changes without compromising the patient. When Baxter in the next generation dialysis machine wanted to implement capacitive level sensors, an extensive evaluation of two sensor models began. The sensors function was to measure the liquid level in chambers where dialysis fluid is mixed and where air bubbles removed.

The two sensor models tested are type A, an in-house developed sensor, and type B, a sensor developed by a third party. A capacitive level sensor uses that air and liquid are able store different amounts of energy. The total difference of energy stored in a chamber can then be measured using various methods. The method used in type A is known, while type B is a so called black box problem, meaning only the output is known. There is however a key difference between the sensors that makes type B more suitable for implementation than type A.
One way of revealing the difference between the sensors is to measure different liquids, where the different liquids are able to store varying amount of energy. Type A sensors uses a method that measures the stored energy straight off and will therefore have different results for different liquids, even if the chamber is filled with the same amount. Much like if you have 1 kg cotton and 1 kg lead, and try to measure the volume of the object only by the weight.

Another method to uncover the difference between the sensor models is to see how they handle changes in temperature. A liquid that is heated will expand causing the liquid level to rise. In the same time is the ability to store energy reduced, causing type A sensors to believe there are lesser liquid in the chamber than before. The type A sensors will thus not only show the wrong measurement, the error will increase as the temperature rises!
Type B doesn’t show problems with the measurements in the same way as type A, but the absence of knowledge about the model gives an unsettling feeling that problems may rise in the future. Problems that can’t be anticipated and without solution. Type B is however build on a more stable foundation and is definitely worth the time spend on more tests to look at the durability and respond to change in climate.

The sensors used in the dialysis machine today are optical and magnetic sensors that can determine if the liquid level is high or low. Capacitive level sensors are able to measure everything in between, and if it the model can sustain 10 consecutive years of work, it will open up new control possibilities in the future. (Less)
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author
Stevander, Viktor LU and Petersson, Viktor LU
supervisor
organization
course
EEM820 20161
year
type
H2 - Master's Degree (Two Years)
subject
keywords
comb-finger, parallel fingers, capacitive sensor, Electrical engineering, Electric measurement, hemodialysis, dialysis machine
language
English
additional info
2016-16
id
8892088
date added to LUP
2016-09-27 13:50:38
date last changed
2016-09-29 09:10:14
@misc{8892088,
  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.},
  author       = {Stevander, Viktor and Petersson, Viktor},
  keyword      = {comb-finger,parallel fingers,capacitive sensor,Electrical engineering,Electric measurement,hemodialysis,dialysis machine},
  language     = {eng},
  note         = {Student Paper},
  title        = {A Technical Evaluation of Capacitive Level Sensors for use in Hemodialysis Machines on Chambers Filled with Dielectric Liquids},
  year         = {2016},
}