Develop a design model for a 4-electrode conductivity sensor.
What you will do
Conductivity is one of the most often determined fluid parameters and is a measure for the total amount of dissolved ions. To determine conductivity, the resistance of a fluid sample located in the area between at least two electrodes is determined by impedance spectroscopy. The conductivity is then given by multiplying the measured conductance with, the so-called cell constant of the sensor (Kcell). The conductivity range that can be determined with a sensor depends on the chosen Kcell. This implies that a correct conductivity sensor has to be chosen, fitting with the expected concentration range in the targeted application. Commercial conductivity sensors embed two parallel planar electrodes (with a certain area and distance that define the Kcell). Such a geometry is difficult to miniaturize. However. Timmer et al  describe how planar interdigitated 2-electrode sensors on chip can be designed and how the Kcell value can be derived. Timmer’s findings we have been reproduced. A disadvantage of the 2-electrode conductivity sensor is the large electrode-electrolyte interface resistance which can no longer be ignored in solutions with a high conductivity. To minimize this interference, a 4-electrode sensor can be used, in which one pair of electrodes are injecting the current, while the other pair are sensing the voltage. We have extended this concept to microfabricated 4-electrodes conductivity sensor. These geometries are tested and the Kcell is determined. For optimizing the sensor design, new designs and sensors have to be made and tested, this process have to be repeated till the sensor with the wright Kcell is found. Designing in such way is a time consuming and expensive process. Therefor a theoretical model has to be derived which can determine the Kcell of a 4-electrode conductivity sensor.  B. Timmer, W. Sparreboom, W. Olthuis, P. Bergveld and A. van den Berg, “Optimization of an electrolyte conductivity detector for measuring low ion concentrations,” Lab Chip, vol. 2, pp 121-124, 2002.
The student will be part of gas and ion sensor for IoT team and develop a new model and design a new 4-electrode conductivity sensors with a specified Kcell.
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