All liquid based calibration scheme for microwave dielectrometry

The calibration of a microfluidic microwave sensor is a challenging field of research. While most of the existing strategies still rely on information out of full wave simulations, this work demonstrates an exact method to calibrate such a sensor with only three transmission measurements of known liquids. In this way, a closed formulation that directly maps S-parameters to the complex permittivity value of the material under test was found. Because this calibration scheme is only based on transmission measurements, it has potentially lower requirements concerning the hardware implementation of a sensor. The theory is validated with measurements of different aqueous NaCl solutions as well as albumin solutions up to a frequency of 10 GHz. It is demonstrated, that due to the accurate data extraction small differences of 0,05mol/l can be discriminated very well. This corresponds to a change in permittivity of Δε = while the error is in average Δ = ±1.5%o.