Multiparametric Measurement of Fluid Properties Utilizing Fringing Field Capacitance Sensing

Accurate measurement of a variety of different fluid intrinsic physical properties has become increasingly important when attempting to optimize the performance of thermal management systems that utilize aqeous ethylene glycol mixtures as a heat transfer medium. An ideal sensing system would non-invasively probe the fluid and rapidly ascertain the key physical properties electronically. In this work, we experimentally demonstrate a noninvasive capacitive proximity sensor, which is able to deduce temperature and chemical composition simultaneously using radio frequency excitation measurements. The sensor consists of interdigitated coplanar electrodes fabricated using traditional printed circuit board techniques on flexible substrates. An excitation signal is provided to the sensor through a vector network analyzer and the signal reflectance at various frequencies ranging from 1 to 500 MHz is measured. Based on the signal reflectance (S
₁₁
), we can distinguish fluid composition, as a function of ethylene glycol content and fluid temperature simultaneously. Interestingly, the position and amplitude of a localized S
₁₁
maximum from 360 to 420 MHz appears to be dependent on both the fluid temperature as well as the composition. We directly observe a shift in the resonant frequency related to the increased dielectric constant caused by the increase in temperature. Simultaneously, we observe a combined frequency shift and S
₁₁
magnitude increase due to the dielectric loss factor increase, which is attributed to the increase in ethylene glycol percentage. Ultimately, we demonstrate that a frequency-dependent capacitance measurement approach can be utilized for multiparameter extraction from a single sensor.