Comparison of invasive and non-invasive cylindrical capacitive sensors for electrical measurements of different water solutions and mixtures

In this study design and operation of invasive and non-invasive cylindrical capacitive sensor (CCS) designs for the electrical measurements of water, water solutions, and water mixtures are reported. Operation of the capacitance measurement module for such probes is based on the charge/discharge method. The measured capacitances and resistances for distilled water, mineral water, tap water and salt water samples are reported by using two sensor types and results are compared. The measured capacitance by invasive CCS for distilled water is about 2.28 μF and by non-invasive CCS is 31.40 pF, which shows a big difference for different probes. Such a difference is due to the electrical conductivity effects on the invasive CCS, which can be used for the conduction study of different liquids. In another study the capacitance values of a binary solution of water plus additive liquids such as antifreeze, petroleum, ethanol, methanol and mixtures of water with fresh and used motor oils are measured. The effects of the guard ring electrodes, and electrode–liquid interactions on the capacitance/resistance measurements for the invasive CCS filled with different water liquids are also investigated.

► For the invasive CCS reactive capacitance is dominating, while for the non-invasive CCS the dielectric capacitance.
► A sample with higher electrical conductivity results in a higher reactive capacitance.
► For non-invasive CCS the capacitance for solutions is a function of the dielectric constant.
► Invasive CCS provides a useful means to study the electrical conductivity of the water liquids, mixtures, and solutions.
► Non-invasive CCS offers a useful means to study the dielectric property of the liquid samples.
► Effects of guard ring electrodes on the capacitance and resistance measurements of the invasive CCS are reported.