Noncontact Dielectric Spectroscopy of Liquids Using Additively Manufactured Coaxial-Type Sensor Matched With an Exchangeable Custom Test Tube

A novel sensing setup for broadband dielectric spectroscopy (BDS) of liquids is proposed. A custom-designed 3-D-printed test tube having a central metal pin is coupled through a spring-loaded pin with a complementary jig that mechanically holds the tube and electrically ensures coaxial transmission line alike operation. This way in-process permittivity determination and monitoring are possible over a wide frequency and permittivity range when the tube geometry is properly designed. Analytical formulas are derived enabling fast calculation of liquid-under-test complex permittivity. Moreover, only one known sample is required for the system calibration. A sensing setup demonstrator is developed and used to characterize a set of test liquids having a permittivity span of 20 yielding usable measurable frequency range up to roughly 5 GHz. An error analysis is provided, and major uncertainty sources are identified with the main one being repeatability of the test tube fabrication. For the presented demonstrator it yields permittivity determination uncertainty of ±0.55 real and ±0.40 imag. (in-band average standard deviation).