Proportional Microvolume Capacitive Liquid Level Sensor Array

A sensor array has been developed for use with laboratory automation to permit closed-loop control of liquid levels in a multiwell microplate geometry. We extended a simple electrical model for non-contact capacitance-based fluid sensors to describe a fluid-level dependency. The new model shows that a charge-transfer based capacitance transducer, employing a liquid-specific calibration, can be used to obtain an output signal that varies linearly with the liquid level. The calibration also compensates for liquid-to-liquid conductivity and permittivity differences. A prototype 3times3 sensor array was built and tested using NaCl and ethanol solutions to simulate the range of conductivity and permittivity typical in biological and chemical research. Calibrated output signals were linear with liquid volume for all tested solutions (R² > 0.92). Measurement error averaged 1.3% (2.0 muI) with a standard deviation of 6.0% (9.0 muI). These results demonstrate the feasibility of developing a microvolume sensor array in essentially any MtimesN microplate geometry