Three-Coil Sensor for Liquid Level Measurement

For sensing the level of a liquid in an insulating container, a three-coil sensor is proposed. One of the coils is shorted and made to float on the surface of the liquid. The other two coils of identical trapezoidal (wedge) shape are fixed on the opposite walls of the container. The two trapezoidal coils are positioned such that the induced emf e
1
in one coil increases with the position of the floating coil and the induced emf e
2
in the other coil decreases with the position of the floating coil. It is shown here that the ratio-metric evaluation of the induced emfs as [(e
1
– e
2
) / (e
1
+ e
2
)] provides an output that linearly varies with the position of the floating coil and thus senses the liquid level. The proposed technique is evaluated analytically, and its efficacy demonstrated through simulation studies. Results obtained from a prototype unit built and tested establishes the practicality of the proffered sensor. It is found that the proposed sensor suffers from end-effects with worst-case error of 3%. The proposed sensor is best suited for liquid level measurement in sealed containers.