A Nonintrusive Magnetically Coupled Sensor for Measuring Liquid Level

A new, linear, nonintrusive, and cost-effective inductive liquid level sensor is designed and developed. The sensor consists of a simple coil that floats on the liquid surface. A couple of printed planar coils partially surround the liquid column and sense the position of the floating coil and hence the liquid level. The planar coils are arranged with overlap such that the floating coil is always in proximity to both the coils. The coils are excited, and their response is processed, such that the final output is proportional to the liquid level. The input-output characteristics of the proposed liquid level measurement are linear, with a worst case percentage nonlinearity <; 1%. A prototype sensor built for a range of 180 mm, had a resolution of 0.73 mm. The output of the prototype has very low sensitivity to the position of the floating coil in the xy plane, indicating that the movement of the floating coil on the surface of the liquid column does not affect the liquid level measurement. Since the sensing part is just made of passive elements, the lifetime of the sensor can be made very high.