Liquid level sensor works with simple electronics

Liquid level sensor works with simple electronics

Although simple in theory – an air bubble and conductive liquid sealed in a chamber surrounded by electrodes – electronic levels are more complex in practice.

If the liquid is an electrolyte, it needs to be driven with ac (and no dc) to avoid chemical decomposition and corrosion. If the liquid is a metal, like mercury or Galinstan (GaInSn), it works with dc drive but sits in a squashy ball that does not flow nicely to the electrodes.

What is needed is a collection of materials and geometry that have the right combination of conductivity wettability and viscosity for the liquid to slosh around in just the right way.

After trying several, including Galinstan, the liquid chosen was water, made conductive by a dilute suspension of carbon nanotubes (black in the photo).

This rests in a flat-bottomed chamber with a number of (12 in the demonstrator) regularly-spaced copper electrodes around the edge, with the rest of the floor covered in a water-repelling fluoro-polymer (‘Cytop’).

Inside, the top of the chamber is a shallow dome on which is deposited a ring-shaped silver electrode, connected to a voltage source.

Dome angle, liquid characteristics and floor repellence combine to make the water sit in a ring around the edge if the sensor is level, and in a crescent around the walls if the sensor is anything other than level, with the crescent stretching out to reach more electrodes when nearly flat, and shortening to reach fewer when tilted heavily. The silver ring (white in the photo) is positioned so that the water always touches it.

The net result is that, using only on-off conductivity, without analogue measurement, the direction of tilt and the angle of tilt can both be approximated by detecting which electrodes, and how many of them, are shorted to the silver ring by the liquid. Adding more electrodes can increase resolution.

“By using a highly fluid conductive liquid, the sensor can measure not only the tilt direction but also the angle in. In the tilted state, the air pocket moves to switch the conduction and insulation of the electrodes – this feature offers the advantage that the inclination direction can be detected without any external calculation mechanisms,” said YNU research engineer Hiroki Ota, adding: “We aim to extend this function to create a multi-physical sensor that can simultaneously measure other physical quantities such as pressure and strain deformation.”

The work has been published as ‘Liquid-based digital readable tilt sensor’ in Advanced Materials Technologies. Only the abstract of the paper is available without payment, although the supplementary material can be downloaded free and reveals geometry, materials and fabrication.