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Point level detection of liquids: Typical systems for point level detection in liquids include magnetic and mechanical floats, pressure sensors, electroconductive sensing or electrostatic (capacitance or inductance) detectors — and by measurement of a signal’s time-of-flight to the fluid surface, through electromagnetic (such as magnetostrictive), ultrasonic, radar or optical sensors. Magnetic and mechanical float See also: Float (liquid level), Float switch, and Magnetic level gauge. The principle behind magnetic, mechanical, cable, and other float level sensors often involves the opening or closing of a mechanical switch, either through direct contact with the switch, or magnetic operation of a reed. In other instances, such as magnetostrictive sensors, continuous monitoring is possible using a float principle. With magnetically actuated float sensors, switching occurs when a permanent magnet sealed inside a float rises or falls to the actuation level. With a mechanically actuated float, switching occurs as a result of the movement of a float against a miniature (micro) switch. For both magnetic and mechanical float level sensors, chemical compatibility, temperature, specific gravity (density), buoyancy, and viscosity affect the selection of the stem and the float. For example, larger floats may be used with liquids with specific gravities as low as 0.5 while still maintaining buoyancy. The choice of float material is also influenced by temperature-induced changes in specific gravity and viscosity – changes that directly affect buoyancy. Float-type sensors can be designed so that a shield protects the float itself from turbulence and wave motion. Float sensors operate well in a wide variety of liquids, including corrosives. When used for organic solvents, however, one will need to verify that these liquids are chemically compatible with the materials used to construct the sensor. Float-style sensors should not be used with high viscosity (thick) liquids, sludge or liquids that adhere to the stem or floats, or materials that contain contaminants such as metal chips; other sensing technologies are better suited for these applications. A variation of magnetic sensing is the “Hall effect” sensor which utilizes the magnetic sensing of a mechanical gauge’s indications. In a typical application, a magnetism-sensitive “Hall effect sensor” is affixed to a mechanical tank gauge that has a magnetized indicator needle, so as to detect the indicating position of the gauge’s needle. The magnetic sensor translates the indicator needle position into an electrical signal, allowing other (usually remote) indication or signalling.
