Magnetic float level gauges, also known simply as magnetic level gauges or magnetic level indicators, provide a continuous visual indication of liquid level in a vessel or tank. Being a robust, relatively cheap level measurement technique suitable for high pressure systems they are found throughout the process, oil & gas, refinery, chemical, petrochemical, and power generation industries.
How does a Magnetic Level Gauge Work?
A magnetic level gauge contains a float, typically made from stainless steel, titanium or plastic which contains an integral, permanent omni-directional magnet. The float is contained in a bypass chamber connected to the vessel. This buoyant float rises and falls with the liquid or interface level whilst the magnet “flips” the magnetic wafers mounted externally to the chamber. As the float rises and falls each wafer rotates 180° and so presents a contrasting colour.
General industry practice is that the wafers above the float show white, whilst those below show red. Hence the indicator then presents a clearly defined and accurate level measurement of the liquid in the chamber.
Float Magnet Construction
The magnetic field is the heart of the magnetic level gauge – the stronger the field, the more reliable the instrument will function. Some manufacturers use a single annular ring magnet, others use a series of single bar magnets in a circular array in their float design. These designs tend to provide better performance than a single bar magnet.
Mounting of Magnetic Level Gauges
Like Reflex Gauges, magnetic level gauges are connected laterally to a vessel via at least 2 process connections. The process connections can be flanged, threaded or welded. The level in the bypass chamber corresponds to the level in the vessel. It is worth noting that the bypass chamber can be manufactured to any length and mounted to suit the best viewing angle therefore bespoke level gauges can be made to suit awkward installations. It is common practice to provide shut off valves between vessel and level gauge to allow removal of the level gauge without emptying the vessel. Depending on the properties of the fluid being measured, and process conditions a vent valve and/or a drain valve may also be specified in the hookup. The highly contrasting colours used on the level gauge flappers means there is normally no need to provide an additional light as is often the case with reflex level gauges.
Float Damage Warning
Many magnetic level gauge manufacturers offer the option of incorporating a damaged float warning system. This is achieved by extending the bypass chamber below the bottom process connection enough to allow the float to pass the process connection. The indicator wafers below the process connection are mounted either with their colours in reverse, or coloured yellow and white. Should the float drop into that zone the wafers will turn presenting a sharp, immediately readable indication of float failure.
The magnetic level gauge is ideally suited for measuring a liquid interface. Floats are available to meet a variety of specific gravities to suit the liquids being monitored – many manufacturers offer floats suitable for specific gravity ranges of 0.33-2.2. See our page on Liquid Level Interface Measurement for mor information on measuring liquid interface.
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