Magnetic Float Level Gauge
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 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 magnet 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 gauges can be made to suit awkward installations. It is common practice to provide shut off valves between vessel and gauge to allow removal of the 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 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 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 liquid interfaces. 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.
Advantages of magnetic level gauges
Magnetic level indicators offer many advantages over other types of level gauge e.g. reflex level gauges. These include:
- No need for a level bridle. The bypass chamber containing the float acts as a bridle allowing accessories to be mounted on it.
- High-pressure capability, with many manufacturers offering gauges suitable for pressures up to 400bar. Bypass chambers certified to P.E.D 97/23/EC Category IV are available from many suppliers.
- High-temperature capability. Applications up to 450°C not uncommon,
- Use in hazardous areas. Because magnetic level indicators do not need an external power supply, not even for an illuminator, they are suitable for intrinsically safe and Exd hazardous areas.
- Maintenance free. Magnetic level gauges are virtually maintenance free because the indicator never touches the process fluid. With sight glasses, the gauges must be periodically checked for leaks and cleaned on a regular basis. Scaling, etching and build-up on the glass from the process fluid can cause the sight glass to become unreadable.
- No worries about broken glass. This makes them ideal for measuring levels of toxic and corrosive liquids.
- Easily heat traced and insulated. An important consideration depending on the fluid being measured. See our page on Electrical Trace Heating for more information on how this can be achieved.
Magnetic Level Gauge Accessories
Magnetic level indicators are available with a range of accessories, the most common being;
- Calibrated Scale
A calibrated scale, attached to the magnetic level indicator allows the level to be read in units or percentage. The scales are most commonly manufactured from traffolyte or 316 stainless steel and engraved with the appropriate units.
- Reed switch
A magnetic reed switch mounted on the external wall of the gauge allows for level alarms and or executive actions to be taken e.g. switching on or off of a pump. Reed switches are widely used and provide a highly reliable and safe operation.
- Reedchain Transmitter
A reedchain transmitter comprises of a chain of switches evenly spaced at intervals of 5mm, 10mm or 20mm gaps. As the float moves along this chain, the magnetic field generated by the float closes the reed switches changing the resistance in the circuit, the resistance output is proportional to the level of the liquid.
- Float Damper Springs
Float damper springs can be installed at either the top, bottom or both ends of the chamber. Damper springs are considered when it is necessary to protect the float from damage in the event that there is a sudden loss or surge of pressure in the system.