A magnetic level gauge, or MLG, shows tank and vessel level by floating a magnetic float inside a sealed bypass chamber and letting that float flip a column of external flags or a shuttle to mark the surface. The chamber connects to the vessel at top and bottom, so liquid rises and falls inside it exactly as it does in the vessel. As the magnetic float rides that surface, its field flips two-color flags on an indicator rail outside the chamber, painting a clear line between the wetted and dry portions. The whole reading happens outside a sealed steel chamber, which means there is no glass to break and nothing of the process is exposed.
Magnetic Level Gauge (MLG) in one line: A magnetic level gauge is a level indicator built around a magnetic float in a sealed bypass chamber piped to the vessel. As the float rides the liquid surface, its magnetic field flips external two-color flags or a shuttle to display the level visibly, and a magnetostrictive or guided-wave transmitter can clamp to the same chamber for a redundant continuous readout.
The core of an MLG is a bypass chamber - a sealed pipe running vertically alongside the vessel, connected to it through process fittings at the top and bottom. Because it is open to the vessel at both ends, the liquid level inside the chamber tracks the level in the vessel by simple communicating-vessels physics. Inside that chamber floats a purpose-built magnetic float, weighted to the product's density so it rides right at the liquid surface, carrying a ring of magnets.
Mounted on the outside of the chamber is the indicator: a rail of small, two-color flags, each free to rotate on its axis, or in some designs a magnetic shuttle. As the float passes behind a flag, its magnetic field flips that flag from, say, red to white. Below the float the flags are flipped one color; above it they show the other; and the sharp boundary between the two colors is the level. Because the flags latch magnetically, they hold their state and give a crisp, high-visibility indication readable from a distance.
Nothing about that display touches the process. The float and the fluid stay sealed inside the steel chamber, and the flags and rail live entirely outside it, coupled only by magnetism through the chamber wall. That is the whole design philosophy of the MLG, and it is what makes it fundamentally different from the sight glass it usually replaces: the indication is external, and the pressure boundary is unbroken metal.
A traditional sight glass or gauge glass shows level through actual glass exposed to process pressure and product. That glass is a weak point: it can crack, craze, or fail, and when it does it releases whatever is in the vessel, which on hot, toxic, or high-pressure hydrocarbon service is a serious hazard. It is also often hard to read - a clear liquid behind glass can be nearly invisible, and the glass fouls and stains over time. The magnetic level gauge was adopted largely to eliminate those problems.
With an MLG the pressure boundary is a solid steel chamber rated for the full process conditions, with no glass in the pressure envelope at all. The failure mode of a broken sight glass simply does not exist. The flag indicator is far easier to read than liquid behind glass - a bold red-and-white column visible across a unit, day or night, regardless of the product's own color or clarity. Longer indication lengths are practical too, since the chamber and rail can run the full height of a tall vessel where a single sight glass could not.
The trade-offs are modest and worth knowing. The float must be matched to the product's density, so a large density change or a very light fluid needs the right float, and heavy buildup or magnetic debris inside the chamber can slow or stick the float. The gauge also reads only inside its own chamber, so an isolation or a plugged process connection can leave it showing a stale level while the vessel moves. Those are manageable and well understood, which is why MLGs became the standard visual level indication across refining, gas processing, and storage.
The same bypass chamber that carries the flag indicator can host a continuous level transmitter, and this is where the MLG earns its place in a modern instrumented site. Because the float is magnetic, a magnetostrictive transmitter can be strapped to the outside of the chamber and track that same float without any additional process penetration - it senses the float already in the chamber. Alternatively, a guided-wave radar probe can be installed in the chamber to give an independent continuous reading. Either way, one chamber yields both a local visual indication and a remote analog signal.
That combination is genuinely redundant because the visual and the transmitted readings come from different physics. The flags follow the float's magnetic field mechanically; the strapped-on transmitter times a torsional pulse to the same float; a guided-wave probe times a microwave reflection off the surface. An operator at the vessel reads the flags, a control room reads the transmitter, and the two can be cross-checked against each other. A disagreement between the flag column and the transmitted value is itself a useful diagnostic - it flags a stuck float, a fouled chamber, or a transmitter problem.
For remote and unmanned operation this piggybacked arrangement is ideal. A cloud SCADA platform such as Merobix historizes the transmitter's continuous output from the MLG chamber, so a vessel that has only a local flag gauge on the ground gains a trended, alarmable remote level without cutting a new nozzle. The visual gauge serves the person standing at the vessel; the strapped-on transmitter serves the dashboard; and having both from a single sealed chamber gives an economical layer of redundancy that is hard to justify with two entirely separate instruments. On a site monitored from afar, that redundant, sealed, glass-free level point is exactly the kind of robust measurement worth having.
It floats a magnetic float in a sealed steel bypass chamber piped to the vessel, so the level inside the chamber tracks the vessel. As the float rides the surface, its magnetic field flips a column of external two-color flags mounted outside the chamber, and the boundary between the two flag colors marks the level. The indication is entirely external, so there is no glass in the pressure boundary to break.
A sight glass exposes actual glass to process pressure and product, and a cracked glass releases the contents - a real hazard on hot, toxic, or high-pressure service. A magnetic level gauge keeps the pressure boundary as solid steel with no glass, gives a bold red-and-white column that is far easier to read, and supports much longer indication lengths. It removes the sight glass failure mode entirely.
Yes. A magnetostrictive transmitter can strap to the outside of the same chamber and track the existing magnetic float, or a guided-wave radar probe can be installed inside the chamber, giving a continuous 4 to 20 mA or digital output. That lets one sealed chamber provide both a local flag indication and a remote, trendable reading, and the two can be cross-checked for redundancy.
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