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Magnetic Level Gauge (Bypass Level Indicator)
A magnetic level gauge reads liquid level from the outside of a vessel. A magnetic float sealed in a bypass chamber flips a bi-colour flag indicator, so the level is visible across the plant with nothing touching the process and no glass to break. Transmitters and switches clamp on without breaking the seal.
- Type: side-mounted bypass chamber, visual indicator
- Indicator: bi-colour rollers or capsule; readable at 30+ m
- Chamber: 316L stainless (other alloys on request)
- Process: to 450 °C; rated to the piping class
- Add-ons: 4–20 mA transmitter, reed switches, GWR
A magnetic level gauge, also called a magnetic level indicator or bypass level gauge, is the modern replacement for a tubular sight glass. It is what most oil, gas, and chemical plants now use above about 150 psi, or whenever the fluid is toxic, flammable, or opaque. The level shows on a bright bi-colour strip on the outside of a sealed metal chamber, so an operator reads it from across the plant, and a transmitter or switches can be added later without opening the process.
Overview
The gauge is a bypass chamber piped into the vessel at two points, so the liquid inside it sits at the same height as the tank. A float carrying a ring magnet rides on that surface, and its field passes through the non-magnetic 316L wall to flip an external flag indicator: red above the liquid, silver below. Because the indicator and any transmitter sit outside the pressure boundary, they never touch the medium. The chamber is built to the height of the vessel and rated to the piping class, and a high-temperature construction extends service to 450 °C. For a continuous electronic reading on the same chamber, add a clamp-on magnetostrictive level transmitter.
Features
What sets a magnetic level gauge apart from a sight glass and from electronic gauges:
No process seal to leakThe float and magnet are sealed in the metal chamber and the readout never touches the medium, so there is no glass to shatter and no leak path.
Readable from 30+ mThe bi-colour flag indicator, red above and silver below the surface, is visible across the plant, where a tubular sight glass forces you to stand at the tank.
Works with no powerThe indicator is purely magnetic and mechanical, so it keeps showing level during a power outage and needs no calibration to read.
Built for tough serviceA 316L chamber, other alloys on request, a high-temperature build to 450 °C, and a pressure rating matched to the piping class.
Clamp on a transmitter, liveA magnetostrictive transmitter (4–20 mA HART, ±0.05% FS) or reed-switch alarms clamp to the chamber outside the seal, and can be added or removed in service.
Replaces a sight glassFor fluids above about 150 psi, or toxic, flammable, or opaque media, where a glass gauge is unsafe or unreadable.
Working principle
Three physical principles work at once. By communicating vessels, the bypass chamber is piped into the tank at two points, so the liquid in it equalises with the tank level. By Archimedes buoyancy, the float is density-matched to ride exactly on the liquid surface, not in the vapour or submerged. By magnetic coupling, the float’s ring magnet flips each external indicator element, a roller or a capsule, as it passes. The chamber wall is non-magnetic 316L, so the field reaches the indicator while the pressure boundary stays sealed. Because the indicator and any clamp-on transmitter are outside the chamber, nothing touches the process.
Indicator types
| Indicator | How it shows level | Best for |
|---|---|---|
| Bi-colour rollers | Each 10 mm roller flips 180 degrees as the float passes: red above, silver below the liquid line | Standard service, long-distance readability (30+ m) |
| Capsule (shuttle) | A single bi-colour capsule tracks the float inside a glass tube | Cold outdoor service (no rollers to freeze), dusty plants |
Technical specifications
| Parameter | Specification |
|---|---|
| Type | Bypass (side-mounted) magnetic level gauge / indicator |
| Indicator | Bi-colour roller (flag) or capsule; red above, silver below the liquid |
| Measuring span | Built to the vessel; sections joined for tall tanks |
| Chamber material | 316L stainless steel standard; other alloys on request |
| Float | Magnetic float, density-matched to the liquid |
| Process temperature | Up to 450 °C with high-temperature construction |
| Process pressure | Rated to the piping class (matched to the flange rating) |
| Readout accuracy | ±10 mm (roller pitch); to ±0.05% FS with an added magnetostrictive transmitter |
| Process connection | Side flanges or threads; top/bottom or side/side arrangement |
| Options | Magnetostrictive transmitter, reed-switch alarms, GWR redundancy, insulation / heat tracing, frost extension |
Transmitter and switches
Because the float carries a magnet, you can pick up its position electronically from outside the chamber, with no wetted parts and nothing breaking into the process. Three common add-ons: a magnetostrictive transmitter clamps to the chamber for a 4–20 mA HART output at ±0.05% FS, the standard choice when the level has to reach the control system; reed-switch alarms clamp on at high, low, high-high, and low-low points with an SPDT relay output and no power on the process side; and a guided wave radar can run in parallel on the same chamber for SIL-rated redundant measurement. All of them can be fitted or removed while the gauge stays in service.
Applications
Magnetic level gauges suit boilers and deaerators, oil and gas separators, LPG and ammonia bullets, chemical reactors and day tanks, asphalt and bitumen, and any toxic, flammable, or opaque liquid where a sight glass is unsafe. They are also chosen where a power-free, visible local indication has to stay readable during an outage, and where a transmitter may be added later.
Challenge: A user with portable cryogenic (liquid nitrogen) tanks needed a level readout that could be checked by eye, with no power and no calibration, on a sealed low-temperature vessel.
Solution: A magnetic level gauge, whose float and flag indicator carry no electronics, reads the level visually through the sealed chamber wall.
Result: A clear local level on each tank that works during an outage and needs no power, with the option to clamp on a transmitter if remote monitoring is added later.
Related products
Magnetostrictive Level TransmitterClamp-on 4–20 mA output, ±0.05% FS.
Guided Wave RadarParallel on the chamber for redundancy.
Hydrostatic Level TransmitterContinuous level from the head of liquid.
Browse all level instruments →
FAQ
How does a magnetic level gauge work?
A bypass chamber is piped into the vessel at two points, so the liquid in it sits at the tank level. A float carrying a ring magnet rides on that surface, and its field passes through the non-magnetic chamber wall to flip an external indicator: red above the liquid, silver below. You read the level from the coloured strip outside the sealed chamber, so nothing touches the process.
How do you calibrate a magnetic level gauge?
The visual indicator itself needs no calibration: the flags track the float mechanically. What you verify is the zero and span of any clamp-on transmitter. Set the transmitter’s empty and full points to the chamber’s measuring range, then confirm two known levels against the flag indicator. If a flag reads wrong, it is usually a stuck roller or a float that has dropped, not a calibration fault.
How do you read a magnetic level gauge?
Read the boundary on the coloured strip: the level is where the red (above the liquid) meets the silver (below). The scale beside the indicator gives the height directly, and the contrast stays readable from across the plant. For a remote or logged reading, take the 4–20 mA signal from a clamp-on transmitter on the same chamber.
Request a quote
Tell us the vessel height, the liquid and its density, the process temperature and pressure, and the flange rating, plus whether you need a transmitter or switches. We build the chamber and indicator to the vessel, not a shelf part. Pricing and lead time follow by return email.