A magnetic level gauge measures liquid level using the buoyancy of a float containing a permanent magnet. As the float rises and falls, its magnetic field rotates bi-colour indicator flags on an external indicator column.
No direct sight glass is needed, and no electrical power is required for the basic visual display.
This guide covers the working principle, construction, accessories, selection criteria, and typical industrial applications. See also the HART protocol guide for how the 4-20 mA accessory is configured remotely.
Magnetic Level Gauge works by magnetic coupling between a float inside the process chamber and an indicator column on the outside. No direct fluid contact occurs at the indicator, making it safe for hazardous, toxic, and high-pressure fluids where a glass sight gauge would be dangerous.
Magnetic Level Gauge Working Principle
Three main parts make up this device: a process chamber on the vessel, a float with a permanent magnet inside the chamber, and an external indicator column with bi-colour flags. Click any term to expand.
Level Gauge Construction and Key Components
| Component | Material Options | Function |
|---|---|---|
| Process chamber tube | 316/316L SS, duplex SS, carbon steel with non-magnetic section, Hastelloy C276, titanium | Sealed process boundary connected to the vessel. Non-magnetic section allows magnetic coupling to the indicator. |
| Float body | 316 SS, Hastelloy C276, titanium, PTFE-lined SS | Buoyant hollow body housing the permanent magnet. Must be compatible with the process fluid. |
| Float magnet | Samarium cobalt or alnico (high-temperature service); neodymium (standard service) | Provides the magnetic field that couples to the external indicator. Sealed permanently inside the float. |
| Indicator column | Anodised aluminium or 316 SS housing; bi-colour flags (red/white or red/silver) or roller balls | Visual level display. Flags rotate magnetically as the float passes each indicator element. |
| Process connections | Flanged (ANSI 150 to 2500, EN PN16 to PN420), threaded, or socket-weld | Top and bottom connections link the chamber to the vessel. Isolation valves are fitted to each connection. |
| Isolation valves | Ball valves or needle valves, same pressure rating as the chamber | Isolate the magnetic level gauge from the vessel for maintenance or float replacement without vessel shutdown. |
Accessories for Remote Output and Alarms
The basic the gauge provides a local visual display only. For remote monitoring, alarms, and control signals, accessories are mounted externally on the indicator column without any additional process penetrations.
Magnetic Reed Switch Transmitter
A reed switch module mounts on the indicator column and tracks the float magnet through the chamber wall, producing a 4-20 mA output proportional to level. No additional vessel nozzle is needed.
See the 4-20 mA signal guide for loop wiring.
Magnetic Reed Switch Alarms
Discrete reed switch alarm modules clamp onto the indicator column at desired high and low positions. When the float magnet reaches the switch, the contact changes state to trigger an alarm relay.
These need no process nozzles and can be repositioned simply by sliding them along the column.
Magnetostrictive Level Transmitter
A magnetostrictive transmitter calculates exact float position using a magnetostrictive wire inside the column, delivering accuracy of typically plus or minus 1 mm. It is used where accurate remote level measurement is needed alongside the visual display.
See the level measurement basics guide.
Insulation and Heat Tracing
For cryogenic and high-viscosity applications, the chamber and indicator column can be insulated or heat traced. The indicator flags are positioned through a purpose-designed window in the insulation jacket, preserving the visual display.
Gauge vs Sight Glass and DP Transmitter vs DP Transmitter
| Parameter | Magnetic Level Gauge | Sight Glass | DP Transmitter |
|---|---|---|---|
| Visible display | Yes: bi-colour flags, readable at distance | Yes: direct view through glass | No: display only on transmitter or DCS screen |
| Pressure rating | Up to 210 bar | Limited by glass pressure rating (typically below 100 bar for borosilicate) | Up to 420 bar (high-static versions) |
| Hazardous fluid safety | Excellent: no glass that can shatter | Risk of glass failure releasing toxic or flammable fluid | Risk of impulse line leak |
| Power required | No (visual display). Yes for 4-20 mA accessory. | No (sometimes illumination added) | Yes: 24 V DC loop power |
| Density dependence | Yes: float must be buoyant (SG of liquid must exceed float SG) | No | Yes: hydrostatic head calculation requires known density |
| Foam and interface | Float sits at the liquid/vapour interface. Foam can affect float position. | Shows whatever is visible in the glass (foam may obscure reading) | Averages the column density; foam adds error |
| Remote output | Via reed switch or magnetostrictive accessory (no extra nozzles) | Not available without adding a separate transmitter | Native 4-20 mA and HART output |
| Maintenance | Low: no wearing parts, float lasts for decades | Regular cleaning; glass replacement if cracked | Impulse line purging, zero shift checking, calibration |
Float Selection: Specific Gravity and Material
The float must be buoyant: its overall density must be less than the liquid density. The minimum liquid SG for a standard float is approximately 0.5.
For lighter liquids such as LPG (SG around 0.5), a larger diameter or lower-density float is required.
Float Buoyancy Check
Where the Gauge Is Used in Industry
The magnetic level gauge is the preferred visual level device wherever a conventional sight glass is unsafe or unreliable. Its no-power local display and zero-leak-path design make it suitable across a wide range of industries.
| Industry | Application | Why Magnetic Level Gauge |
|---|---|---|
| Oil and gas | LPG storage bullets, crude oil separators, gas scrubbers, produced water vessels | No glass to shatter; handles low SG hydrocarbons; safe in hazardous areas |
| Power generation | Steam drum level, boiler feed water tanks, condensate receivers, deaerators | High-pressure and high-temperature rated; critical boiler drum level requires reliable visual indicator |
| Chemical processing | Acid storage tanks, solvent reactors, chlorine vessels, styrene and polyol tanks | All-metal wetted parts with no glass; compatible with corrosive and toxic fluids |
| Pharmaceuticals and food | Sterile tanks, bioreactors, CIP vessels, edible oil storage | Clean-in-place compatible; no crevices in the indicator; hygienic flanged connections |
| Cryogenic | Liquid nitrogen, liquid oxygen, LNG storage | Cryogenic-rated floats and chamber; insulated jacket with indicator window |
| Water and wastewater | Sump tanks, chemical dosing vessels, holding tanks | Low maintenance, no power, long service life in outdoor installations |
Watch: Magnetic Level Gauge Working Principle and Applications
Level Gauge Questions
External References
- Magnetic Level Gauge Selection Guide | K-TEK Instruments (Magnetrol)
- Magnetic Level Indicators: Products and Selection | WIKA
What We Learn Today
- The magnetic level gauge works by magnetic coupling between a float (with internal permanent magnet) inside a sealed non-magnetic chamber and bi-colour indicator flags on an external column.
- No power is required for the visual display. The float has no wearing parts and typically lasts decades.
- Rated up to 210 bar and 370°C, the magnetic level gauge is suitable for high-pressure steam drums, cryogenic vessels, toxic chemical tanks, and LPG storage where sight glass is unsafe.
- Float buoyancy requires the liquid SG to exceed the float SG. Standard floats work from SG 0.5. Larger floats are needed for very light liquids.
- Remote 4-20 mA output is available via a reed switch or magnetostrictive transmitter clamped to the indicator column, needing no extra vessel nozzles.
- Discrete alarm reed switches can be repositioned on the indicator column in the field without any process penetration or vessel entry.