A magnetic level gauge (MLG) is a bypass chamber fitted with a float containing a permanent magnet array, where the float's position is read externally through a coupled indicator column and is independent of electrical power at the chamber wall [S3][S4].
ABB's KM26, in production since 1975 under the K-TEK brand, holds a documented installed base above 350,000 units across oil and gas, refinery, chemical, petrochemical, and power generation service, and supports both total and interface level detection in toxic, corrosive, high pressure, and high temperature processes [S3][S4].
Operating Principle and Where MLGs Fit vs Sight Glasses
Magnetic level gauges use a float with an integrated permanent magnet traveling inside a process-wetted chamber, and the external indicator flips colored flags or follower tabs through non-contact magnetic coupling, so the process fluid never contacts the glass or external indicator [S3]. This bypass arrangement is mechanically simpler than a reflex or transparent sight glass because the only pressure-retaining components are the chamber pipe and end connections, not a glass plate. Compared with tubular, reflex, transparent, and two-color sight glasses, the magnetic variant is the only one that natively produces a repeatable electrical signal at any point along the level span [S2]. When a sight glass is sufficient for a non-toxic, low-pressure liquid at a single local reading, an MLG is over-specified; when toxic, corrosive, hot, or interface duty appears, an MLG becomes the baseline choice [S2][S3].
Selection Criteria 1-2: Chamber Material and Process Connection
Chamber material selection drives corrosion allowance: KM26 chambers are offered in virtually any machinable alloy, while KROHNE's BM26A-3000 uses PVC and PP for corrosive liquids where stainless steel is insufficient [S2][S3]. Process connection sizing follows the chamber ID, which on KM26 is normally matched to the vessel nozzle size to avoid flow restriction at the bypass tap [S3]. For vacuum service, the chamber must be specified with vacuum-rated wall thickness, and for high temperature service, the magnet material grade must be selected to retain field strength at the design temperature [S3][S4]. Buyers often over-spec flange rating when the actual control is float specific gravity relative to the process liquid, not the pressure class.
Selection Criteria 3-4: Pressure-Temperature Class and Certifications
The KM26 carries optional PED, ASME (U, UM, or S stamp), ATEX constructional safety, and IP68 hermetically sealed indicator certifications as documented on the current ABB product page, and these are the four codes most European and North American EPCs require on a datasheet [S2][S3]. ASME U-stamp applies to a single pressure vessel, UM-stamp to a multi-vessel shop assembly, and S-stamp to a boiler-only code scope, so a buyer who conflates them risks an un-stamped chamber being rejected at inspection. IP68 on the indicator column, not just on cable entries, is the line item that lets the gauge survive a flooded bund or wash-down area; datasheets that show only IP65 on the indicator should be queried before acceptance [S2][S3].
Selection Criteria 5-6: Switch and Transmitter Integration
MLG switch integration: external reed or proximity switches clamp onto the indicator rail at setpoints and can be added or moved without breaking the process seal, which is the single biggest operational advantage over a tapped capacitance or differential pressure switch [S1][S4]. Continuous transmitter output is achieved in two ways, either by a guided-wave radar (GWR) probe inserted into the same bypass, as in the MagWave MW05 hybrid, or by an external magnetostrictive or reed-chain transmitter strapped to the chamber [S2]. For interface detection between two liquids, a single float with a defined specific gravity band sits at the phase boundary and reports interface level, while a second float can be added for total level, and this dual-float arrangement is the standard approach for separator and decanter service [S3][S4]. Buyers comparing continuous options should weigh the MagWave hybrid against a separate external MLI transmitter, since the hybrid shares one chamber and one process tap, which cuts one potential leak point.
Criteria Comparison: Four Main MLG Variants
Four variants cover most industrial duty and can be lined up against four decision criteria: standard metal-chamber MLG (e.g. KM26, BM 26 KP), plastic-chamber MLG (e.g. BM26A-3000 in PVC/PP), radar-hybrid MLG (e.g. MagWave MW05), and top-mounted MLI (e.g. UTN) [S2]. On corrosion resistance the plastic-chamber variant wins for aggressive media, on temperature-pressure the metal-chamber variant is the only choice, on interface detection the metal-chamber and radar-hybrid are equally capable, and on retrofit simplicity the top-mounted UTN-type MLI installs on an existing nozzle without a full side-mounted bypass [S2][S3]. A standard metal MLG is the right default for clean hydrocarbon and steam-boiler service; a plastic-chamber MLG is the right default for HCl, hypochlorite, or HF-trace service where stainless still fails; a radar-hybrid MLG is justified only when a 4-20 mA level output is required at the same physical location as the local visual reading; and a top-mounted MLI is justified for tall atmospheric storage tanks where a side-mounted bypass would need long unsupported piping.
Limits, Failure Modes, and When NOT to Specify an MLG
MLG failure modes concentrate at the float, not at the chamber, and the most common field issue is a float that sinks because process density crept outside the specified specific-gravity band, so the datasheet's SG window must bracket the real operating density with margin [S3]. Solids-laden or fouling service plugs the bypass tap and the chamber, and the gauge will read the tap, not the vessel, so MLGs are not for slurry, asphalt, or coker service. Cryogenic service below the float magnet's rated temperature loses the magnetic coupling, and high-vibration service can shake follower flags; both require a vendor-confirmed temperature and vibration envelope, not a generic "industrial" rating [S3][S4]. For interface detection on a very narrow SG difference under 0.05, a guided-wave radar or capacitance probe is more repeatable than an MLG float.
Related Selection Guides and Where to Read Next
For broader level technology context, the radar vs ultrasonic level selection map lines non-contact electronics against the contact MLG family, and the radar level meter 2026 buying guide covers the hybrid transmitter option that bolts onto an MLG chamber. For switch-level selection specifically, the level switch technology map covers reed, capacitance, and float switches that can be specified alongside an MLG chamber. For foundational MLG mechanics and how the float magnet couples through the chamber wall to the indicator, the magnetic level gauge reference defines the bypass principle and SG window in detail. [S2]
Two trackable signals to watch: the next ABB KM26 product-page revision (last updated 2026-06-24 [S3]) for any new material or certification addition, and the DirectIndustry MLG category (32 manufacturers, 70-72 products on the listing as of 2026-07-18 [S2]) for new entrants in plastic-chamber and radar-hybrid variants.
Component reference pages worth checking: magnetic material, and magnetic sensor.