Magnetic Level Gauges and Magnetostrictive Level Transmitters

What Is a Magnetic Level Gauge?

A magnetic level gauge consists of a chamber mounted in parallel with the process vessel through top and bottom connections, a magnetic float that rises and falls with the liquid inside this chamber, and a magnetic indicator fitted to the outside of the chamber. Following the principle of communicating vessels, the level in the chamber matches the level in the vessel. The float magnets produce a field that passes through the non-magnetic chamber wall and turns colored flags or drags a shuttle on the outside, so the operator sees the level from a distance as a clear change of color.

Unlike conventional gauges in which the liquid is visible directly behind glass, this design keeps the process fluid entirely inside a closed metal chamber. There is no glass under pressure or temperature in the indicator and fewer potential leak points, which is why the design is preferred for flammable, toxic, corrosive, hot and high-pressure fluids.

When a continuous level signal is needed, an externally mounted magnetostrictive level transmitter is added to the same chamber. Its sensor tube is clamped along the chamber and detects the magnetic field of the same float, so no additional process opening or pipework change is required. Local indication, electronic measurement for the control system and, if required, level switches are thus combined on a single chamber. For applications that need measurement from the tank top without a chamber, non-contacting radar level transmitters can be considered as an alternative.

Working Principle: Float, Magnetic Coupling and Magnetostrictive Measurement

The system combines a mechanical buoyancy element with two different reading methods: magnetic coupling for the visual indicator and magnetostrictive time-of-flight measurement for the electronic signal. The chain can be summarized in five steps:

1.      Level equalization in the chamber: The chamber is connected to the vessel through top and bottom process connections; as with communicating vessels, the liquid level in the chamber follows the level in the vessel. A float stop spring or disc at the chamber ends keeps the float within the measuring zone.

2.      The float follows the level: The float is calculated for the specific gravity, pressure and temperature of the fluid and weighted to provide at least 75 g of upward buoyancy. Magnets arranged at precise intervals generate a magnetic field covering 360° around the chamber.

3.      Visual indication through magnetic coupling: As the float rises, its magnetic field turns the two-colored flags inside a hermetically sealed tube one after another, or drags a shuttle-type indicator. The liquid zone and the gas zone appear in different colors, and a scale next to the indicator gives the level as length, volume or percentage.

4.      Torsional wave in the magnetostrictive wire: At regular intervals, the electronics of the external transmitter send a low-energy current pulse into a special wire inside the sensor tube. Where the magnetic field created by this pulse interacts with the field of the float, a torsional strain (Wiedemann effect) forms in the wire, and this twist travels along the wire at a known speed.

5.      From time of flight to level: A sensing element in the transmitter head turns the mechanical twist into an electrical return pulse. The microprocessor measures the time between the start pulse and the return pulse, calculates the float position and therefore the level, and updates the output at a rate of 10 measurements per second.

Total Level and Interface Measurement

When the float is weighted to sink in the upper liquid and float on the lower liquid, it tracks the interface between the two; in this way interfaces can be followed at specific gravity differences as small as 0.03. If both total level and interface are required, two floats are used in the chamber. The indicator is then ordered as a dual-level version, and the transmitter is calibrated for the two-float application and, with the dual level display option, presents both values.

What External Mounting Means for the Process

The sensor tube of the magnetostrictive transmitter sits outside the chamber and is secured with stainless steel clamps. The float is the only moving part in contact with the process liquid. As a result, the transmitter, the indicator scale and level switches can be added, adjusted or serviced without modifying the piping and without stopping the process, and the electronics module can be replaced or upgraded in the field.

Factors Affecting Transmitter Accuracy

The accuracy figure of the transmitter is determined under factory ambient conditions with a calibration magnet. In the field, the actual system accuracy is also influenced by float hysteresis, the chamber installation, changes in the specific gravity of the fluid and ambient conditions. Float design and transmitter selection should therefore be treated together.

Because the indicator works without electricity, the level can still be read during a power failure, while the transmitter uses the same float to supply a continuous signal to the control system. Since both measurement paths rely on the same float, the local reading and the control room value are easy to compare.

Key Features

·         Indicator isolated from the process liquid: The level is read from a hermetically sealed indicator outside the closed metal chamber; the process fluid does not touch glass and the indicator is not under pressure.

·         Float engineered for the application: Each float is calculated for the specific gravity, pressure and temperature of the fluid, providing at least 75 g of buoyancy and 360° magnetic field coverage.

·         Wide pressure and temperature envelope: The chamber design covers process conditions from full vacuum to 345 bar and from –196 °C to 538 °C.

·         Low density and interface: Designs are possible for liquids with a specific gravity down to 0.25, for all viscosity ranges and for interfaces with a specific gravity difference down to 0.03.

·         Choice of indicator: A high-visibility shuttle indicator or a flag indicator in yellow/black, red/white, red/green or red/black; dual-level and float failure indication options.

·         High-resolution magnetostrictive transmitter: Measurement accuracy of ±0.01% of full scale (FS) or 1.27 mm (whichever is greater), repeatability of ±0.005% or 0.315 mm, and a factory-calibrated design that needs no recalibration.

·         Total and interface level in one transmitter: The transmitter can measure total level, interface level or both; a 21-point linearization table allows volume characterization.

·         Local interface with waveform display: The optional display shows the signal waveform, allowing performance to be verified without an oscilloscope; settings are made with through-the-glass buttons without opening the cover, and faults are shown as NE 107-compliant plain text.

·         Digital communication: 4…20 mA HART (revision 7) or FOUNDATION Fieldbus (ITK 6.3.0) output; alarm direction to NE 43 selectable by software or hardware.

·         Sensor options for extreme temperatures: Standard probes are used between –80 °C and 232.22 °C or 426.66 °C, and nitrogen-purged cryogenic probes between –195.55 °C and 121 °C; a 90° bent housing extension and remote electronics up to 30 m are available.

·         Comprehensive chamber accessories: Level switches, frost extensions, steam or electric heat tracing, high-temperature and cryogenic insulation, magnetic particle traps and drain/vent valves can be factory-mounted together with the chamber.

Technical Specifications

The values below are typical technical data for the magnetic level gauge with bypass chamber and the externally mounted magnetostrictive level transmitter. Because the chamber, float and connections are designed for each process, final pressure, temperature and measuring length values should be confirmed at project stage for the selected material and configuration.

Lined and plastic chambers have lower maximum temperatures and measuring lengths than metal chambers (e.g. 5.48 m for PVC, CPVC, PVDF and fiberglass chambers). The temperature limit of the transmitter may be lower than that of the gauge chamber; the system limit is always set by the lower value.

Key Advantages

·         Safe local reading: With no glass under pressure and the fluid kept inside a closed chamber, the risk of leaks and breakage is reduced for flammable and toxic fluids.

·         Level visible during power failures: Because the indicator needs no electrical power, the level can still be read after a loss of power and before commissioning.

·         Expansion without process interruption: The transmitter, scale and switches can be added, adjusted or replaced on the chamber later, with no changes to the piping.

·         One float, two independent readings: The visual indicator and the electronic transmitter use the same float, so the field reading and the control room value are easy to compare.

·         Low maintenance, no field calibration: A robust design, an indicator that needs no calibration on site and a factory-calibrated transmitter reduce the maintenance workload; the chamber can be blown down or flushed without being removed.

·         Fully factory-assembled and tested delivery: Chamber, indicator, transmitter and accessories are assembled and tested at the factory and shipped with quality reports, reducing fabrication and installation work on site.

·         Material flexibility for difficult fluids: A material range extending from stainless steels to nickel alloys, and from titanium to lined and plastic chambers, allows a suitable choice for corrosive fluids.

Application Areas

Magnetic level gauges and externally mounted magnetostrictive transmitters are used in plants handling corrosive, flammable, toxic, hot and high-pressure fluids, above all in oil and gas, petrochemicals, chemicals and power generation.

Oil and Gas Processes

Used for local indication of total level and interface level in pressure vessels and for transferring these values to the control system. Hardness certificates to NACE MR0103 and MR0175/ISO 15156 can be supplied for sour service conditions.

Petrochemical and Chemical Plants

For corrosive, toxic and flammable fluids, nickel alloy, titanium, zirconium, PTFE-lined or plastic chamber options provide a fluid-compatible material; the fluid remains fully isolated from the indicator.

Power Generation and Boilers

A marine and industrial type approval option is available for high-pressure boilers. For high-temperature service, the shuttle indicator with a hermetically sealed glass tube can be used up to 538 °C, and insulation and heat tracing accessories can be added.

Cryogenic Applications

Indicators with a frost extension prevent ice build-up from blocking the view and can be used down to –196 °C; cryogenic insulation and nitrogen-purged transmitter probes support stable measurement at low temperatures.

High-Vibration Environments

Near high-vibration equipment such as compressors, the transmitter is attached to the chamber with vibration isolators whose number depends on the measuring length; a minimum of two isolators is recommended for measuring lengths up to 914.4 mm.

Valve Position Monitoring

Used together with a magnet assembly attached to the valve stem, the magnetostrictive transmitter can provide continuous position measurement in applications such as hydraulic control valves.

How to Select a Magnetic Level Gauge and Transmitter

Magnetic level systems are engineered to order, and correct performance depends largely on the float and chamber design. For a technical assessment, the following information needs to be clarified:

Process Fluid

·         Operating and minimum specific gravity of the fluid; for interface, the specific gravities of the upper and lower liquids

·         Operating and design temperature and pressure; minimum ambient temperature

·         Measurement type: total level, interface or both (dual level)

·         Whether the service is water/steam and whether high vibration is present

Chamber and Process Connections

·         Chamber and connection material; if a lined chamber is selected, the connections are produced with the same lining

·         Measuring length and distance between connections (center-to-center, face-to-face, etc.)

·         Size, type, pressure rating and orientation of the process connections (side–side, top–bottom, etc.)

·         Vent and drain connections, need for valves

·         Support brackets for lengths above 12 ft; flanged sections or a one-piece welded chamber for lengths above 20 ft

Indicator and Scale

·         Indicator type: shuttle or flag; flag color combination

·         Scale unit: ft/in, m/cm (1 cm graduations), percentage or volume; custom scales

·         Suitability of the indicator tube material for the process temperature (plastic or hermetically sealed glass)

·         Need for a frost extension in cryogenic applications

Transmitter and System Integration

·         Output: 4…20 mA HART or FOUNDATION Fieldbus

·         Probe type according to ambient and process temperature; electronics housing mounted at the top or bottom; remote electronics if needed

·         Calibration for total, interface or two-float measurement; 3-point or 5-point calibration verification certificate

·         Hazardous-area type of protection and SIL requirement

·         Optional local display, surge protection and housing material

Accessories and Site Conditions

·         Level switches mounted on the chamber for point level

·         Steam or electric heat tracing, steam jacket, high-temperature or cryogenic insulation

·         Magnetic particle traps and mounting supports

·         Need for chamber insulation for personnel protection

Where the specific gravity is very low or variable and float design becomes difficult, alternative technologies such as differential pressure or guided wave radar should also be compared.

Manufacturing Standards, Tests and Approvals

Chambers are fabricated in-house by code-certified welders. The main standards, tests and documents available on request are summarized below.

The scope of documentation and approvals is defined per order; clarifying requirements such as PED, ASME stamp and NACE at the start of the project is important for delivery time and fabrication planning.

Magnetic Level Measurement Solutions from TLY Enerji

Because a magnetic level gauge is pressure equipment engineered to order, TLY Enerji starts each project by collecting data. The specific gravities of the fluid, design pressure and temperature, measuring length, connection positions and material requirements are clarified through a process data form, and the float, chamber, indicator type and transmitter are selected in one technical assessment.

Our engineering team supports customers with defining documentation requirements such as ASME, PED and NACE at quotation stage, following up approval drawings and test plans, PLC/DCS integration via HART or FOUNDATION Fieldbus, commissioning and field support. For plants that need a transmitter or switches added to existing gauges, we plan conversions that do not require a process shutdown. The system of indicator, transmitter and switches is then commissioned on site under a single supply and engineering responsibility.

Frequently Asked Questions

How does a magnetic level gauge work?

The liquid level in a bypass chamber connected in parallel with the vessel follows the vessel level on the principle of communicating vessels. A magnetic float inside the chamber moves with the liquid, and its magnetic field turns colored flags or drags a shuttle outside the chamber wall. The level is therefore read as a change of color on an indicator that has no contact with the fluid and needs no electricity.

How is a magnetostrictive level transmitter attached to the gauge?

The sensor tube of the transmitter is mounted externally, parallel to the gauge chamber, with stainless steel clamps and detects the magnetic field of the same float inside the chamber. The electronics calculate the float position from the time of flight of the torsional wave generated in the sensor wire. Since no additional process opening is needed, the transmitter can also be added to existing gauges without stopping the process.

What is the difference between a magnetic level gauge and a sight glass?

In a sight glass the process liquid sits directly behind the glass, which is exposed to pressure and temperature. In a magnetic level gauge the fluid stays inside a closed metal chamber, the indicator is hermetically isolated and there is no glass under pressure. This design reduces potential leak points, improves safety with flammable and toxic fluids and offers a color contrast that is easier to read from a distance.

Can a magnetic level gauge measure interface level?

Yes. When the float is weighted to sink in the upper liquid and float on the lower liquid, it tracks the interface between them; designs are possible for specific gravity differences as small as 0.03. If both total level and interface are needed, two floats and a dual-level indicator are used. The magnetostrictive transmitter can also be calibrated for the two-float application and deliver both levels at the same time.

Why is the float designed for each process?

Whether the float rides at the correct level depends on the specific gravity of the fluid and on the weight and volume of the float. Pressure and temperature determine its wall thickness and material. Each float is therefore calculated for the operating conditions and designed with at least 75 g of upward buoyancy. If the specific gravity falls well below the design value, the float may not produce enough buoyancy, so the minimum specific gravity must always be specified.

How accurate is a magnetostrictive level transmitter?

The measurement accuracy of the externally mounted magnetostrictive transmitter is ±0.01% of full scale (FS) or 1.27 mm (whichever is greater), and repeatability is ±0.005% or 0.315 mm. These values are determined under factory conditions with a calibration magnet. Because system accuracy in the field is also affected by float hysteresis, installation, fluid properties and ambient conditions, float and transmitter should be assessed together.

Can it be used in cryogenic and high-temperature processes?

Yes. The chamber design covers –196 °C to 538 °C. In cryogenic service, indicators with a frost extension and cryogenic insulation are used; on the transmitter side, nitrogen-purged cryogenic probes work down to –195.55 °C. For high temperatures there are hermetically sealed glass tube indicators, insulation pads and standard probe options up to 426.66 °C. As the transmitter's temperature limit may be lower than that of the gauge chamber, the system limit should be checked separately.

Can a transmitter or level switch be added without shutting down the process?

Yes. Because the transmitter, the indicator scale and magnetic level switches are clamped to the outside of the chamber, no piping changes or process interruption are needed. This flexibility makes it easy to add a control system signal to an existing gauge later, to reposition alarm points on site and to replace the electronics module. Installation and calibration only need to match the measuring length.

Does a magnetic level gauge need electrical power?

No. The indicator works entirely through the magnetic field of the float and needs no external power, so the level can still be read during a power failure. Electricity is only required for a magnetostrictive transmitter and electronic switches added to the system. The transmitter is two-wire loop powered; the HART version operates on 12–42 V DC and the Fieldbus version on 9.0–32 V DC.

Which tests and documents can be supplied?

On request, radiographic and liquid penetrant examination, hydrostatic testing, positive material identification, EN 10204 3.1 or 3.2 material certificates, ASME U, UM or S stamp, a certificate of conformance to ASME B31.1/B31.3, PED conformity, NACE hardness certificates, CRN, third-party inspection and float curves can be supplied. Approval drawings, a weld map and an inspection and test plan can also form part of the project documentation.