When the level in a tank, process vessel or storage tank is not only to be indicated roughly, but measured continuously, precisely and automatically, a magnetostrictive level transmitter is a very powerful solution. It combines the proven float principle with a high-resolution measuring method and provides a continuous output signal for PLC, process control system, display or SCADA system.
Unlike simple point level detection, a level transmitter does not only detect “full” or “empty”, but records the current level across the entire measuring range. This allows levels to be controlled, tank contents to be monitored, limit values to be evaluated, trends to be identified and processes to be documented more effectively.
This article explains how magnetostrictive level transmitters work, when they are useful compared with reed chain transmitters, float switches or simple indicators, and what should be considered regarding float selection, medium density, 4–20 mA signal, HART, Modbus, local display, limit values and interface measurement.
Table of contents
- Basics: What is a magnetostrictive level transmitter?
- Measuring principle: How magnetic float and magnetostriction work together
- Advantages: Why magnetostrictive transmitters are particularly interesting for tanks
- Output signal: 4–20 mA, HART, Modbus and local display
- Float selection: Why density, medium and process conditions are decisive
- Interface measurement: Detecting oil/water separation and interfaces
- Installation in the tank: Direct mounting, bypass and process connection
- Limit values and alarms: Monitoring level safely
- Typical sources of error in continuous level measurement
- Tables: Selection, signal types and typical applications
- Practical example: Continuous level measurement in a storage tank
- Which measuring instruments / products are suitable?
- Conclusion: High-precision level measurement starts with the right design
- FAQ: Frequently asked questions about magnetostrictive level transmitters
Basics: What is a magnetostrictive level transmitter?
A magnetostrictive level transmitter is a measuring instrument for continuous level measurement of liquids in vessels, tanks or process plants. It typically consists of a guide tube, a float with an integrated magnetic system and a sensor head with electronics.
The float moves along the guide tube with the liquid level. Its position is detected contactlessly using the magnetostrictive measuring principle. From this position, the electronics calculate the current level and output it as an analog or digital signal.
The essential difference compared with a simple float switch is that not only a single switching point is detected. Instead, the level is measured continuously across the measuring range. This makes the transmitter particularly interesting for applications in which the level needs to be controlled, documented or integrated into automation.
Typical applications include storage tanks, process vessels, day tanks, hydraulic tanks, chemical tanks, water and wastewater tanks, oil tanks, dosing tanks or systems with PLC/SCADA integration. The decisive factor is that the medium, float, materials, pressure, temperature and process connection match the application.
Measuring principle: How magnetic float and magnetostriction work together
The magnetostrictive measuring principle uses the position of a magnetic float. Inside the guide tube there is a sensor element or waveguide. The electronics send a short current pulse through this conductor. This creates a magnetic field along the measuring rod.
The float contains a permanent magnet. At the position of the float, the magnetic field of the current pulse and the magnetic field of the float overlap. This creates a mechanical wave whose travel time is measured. From this travel time, the exact position of the float is calculated.
Because the float directly follows the liquid level, its position corresponds to the level in the vessel. This principle enables very fine position resolution and is therefore particularly suitable for applications where simple step-by-step detection is not sufficient.
One advantage is that the measurement does not directly depend on electrical properties of the medium such as conductivity or dielectric constant. The main requirement is that the float matches the density, viscosity, temperature, pressure load and chemical resistance of the medium.
Advantages: Why magnetostrictive transmitters are particularly interesting for tanks
Magnetostrictive level transmitters are particularly suitable when high resolution, a continuous output signal and robust mechanical measurement are required. In many tank applications, they provide a very reliable connection between on-site measurement and automated evaluation.
Compared with simple float switches, they offer the advantage that the entire level profile becomes visible. A PLC can therefore not only evaluate limit values, but also monitor the current tank content, detect trends or implement control strategies.
Compared with reed chain level transmitters, magnetostrictive systems often offer finer resolution because the position is not detected in discrete steps via reed contacts. This is particularly interesting for small vessels, precise dosing processes or applications where level changes must be tracked accurately.
Another advantage is good integration capability. Depending on the version, 4–20 mA, HART, Modbus, RS-485, IO-Link, local displays or limit contacts may be available. This allows level measurement to be adapted to different automation environments.
Output signal: 4–20 mA, HART, Modbus and local display
For many industrial applications, the 4–20 mA signal remains the most important interface. It is robust, well established and can be easily connected to PLC input cards, displays, data loggers or process control systems. The measuring range is scaled so that 4 mA corresponds to the lower and 20 mA to the upper measuring range.
With HART-capable versions, digital communication can be used in addition to the analog signal. This is helpful when parameters, diagnostic information, measuring range or other device data need to be read out or adjusted.
Depending on the device family and version, digital interfaces such as Modbus/RS-485 or IO-Link may also be of interest. They are particularly useful when several measured values, diagnostic information or status data are to be integrated into higher-level systems.
A local display can be useful if the level should be readable directly at the tank. In many systems, the combination of local measured value and remote signal is particularly practical: Operating personnel can see the level on site, while the PLC or SCADA system processes the value in parallel.
Float selection: Why density, medium and process conditions are decisive
The float is a central component of the measurement. It must float reliably on the medium and change its position stably with the level. The density of the medium is therefore a decisive design factor. If the float is unsuitable for the medium density, it may immerse too deeply, operate unstably or, in the worst case, not float correctly.
In addition to density, viscosity, temperature, pressure, chemical resistance and tendency to form deposits also play an important role. A low-viscosity oil places different demands on the float than water, a solvent, an aggressive chemical mixture or a medium containing solids.
The material of the float and guide tube must also match the application. Stainless steels are often used, but depending on the medium, other materials may be required. For aggressive media, hygienic requirements or high temperatures, the selection should be checked carefully.
For small tanks, narrow installation conditions or strongly moving liquids, the float size must also be considered. The float must not get stuck on internals, tank walls, stilling tubes or process connections. Clean mechanical guidance is a prerequisite for stable measured values.
Interface measurement: Detecting oil/water separation and interfaces
A special application of magnetostrictive level measurement is interface measurement. This is not only about the total level in a vessel, but about the position of an interface between two liquids, for example oil and water.
The prerequisite is that the liquids differ sufficiently in density and that a suitable float or suitable float combination is used. One float can then follow the total level, while another detects the interface.
Interface measurement is particularly interesting in tank farms, separation processes, oil/water separators, process vessels or applications where phase separation must be monitored. The measurement can help avoid product losses, optimize separation processes or detect unwanted water content.
In practice, the design must be carried out very carefully. Density values, temperature range, medium composition, emulsions, foam formation, turbulence and deposits can influence the measurement. For interface applications, it should therefore always be checked whether the medium and process conditions are stable enough for reliable interface measurement.
Installation in the tank: Direct mounting, bypass and process connection
Depending on the version, magnetostrictive level transmitters can be installed directly in the tank or used in a bypass or measuring chamber. Which variant is useful depends on tank geometry, installation situation, process conditions and maintenance concept.
With direct mounting, the guide tube is usually inserted into the vessel from above. This solution is compact and well suited if the vessel has a suitable connection and the float can move freely. Installation length, dead zones, vessel height, medium density and any internals must be considered.
A bypass can be useful if the level is to be measured outside the main vessel in a separate measuring chamber. This often makes maintenance, reading or retrofitting easier. In addition, a bypass can help calm the measurement in turbulent media.
The process connection must match the system. Thread, flange, hygienic connection or special connection must be suitable mechanically and with regard to pressure and temperature. For tanks with pressure, aggressive media or safety-relevant levels, the measuring point should be planned carefully.
Limit values and alarms: Monitoring level safely
A continuous level transmitter provides an ongoing measured value. From this, limit values, pre-alarms and switching functions can be derived in the control system. This is particularly useful for minimum level, maximum level, overfill protection, dry-run protection or process releases.
Depending on the version, limit contacts can also be added directly to the level system. Alternatively, the limit values are generated in the PLC, process control system or display unit. Which solution is better depends on the safety concept and the system structure.
It is important not to set limit values too close to the mechanical ends of the measuring range. Float movement, dead zones, process dynamics, sloshing movement and system response time must be taken into account. Especially during filling and emptying processes, a safety margin should be planned.
The behavior in case of errors must also be defined. With 4–20 mA signals, fault currents, wire breaks or invalid measured values can be evaluated. The control system should be able to distinguish whether a real level limit has been reached or whether there is a fault in the measuring chain.
Typical sources of error in continuous level measurement
Many problems in level measurement are not caused by the electronics, but by unsuitable mechanical or process-related design. An incorrectly selected float, too narrow an installation space, deposits or strong turbulence can impair the measurement.
The scaling of the output signal is also a frequent source of error. If 4 mA and 20 mA are not correctly assigned to the actual measuring range, the PLC displays an incorrect level even though the transmitter is electrically working correctly.
In interface measurements, emulsions, foam layers, changing densities or unclear phase boundaries can lead to uncertain measurement results. The measurement requires that the interface in the process is sufficiently stable and physically clearly detectable.
Finally, installation, wiring and parameterization should be checked. Incorrect supply voltage, wrong input type, missing load, damaged cable or unsuitable communication parameters can cause a correct level value not to be processed correctly.
Tables: Selection, signal types and typical applications
The following tables help with selecting a magnetostrictive level transmitter and evaluating typical applications.
| Requirement | Why important? | Practical effect |
|---|---|---|
| Medium density | The float must float safely on the medium | Select float according to density and process conditions |
| Measuring range / guide tube length | The entire relevant tank range must be detected | Consider installation length, dead zones and vessel height |
| Temperature and pressure | Sensor, float and process connection must withstand the load | Select version suitable for the application |
| Media compatibility | Materials must be chemically resistant | Check stainless steel, special materials or hygienic version |
| Signal and integration | Measured value must match PLC or control system | Select 4–20 mA, HART, Modbus, IO-Link or local display |
| Interface measurement | Interfaces require suitable floats and stable densities | Design oil/water separation or multiphase measurement separately |
| Signal type / function | Typical use | Advantage |
|---|---|---|
| 4–20 mA | Standard connection to PLC, display or control system | Robust, widely used, easy to scale |
| HART | Parameterization, diagnostics and digital additional information | Analog signal plus digital communication |
| Modbus / RS-485 | Digital integration into automation systems | Multiple data points and diagnostic information possible |
| IO-Link | Integration into modern machine and plant automation | Parameterization and status data via digital interface |
| Local display | Reading the level directly at the tank | Quick on-site check without control system |
| Limit contacts | Alarm, dry run, overfill or process release | Additional switching function in addition to continuous measured value |
| Application | Why magnetostrictive is useful? | Especially to check |
|---|---|---|
| Storage tank | Continuous level for inventory, refilling and monitoring | Tank height, medium density, signal to control system |
| Process vessel | Accurate level control during process steps | Temperature, pressure, turbulence, cleanability |
| Hydraulic tank | Continuously monitor oil level and evaluate limit values | Oil viscosity, temperature, foam formation |
| Oil/water separator | Detect interface between two media | Density difference, emulsions, float selection |
| SCADA integration | Monitor and document measured value centrally | Signal type, scaling, diagnostics and fault behavior |
Practical example: Continuous level measurement in a storage tank
In a plant, the level of a storage tank is no longer to be monitored only by visual inspection or individual limit switches. The tank content is to be transmitted continuously to the PLC and displayed in the control system. In addition, a pre-alarm at low level and a maximum alarm during filling are to be implemented.
A magnetostrictive level transmitter is used for this task. The float is selected to match the density of the medium. The guide tube is adapted to the tank height, and the 4–20 mA signal is scaled so that the relevant level range is correctly represented in the PLC.
After commissioning, the visualization no longer shows only limit states, but the actual level profile. Operating personnel can see how quickly the tank is being emptied, when it needs to be refilled and whether the level fluctuates unusually under certain process conditions.
In addition, limit values are stored in the control system. This allows a low level to be reported in good time before a process is interrupted. During filling, an upper limit value is evaluated so that the tank is not accidentally overfilled. Continuous measurement therefore does not necessarily replace all safety devices, but significantly improves monitoring, documentation and process control.
Which measuring instruments / products are suitable?
For high-precision, continuous level measurement in tanks and process vessels, the WIKA Type FLM-CA magnetostrictive level transmitter is suitable. It is based on determining the position of a magnetic float using the magnetostrictive measuring principle and is suitable for applications where a compact design and a continuous output signal are required.
For a broader selection of float-based continuous level measuring instruments, the category Continuous measurement with float is also relevant. Depending on the application, magnetostrictive transmitters, reed chain transmitters and other float-based systems for different process conditions can be found there.
When selecting a device, not only the desired measuring range should be considered. Decisive factors are medium density, viscosity, temperature, pressure, material compatibility, process connection, installation position, output signal, desired resolution, local display, limit values and possible interface measurement.
For systems with PLC or SCADA integration, it is particularly important that signal type, scaling and fault behavior are clearly defined. This helps prevent a correctly operating transmitter from being displayed incorrectly in the control system due to incorrect parameterization.
Conclusion: High-precision level measurement starts with the right design
Magnetostrictive level transmitters are a very good solution when levels in tanks, vessels or process plants need to be measured continuously, with high resolution and automatically. They offer clear advantages over pure point level detection when measured value profile, control, documentation or process control integration are important.
The measuring principle with magnetic float is robust and precise, but requires careful design. Float selection, medium density, temperature, pressure, materials, installation situation and signal processing must all match the application.
The technology becomes particularly valuable in applications with 4–20 mA signal, HART communication, digital integration, local display, limit value monitoring or interface measurement. Anyone who considers these points early obtains a level measurement that not only provides values, but reliably supports plant operation.
FAQ: Frequently asked questions about magnetostrictive level transmitters
What is a magnetostrictive level transmitter?
A magnetostrictive level transmitter is a measuring instrument for continuous level measurement of liquids. It detects the position of a magnetic float and outputs a level value from it.
How does the magnetostrictive measuring principle work?
A current pulse creates a magnetic field along a sensor element. The magnetic field of the float triggers a mechanical wave at its position. The float position is calculated from the travel time of this wave.
What is a magnetic float needed for?
The float follows the liquid level. Its magnet enables contactless position detection by the measuring system.
What is the difference compared with a float switch?
A float switch usually detects individual limit levels. A magnetostrictive level transmitter measures the level continuously across the entire measuring range.
What is the difference compared with a reed chain level transmitter?
A reed chain transmitter works in steps along a resistor chain. A magnetostrictive transmitter can detect the float position continuously and with high resolution.
Which output signals are possible?
Depending on the version, 4–20 mA, HART, Modbus, RS-485, IO-Link, local display or limit contacts may be possible. The specific availability depends on the device type.
When is 4–20 mA useful?
4–20 mA is useful when the level is to be transmitted robustly and simply to a PLC, display or process control system.
When is HART useful?
HART is useful when digital diagnostic, parameter or device data are to be used in addition to the analog measured value.
What must be considered when selecting the float?
The float must match the density, viscosity, temperature, pressure load and chemical resistance of the medium. It must also be mechanically free to move.
Can a magnetostrictive level transmitter distinguish oil and water?
With a suitable version, interface measurement may be possible. Prerequisites are sufficiently different densities, suitable floats and stable process conditions.
What does interface measurement mean?
Interface measurement means that not only the total level, but also the interface between two liquids is detected, for example between oil and water.
Can the transmitter be mounted directly in the tank?
Yes, depending on the version, direct mounting in the tank may be possible. Alternatively, measurement can also take place in a bypass or measuring chamber.
Why is medium density so important?
The medium density determines whether the float floats correctly on the medium. An incorrectly designed float can cause measurement errors or unstable behavior.
Which errors often occur in practice?
Typical errors include incorrect float selection, unsuitable scaling, mechanical blockage, deposits, incorrect wiring, unsuitable process conditions or an unsuitable interface to the control system.
For which applications is a magnetostrictive level transmitter particularly suitable?
It is particularly suitable for tanks, process vessels, storage tanks and applications with PLC or SCADA integration where a continuous and accurate level value is required.