A float riding on a stainless steel tube, a current pulse and a stopwatch that counts microseconds together give one of the most accurate level readings in any plant. Learn how the waveguide turns a twist into millimetre level and interface data.
Magnetostrictive transmitters measure level by timing a torsional wave that travels along a wire from a magnetic float to the head. This guide covers the physics, interface measurement, installation and field troubleshooting.
What Is a Magnetostrictive Level Transmitter?
A magnetostrictive level transmitter is a continuous level instrument that finds the position of a magnetic float on a vertical probe by timing a mechanical wave in a special wire called the waveguide. It belongs to the float based family in types of level transmitters, but it has no moving parts other than the float itself.
The float follows the liquid surface, and its ring magnet marks a point on the waveguide. The electronics in the head send a current pulse, wait for the returning twist and convert the time into distance and then into level.
The same probe is often clamped to the outside of a magnetic level gauge, where the float inside the chamber drives both the local flags and the transmitter. SOR notes that this arrangement needs no extra vessel penetration and has no process contact with the transmitter.
The Wiedemann Effect Behind the Measurement
Magnetostriction is the property of ferromagnetic materials that makes them change shape slightly in a magnetic field. The Wiedemann effect is a special case, in which a wire carrying current twists where a second, axial field crosses the circular field around the wire.
In the transmitter, the current pulse creates the circular field along the whole waveguide, and the float magnet supplies the axial field at one point. The waveguide twists only at the float, and that twist travels along the wire as a torsional stress wave.
The torsional wave moves at the speed of sound in the waveguide alloy, roughly 2,800 metres per second, which is far slower than the electrical pulse. That slow speed is exactly what makes microsecond timing give millimetre resolution.
How It Works in 5 Simple Steps
ABB describes exactly this sequence for its K TEK AT200: a current pulse along a sensor wire under tension, a torsional vibration where the float field acts, and a piezo ceramic sensor that measures the delay. The end of the wire has a damping element so the wave travelling away from the head does not reflect back.
Because the measured quantity is time, the reading does not depend on dielectric constant, conductivity, vapour or foam above the liquid, unlike some methods discussed in why foam affects level sensors. Only the float must follow the true surface.
Level From Transit Time: The Formula
The distance from the head reference to the float equals wave speed multiplied by transit time. Level is then the reference height to the tank bottom minus that distance, and the output is scaled over the calibrated range.
Level, L = H minus d
Output = 4 + 16 × L ÷ span
v in m/s and t in µs give d in mm
Example:
v = 2800 m/s, t = 1000 µs, H = 4000 mm, span = 3500 mm
d = 2800 × 1000 ÷ 1000 = 2800 mm
L = 4000 minus 2800 = 1200 mm
Output = 4 + 16 × 1200 ÷ 3500 = 9.49 mA
Real transmitters store the waveguide speed as a factory gradient, so the user rarely enters it. The calculator below helps students check a reading or understand what the electronics do internally.
Magnetostrictive Level Calculator
Probe Styles of a Magnetostrictive Level Transmitter
A stainless steel tube holds the waveguide, usually up to several metres long.
A flexible hose carries the waveguide, with a weight at the bottom.
The probe is strapped to a magnetic level indicator chamber.
Polished probe and sanitary fittings for clean in place duty.
The float is chosen for the liquid density, pressure and temperature, and it must be lighter than the fluid it follows. Float design rules are similar to those in float level switch working principle, with extra care for wall thickness at high pressure.
The head of a magnetostrictive level transmitter holds the pickup, timing electronics and output board. Many models also carry an RTD string inside the probe, so the same instrument reports average product temperature for volume correction.
Interface Measurement With Two Floats
A second float, weighted to sink through the light liquid but float on the heavy one, lets one probe report both total level and interface level. This is a common solution for oil and water separators, as compared in interface level measurement.
The ABB AT200 manual lists dual float configurations for interface measurement and states that specific gravity differences as low as 0.03 can be handled. The interface float is ballasted to sit between the two densities, so exact density data is essential at ordering time.
Second Worked Example: Separator Interface
A separator probe has H = 4000 mm and v = 2800 m/s. The top float returns after 700 µs and the interface float after 1100 µs, giving distances of 1960 mm and 3080 mm.
Total level is 4000 minus 1960 = 2040 mm, and interface level is 4000 minus 3080 = 920 mm. The oil layer thickness is therefore 2040 minus 920 = 1120 mm, a value the DCS can use for draw off control.
Because each float produces its own return pulse, a single magnetostrictive level transmitter can track total level and interface level at the same time. Some models also report the thickness of the upper layer directly.
Comparison With Other Level Technologies
| Feature | Magnetostrictive | Guided Wave Radar | DP Transmitter | Displacer |
|---|---|---|---|---|
| Principle | Float and timed torsional wave | Microwave pulse on probe | Hydrostatic head | Buoyancy force |
| Density effect | Float sinks or rises slightly | None on level | Directly affected | Directly affected |
| Interface | Yes, second float | Yes, with clear layers | Needs two densities | Yes, with proper sizing |
| Moving parts | Float only | None | None | Displacer and torque tube |
| Weak point | Sticky or coating liquids | Low dielectric, heavy build up | Impulse lines | Mechanical wear |
For sticky or crystallising liquids, a probe without a float, such as a guided wave radar level transmitter, is often safer. Where density changes widely, a displacer level transmitter or DP instrument needs compensation, while the float method changes only slightly.
Installation and Commissioning Checklist
- Confirm the float matches the liquid density and pressure on the datasheet.
- Keep the probe vertical and away from inlets and agitator paths.
- Allow a stilling well or chamber in turbulent or agitated tanks.
- Check the dead zones at the top and bottom of the probe.
- Set the reference point and span to the tank drawing.
- Verify level at two or more points against a dip tape or sight gauge.
- Record the HART configuration and float offset in the loop file.
Agitated vessels can make the float swing or strike the wall, so use a stilling well as recommended in level measurement in agitated vessels. The dead zones at the ends of the probe must lie outside the required measuring range.
Before installing a magnetostrictive level transmitter, slide the float by hand along the probe and watch the reading change. It is the quickest way to confirm the float orientation, the output direction and the dead zones.
For calibration, move the float or the liquid to known points and compare them with the output, as explained in level transmitter 5 point calibration. ABB states that the AT200 needs no routine recalibration, but a periodic verification is still good practice.
Troubleshooting a Magnetostrictive Level Transmitter
A frozen reading usually means a stuck float from coating, debris or a dented probe, and the fix is cleaning or float replacement. A reading that jumps to the end of range often shows the float has entered a dead zone or the signal has been lost.
An offset reading with correct movement points to a wrong float offset, a changed density or a wrong reference height. When the transmitter sits on a chamber, also check the chamber itself using magnetic level gauge troubleshooting.
If a float is suspected of leaking, weigh it and compare with the datasheet weight. A float that has taken in liquid sinks lower and gives a steady negative level error.
- Very high accuracy and resolution.
- Level, interface and temperature on one probe.
- Not affected by foam, vapour or dielectric.
- No recalibration drift in normal service.
- Float can stick in coating or dirty liquids.
- Density change shifts the float slightly.
- Probe length and headroom limit installation.
- Dead zones at both ends of the probe.
Where Magnetostrictive Level Measurement Is Used
In storage, a magnetostrictive level transmitter feeds the level and temperature needed for tank volume from level measurement and larger automatic tank gauging systems. Output is usually 4 to 20 mA with HART, and SOR also lists Modbus over RS 485 for its model.
ABB AT200 Magnetostrictive Transmitter Manual
Magnetostrictive Working Principle Video
Magnetostrictive Level Transmitter FAQ
It is a continuous level instrument that locates a magnetic float on a probe by timing a torsional wave in a waveguide. The float follows the liquid surface, so its position gives the level.
The head sends a current pulse and measures the delay of the returning twist. It then converts that delay into distance, level and a 4 to 20 mA output.
It is the twisting of a ferromagnetic wire when a current in the wire meets an axial magnetic field. The circular field of the current and the axial field combine to produce a helical field.
In the transmitter, the float magnet supplies the axial field at one point only. The waveguide twists there, and the twist travels to the head as a stress wave.
Accuracy is very high because a magnetostrictive level transmitter measures time, which electronics can count very precisely. ABB states 0.01 percent of full scale for its AT200 series.
SOR describes its model as accurate to within millimetres. Real field accuracy also depends on float condition, density match with the liquid and a correct reference height.
Yes, a second float weighted to sit between the two liquids provides the interface position. The electronics detect two separate return pulses travelling on the same waveguide.
ABB lists specific gravity differences as low as 0.03 for its dual float design. Accurate density data for both liquids at operating temperature is needed when ordering the floats.
No, foam and vapour above the liquid do not change the transit time of the torsional wave. Only the float position matters for the reading of a magnetostrictive level transmitter.
The float must still rest on the real liquid surface. Very heavy foam can occasionally hold a light float up, so check the float density for foaming duty.
The most common failure is a stuck float caused by coating, debris or a bent probe. A leaking float that sinks lower is another frequent cause of error.
Electronics faults are less common and usually show as lost signal or end of range readings. Cleaning, float checks and a two point verification solve most problems.
Avoid it on sticky, crystallising or heavily fouling liquids where the float cannot move freely along the probe. Very tall tanks with little headroom above the nozzle can also make probe installation hard.
For those cases, guided wave radar or non contact radar is often a better choice. Always compare options using real process data before final selection.
Related Articles
- Magnetic Level Gauge Working Principle
- Interface Level Measurement
- Guided Wave Radar Level Transmitter
- Automatic Tank Gauging Explained
- Displacer Level Transmitter Working Principle
External References
- K TEK AT200 Magnetostrictive Level Transmitter Manual, ABB
- What Is a Magnetostrictive Level Transmitter and How Does It Work With an MLI, SOR
- Magnetostriction, Wikipedia
What We Learn Today
- A magnetostrictive level transmitter times a torsional wave created where the float magnet meets the current pulse field, and converts that time into level.
- The Wiedemann effect twists the waveguide only at the float, and the twist travels to a pickup in the head at a known speed of sound.
- A second float gives interface level on the same probe, and ABB lists specific gravity differences as low as 0.03 for dual float designs.