How to Choose the Permanent Magnet (Position Magnet): Remanence, Temperature Coefficient, Mounting
The position magnet (permanent magnet) is the magnetic coupling between a magnetostrictive sensor and the object being measured. Its remanence, temperature coefficient and mounting method directly decide whether a torsional-wave echo strong enough and stable enough can be launched on the waveguide. A wrong magnet or a skewed mounting is a far more common root cause of a weak site signal or jumping values than a fault in the sensor body.
Remanence and field strength
The magnet must provide a strong enough axial static field so that a detectable torsional wave is produced when the current pulse passes. Too little remanence yields a weak echo that is easily lost; too much remanence brings no extra benefit and makes the installation clearance more critical. Therefore choose a magnetic material and size matched to the sensor, not "the stronger the better".
Temperature coefficient
A permanent magnet's remanence falls as temperature rises (negative temperature coefficient). In high-temperature duty (for example high cylinder oil temperature, metallurgical sites), remanence decay can weaken the echo or even cause failure. Choose a magnetic material with a low temperature coefficient, or leave enough field margin in the design.
Mounting points
- Coaxial fit: the magnet rides on the outside of the rod and stays coaxial with the waveguide;
- Uniform clearance: too large a radial gap weakens the signal; too small risks mechanical interference;
- Anti-loosening fixings: machines with high vibration need an anti-loosening arrangement so the magnet cannot shift axially.
Specific pitfalls are in five pitfalls of position-magnet mounting; overall installation and commissioning is in installation in practice.
Typical magnet sizes
Choose the magnet by series and inner diameter — a Series 17 ring magnet is not a Series 18 profile magnet.
| Size | What it is | Typical use |
|---|---|---|
| 1700 951 001 | Ø33 mm ring magnet | Series 17 / 19 in-cylinder |
| 1700 951 003 | Ø25 mm ring magnet | Series 17 / 19 in-cylinder |
| 1700 951 020 / 021 | Ø32 mm ring magnet | In-cylinder option |
| 1700 951 030 | Ø60 mm ring magnet | Larger piston ID |
| 1700 951 005 | Block magnet, ID 15 mm, 52×52 mm, 304 housing | External / special mounting |
| 1700 951 013 | Compact magnet, ID 10 mm, 20×13 mm, 304 housing | Tight radial space |
| 1800 951 001 / 002 / 003 / 004 | Series 18 / 19 external magnets (mass and air-gap classes differ) | Profile / floating external |
| 1900 951 002 / 003 | 10 mm float-style magnet | Series 17 / 19 |
| 1900 951 005 | 12.7 mm float-style magnet | Series 19F |
Fix magnets with non-magnetic screws. Confirm magnet size against the model when ordering.
Pick the ring magnet by piston inner diameter. Common IDs are Ø25 / Ø33 / Ø60 mm (1700 951 003 / 001 / 030). Measure the piston bore first. Profile and carriage types use the 1800 magnets; do not fit a Series 17 / 19 in-cylinder ring onto Series 18 — ID, air gap and field direction will not match, and the head will report no magnet.
How magnet parameters affect the signal
| Parameter | Effect | Abnormal behaviour |
|---|---|---|
| Remanence | Echo amplitude | Too low → lost echo |
| Temperature coefficient | High-temperature stability | High temperature → weak signal |
| Concentricity/clearance | Signal consistency | Offset/gap error → jumps |
The measuring principle is in the magnetostrictive effect and the Wiedemann effect; dead-zone constraints are in measuring dead zones; fault symptoms caused by the magnet are in troubleshooting.
Practical tips for engineers
- The position magnet must not touch the measuring rod; the designed clearance between them must be maintained.
- Fix the magnet with screws, spacers and similar parts made of non-magnetic material, so that ferromagnetic parts do not disturb the measurement.
- An M6 hexagon-socket set screw is recommended for locking the magnet — this screw is not supplied with the sensor and must be provided by the user.
Frequently Asked Questions
Q: Can an ordinary magnet be used as the position magnet?
No. A magnetic material with specific remanence and temperature coefficient is required; ordinary magnets do not match.
Q: Will the position magnet fail at high temperature?
Remanence falls as temperature rises; choose a temperature-rated magnetic material or leave field margin in the design.
Q: What happens if magnet clearance is too large?
The signal weakens and the echo is easily lost; keep a uniform radial clearance.
