Hall effect or ultrasound: how to measure thickness.
Both measure wall thickness without damaging the part, but they don't travel the same way through every material. The choice depends on the material, the thickness and whether you can access the opposite face of the part — not on one being "better" than the other.
Hall effect, for plastic, glass, rubber and composites, especially on very thin walls or closed curved geometries — bottles, drums, cans — where ultrasound loses accuracy or doesn't work at all.
Ultrasound, for metal with access from one side only — pipes, tanks, plate — and also for plastic or glass when the wall is thick enough and the part is reasonably flat.
A magnetic ball versus a sound echo.
Hall effect
Inside the probe is a magnetic sensor that locates a small steel ball placed on the other side of the part, drawn towards the probe by its own magnet. As you move the probe across the surface, the ball follows it on the opposite face, and the instrument calculates thickness from the distance between the two.
It doesn't depend on sound propagating through the material or on a couplant gel, so it measures just as reliably on plastic, glass, rubber and composites, even on very thin walls and complex geometries. The real condition is being able to place the ball on the opposite face of the part — on a solid or fully closed part, the Hall effect method doesn't apply.
Ultrasound
The transducer emits an ultrasonic pulse that travels through the material and bounces off the opposite face; the instrument measures the time it takes to return and, knowing the sound's propagation velocity in that material, calculates thickness. The transducer is coupled to the surface with gel to remove the air gap between them, essential for the sound to transmit.
It's the most accurate method when you only have access to one side of the part. But it needs the sound to propagate predictably: on very thin walls, the echo from the opposite face arrives almost on top of the initial pulse and the instrument can't separate them; on materials that heavily damp sound (rubber, composites, fibre) or on very curved geometries, the signal scatters and the reading loses reliability or disappears altogether.
By material and situation.
| Material / situation | Method | Why |
|---|---|---|
| Steel or other ferrous metal | Ultrasound | The Hall effect method needs the ball to respond only to the probe's magnet; in ferrous metal, the part itself competes to attract it |
| Rubber, elastomers or composites / carbon fibre | Hall effect | These damp or scatter ultrasound too much for a reliable reading |
| Plastic or glass, thick wall, reasonably flat part | Ultrasound | Good sound transmission, no need for access to the opposite face |
| Plastic or glass, very thin wall or closed curved geometry (bottles, drums, cans) | Hall effect | A thin wall's echo arrives too close to the initial pulse; the closed curvature scatters the sound |
| Aluminium, flat part with access from one side | Ultrasound | Good transmission, no need for access to the other side |
| Aluminium, hollow part with access from both sides (e.g. a can) | Hall effect | Allows the calibration ball to be inserted from the other side |
What usually goes wrong, and why.
Using ultrasound without adjusting the material's propagation velocity
Every plastic has its own sound propagation velocity. Measuring with steel's velocity, or a default value, gives a completely wrong thickness, even though the instrument shows a reading that looks perfectly normal.
Attempting Hall effect measurement without access to the opposite face
If you can't place the calibration ball on the other side of the part — because it's solid, closed or inaccessible — the Hall effect method doesn't apply, however good the probe is. It's a real limitation of the method, not a setting to adjust.
Too little couplant gel or trapped air in ultrasonic testing
Without enough gel, or with bubbles between the transducer and the part, sound doesn't transmit well and the reading comes out erratic, or the instrument gives no reading at all.
Measuring rubber with the standard calibration sphere
On rubber or elastomers, the Hall effect method needs the aluminium sphere with an internal magnetic ball, not the standard steel sphere — otherwise the calibration isn't correct for that material.
One line for each method.
I need Hall effect
Plastic, glass, rubber, composites, very thin walls or closed curved geometries. QB7 series, three models depending on range and whether you need to measure rubber.
See the QB7
I need ultrasound
Metal with access from one side only, or thick-walled plastic/glass. QS5 (with corrosion mapping and through-paint measurement) and QS3 series.
See the QS instruments
Need to inspect 100% of a plastic pipe on an extrusion line, rather than spot-checking parts already made? That's a different case.
Hall effect or ultrasound?
Tell us what material you need to measure, the approximate thickness, and whether you have access to the opposite face of the part. We'll confirm the method and the model that fits.
