Magnetic wall thickness gauges

Magnetic wall thickness gauges are non-destructive instruments for measuring the wall thickness of vessels, tanks, containers and structures made of ferromagnetic materials, where access to both sides of the wall is impossible. They exploit the way a magnetic field varies with the distance between a magnetic source and a ferromagnetic target placed on the other side of the wall. They complement ultrasonic thickness gauges: magnetic measurement needs neither contact nor couplant, and can be carried out even through non-magnetic coatings (paint, plastic).

Measuring principle

A magnetic thickness gauge consists of a measuring probe, containing a magnet and a Hall or inductive sensor, and a ferromagnetic target (target ball). The probe is placed on one side of the wall and the target on the other. The magnetic field generated by the probe’s magnet interacts with the target, drawing it towards the probe; the Hall sensor built into the probe measures the strength of the magnetic field, which varies with the distance between magnet and target — that is, with the thickness of the wall. A characteristic calibration curve relates field strength to actual thickness.

Features and limitations

The typical measuring range is 0-25 mm for standard thicknesses, with an accuracy of ±0.1-0.5 mm. Resolution is 0.01-0.1 mm. The instrument works only on ferromagnetic materials (carbon steels, cast irons) and where curvature is not excessive. For non-ferromagnetic materials (aluminum, plastic, 304/316 stainless) alternative solutions based on alternating fields are available, but with lower accuracy. The principle requires the target to be kept in stable contact with the opposite wall, a condition critical to a correct measurement.

Applications

The main applications are the thickness checking of closed tanks and vessels that cannot be entered (cisterns, autoclaves, shell-and-tube exchangers), of piping that cannot be drained during the measurement, of the hulls and sides of ships accessible only from outside, of industrial steel containers and of some automotive components. Magnetic measurement is preferred to UT when the inner surface is inaccessible and when the external coating (paint, enamel) cannot be removed in order to apply UT couplant.

Standards and calibration

Calibration of magnetic thickness gauges is carried out with sets of non-magnetic shims (PET, PVC) of certified thickness, placed between probe and target in the laboratory. The specific test procedures follow the manufacturer’s instructions and best practice for NDT methods. For qualitative applications outside any standard, it remains important to validate the measurements on walls of known thickness, representative of the real application, before use in the field.

Selection criteria

The choice of magnetic thickness gauge takes into account the thickness range needed, the accuracy required, the wall material (standard carbon steel, magnetic stainless), the curvature of the vessel, the accessibility of the target on the opposite side, and reporting requirements. Portable models suit on-site inspection and maintenance; bench models offer greater precision for the laboratory. Comparative assessment against other methods (UT, X-ray) is recommended before adopting magnetic measurement for critical applications.

Using UT, magnetic and visual methods together allows a complete inspection of industrial vessels, with each technique providing complementary information. Planning a periodic inspection program in accordance with sector standards (UNI EN 12572, API 653, ASME Section VIII) makes it possible to keep the progress of corrosion under control and to plan preventive maintenance work, reducing the risk of failure in service.