Magnetic performance is measured with big, expensive magnetometers. But the rest of the datasheet — density, strength, expansion — comes from much simpler equipment. Density in particular is one of the cheapest, fastest quality checks available on a magnet, and it catches real problems.
Density: the classic water-displacement test
The method is thousands of years old — Archimedes' principle. Weigh the magnet in air, then weigh it suspended in water (or alcohol). The difference between the two weights is the buoyancy, which equals the weight of the water the magnet pushes aside — and that gives you the volume. Weight divided by volume is density. A balance, a beaker and a piece of wire are all you need.
Why density matters
A well-made sintered NdFeB magnet reaches about 7.5 g/cm³, close to the theoretical maximum of 7.62. If a magnet measures lower, it is porous — not fully sintered. Porosity means two problems at once: weaker magnetic output and weaker mechanical strength. A ten-minute weighing catches under-sintered or cracked parts long before expensive magnetic testing would, which is why many buyers add it to incoming inspection.
Other physical tests, one line each
- Electrical resistivity — pass a known current through a bar and measure the voltage drop. Low resistivity is why magnets heat up from eddy currents inside motors.
- Thermal expansion — heat a sample and watch how much it grows. This explains why rings can crack and why motor assemblies are checked at operating temperature.
- Flexural strength — bend a bar until it snaps. NdFeB is brittle, and this number tells you how careful handling must be.
Read on: physical properties explained → · magnet testing overview →