Magnetic particle work mixes two unit systems and two different physical quantities, which is where most of the confusion comes from. Field strength H – the magnetising force applied by the current – is measured in amperes per metre in SI and in oersteds in the older CGS system. Flux density B – the resulting flux per unit area in the material – is measured in tesla in SI and in gauss in CGS. They are linked by the permeability of the material, B = mu H.

In air, and therefore for a Hall-effect probe held just above the surface measuring the tangential field, permeability is that of free space and the two scales coincide numerically in CGS: 1 oersted of H corresponds to 1 gauss of B. That is why field indicators and acceptance criteria are quoted interchangeably in gauss and oersted. Inside the steel the relationship no longer holds, because the permeability of ferritic steel is hundreds to thousands of times that of air.

The fixed conversions are 1 Oe = 1000/(4 pi) = 79.577 A/m and 1 T = 10 000 G. A common acceptance band for a verified tangential field on the surface of a part is 30 to 60 G, equivalent to about 2.4 to 4.8 kA/m. Below that the particles will not gather at a fine discontinuity; above it the background furring makes real indications hard to see.

Field indicators such as the pie gauge and the artificial flaw shim confirm that a field exists and roughly which way it runs, but they do not measure it. Only a calibrated Hall-effect gaussmeter with the probe held tangential to the surface gives a number that can be compared with these limits.

Worked example

Measured value	40
Unit of the reading	Oe
Field strength H	3183.1 A/m
Field strength H	3.183 kA/m
Field strength H	40 Oe
Flux density B in air	40 G
Flux density B in air	4 mT
Flux density B in air	0.004 T
Tangential field minimum	30 G
Tangential field maximum	60 G
Inside 30 to 60 G band	1

A gaussmeter reads 40 Oe tangential to the surface. H = 40 x 79.57747 = 3 183.1 A/m, which is 3.183 kA/m. In air the flux density matches numerically at 40 G, and 40 G / 10 = 4 mT, or 40/10 000 = 0.004 T. The reading sits inside the usual 30 to 60 G acceptance band, so the field is adequate without being so strong that background furring hides indications.