The pie gauge is a disk of highly permeable material divided into four, six, or eight sections by non-ferromagnetic material. The divisions serve as artificial defects that radiate out in different directions from the center. The diameter of the gauge is 3/4 to 1 inch. The divisions between the low carbon steel pie sections are to be no greater than 1/32 inch.
The sections are furnace brazed and copper plated. The gauge is placed on the test piece copper side up and the test piece is magnetized. After particles are applied and the excess removed, the indications provide the inspector the orientation of the magnetic field. The principal application is on flat surfaces such as weldments or steel castings where dry powder is used with a yoke or prods.
The pie gauge is not recommended for precision parts with complex shapes, for wet-method applications, or for proving field magnitude. The gauge should be demagnetized between readings. Several of the main advantages of the pie gauge are that it is easy to use and it can be used indefinitely without deterioration.
The pie gauge has several disadvantages, which include: it retains some residual magnetism so indications will prevail after removal of the source of magnetization, it can only be used in relatively flat areas, and it cannot be reliably used for determination of balanced fields in multi-directional magnetization.