Specifically addressed in the BS 6072 Specification
Used to check if an induced field will detect a certain orientation (direction) of defect
The Western Instruments W-PG Pie Gauge is used to measure Magnetic Field Direction, and unfortunately it is often used to ensure the presents of an adequate magnetic field. Outside North America, the Pie Gauge is often referred to as an ASME Magnetic Field Indicator. The design dates to the earliest versions of industry specifications, and is still outlined in ASME, ASTM, and EN versions. The Pie Gauge is very robust, but due to the size (thickness), it is not that effective as a representative of artificial defects. A QQI (CX type) is far superior to a Pie Gauge and even a Castrol Strip can be used to measure field direction.
The overall thickness of the octagon disk is approximately 3mm (0.125"), with it's copper backing plate being almost a third of its thickness. While the magnetic side (low carbon steel) is placed on the workpiece, it means the separation of the 8 pie segments are well off the workpiece. Where a QQI or Castrol Strip is flexible and virtually conforms to the workpiece, the field on a Pie Gauge must extend though its copper backing. With all Flux Sharing Devices, intimate contact with the surface is absolutely necessary. It is recognized that Pie Gauges are most suited to Dry Method Inspection, and to be used on fairly flat surfaces.
Western Instruments W-PG Pie Gauge are supplied with a vinyl storage sheath and a Certificate of Conformance. The Western Instruments W-PG Pie Gauge can be re-certified, when necessary, by an authorized agency, however this is only required when there are signs of physical damage.