Pie gauge for magnetic particle or field indicator sometimes confused with a gauss meter, is called out in most NDT Nondestructive Testing (MT) Magnetic Particle Testing procedures yet they seem to be overlooked by many new and experienced NDT technicians. The sentiment by an inexperienced inspector seems to be that if NDT magnetism is induced in the part that the part must be properly magnetized.
The pie gauge in ASTM E-709, ASTM E-1444 and ASME Section V relies on slots between the pie shaped segments to show the presence and the approximate direction of the external magnetic field during an NDT examination. The lines you see represents the direction in which flaws should be detectable and the lines that you do not see represent the direction of the magnetic field. It is important to remember that a pie gauge is highly permeable and it is easily affected by external fields. For this reason the presence of the lines on the pie gauge do not represent adequate magnetization of the part for the detection of flaws. Adequate magnetization is more appropriately determined with a magnetism hall effect probe.
How to use the pie gauge for MT? The pie gauge is used notches down or towards the surface of the part with the copper face up in the continuous method of magnetization. When you are looking at the pie gauge during use you should not see the pie lines, you should see a solid copper face. When utilizing a pie gauge it should be used in multiple areas of the part. If the surface is not flat qqi shims or artificial flaw standard should be used instead of a pie gauge.
It is imperative when using a pie gauge, qqi or shim to reapply particle under the area which was covered by the pie gauge after you verify your field direction. For hands on training OJT and education by and NDT MT instructor contact Aqualified.
To encourage the use of a pie gauge, a good question to ask of the ndt magnetic particle Level 2-3 technician is, how do you know you are using enough dry or fluorescent magnetic particles. This question is often followed by silence and a deep thought. The simple answer is with a pie gauge or shims. If you are using the pie gauge correctly and an indication forms you have used enough particles to adequately perform the inspection. It is fair to say that most technicians over apply dry particles with a manual puffer and under apply fluorescent particles from a can. When inspecting with a yoke covering a 4” square area the 14AM Magnaflux-magnaglo can should usually be sprayed for 2-3 seconds over one area and if using 8A Magnaflux Magnavis dry particle the inspection surface should be lightly puffed to create a light cloud over the area.
Over application of particle may cause excessive background noise and under application may cause for a flaw to not have enough particles flow or blow over it to attract to the magnetic flux leakage field. When performing MPI magnetic particle inspection with fluorescent particles the surface covered should consistently lightly fluoresce. If the surface appears like stars in the sky at night or random spots of fluorescents the 14am prepared bath can probably needs to be agitated or shaken more. If after shaking the can vigorously for 1 minute the surface still looks like stars in the night sky the can is probably bad. The easiest way to verify a bad can of fluorescent magnetic particle is to use a pie gauge. If the lines on the pie gage are not forming, you probably have a problem and need a new can. To purchase your pie gauge, gauss meter or other NDT Magnetic particle products you may contact Newco.
When performing NDI Nondestructive Inspection dry magnetic particle inspection using dry particles the surface of the part after application should look like peach fuzz. If large areas of clumping are noted on the surface to much particle is probably being used. Proper application requires light air puffing after application. If the inspector is applying dry particles then spending 30 seconds puffing on the surface to get excess particles off prior to evaluation, excessive particles are definitely being used and the NDT inspector may need additional OJT on the job training hours.
For NDT Magnetic Particle Testing, Training and 20-32 hr NDT education training class from a quality driven company with Certified Nondestructive Services Call Aqualified an international NDT company.