MPI using the AC Yoke on site can be challenging when power supply is limited, especially in remote areas. Long extension cords reduce the “strength” of the yoke and results in small cracks being missed. This is why we perform a lift test with the yoke connected to those long extensions, we plan to use.
So the answer is permanent magnets.
I personally, do not like these since I have missed a crack some years ago and was subsequently kicked off site.
What is the trick when using permanent magnets?
AC Yokes uses alternating current to magnetize the test surface. The alternating field magnetizes the particles (ink) in one direction then stops and re-magnetizes the particles in the opposite direction. This, of course, happens 50 times a second. The alternating magnetic field in the yoke also causes a light vibration of the test surface, adding to the mobility of the particles.
The result is excellent particle mobility and cracks are shown within seconds.
Permanent magnets do not have this luxury. No particle mobility, no vibration.
This means that you need to allow at least 10 seconds for the magnet to magnetize the test surface almost to the point of saturation – that’s why these magnets have to be able to lift more than 18 kg!
Only then will the particles start to move – slowly – to where the cracks are.
How can you speed this up?
The steps are:
Place the magnet. Apply a light coat of particles. Re-apply a little more particles after a few seconds, blow onto the surface and re-apply some more particles. Be careful not to apply too much ink – this will mask small cracks. Then wait. Only move the magnet after 10 seconds, or you may miss a crack and be kicked off site, like me.
Using permanent magnets work when used correctly, but it is not recommended for the impatient technician. Slow and steady, permanent magnets are therefore recommended for the older technicians.
Hennie de Wet