Some handy tips for beginners who are new to magnetic particle testing or NDT pros who want an easy reference guide.
July 11, 2017
How to Prepare A Wet Method Bath
Oil Bath
Empty the old bath, if necessary.
Wipe any debris out of the tank and pump.
Add Carrier II oil to the tank, making sure to keep track of how many gallons you use.
Turn on the pump.
Measure out the magnetic particles per the concentration recommendation on the Product Data Sheet.
Add particles directly over the pump for faster distribution.
Mix for a minimum of 15 minutes, until the particles are completely and evenly dispersed in the suspension.
Check concentration before use.
Water Bath
Empty the old bath, if necessary.
Wipe any debris out of the tank and pump.
Add water to the tank, making sure to keep track of how many gallons you use.
Turn on the pump.
If you are using 14A or MG-410 particles only: Measure out the water conditioner, add directly over the pump and mix for 5 minutes.
Measure out the magnetic particles, powder or liquid per the concentration recommendation on the Product Data Sheet.
Add magnetic particle material directly over the pump for faster distribution.
Mix for a minimum of 15 minutes, until the particles are completely and evenly dispersed in the suspension.
Check concentration before use.
How to Check Particle Concentration
Check particle concentration after making a new bath before running parts and at the beginning of every shift.
Let pump motor run for several minutes to agitate a normal mixture of particles and liquid.
Flow the bath mixture through hose and nozzle for a few moments to clear hose.
Fill the centrifuge tube to the 100 mL line.
Place centrifuge tube in stand in location free from vibration.
Let tube stand for 60 minutes for oil baths and 30 minutes for water baths.
If reading is higher than recommended, add more suspension liquid (water or oil depending on your bath). If the reading is lower than recommended, add magnetic particles or mag particle concentrate.
Recommended Particle Concentration
In a 100 mL bath sample in a centrifuge settling tube.
Fluorescent magnetic particles: 0.1 to 0.4 mL
Nonfluorescent magnetic particles: 1.2 to 2.4 mL
Best Practices for Checking Particle Concentration
Do not include dirt particles when reading the settling volume of your magnetic particles.
For fluorescent particles: check the centrifuge tube with bath under UV light after measuring the settling volume to make sure there is not loose fluorescent pigment in the liquid portion.
Check particle concentration once a day at minimum, more if you process a lot of parts or have a lot of particle drag-out.
Change the bath regularly for contamination and particle integrity – about once a week – or up to once a month if bath is not contaminated or if the inspection volume is low.
How to Demagnetize a Part
After inspection, parts are usually demagnetized to remove the magnetic field.
Demagnetization is accomplished by a reversing polarity field in the part. This field is progressively reduced from high to low strength in small steps.
Most industry standards require the magnetic flux to be reduced to less than 3 gauss.
Put the part in demag coil.
Turn on the demagnetizing current.
Pull the part out and at least 3 feet away from the coil while the current is on. For greater effect, rotate the part end-over-end while withdrawing.
Check for residual magnetism with a Gauss Field Meter.
