MFL (Magnetic Flux Leakage) is it an ET, MT or a method alone?
I see this topic is being revisited, so I have re-posted in the 2001 fourm so we don't have to go searching.
A question was asked by Jasmine in NZ regarding Hall effect sensors and the effect of speed. I have worked with a few different types of MFL equipment, steel wire rope testers, tank floor testers and pipe scanners. Some equipment show variations with speed, some have speed compensation software etc.
If the instrument only uses coils with no speed compensation the output signal will vary with speed which is Faraday's law.
If the instrument only uses hall effect sensors what would cause the same effect, this was a dilemma I had with a particular machine. I have been told fom the manufacturers of hall effect sensors that because they are solid state semiconductors and the measure of the deflection of electron flow in a magnetic field is instantaneous. The cause in my particular case was a fixed sampling rate on the equipment being used not any measurable effect from the hall effect sensors.
To find out more about hall effect sensors it is better to ask the actual sensor manufaturer's.
I hope this helps and as there is such small literature on MFL that the disscussion continues.
Re: MFL (Magnetic Flux Leakage) is it an ET, MT or a method alone? As a tube inspection man I feel very strongly that MFL should become a "stand alone technique" instead of been lumped with ECT as RFT was in the not so distance past. After all how long did it take for "the power that be" to remove the words "eddy Current" from "REMOTE FIELD EDDY CURRENT TESTING", to read "Remote Feild Testing" which is now a "stand alone technique".
MFL is mainly a volumemetric inspection technique used on carbon steel tubing. There is no changes in the phase or signal shape as there is for RFT and ECT.
For wall thickness measurements you have a good responce from the hall element and the probe has to be pulled at a steady & constant speed. I would only use the MFL technique where internal pitting was evident and due to customer or other restraints the use of other techniques was NOT an option.
The uses and aplications of MFL have also greatly increased along with the use of computerised software. After all why does anybody who specialises in tank floor inspections wants to sit through a 40hour ECT course when maybe 2hrs (if he's lucky)are spent on MFL?
I have also noted that European market has placed a greater emphasis on "saturated eddy current" instead of MFL for the inspection of carbon steel tubes. Does anybody know the reason for this?
Re: MFL (Magnetic Flux Leakage) is it an ET, MT or a method alone? MFL is used in the NDT of steel wire ropes, tank floor scanning, plate scanning, pipe scanning (internal/external) and tube scanning as you mentioned. I too have used some MFL on the inspection of tubing and concur with your limitations however I consider them to be a equipment limitation not a fault of the method. MFL used on other products such as tank floor scanners and rope testing can show a high degree of accuracy given good software and operator training.
MFL and RFT have a few things in common and that is:
1. There is a very limited amount of reference material on both methods. 2. There are many known and unknown variables that affect the signal 3. The operator must know what he is doing
My reason behind posting this topic was the above two points. I am very interested in all questions and comments on both MFL and RFT as with these comments may come a better understanding for everybody concerned.