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Technical Discussions
MANIVANNAN.P
MANIVANNAN.P
13:12 Aug-22-2016
Use of hall effect probe

Please clarify me the following:

Why Hall effect probe is not permitted to measure field strength in encircling coil magnetization technique?

Regards,
MANIVANNAN.P

 
 Reply 
 
James Scalf
NDT Inspector,
Global Integrity, Canada, Joined Oct 2012, 273

James Scalf

NDT Inspector,
Global Integrity,
Canada,
Joined Oct 2012
273
17:10 Aug-25-2016
Re: Use of hall effect probe
In Reply to MANIVANNAN.P at 13:12 Aug-22-2016 (Opening).

I believe it is because of the nature of magnetic fields the Hall Effect Detector measures.
The Hall Effect Detector Measures the magnetic fields generated in materials conducting electrical currents. The Hall effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current.
These can be used if for example you are performing a head shot on a magnetic particle wet bench as we are sending the current applied directly through the component being examined.
In a coil shot or Parker Probe (Magnetic Yoke) application the electrical field is contained in the coil wrap and/or the wrap around the Yoke Legs and never enters the component under examination.
There is much more to Hall Effects if you google it there are many pages on this and the various applications for these detectors and the Hall Effect Phenomena.
This Webpage gives a fairly good explanation of what this effect is and how it works:
http://www.explainthatstuff.com/hall-effect-sensors.html
Hope this helps...

 
 Reply 
 
Bruce McPherson
Consultant
MFL Services, United Kingdom, Joined Mar 2009, 26

Bruce McPherson

Consultant
MFL Services,
United Kingdom,
Joined Mar 2009
26
08:47 Aug-29-2016
Re: Use of hall effect probe
In Reply to MANIVANNAN.P at 13:12 Aug-22-2016 (Opening).

A hall probe gaussmeter can be used to measure the magnetising force of an encircling coil to check that the coil produces sufficient magnetising force to magnetise the component being inspected to an acceptable flux density.
In the oilfield tubular inspection business, encircling coils are frequently used to magnetise tubulars as part of an MPI procedure to detect circumferential flaws.
Some of the standards which govern this type of work specify a minumum magnetising force in air at the centre of the coil and typically this is measured using a hall effect gauss meter and longitudinal probe.

Regards

Bruce McPherson

 
 Reply 
 
George Hopman
Consultant,
NDE Solutions, USA, Joined Apr 2009, 44

George Hopman

Consultant,
NDE Solutions,
USA,
Joined Apr 2009
44
18:03 Aug-29-2016
Re: Use of hall effect probe
In Reply to MANIVANNAN.P at 13:12 Aug-22-2016 (Opening).

Sir, I have done three papers on this subject at ASNT Fall Conferences, so I can speak with some authority on this subject since I make my conclusions based upon empirical data. I am working on a paper to be published in Materials Evaluation. To answer your question directly, I am not sure why industrial codes with the exception of ASTM E709 do not allow this practice. Aerospace standards such as ASTM E1444/E1444M have allowed this practice for years. Boeing and other major aerospace prime manufacturers allow one to use a Hall Effect probe/meter in the coil. The probe and meter are tools and as such, it must be used with understanding. It measures the flux density that is flowing across the surface of the part while the current is on. That distance from the surface is dependent upon the "stand-off" distance of the actual Hall sensor from the tip of the probe. I X-rayed a Bell 4048 probe and measured 0.047" as the standoff distance. The applied (current on) tangential (parallel) field is measured. The tangential field can be circular or longitudinal. The probe itself is non-magnetic so it does not concentrate flux. A part in a coil concentrates or draws the magnetic field into the part at the poles. With the current on, one should place the probe toward the middle of the part where there is no flux leakage from the flux (magnetic domain generated) field. If on places the probe at either pole with the current on, one will get a measurement of the combined applied and flux field. That measurement is not relevant and is deceptive. Also, do not place the probe near non-relevant flux leakage such as sub-surface keyways, etc. that will also provide a combination of flux and applied fields. I trust I have been helpful.

George Hopman
ASTM E07.03 (Magnetic Particle-Liquid Pentrant) Chairman

 
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