French (Fr)English (United Kingdom)

DPC NEWS: a website dedicated to Penetrant Testing and Magnetic Testing



visits on site since April 2008

Log in


Receive HTML?

Some clues to choose between PT and MT

Written by Administrator
Friday, 01 April 2011 10:46

April 2011

1 – Introduction

Though PT and MT are two non destructive testing (NDT) methods designed for detection of surface discontinuities, they both have each their own respective applications and limits for use.

We have already published papers dealing with this topic(2) (3) (4) (5) on our Website (1).

However, ferromagnetic materials may be checked by both methods when looking for open-to-surface discontinuities.

2 – Certification of personnel

2009 metrics of the French NDT Society (COFREND)(6) show that, all Levels and industrial sectors combined, the numbers of PT- and MT-certified people are 37% and 18% respectively of the total of COFREND certified people. Within one year, the number of PT-COFREND certified people even increased by 14%, whereas that of MT decreased by 6%.

This means that not only PT is the most widely used NDT method, but also that it is more and more used when MT seems to be on a descending path.

These figures confirm what we wrote in the papers(2) (4) published on our website regarding the factors that contributed to changes in each of these methods.
3 - Why is pt used in lieu of mt in some companies?
Throughout our careers, we have seen many companies which carried out PT when obviously, MT would have been the best choice. Why?

Quite often, the answers were as follows:

• Our Customers require MT to be performed by COFREND-, ASNT- or any other - certified personnel while we have PT-certified only inspectors and no one in MT. Training and certifying people is costly for us; further, we would have to send our PT-certified personnel for a week or more out of the company. No PT then could be performed while they are away. This would stop manufacturing, or at least inspection; hence delays in shipping, invoicing, even some penalties for delays.

• We have budget for consumables, but in no way for any investment in magnetic equipment.

• Very few of our customers ask for MT: investment in MT equipment is not a priority.

• We have just bought new machines, and there is no room left for an MT unit.

• Etc.

Therefore, when MT is mandatory, these companies must subcontract to service companies. These services increase both costs and delays: the service company should perform some inspection during several, if not all the manufacturing steps, just to prevent, for instance, a defective part going down to the next steps.

Some Companies, trying to lower inspection costs and delay, play ''Russian roulette'', kind of, by asking for a final inspection only. Consequences may be dramatic: faulty parts would need to be repaired, the service company called back for a second final inspection. Worst case: a part beyond repair shall be scrapped!

Nevertheless, many companies opt for some effort. They buy small equipment, such as hand-held electromagnets, ASME magnetic field indicator (pie gauge or Pie Gage) or Berthold penetrameter, residual magnetization indicator and some detection media.

Personnel’s training is then provided by:

• Either the supplier, for half a day or a full day, no more generally. This training does not allow for the passing of the exam for certification. Only how to use equipment and materials the right way...if the supplier gives the right information - it may be that the supplier is mainly “sales-oriented” and not familiar with all the technical details!

• Or a training centre for a week, generally in order to pass the certification exam. This implies also that the trainee has used the method for some hours and that this “on-the-spot” training has been validated.

4 – Choice based on part’s size

MT inspection of a large surface may be a rather tedious task. Using a hand-held electromagnet or a current generator the power of which is not enough for magnetizing the part in one shot leads to magnetize and inspect the part small area after small area. The area’s dimensions are determined by the capabilities of the MT equipment. It is generally the maximum distance, according to the required minimum tangential magnetic field strength:
• Of poles when using a hand-held electromagnet.
• Of prods when using the current flow technique.

Further, discontinuities direction is likely to be unknown; that’s why two successive magnetizations shall be performed more or less at right angle of each other.

On the other hand, PT, a global method, allows for the inspecting of large surfaces in one process.

However, suitable-sized parts may be quickly and completely inspected in one step using the swinging field technique on a magnetic bench or a 2D or 3D non-contact chamber.

5 - Choice based on parts geometry

Parts’ shape shall be considered. As a matter of rule, PT is more suitable than MT to check complex-shaped parts.

This is particularly true in areas where sharp angles, sharp changes of diameters may lead to non-significant MT indications.  Better to perform PT then.

6 – Choice based on the surface condition

Surface porosity or roughness may be so high that PT cannot be used.
Then, MT is ideal (once again, given the material is ferromagnetic) if a contrast aid paint is applied.

Oxide scale formed at high temperatures or heavy carbon deposits may be found on some areas and may require a mechanical etching (sand blasting, shot blasting, etc.) which can induce metal smearing and close an open-to-surface discontinuity. When facing this problem, two options:
• Either PT, after a chemical etching, when possible.
• Or MT which allows also for the detecting of closed discontinuities.

When parts chafe on each other when in use, metal smearing occurs. For maintenance, it is better to use MT. As an example: crane hooks, due to the friction of chains.

Another example of “no-choice available”: parts with non-ferromagnetic coatings (organic or inorganic). Except for the few exceptions described in our paper(3), PT is not suitable. Nevertheless, MT may then be considered if the coating is less than 50µm thick.

7 - Choice based on the ambient temperature and/or the part surface temperature

Our previous papers have dealt with this topic (2) (3) (4) (5).

Keep in mind that the so-called standard PT materials may be used between 10 and 50°C (ca 50-120°F), the high temperatures materials, between 50 and 200°C (ca 120-390°F) and the low temperatures materials, between -30°C and +10°C (ca -22 and 50°F).

Water-based magnetic inks may generally be used between 0 and 80°C (ca 30-175°F).

Oil-based magnetic inks generally may be used between -40°C and +80°C (ca -40°F and +175°F).

Colour contrast dry magnetic powders, depending on type and colour, may be used at very low temperature and up to 200-300°C (ca 390-570°F).

8 – Weld beads inspection

MT often outweighs PT due to an overall quicker inspection.

9 – On-site inspection

On site, the choice of means for inspection is more difficult.

Materials are applied from spray cans, or sometimes from hand-held sprays. Inspection is performed, as appropriate, with white light or ultraviolet (UV-A) radiation sources plugged to batteries if no electric mains is available.

Hand-held electromagnets or sometimes portable or mobile generators are used.

If no electrical mains is available, a hand-held electromagnet powered by a battery is used, or even a permanent magnet if allowed.

A permanent magnet is very useful especially in areas where mains is forbidden for safety reasons.

Lack of water on site is another problem. The excess of penetrant removal with a solvent is of course possible, provided that it is not forbidden by applicable specifications/codes. If it is forbidden, then MT is probably the best choice.

Environment also is a concern. Removing the excess of penetrant with water gives a noticeable volume of waste water which cannot be left on the ground or be left going to sewers, rivers, etc. If no waste water treatment equipment is available on site, two alternatives are to be considered:

• Either PT: most of the excess of penetrant is removed using a dry, clean, lint-free and non-absorbent rag or paper; cleaning towels impregnated with isopropyl alcohol are also acceptable. Indeed, we know at least one solvent listed in the QPL (Qualified Product list) of the American SAE- AMS 2644E(7) specification made of isopropyl alcohol only. Therefore, such towels should be allowed; they shall be disposed of after local regulations.

• Or MT, the best example being ski lifts inspection.

10 - Conclusion

Each of these two complementary methods, useful to detect open-to-surface discontinuities, may sometimes be used as an alternative to the other, depending on criteria such as: where inspection is carried out, part’s size, shape, surface condition (roughness, porosity), temperature, utilities (water, electricity) availability , etc ... which are difficult to combine in a simple paper.

That’s why the person in charge, Level 3 or not, will make the sound choice if using his (her) expertise, thought and knowledge of the limits of both methods.

Further, don’t forget to ask renowned suppliers of both methods the right questions.


(1) Address of our Website:

(2) Pierre CHEMIN and Patrick DUBOSC: Some Penetrant Testing industrial uses.

(3) Pierre CHEMIN and Patrick DUBOSC: Some of the limits for industrial uses of PT.

(4) Pierre CHEMIN and Patrick DUBOSC: Some Magnetic Particle Testing industrial uses.

(5) Pierre CHEMIN and Patrick DUBOSC: Some of the limits for industrial uses of MT.

(6) Chiffres 2009 de la certification, Annuaire 2010 de la COFREND (Confédération Française pour les Essais Non Destructifs), 1 rue Gaston Boissier, 75724 PARIS Cedex 15, France.
(Editor’s note: 2009 figures on certification, 2010 COFREND, the French Confederation for Non-Destructive Testing, Directory.)

(7)S AE-AMS 2644E: Inspection Material, Penetrant, Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pennsylvania 15096, 2006.

Last Updated ( Sunday, 22 May 2011 09:42 )