MPI

Question 1

Name 3 types of magnetism

Answer 1

There are Three Types of Magnetism
Ferromagnetism - Materials which can be strongly magnetised & which show good magnetic properties

Paramagnetism - Materials which are weakly attracted by strong magnetic fields

Diamagnetism - Materials that are repelled by a strong magnetic field. An externally applied magnetic field induces a “like” magnetic field within the material & repulsion occurs.

Question 2

Explain the theory of magnetism

Answer 2

In ferromagnetic materials, the atoms are gathered together in groups called Domains.
 These domains have a magnetic moment, one end acting as a North pole, the other as a South pole.
 Created by the combined effort of the motion of electrons around the nucleus of the atom & by the electron spin around its own axis.

When a material is un-magnetised, the domains are distributed randomly & their magnetic effects cancel
each other out

When an external magnetic field is introduced, the domains align themselves North to South in a common direction.

When all the domains in a material are fully aligned, the material is said to be magnetically saturated.

Even after the external magnetic field is removed, there will be some residual magnetism left in the material, as the domains will not be totally randomised again.

Question 3

Describe a magnetic field

Answer 3

The Magnetic Field Is described as the area surrounding the magnet, in which the magnetic forces exist.
 Lines of force or lines of magnetic flux, represent the magnetic field.
 These purely imaginary lines were introduced by Faraday as a means of visualising the distribution &
density (flux density) of a magnetic field.
 The SI unit used to measure flux density is the Tesla (T).
 For practical MPI the min. flux density required is 1.0(T).

Question 4

Describe the characteristics of the magnetic lines of force

Answer 4

Magnetic lines of force :
 Have direction as if flowing, but no actual movement occurs.
 Travel North to South externally, South to North internally.
 Form a closed loop.
 All have the same strength.
 Do not cross each other.
 Seek the path of least resistance.
 Are in constant tension.
 Decrease in density with increasing distance from the poles.
 Decrease in density when moving from an area of higher
permeability to an area of lower permeability.
 Prefer to travel in materials that easily accept magnetic
fields.

Question 5

Describe Longitudinal Magnetism

Answer 5

Magnetic field around a bar magnet produces longitudinal magnetism

Question 6

Describe Electromagnetism

Answer 6

In electromagnetism, when an electric current flows through a conductor (copper wire or rod), a magnetic field is set up around the conductor, in a direction at 90o to the electric current

When a conductor carries an electrical current, strong magnetic flux lines are created this is called Circular Magnetism.
 It is not polar, so cannot be detected externally on a round symmetrical specimen.
 If the original conductor carrying the current is bent into a loop, the magnetic field around the conductor will pass through the loop in one direction.

The field within the loop has direction:
 One side will be a North pole, the other a South pole.
 By increasing the number of loops, a coil or solenoid is created.
 The strength of the field passing through the coil is proportional to the current passing through the conductor in amperes, multiplied by the number of turns in the solenoid.

When a ferromagnetic specimen is placed in an energised coil, the magnetic field is concentrated in the specimen.
 One end is a North pole, the other a South pole.
 This is called Longitudinal Magnetism.
 It has polarity & is, therefore, readily detectable.

Question 7

Describe Magnetic Hysteresis

Answer 7

When a ferromagnetic material is influenced by an alternating magnetising force (H), the variation of magnetic flux density (B) in it is related to a phenomenon known as Magnetic Hysteresis.
 Hysteresis, is derived from the Greek word for delayed & is used to describe one quantity lagging behind another.
 The variation of B-H follows a Hysteresis Loop & is characteristic to particular ferromagnetic materials.

Question 8

Low Permeability

Hard Ferromagnetic

Answer 8

1) High Retentivity
2) High Remanence/Residual Magnetism
3) High Reluctance
4) High Coercivity to demagnetise

Question 9

High Permeability

Soft Ferromagnetic

Answer 9

1) Low Retentivity
2) Low Remanence/Residual Magnetism
3) Low Reluctance
4) Low Coercivity to demagnetise

Question 10

Magnetic hysteresis terms

Answer 10

Flux Density (B) - Number of magnetic flux lines per unit area.
Magnetising Force (H) – Force used to set up a magnetic circuit.
Permeability – Ease with which a material can be magnetised.
Saturation - Stage at which any increase in H produces no gain in B.

Coercive Force – Measure of the amount of reverse magnetic force needed to return a material’s magnetic field to zero.
Residual Magnetism or Remanence –Magnetic flux density remaining in a material when the magnetising force is zero.
Reluctance – Opposition that a ferromagnetic material shows to the establishment of a magnetic field.
Retentivity – Flux density remaining when H is at zero

Question 11

Explain flux leakage

Answer 11

A flux leakage is a discontinuity in a magnetic circuit.
 Any abrupt change of permeability within a magnetic specimen will change the number of flux lines that can flow, there will be a diversion of the field.
 Magnetic particle inspection relies on flux leakage fields being seen on the surface of a ferromagnetic specimen.
 All defects produce flux leakage, but not all flux leakage fields are created by defects.

A flux leakage is a discontinuity in a magnetic circuit.

Magnetic Particle Inspection relies on:
 Magnetising the specimen to an adequate flux density.
 Applying fine ferromagnetic particles over the surface.
 Being able to see the magnetic particles that gather at flux leakage fields.

Question 12

Why is the direction of the magnetic field important when looking for flux leakage

Answer 12

The magnetic field.
 Must run in a direction in which, it can be interrupted by the defect, thus producing a flux leakage field.
 The degree of distortion at the leakage must allow the magnetic particles to provide an adequate degree of contrast between the leakage & the adjacent material surface, so that it is readily visible.

Flux lines flowing in a ferromagnetic bar but having to divert around an air gap, create a flux leakage
If ferromagnetic particles are sprinkled on the bar they’ll start to form a magnetic bridge across the
flux leakage

Question 13

What are the factors that make flux leakage visible

Answer 13

Whether a flux leakage is made into a visual indication depends on a number of factors:
 Size of defect.
 Shape of defect.
 Volume of defect.
 Orientation of defect.
 Depth below surface.
 Permeability of material.

Question 14

What are indications

Answer 14

Indications are any particle indications that are seen on the specimen under test.
Just as not all flux leakage fields are defects, not all indications are due to flux leakage.
Indications can be further subdivided into:
 Relevant.
 Non-relevant.
 Spurious.

Question 15

What is relevant indications

Answer 15

Relevant Indications are discontinuities or flaws, which in turn, are unwanted imperfections.
When it is considered that a relevant indication will affect the fitness-for-purpose of a test
specimen, then it is classified as a defect, but not all defects are cracks

Question 16

What are non relevant indications

Answer 16

Non-Relevant Indications
 True magnetic particle patterns formed & held in place by leakage fields.
 Caused by design features & the structure of the specimen.
 Only in exceptional cases will they affect the fitness-for-purpose.
A non-exhaustive list:
 Tool marks.
 Abrupt changes of geometry.
 Dissimilar magnetic material.

Question 17

What are Spurious Indications

Answer 17

 Indications that are not held on the surface by a flux leakage are termed Spurious, i.e. scale or dirt.
 Magnetic Writing is when two pieces of steel touch when one of them is in a magnetised condition, local poles are created at the areas of contact.

Question 18

What direction must the indications be to be seen

Answer 18

Indications that are Transverse will show (90 deg to the lines of magnetism)
Indications that are up to at least 30 deg will show
Defects running in the line of magnetism will not show

Question 19

Methods of Magnetisation

Answer 19

The equipment used for MPI can be divided according to size & purpose.
 Electricity used to magnetise, is transformed into a low voltage, high amperage supply.
 There is no danger from electrocution.
 Magnetising equipment must meet the requirements of & be used in accordance with, BS EN ISO 9934-3.

Question 20

Permanent Magnets

Answer 20

Permanent Magnets
 Produce a longitudinal magnetic field between the poles.
 Horseshoe magnets have adjustable arms & may have variable-geometry, removable pole ends.
 The optimum defect detectability is at 90deg to the poles.

On straight work pieces, like plates & cylinders, good contact between the pole pieces & the work piece is easily obtained by having shaped pole pieces, flat for plate & radiused for cylindrical-shaped work pieces.
For more complicated shapes, e.g., the weld at 12o’c on the joint of a VDM on a node, the pole pieces need to rotate as well as being shaped in order to make good contact

BS EN ISO 9934-1 states that a lift test should be carried out before a magnet is used for MPI.
 The lift test should confirm that the magnet can lift 18kg with a pole spacing of between 75 & 150mm.
 The inspection area is between the poles & half the pole spacing either side.

Question 21

What are the advantages and disadvantages of permanent magnets

Answer 21

Advantages
No power supply needed 
Clings to vertical surfaces 
No electrical contact problems
Cheap & ready available
No damage to test piece 
Lightweight 

Disadvantages
Direct longitudinal field only
Deteriorate with wear
Have to be pulled from test piece
No control over field strength
Ink particles attracted to poles
Poles must have good contact

Question 22

Electromagnets

Answer 22

Electromagnets are made from soft iron laminates to reduce eddy current losses, if powered by alternating
current (AC)
The yoke laminates are encased in a multi-turn coil, usually powered by mains electricity, stepped down to
6 or 12v
The legs of modern equipment are normally articulated to allow contact on uneven surfaces

Electromagnets produce a longitudinal field, with the test area being a circle inscribed by the poles.
BS EN ISO 9934-1 states that a lift test should be carried out prior to using for MPI.
The lift test should confirm that the electromagnet can lift 4.5kg with a pole spacing of up to 300mm.
Defect orientation is the same as with a permanent magnet.

Question 23

What are the advantages and disadvantages of electromagnets

Answer 23

Advantages
AC, or rectified DC operation
Controllable field strength
Can be switched off allowing easy removal
No harm to test piece
Lightweight
Can be used to demagnetise

Disadvantages 
Needs a power supply
Longitudinal field only
Poles may attract magnetic particles
Poles must have good contact

Question 24

Current Flow Prods

Answer 24

Current flow Prods induce a circular magnetic field by sending a high amperage current (typically 1000A)
through the test piece.
 Prods produce a circular magnetic field with defects showing at a maximum when orientated along a line
between the prod tips.

Question 25

Advantages and disadvantages of prods

Answer 25

Advantages
Either AC or DC may be used
Low voltage used (Typically 3V)
Field strength can be adjusted
No collection of ink around the prods

Disadvantages 
Good prod contact is required
Heavy duty transformer required
Danger of arcing on test piece
If arcing does occur inclusions from the prod tips may result

Question 26

Flexible Coil

Answer 26

Flexible Coil Technique
A current-carrying cable is wound around the component.
 It is a longitudinal magnetisation method & will find defects lying parallel to the cable.
 The area to be tested is considered to lie between the turns of the coil.

Evenly Spaced Coil

A flexible cable is wound around the test piece so that the area under inspection is contained within the encircling coil. Individual wraps of the coil are spaced equidistant to each other.

Close Wrapped Coil

In this technique, the inspection area does not have to lie inside the encircling coils, however, the test zone must still lie close enough to the ends of the coil to ensure an adequate flux density.

Parallel Loops

This produces a transverse field between the two sides of the loop. The loop has to be positioned so that the current in the two sides is moving in the same direction, otherwise the magnetic fields will tend to cancel instead of reinforce.

Question 27

Advantages and disadvantages of parallel loops

Answer 27

Advantages
Cables can be positioned relatively easily
Large areas can be inspected with one placement of the coil
Field strength & flux density can be adjusted
Demagnetising is easily achieved

Disadvantages
A Relatively weak Field is produced
Large, heavy step-down transformers are required
Isolating transformers are required on deck
Long cables are required

Question 28

Continuous & Residual Magnetisation

Answer 28

Continuous & Residual Magnetisation Techniques
 Both techniques are used for land base inspections.
 The continuous technique is used almost exclusively for underwater applications, because of the possibility of wash out or dilution of the indicating medium in the water.

Continuous Magnetisation Technique
 The work piece is magnetised at the same time as the indicating medium is applied.
 The area under inspection is then examined while the magnetic field is maintained.
 This method is generally regarded as being the more sensitive, but indications of defects other than actual damage can be given.

Residual Magnetisation Technique
 The work-piece is magnetised then made cold before the indicating medium is applied.
 The inspection is completed with the magnetising force removed; the work piece is examined with only any residual field producing the flux leakage.
 These fields are much weaker than those by the continuous method, so much lower in sensitivity.

Question 29

Detecting Media

What factors are involved when considering the sensitivity of MPI

Answer 29

Detecting Media
 The effectiveness of the magnetic particle inspection is determined by the sensitivity with which it is possible to detect the change in the magnetic field due to the presence of a defect causing flux leakage.
 This is done by visual observation, using an indicating medium to show the distortion.

Question 30

Visual Detection
How is the defect seen

Answer 30

Visual Detection
 Achieved by observing the distortion of the magnetic field as shown by the patterns in
magnetic particles distributed over the test surface.
 Magnetic particles are available in dry powder form, as a liquid slurry or fully mixed with water & ready to use.

The Particles
 Fine ferrous-oxides that are much lighter than iron filings & will go into suspension in the water more easily & remain suspended much longer.
 A fluorescent powder (usually green coloured) is mixed with the particles so that they will fluoresce under ultraviolet light.

The Method
 Apply the magnetic field & then apply the ink.
 Any flaws cause flux leakage with a resultant build-up in flux density.
 The ferrous-oxide in the ink is attracted to this strong magnetic field & will collect there.
 Using a suitable ultraviolet light, the indication can be viewed, interpreted & reported.

Question 31

What properties must the ink have

Answer 31

The standard for MPI ink is BS EN ISO 9934-2
 Ink must be non-toxic, free from contaminants, not cause discomfort to users & be non-corrosive to the
work piece.
 Grains should be fine enough to reduce gravitational effects.
 Go easily into suspension, but not coagulate in the liquid.
 Have elongated shape to facilitate polarisation.
 Have high permeability so as to be easily magnetised.
 Have low retentivity to facilitate removal.
 Have good contrast against the test background.

The concentration of ferrous-oxide particles, additives & fluorescent powder is specified in the standard, as is the method for testing the correct concentration of ink.
The ink or powder must be supplied as certified manufactured to the ISO standard with a batch number that should be recorded.

The correct quantity of powder should be measured & put into a suitable mixing vessel
 Inks are available which include wetting agents.
 It’s more common to add such an agent at this stage.
 Enough liquid is added to make it up into a slurry, which is put into a bucket & made up to the correct quantity with water.

Once the ink is mixed, it should be constantly agitated to ensure the suspension is maintained & a 100ml sample drawn off.The sample is required to undergo a settling test to confirm the ink has been made up to the correct concentration.

Question 32

Explain the settlement test

Answer 32

 Draw off the 100 ml sample into a calibrated settling flask.
 Either a Sutherland or Thistle type flask is suitable.
 Allow the sample to settle for 60 minutes.
 Record the quantity of solids collected in the bottom.

Of paramount importance is the maintenance of the ink strength.
 The solid content must be constantly monitored.
 The ink must be constantly agitated to keep the solid content in suspension. Not stated in current standards, but previously that the solid content should be between:
 0.1-0.3% by volume for fluorescent ink.
 1.25-3.5% for visible (black) ink.
 No more than 10% by weight of other solids.

Question 33

What are the requirements for lighting and viewing conditions

Answer 33

As the chosen method for detecting MPI indications is vision, the ambient light conditions & the quality
& quantity of ultraviolet light must be at correct levels
 Standards for lighting are stated in BS EN ISO 3059. The minimum requirements for ambient & ultraviolet
light as stated in the standard are:
 Visible Light Inspection (Using non-fluorescent Inks).
 Minimum of 500 lux of white light in the viewing area.

Background & Ultraviolet Light Levels (Fluorescent Inks).
 Ambient light must not exceed 20 lux, so that the fluorescence from the ink particles has sufficient contrast to be easily seen.
 The ultraviolet light must be a minimum of 1000µW/cm2.
 The wavelength of the light should be between 365 to 400nm (A nanometre (nm) is 1 millionth of a mm).

Question 34

What are the safety considerations when using UV light

Answer 34

Ultraviolet light covers a range of frequencies
 These are split into UVA & UVB with UVA being the less harmful wavelengths, which is why they are used for MPI.
 Mercury vapour bulbs, which produce light by discharging an arc in mercury vapour, produce a large amount of ultraviolet light along with visible white light.
 This type of bulb is put into a housing with a Woods filter in front of it, which blocks the harmful UVB wavelengths.

Question 35

Testing the UV light

Answer 35

Reasons to test ultraviolet lights used in MPI:
 Intensity must be high enough.
 Wavelength must be in the correct part of the
spectrum.
 Mercury vapour bulbs degenerate with use.
 Requirements stated in the standard must be met.

The procedure for testing the ultraviolet light:
 Switch on & allow it to warm up for 15 minutes.
 Lamps for underwater use must be immersed in cooling water during this test, otherwise the heat generated will damage the seals.
 Shine the light onto a photometer (Black Light Meter) holding it at a distance of about
400mm.
 Record the results.

Question 36

What is Residual Magnetic Field

Answer 36

At the end of a magnetic particle inspection when the magnetising force is removed, a residual field will remain
 The strength will depend on the material retentivity.
 Applying a field strength indicator to the work piece will assess the magnitude of the residual field.
 Also known as gauss meters.

Offshore structures are fixed in Earth’s magnetic field, subject to vibration so may be weakly magnetic.
Any residual field should be removed:
 Prior to MPI to prevent the possibility of vector fields.
 May interfere with sensitive electronic equipment.
 Prior to welding so as to avoid arc blow.

Question 37

Explain demagnetism and why it’s important

Answer 37

Looking at a typical Hysteresis Loop, after the initial magnetising force is applied & then removed, it is virtually impossible to end the test with zero flux density.
 Even if a negative coercive force is applied it will only keep the flux density at zero, as long as it continues to be applied.
 The following diagram shows that the key to demagnetisation is that a reversing & reducing magnetising force must be applied.

The energised yoke is pulled over & off the component, to a distance of about 450mm & then
switched off The operation is repeated in the same way & direction until the residual field is removed

Question 38

Alternative Forms of Current Applied in MPI

Answer 38

Inducing a magnetic field was examined earlier where it was stated that either AC or DC may be used.
 The type of current must be considered as the induced field strength & characteristics are determined by this.
 Either current or voltage may be measured & for the purpose of inducing a magnetic field, the amperage is the more important, up to 1000amps may well be used.
 Underwater, the voltage is kept as low as possible for safety, only about 4 volts potential is applied.

Question 39

Talk about using DC current for MPI

Answer 39

DC is an electrical current flowing in one direction only & effectively free from pulsation
After a small build-up period, the current is at a constant peak value & this is what the meter reads

Direct current is either supplied from a permanent magnet, a battery pack or a DC generator.
Early days of MPI, DC was almost universally used.

Advantages
Finds sub-surface defects
Availability from batteries

Disadvantages
No agitation
Less sensitive to surface defects

Question 40

Talk about using AC current for MPI

Answer 40

Alternating current
 In the UK, 50Hz is the commercial norm, & as AC is readily available, it is convenient to make use of it MPI.
 When AC is used; the magnetic field will be limited to a narrow region at the surface of the component.
 This is known as the ‘Skin Effect’.
 Induction is not a spontaneous reaction & the rapidly reversing current does not allow the domains down in the material time to align.

 As the current is constantly reversing, no single value for the current can be measured.
 To determine a single value for an AC current the Root Mean Square (RMS) value is calculated.
 To calculate the RMS value, all that is required is to divide the peak value by the constant 1.414.

Advantages 
Availability
Sensitivity to surface defects
Agitation of particles
Demagnetisation

Disadvantages
Will not detect sub-surface defects

Question 41

How do we verify if there’s enough magnetism

Answer 41

BS EN ISO 9934-1 specifies that the adequacy of the surface flux density should be established.
 Flux Indicators, such as the Burmah Castrol Strip are common & simple to use, giving a clear visual indication of the direction of the surface field.
 They’re a rough guide to the magnitude of the field.

Flux Indicators consist of a magnetic material that is interrupted by non-magnetic spacers.
 When the flux indicator is placed on the surface of a magnetised specimen, flux is induced in it.
 The non-magnetic spacers behave as artificial flaws.
 If the magnetic field at the surface of the specimen is sufficiently high, flux leakage above the artificial flaws can be detected.

Question 42

What are BC foil strips made of

Answer 42

Flux Indicators are made with high permeability magnetic materials with low coercivity & low remanence, so that a flux can be easily induced into them, yet without permanently magnetising them.
Flux indicators may be divided into two main types:
 Segment type
 Foil type

Question 43

Explain Berthold Penetrameter

Answer 43

Berthold Penetrameter

 A four section indicator with an adjustable foil, giving a varying air gap between them.
 Designed to indicate flux direction & sensitivity.
 The central, cylindrical iron piece is cut into quadrants to provide indications at 0 & 90o.

When the penetrameter is placed on a magnetised test surface, magnetic lines of flux will pass through the cut quadrants of the iron cylinder.
These lines will be visible when using MPI testing media.

Question 44

Explain Burma’s Castro Strips

Answer 44

The most common foil type indicator is the Burmah Castrol Strip or as it is more correctly now called, a magnetic flux indicator.
 Consist of a magnetic foil containing linear slots of different widths to simulate discontinuities, sandwiched between non-magnetic foils.
 The simulated discontinuities are arranged in 3 parallel lines.
 These foils are less than 0.2mm thick & flexible, which gives them a significant advantage over the segment type.

 BCS’s are placed on the test object as it is inspected, ideally at 90o to the possible defect orientation.
 The number of linear indications gives the Inspector a general idea of the magnetic field strength.
 The results are fairly repeatable as long as the same orientation of the magnetic field maintained.

Disadvantages of these strips are that they cannot be bent to complex shapes & are not suitable for multi-
directional field systems as they indicate defect indications in one direction only

Question 45

Explain Gauss meters

Answer 45

An electric powered meter that will read the field strength
directly from the input of a probe that is applied to the
test area. The meter gives a digital readout in Tesla.

Question 46

List the surface checks when doing MPI

Answer 46

Surface Checks:
 Obtain the necessary work permits.
 Ensure an isolating transformer is in the circuit.
 Test all circuit breakers or residual current devices .
 Check all electric cables for integrity.
 Confirm rigging & buoyancy is correct.
 Ensure the UV light is to BS EN ISO 3059 specifications.
 Ensure the ink is to BS EN ISO 9934-2 (settling test).
 Confirm ink distribution system is functioning correctly.
 Function test all the MPI equipment.
 Ensure all ancillary equipment is ready.
 Prepare the recording equipment.

Question 47

What is the in water preparation and pre inspection procedures

Answer 47

In Water Preparation:
 Establish the down lines on the correct site
 Clean the site to SA2½ for 75mm either side
Pre-inspection:
 Establish a datum & place the tape, mark up the weld.
 Complete a CVI to identify any areas that may cause spurious indication during the MPI.

Question 48

What must be done before testing can take place

Answer 48

Magnetic Particle Inspection:
 Rig the transformer as close as possible to the test site.
 Switch on the UV light.
 Confirm the ambient light is less than 20lux.
 Rig & lay out the coils, or magnet on the test site.
 Demagnetise the work site.
 Confirm the ink is constantly agitated.
 Position the gauss meter to monitor the induced field strength.
 Switch on the current on the diver’s command, record current.
 Record the field strength on the gauss meter (1.0T minimum).
 Apply ink as required to the weld & complete the inspection.
 Confirm adequate field strength at each clock position.

Question 49

What is the procedure when testing

Answer 49

Magnetic Particle Inspection
If any indications are identified, re-inspect to confirm
With confirmed indications record:
 Location (distance from datum).
 Length
 Orientation & position on weld
 Continuous or intermittent
 Branching or not
 Weak or strong indication
Complete remedial grinding as per the procedure.
Retest after grinding.
Mark the ends of the feature with a punch mark if required.

Question 50

What are the post inspection procedures

Answer 50

Post Inspection:
 Demagnetise.
 De-rig and recover all equipment to the surface.
 Wash all equipment with fresh water.
 Flush out the ink system.
 Report the results to the client.
 Cancel work permits.

Question 51

In what ways can indications be recorded

Answer 51

MPI indications can be recorded in a number of ways.
 Ultraviolet Photography. A conventional photograph can be taken using either:
 An ultraviolet lamp with a long exposure time on the camera.
 An appropriate ultraviolet filter on the strobe.

Foil Packets (Magfoil)
 Supplied in ready-made, two-compartment packets.
 These contain magnetic particles & the mixing liquid.
 A flux indicator is also mounted inside the packet.
 Break the internal barrier & mix the contents.
 After 45 seconds, the contents will take on a grey colour.
 Apply the bag to the test site.
 Contents remain liquid for 100 seconds.
 Apply the magnetising force during this period.
 Indication will be recorded as a white mark in the packet.
 Leave the packet in place until the liquid sets (5 minutes).
 Remove & measure the indication length & breadth.

Video
Some video cameras will capture the visible light of the fluorescent particles.

Rubberised Tape Transfer:
 Rubberised tape can be applied to a dry indication.
 This cannot be applied underwater.
 It may have a possible use in a welding habitat.

Question 52

Name Factors Affecting MPI Sensitivity

Answer 52

The sensitivity of a magnetic particle inspection is the effectiveness with which the test will discover defects & crack-like features in the material.
This will depend, in the main, on three factors:
 Diver Inspector.
 Equipment being used.
 Conditions on site.

For maximum efficiency, the diver inspector should, as well as being comfortable & as well equipped for the dive as possible:
 Have confidence in his ability to use the equipment.
 Be confident in his own ability to detect defects with it.
 Be sure of the value of his contribution to the efficiency & safety of the structure he is inspecting.

Those last three points are acquired from the competence & confidence imparted by good training & experience.
The sensitivity of the MPI will depend on several factors, some within the control of the inspector, others not.
Sensitivity of detection depends ultimately on the contrast produced between the defect & its surroundings & the definition, which tells the size, shape & orientation.