Electromagnetic Testing-ASNT Level III Study Guide ET

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Electromagnetic Testing

Study Guide Electromagnetic Testing Part 1 My ASNT Level III

Pre-Exam Preparatory Self Study Notes

17th April 2015

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Charlie Chong/ Fion Zhang

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http://independent.academia.edu/CharlieChong1 http://www.yumpu.com/zh/browse/user/charliechong http://issuu.com/charlieccchong

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Fion Zhang at Shanghai

17th April 2015

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IVONA TTS Capable.

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Chapter 1

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EDDY CURRENT an Overview

Description of Eddy Current Detectors

Coil configurations

Appropriate coil selection is the most important part of solving an eddy current application, no instrument can achieve much if it doesn’t get the right signals from the probe.

Coil designs can be split into three main groups:

1. Surface probes used mostly with the probe axis normal to the surface, in addition to the basic ‘pancake’ coil this includes pencil probes and special-purpose surface probes such as those used inside a fastener hole.

2. Encircling coils are normally used for in-line inspection of round products, The product to be tested is inserted though a circular coil.

3. ID probes are normally used for in-service inspection of heat exchangers. The probe is inserted into the tube. Normally ID probes are wound with the coil axis along the centre of the tube.

Absolute probes

These categories are not exhaustive and there are obviously overlaps, for example between non-circumferential wound ID probes and internal surface probes. To this point we have only discussed eddy current probes consisting of a single coil. These are

commonly used in many applications and are commonly known as absolute probes because they give an ‘absolute’ value of the condition at the test point. Absolute probes are very good for metal sorting and detection of cracks in many situations, however they are sensitive also to material variations, temperature changes etc.

Differential’ probe

Another commonly used probe type is the ‘differential’ probe this has two sensing elements looking at different areas of the

material being tested. The instrument responds to the difference between the eddy current conditions at the two points. Differential probes are particularly good for detection of small defects, and are relatively unaffected by lift-off (although the sensitivity is

reduced in just the same way), temperature changes and external interference. (assuming the instrument circuitry operates in a "balanced“ configuration)

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Note the characteristic "figure of eight for differential probe" response as first one probe element, then the other, move over the defect. In general the closer the element spacing the wider the "loop" in the signal. Lift-off should be cancelled out assuming that the probe is perfectly balanced, but there will still be a "wobble" response as the probe is moved and tilted slightly.

Reflection or driver pick-up probes have a primary winding driven from the oscillator and one or more sensor windings connected to the measurement circuit. Depending on the configuration of the sensor windings reflection probes may give response equivalent to either an absolute or differential probe. The two coils

(differential or absolute plus balancing coil) form the ‘legs’ of a bridge. When the bridge is balanced the

measured voltage will be zero. Any change in the condition of either coil will result in an unbalanced bridge, the degree of imbalance corresponds to the change in coil impedance.

Driver pick-up: As can be seen the essential elements are the same for a driver pick-up configuration as for a bridge, the necessary The diagram shows a typical response from a

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Tangential Probe

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Electromagnetic Testing Advantages

The following characteristics of the method can be used to advantage :

 it can be used without making physical contact with the product ;

 it does not need a coupling medium such as water ;

 it is capable of being used at high throughput speeds.

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Factors Affecting Eddy Current Responses

The basic parameters which influence the measured quantity are all of the following properties of the product to be tested, alone or in combination :

 the conductivity of the material ;

 the magnetic permeability of the material ; (magnetic factor)

 the size and geometry of the product to be tested ; (magnetic factor)

 the geometry between the eddy current probe and the product to be tested.

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Factors Affecting Eddy Current Response

Material conductivity

The conductivity of a material has a very direct effect on the eddy current flow: the greater the conductivity of a material the greater the flow of eddy currents on the surface. Conductivity is often measured by an eddy current technique, and inferences can then be drawn about the different factors affecting conductivity, such as material composition, heat treatment, work hardening etc.

Permeability

This may be described as the ease with which a material can be magnetised. For non-ferrous metals such as copper, brass, aluminum etc., and for austenitic stainless steels the permeability is the same as that of ‘free space’, i.e. the relative permeability (μ_r) is one. For ferrous metals however the value of μ_r may be several hundred, and this has a very significant influence on the eddy current response, in addition it is not uncommon for the permeability to vary greatly within a metal part due to localised stresses, heating effects etc.

Frequency

As we will discuss, eddy current response is greatly affected by the test frequency chosen, fortunately this is one property we can control.

Geometry

In a real part, for example one which is not flat or of infinite size, geometrical features such as curvature, edges, grooves etc. will exist and will effect the eddy current response. Test techniques must recognise this, for

example in testing an edge for cracks the probe will normally be moved along parallel to the edge so that small changes may be easily seen. Where the material thickness is less than the effective depth of penetration (see below) this will also effect the eddy current response

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Proximity / Lift-off

The closer a probe coil is to the surface the greater will be the effect on that coil. This has two main effects: The "lift-off" signal as the probe is moved on and off the surface. A reduction in sensitivity as the coil to product spacing increases.

Depth of penetration

The eddy current density, and thus the strength of the response from a flaw, is greatest on the surface of the metal being tested and declines with depth. It is mathematically convenient to define the "standard depth of penetration" where the eddy current is 1/e (37%) of its surface value. The standard depth of penetration in mm is given by the formula:

Where:

δ is standard depth in mm ρ is resistivity in μΩ.cm f is frequency in Hz

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from this it can be seen that depth of penetration: 1. Decreases with an increase in frequency 2. Decreases with an increase in conductivity

3. Decreases with an increase in permeability: this can be very significant penetration into ferrous materials at practical frequencies is very small.

The graph above shows the effect of frequency on standard depth of penetration.

It is also common to talk about the "effective depth of penetration" usually defined as three times the standard depth, where eddy current density has fallen to around 3% (5%?) of its surface value. This is the depth at which there is considered to be no influence on the eddy current field.

δ δ

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The Impedance Plane

Eddy current responses of a single coil may be conveniently described by reference to the "impedance plane". This is a graphical representation of the complex probe impedance where the abscissa (X value) represents the resistance and the ordinate (Y value) represents the inductive reactance.

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The Impedance Plane

http://ww w .eng.mo rgan.edu/~hub e rt/IEGR470/ed d ycu rr ent.htm l

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Note that, while the general form of the impedance plane remains the same, the details are unique for a

particular probe and frequency. The display of a typical CRT eddy current instrument represents a ‘window’ into the impedance plane, which can be rotated and "zoomed" to suit the needs of the application. For example in the above impedance plane diagram a rotated detail of the "probe on aluminum" area would appear as below:

This shows the display when moving over a series of simulated cracks of varying depths. Note that in the example shown both the amplitude and the phase of response from the different sized cracks varies.

Reliability

Eddy currents are often generated in transformers and lead to power losses. To combat this, thin, laminated strips of metal are used in the construction of power transformers, rather than making the transformer out of one solid piece of metal. Insulating glue, which confines the eddy currents to the strips, separates the thin strips. This reduces the eddy currents, thus reducing the power loss. Beside that, Eddy-Current Detectors are very reliable as far as their industrial usage. They are so reliable that nuclear plants are using robots to the tests,

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Measurement Techniques (

EN!

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a) Absolute measurement.

The measurement of the deviation from a fixed reference point. The reference point is defined by a calibration procedure and can be generated by a reference voltage or coi l. This technique can be used for sorting the product into classes based on physical properties such as hardness, dimensions or chemical composition. It can also be used for the identification of continuous or gradually changing discontinuities.

b) Comparative measurement.

The subtraction of two measurements, one of which is taken as a reference. This technique is normally used to sort the product into classes.

c) Differential measurement.

The subtraction of two measurements made at a constant distance between the measurement locations and on the same scanning path. This measurement technique reduces the background noise due to slow variations in the product to be tested. (?)

d) Double differential measurement.

The subtraction of two differential measurements. This measurement technique provides high-pass filtering of a differential measurement independent of the relative speed between the probe and the product to be tested. e) Pseudo differential measurements

The subtraction of two measurements made at a constant distance between the measurement locations.

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Historical Background

Before discussing the principles of eddy current testing, it seems appropriate to briefly discuss the concept of magnetism and electromagnetism that serve as the foundation for this study. In the period from 1775 to 1900, scientific experimenters Andre Marie Ampere, Françios Arago, Charles Augustin coulomb, Michael Faraday, Lord William Thomson Kelvin, James Clerk Maxwell and Hans Christian Oersted had investigated and

cataloged most of what is known about magnetism and electromagnetism. Arago discovered that the oscillation of a magnet was rapidly damped when a nonmagnetic conductor disk was placed near the magnet. He also observed that by rotating the disk, the magnet was attracted to the disk. In effect, Arago had introduced a varying magnetic field into the metallic disk causing eddy currents to flow in the disk. This produced a secondary magnetic field in the disk that affected the magnet. Arago's simple model is a basis for many automobile speedometers used today. This experiment can be modeled as shown in Figure 1.1.

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Arago’s Disk Experiment

Arago discovered that the oscillation of a magnet was rapidly damped when a nonmagnetic conducting disk was placed near the magnet. He also observed that by rotating the disk, the magnet was attracted to the disk. In effect, Arago had introduced a varying magnetic field into the metallic disk causing eddy currents to flow in the disk. This produced a secondary magnetic field in the disk that affected the magnet. Arago's simple model is a basis for many automobile speedometers used today.

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Oersted discovered the presence of a magnetic field around a current carrying conductor and observed magnetic field developed in a perpendicular plane to the direction of current flow in a wire. Ampere observed that equal and opposite currents flowing in adjacent conductors cancelled this magnetic effect. Ampere's observation is used in differential coil applications and to

manufacture non inductive precision resistor. Faraday's first experiments investigated induced currents by the relative motion of magnet and a coil (Figure 1.2). Faraday's major contribution was the discovery of electromagnetic induction. His work can be summarized by the example shown in Figure 1.3.

A coil "A" is connected to a battery through a switch, "S", A second coil, B, connected to a voltmeter is near by. When switch S is closed it produces a current in coil A in the direction shown (a). A momentary current is also induced in coil in direction (b) opposite to the current flow in coil A. If S is now opened, a momentary current will appear in coil B having the direction of (c). In each case current flows in coil B only while the current in coil A is changing.

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Figure 1.3: Induced current electromagnetic technique

A coil "A" is connected to a battery through a switch, "S", A second coil, B, connected to a voltmeter is near by. When switch S is closed it produces a current in coil A in the direction shown (a). A momentary current is also induced in coil in direction (b) opposite to the current flow in coil A. If S is now opened, a momentary current will appear in coil B having the direction of (c). In each case current flows in coil B only while the current in coil A is changing.

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Electromagnetic induction

is the production of an electromotive force across a conductor when it is exposed to a varying magnetic field. It is described mathematically by Faraday's law of induction, named after Michael Faraday who is generally credited with the discovery of induction in 1831.

Electromagnetic induction was first discovered by Michael Faraday, who made his discovery public in 1831. It was discovered independently by Joseph Henry in 1832.

In Faraday's first experimental demonstration (August 29, 1831), he wrapped two wires around opposite sides of an iron ring or "torus" (an arrangement similar to a modern toroidal transformer). Based on his assessment of recently discovered properties of electromagnets, he expected that when current started to flow in one wire, a sort of wave would travel through the ring and cause some electrical effect on the opposite side. He plugged one wire into a galvanometer, and watched it as he connected the other wire to a battery. Indeed, he saw a transient current (which he called a "wave of electricity") when he connected the wire to the battery, and another when he disconnected it. This induction was due to the change in magnetic flux that occurred when the battery was connected and disconnected. Within two months, Faraday found several other manifestations of electromagnetic induction. For example, he saw transient currents when he quickly slid a bar magnet in and out of a coil of wires, and he generated a steady (DC) current by rotating a copper disk near the bar magnet with a sliding electrical lead ("Faraday's disk"). Faraday explained electromagnetic induction using a concept he called lines of force. However, scientists at the time widely rejected his theoretical ideas, mainly because they were not formulated mathematically. An exception was Maxwell, who used Faraday's ideas as the basis of his quantitative electromagnetic theory. In Maxwell's model, the time varying aspect of

electromagnetic induction is expressed as a differential equation which Oliver Heaviside referred to as Faraday's law even though it is slightly different from Faraday's original formulation and does not describe motional EMF. Heaviside's version (see Maxwell– Faraday equation below) is the form recognized today in the group of equations known as Maxwell's equations.

Heinrich Lenz formulated the law named after him in 1834, to describe the "flux through the circuit". Lenz's law gives the direction of the induced EMF and current resulting from electromagnetic induction (elaborated upon in the examples below).

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Faraday's Law - Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil

relative to the magnet, etc. Faraday's law is a fundamental relationship which comes from Maxwell's equations. It serves as a summary of the ways a

voltage (or emf) may be generated by a changing magnetic environment. The induced emf in a coil is equal to the negative of the rate of change of

magnetic flux times the number of turns in the coil. It involves the interaction of charge with magnetic field.

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The law of physics describing the process of electromagnetic induction is known as Faraday's law of induction and the most widespread version of this law states that the induced electromotive force in any closed circuit is equal to the rate of change of the magnetic flux enclosed by the circuit. Or mathematically,

ε = dф_B/ dt

where ε (epsilon) is the electromotive force (EMF) and ΦB (Φ= BA) is the magnetic flux. The direction of the electromotive force is given by Lenz's law. This version of Faraday's law strictly holds only when the closed circuit is a loop of infinitely thin wire, and is invalid in some other circumstances. A different version, the Maxwell–Faraday equation (discussed below), is valid in all circumstances. For a tightly wound coil of wire, composed of N identical turns, each with the same magnetic flux going through them, the resulting EMF is given by

ε = -N dф_B/ dt

Faraday's law of induction makes use of the magnetic flux ΦB through a hypothetical surface Σ whose

boundary is a wire loop. Since the wire loop may be moving, we write Σ(t) for the surface. The magnetic flux is defined by a surface integral:

фB =

∫

_Σ(t) B(r,t)∙dA

where dA is an element of surface area of the moving surface Σ(t), B is the magnetic field, and B·dA is a vector dot product (the infinitesimal amount of magnetic flux). In more visual terms, the magnetic flux through the wire loop is proportional to the number of magnetic flux lines that pass through the loop.

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Lenz's Law

When an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it. The

induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop constant. In the examples below, if the B field is increasing, the induced field acts in opposition to it. If it is decreasing, the

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Magnetic Force

The magnetic field B is defined from the Lorentz Force Law, and specifically from the magnetic force on a moving charge:

The implications of this expression include:

1. The force is perpendicular to both the velocity v of the charge q and the magnetic field B.

2. The magnitude of the force is F = q∙v∙B sin θ where θ is the angle < 180 degrees between the velocity and the magnetic field. This implies that the magnetic force on a stationary charge or a charge moving parallel to the magnetic field is zero.

3. The direction of the force is given by the right hand rule. The force relationship above is in the form of a vector product.

When the magnetic force relationship is applied to a current-carrying wire, the right-hand rule may be used to determine the direction of force on the wire. From the force relationship above it can be deduced that the units of magnetic field are Newton seconds /(Coulomb meter) or Newtons per Ampere meter. This unit is named the Tesla. It is a large unit, and the smaller unit Gauss is used for small fields like the Earth's magnetic field. A Tesla is 10,000 Gauss. The Earth's magnetic field at the surface is on the order of half a Gauss

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Lorentz force

In physics, particularly electromagnetism, the Lorentz force is the combination of electric and magnetic force on a point charge due to electromagnetic fields. If a particle of charge q moves with velocity v in the presence of an electric field E and a magnetic field B, then it will experience a force

F = - q

∙ [ E + (v x B) ]

(in SI units). Variations on this basic formula describe the magnetic force on a current-carrying wire (sometimes called Laplace force), the electromotive force in a wire loop moving through a magnetic field (an aspect of

Faraday's law of induction), and the force on a charged particle which might be traveling near the speed of light (relativistic form of the Lorentz force).

The first derivation of the Lorentz force is commonly attributed to Oliver Heaviside in 1889, although other historians suggest an earlier origin in an 1865 paper by James Clerk Maxwell. Hendrik Lorentz derived it a few years after Heaviside.

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Generation of Eddy Currents

I_p = Primary Current

Ф_p =Primary magnetic flux

Ф_E = Secondary Eddy current magnetic flux I_E = Secondary Eddy current

When a conductor is place in the area

influence by the primary field, eddy current is induced in the conductor, see Fig. 1.4.

Following Lenz’s law, the induced eddy current IE will produce a secondary field ф_E that oppose the ф_P. The magnitude of ф_E is proportional to IE.

The test objet, conductor B’s characteristic like, material conductivity, permeability and geometry will affect the IE, this in turn cause variation in ф_E. The variation in ф_E is reflected in conductor CA by ф_E influences on ф_p. The variations are recorded in media like meter, CRT, digital read out or chart. The

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Factors Affecting Inductance

There are four basic factors of inductor construction determining the amount of inductance created. These factors all dictate inductance by affecting how much magnetic field flux will develop for a given amount of magnetic field force (current through the inductor's wire coil):

NUMBER OF WIRE WRAPS, OR "TURNS" IN THE COIL: All other factors

being equal, a greater number of turns of wire in the coil results in greater inductance; fewer turns of wire in the coil results in less inductance.

Explanation: More turns of wire means that the coil will generate a greater amount of magnetic field force (measured in amp-turns!), for a given amount of coil current. L ∝ N2

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COIL AREA: All other factors being equal, greater coil area (as measured

looking lengthwise through the coil, at the cross-section of the core) results in greater inductance; less coil area results in less inductance.

Explanation: Greater coil area presents less opposition to the formation of magnetic field flux, for a given amount of field force (amp-turns). L ∝ A

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COIL LENGTH: All other factors being equal, the longer the coil's length, the

less inductance; the shorter the coil's length, the greater the inductance.

Explanation: A longer path for the magnetic field flux to take results in more opposition to the formation of that flux for any given amount of field force (amp-turns). L ∝ (l)-1

COIL LENGTH COIL LENGTH

L ∝ (l)-1 L ∝ (l)-1

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CORE MATERIAL: All other factors being equal, the greater the magnetic

permeability of the core which the coil is wrapped around, the greater the inductance; the less the permeability of the core, the less the inductance. Explanation: A core material with greater magnetic permeability results in greater magnetic field flux for any given amount of field force (amp-turns). L ∝ μ

μ₀ = 4π x 10-7 H.m-1 μ_r = 600, μ_iron = 600 x μ₀

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Coil Inductance L

An approximation of inductance L, for any coil of wire can be found with this formula: The electromagnetic field produced about an unloaded test coil can be described as decreasing in intensity with distance from the coil and also varying across the coil's cross section. The field is most intense near the coil's surface. The field

produced about this coil is directly proportiona1 to the magnitude of applied current, rate of change of current or frequency and the coil parameters. Coil parameters inc1ude inductance, diameter, length， thickness, number of turns of wire and core material.

L = μ_r• (N2 _{x A /l)}• 1.26 x 10-6 _Henry

μ₀ = 4π x 10-7 _H.m-1 _{or 1.26 x 10}-6 _H.m-1

EMF = L di/dt Volt

Where:

L= inductance in Henry H

N = Numbers of turn in coil wire (straight wire N=1) μ_r= relative permeability

l = average length of coil in m

A = area of coil (not wire area?) in m2

μ_o = relative permeability in air 4π x 10-7 H.m-1 _{or 1.26 x 10-6 H.m}-1

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Note the direction of the primary current (I_p) and the resultant eddy current (I_E). I_E extends some distance into the test object. Another important observation is that I_E is generated in the same plane in which the coil is wound. Figure 1.6 emphasizes this point with a loop coil surrounding a cylindrical test object (4).

Figure 1.6 Induction current flow in a cylindrical part.

Important observation is that I_E is generated in the same plane in which the coil is wound.

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Note the direction of the primary current (I_p) and the resultant eddy current (I_E). I_E extends some distance into the test object. Another important observation is that I_E is generated in the same plane in which the coil is wound. Figure 1.6 emphasizes this point with a loop coil surrounding a cylindrical test object (4).

Important observation is that I_E is generated in the same plane in which the coil is

wound & in opposite direction of I_p

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Generation of Eddy Current

With a primary current 1p flowing through the coil, a primarr electromagnetic field фp is produced about the coil. When this excited test coil is placed on an electrically conductive test object, eddy currents IE will be generated in that test object Figure 1.5 illustrates this concept.

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It must be understood that this formula yields approximate figures only. One reason for this is the fact that permeability changes as the field intensity

varies (remember the nonlinear "B/H" curves for different materials).

Obviously, if permeability (µ) in the equation is unstable, then the inductance (L) will also be unstable to some degree as the current through the coil

changes in magnitude. If the hysteresis of the core material is significant, this will also have strange effects on the inductance of the coil. Inductor designers try to minimize these effects by designing the core in such a way that its flux density never approaches saturation levels, and so the inductor operates in a more linear portion of the B/H curve.

If an inductor is designed so that any one of these factors may be varied at will, its inductance will correspondingly vary. Variable inductors are usually made by providing a way to vary the number of wire turns in use at any given time, or by varying the core material (a sliding core that can be moved in and out of the coil). An example of the former design is shown in this photograph:

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Permeability changes as the field intensity varies (remember the nonlinear "B/H" curves for different materials).

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Figure 1: This unit uses sliding copper contacts to tap into the coil at different points along its length. The unit shown happens to be an air-core inductor used in early radio work.

Figure 2: A fixed-value inductor is shown in the next photograph, another antique air-core unit built for radios. The connection terminals can be seen at the bottom, as well as the few turns of relatively thick wire:

Figure 3: Here is another inductor (of greater inductance value), also intended for radio applications. Its wire coil is wound around a white ceramic tube for greater rigidity:

Figure 4: The two inductors on this circuit board are labeled L1 and L2, and they are located to the right-center of the board. Two nearby components are R3 (a resistor) and C16 (a capacitor). These inductors are called "toroidal" because their wire coils are wound around donut-shaped ("torus") cores.

Figure 5: Like resistors and capacitors, inductors can be packaged as "surface mount devices" as well. The following photograph shows just how small an inductor can be when packaged as such: A pair of inductors can be seen on this circuit board, to the right and center, appearing as small black chips with the number "100" printed on both. The upper inductor's label can be seen printed on the green circuit board as L5. Of course these inductors are very small in inductance value, but it demonstrates just how tiny they can be manufactured to meet certain circuit design needs.

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A Dual Variable inductors

Figure 1: This unit uses sliding copper contacts to tap into the coil at different points along its length. The unit shown happens to be an air-core inductor used in early radio work.

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Figure 2: A fixed-value inductor is shown in the next photograph, another antique air-core unit built for radios. The connection terminals can be seen at the bottom, as well as the few turns of relatively thick wire:

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Figure 3: Here is another inductor (of greater inductance value), also intended for radio applications. Its wire coil is wound around a white ceramic tube for greater rigidity:

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Figure 5: Like resistors and capacitors, inductors can be packaged as "surface mount devices" as well. The following

photograph shows just how small an inductor can be when packaged as such: A pair of inductors can be seen on this circuit board, to the right and center, appearing as small black chips with the number "100" printed on both. The upper inductor's label can be seen printed on the green circuit board as L5. Of course these inductors are very small in inductance value, but it demonstrates just how tiny they can be manufactured to meet certain circuit design needs.

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Grundig radio satellit 750

http://www.universal-radio.com/catalog/portable/0750.html

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Phasor Vector Diagram of Coil Voltage

A more precise method of describing the relationships of magnetic flux, voltage and current is the

phase vector diagram or phasor diagrams (4). Figure 1.7 compares the electromagnetic events

associated with an unloaded test coil and what happens when that same coil is placed on a nonferromagnetic test object. The components of phasor diagrams are as follows:

Fig.17(b)

E_p = Primary coil voltage

I = Exciting current (Primary coil current) Ф_p = Primary flux

Ф_s = Secondary flux

Fig.17(b)

E_p = Primary coil voltage

I = Exciting current (Primary coil current) Ф_p = Primary flux

Ф_s = Secondary flux E_s = Secondary voltage E_T= Total voltage

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Figure 1.7: Phasor Diagram of Coil Voltage (?)

In Figure 1.7(a) the current (I) and primary magnetic flux (фp) are plotted in phase. The primary voltage (Ep)

is shown separated by 90 electrical degrees. The secondary magnetic flux (ф_s) is plotted at zero

because without a test object no secondary flux exists.

Figure 1.7(b) represents the action of placing the coil on a

nonferromagnetic test object Observing the figure, one can see by vectorial addition of E_p and E_s that a new coil voltage (E_T) is arrived at for the loaded condition. The primary magnetic flux ф_p and

secondary magnetic flux ф_s are also combined by vectorial addition to arrive at a new magnetic flux (ф_T) for the loaded coil.

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In Figure 1.7(a) the current (I) and primary magnetic flux (ф_p) are plotted in phase. The primary voltage (E_p) is shown separated by 90 electrical degrees. The secondary magnetic flux (фs) is plotted at zero because without a test object no secondary flux exists.

Figure 1.7(b) represents the action of placing the coil on a nonferromagnetic test object Observing the figure, one can see by vectorial addition of E_p and Es that a new coil voltage (E_T) is arrived at for the loaded condition. The

primary magnetic flux ф_p and secondary magnetic flux фs are also combined by vectorial addition to arrive at a new magnetic flux (ф_T) for the loaded coil. Notice that for the condition of the test object in the test coil, ф_T is no longer in phase with the excitation current I. Also observe that the included angle

between the excitation current and the new coil voltage E_T is no longer at 90 electrical degrees. These interactions will be discussed in detaillater in this study guide.

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Current Density

The distribution of eddy currents in a test object varies exponentially. The current density in the test object is most dense near the test coil. This

exponential current density follows the mathematical rules for a natural

exponential decay curve (1/ e) where

ε

(epsilon) is 2.718. Usually a natural exponential curve is illustrated by a graph with the ordinate (Y axis)

representing magnitude and the abscissa (X axis) representing time or

distance. A common point described on such a graph is the knee of the curve. The knee occurs at the 37% value on the ordinate axis.

This 37% point is chosen because changes in X axis values produce significant changes in Yaxis values from 100% to 37% and below 37%

changes in X axis values þroduce less signlficant changes in Y axis values (?).

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Applying this logic to eddy current testing, a term is developed to describe the relationship of current distribution in the test object. The eddy current

generated at the surface of the test object nearest the test coil is 100%. The point in the test object thickness where this current is diminished to 37% of its previous strength is known as the standard depth of penetration. The term δ (delta) is used to represent this point in the material. Figure 1.8 is a relative eddy current density curve for a plane wave of infinite extent with magnetic field parallel to the conducting test object surface.

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The current density at any depth can be calculated as:

J

_x

=J

₀

e

- x√(πfμσ)

Where:

J_x = Electrical density at depth x in A∙m-2 J₀ = Electrical density at the surface x=0 x = distance fro surface in meter m

f = Frequency of the AC primary current Hz μ = Permeability of the test object in H∙m-1

σ = Conductivity of the test object in Siemen∙m-1

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Relative Magnetic Permeability

Permeability of free space μ₀ = 4π x 10-7 HM-1

Permeability of material can be expressed as relative to μ₀ μ_material = μ_r∙μ₀

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The Standard Depth of Penetration

δ

The Standard Depth of Penetration can be expressed as:

δ = (πfμσ)

-½

Where:

δ = One standard depth of penetration; 1/e of the surface current density (37%) in meter, m

f = Frequency of the AC primary current in Hz

μ = Permeability of the test object in Henry per meter, H∙m-1

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It should be observed at this point that as frequency, conductivity or

permeability is increased, the penetration of current into the test object will be decreased. The graph in Figure 1.8 is used to demonstrate many eddy

current characteristics.

Using an example of a very thick block of stainless steel being interrogated with a surface or probe coil operating at a test frequency of 100 kHz, the standard depth of penetration can be determined and current densities

observed at other depths. Stainless steel (300 Series) is nonferromagnetic. Magnetic permeability (μ) is 4πX 10-7 H∙m-1, the conductivity σ is 0.14 X 107

siemens (mhos) per meter for 300 Series stainless steel. δ = (πfμσ) -½

δ = (π x 100 x 103 x 4 x π x 0.14)-½

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Charlie Chong/ Fion Zhang http://fooplot.com/

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Using 1.35 mm as depth x from surface, a ratio of depth/depth of penetration would be 1 Referring to Figure 1.8, a depth/ depth of penetration of 1

indicates a relative eddy current density of 0.37 or 37%. What is the relative eddy current density at 3 mm?

The relative standard depth D_relative of x = 3mm is: D_relative = 3/δ = 3/1.53 mm = 2.22δ

This ratio indicates a relative eddy current densityof about 0.1 or 10%

[ (1/e)2.22 _{= 10.9%} _{]. With only 10% of the available current flowing at a depth}

of 3 mm, detectability of variables such as conductivity, permeability and discontinuities would be very difficult to detect. The obvious solution for

greater delectability at a depth of 3 mm depth is to lower the test frequency. Frequency selection will be covered in detaillater in this text.

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f(x) = (1/e)x _{where x = depth/}δ

R

e

lative current den

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Standard Depth for Different Conductive Materials

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Phase/Amplitude and Current Time Relationships

Figure 1.9 reveals another facet of eddy current. Eddy currents are not generated at the same instant in time throughout the part. Eddy currents require time to penetrate the test part. Phase and time are analogous

meaning - phase is an electrical term used to describe timing relationships of electrical waveforms.

Phase Lag = x/δ radian

Where:

x =depth below surface δ = Standard depth

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Current (?) Lagging

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Current (?) Lagging

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Phase is u,sually expressed in either degrees or radians. There'are

2πradians per 360 degrees. Each radian therefore is about 57 degrees

(360/2π). Using the surface eddy current near the test coil as a reference, the deeper the eddy current the greater the phase lag. The amount of phase lag is determined by:

β = x/δ = x∙√(πfμσ)

β or Φ = Phase lag angle in radian. Others as defined earlier

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Figure 1.9 should be used as a relative indicator of phase lag. The exact phase relationship for a particular system may be different due to other variables, such as coil parameters and excitation methods.

The amount of phase lag for a given part thickness is an important factor when considering resolution. Resolution is the ability to separate variables occurring in the test object; for example, distinguishing two discontinuities occurring at different depths in the same test object. As an example, using a standard depth of penetration at 1 mm in a 5 mm thick test object. Refer to Figure 1.9 and observe the phase lag of the current at one standard depth of penetration. Where depth of interest (x) is 1 mm and depth of penetration (δ) is 1 mm, the x/ δ ratio is 1 and the current at depth x lags the surface current by 1 radian or 57 degrees.

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Projecting this examination, observe the phase lag for the entire part

thickness. The standard depth of penetration is 1 mm, the part thickness is 5 mm; therefore, the ratio x/δ equals to 5. This produces phase lag of 5 radians or about 287 degrees for the part thickness. Having a measurement capability of 1 degree increments, the part thickness could be divided into 287 parts

each part representing 0.017mm. That would be considered excellent resolution.

There is an obvious Iimitation. Refer to Figure 1.8 and observe the resultant relative current density with an x/δ ratio of 5. The relative current density is near 0. It should become apparent that the frequency can be adjusted to achieve optimum results for a particular variable. These and other variables will be discussed in Chapter 5 of this Study Guide.

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Chapter 1

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Q.1.1 Generation of eddy currents depends on the principle of: A. wave guide theory.

B. electromagnetic induction.

C. Magnetostriction force D. All of the above

Q.1.2 A secondary field is generated by the test object and is; A. Equal and opposite to the primary field

B. Opposite to the primary field but much smaller

C. In the same plane as the coil is wound. D. In phase with the primary field.

Q.1.3 When a non ferromagnetic part is placed in the test coil, The coil' s voltage:

A. increases

B. remains constant because this is essential.

C. decreases.

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Q.1.4 Refer to Figure 1.7(b). If ET was produced by the test object being stainless steel, what would the effect be if the test object were copper?

A. ET would decrease and be at a different angle.

B. ET would increase and be at a different angle.

C. Because both materials are non-ferromagnetic, no change occurs D. None of the above.

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Q.1.5 Eddy current generated a test object flow; A. in the same plane as magnetic flux

B. in the same plane as the coil is wound

C. 90 degrees to the coil winding plane.

D. eddy currents have no predictable direction.

Q.1.6 The discovery of electromagnetic induction is credited to A. Arago

B. Oersted. C. Maxwell.

D. Faraday.

Q.1.7 A standard depth of penetration is defined as the point in a test object where the relative current density is reduced to:

A. 25%.

B. 37%

C. 50%. D. 100%

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Q.1.8 Refer to Figure 1.8. If one standard depth of penetration was

established at 1 mm in an object 3 mm thick, what is the relative current density on the far surface?

A. 3

B. <0.1

C. 1/3

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Q.1.9 Refer to Figure1.9 using example in question 1.8, what is the phase difference between the near and far surfaces?

A. the far surface current leads the near surface current by 57 degrees. B. the far surface current leads the near surface current by 171 degrees.

C. the far surface current lags the near surface current by 171 degrees.

D. the far surface current lags the near surface current by 570 degrees.

Q.1.10 Calculate the standard depth of penetration at 10KHz in Copper with σ = 5.7 x 107 _{Siemens per meter.}

A. 0.1 mm (3.9 x10-3 _in.) B. 0.02 mm (7.9 x10-4 _in.) C. 0.66 mm (0.026 in.) D. 66 mm (2.6 in.) β = x/ δ x 57.3º δ = (πfμσ) -½ = √(10 x 103 x π x 4 π x 10-7 _{x 5.7 x 10}7_{) x 1000 mm}

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Chapter 2

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Test Coil Arrangement

Test coils can be categorized into three main mechanical groups: probe coils, bobbin coils and encircling coils.

(Surface coil, internal bobbing coil, encircling coil)

Probe Coils

Surface coil, probe coil, flat coil or pancake coil are all common terms used to describe the same test coil type. Probe coils provide a convenient method of examining the surface of a test object. Figure 2.1 below illustrates a typical set of probe coils used for several surface scanning applications.

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Probe coils and probe coil forms can be shaped to fit particular geometries to solve complex inspection problems. As an example, probe coils fabricated in a pencil shape (pencil probe) are used to inspect threaded areas of mounting studs and nuts or serrated areas of turbine wheels and turbine blade

assemblies. Probe coils may be used where high resolution is required by adding coil shielding (2). When using a high resolution probe coil, the test object surface must be carefully scanned to ensure complete inspection coverage. This careful scanning is very time consuming. For this reason,

probe coil inspections of large test objects are usually limited to critical areas. Probe coils are used extensively in aircraft inspection for crack detection near fasteners and fastener holes. In the case of fastener holes (bolt holes, rivet holes), the probe coil may be rotated either manually or mechanically to provide a helical scan of the hole using a spinning probe technique (Figure 2.2).

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Encircling Coils

Encircling coil, outside diameter coil and feed through coil are terms

commonly used to describe a coil that surrounds the test object. Figure 2.3 illustrates a typical encircling coil. Encircling coils are primarily used to inspect tubular and bar-shaped products. The tube or bar is fed through the coil (feed through) at relatively high speed. The cross section of the test object within the test coil is simultaneously interrogated. For this reason, the

circumferential location of discontinuities cannot be determined with an encircling coil.

The volume of material examined at one time is greater using an encircling coil than a probe coil; therefore, the relative sensitivity is lower for an

encircling coil. The additional advantage that a probe coil would have over the encircling coil is that the probe coil could define where within the

circumferential plane the discontinuity exists. The encircling coil cannot make that distinction. If there are multiple signal sources within the coil's field of

view the encircling coil response will indicate the average of all of those events.

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Discussion

Subject; Encircling Coil

If there are multiple signal sources within the coil's field of view the encircling coil response will indicate the average of all of those events.

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When using an encircling coil, it is important to keep the test object centered in the coil. If the test object is not centered, a uniform discontinuity response is difficult to obtain. To ensure proper centering it is corrunon practice to run the calibration standard several times, each time indexing the artificial

discontinuities to a new circumferential location in the coil. As in all discontinuity detection schemes, it is essential to select a reasonable operating frequency, properly adjust the system display parameters and

ensure that the tube is centered in the coil at all times to achieve optimum test sensitivity.

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Bobbin Coils

Bobbin coil, inside diameter coil and inside probe are terms that describe

coils used to inspect from the inside diameter or bore of a tubular test object. Bobbin coils are inserted and withdrawn from the tube inside diameter by long, semi flexible shafts or simply blown in with air and retrieved with an attached pull cable. These mechanisms will be described later in the text. Bobbin coil information follows the same basic rules stated for encircling coils. Figure 2.4 illustrates a typical bobbin coil.

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Coil Arrangements

Probe coils, encircling coils and bobbin coils can be additionally classified. These additional classifications are determined by how the coils are

electrically connected. The three coil categories are absolute, differential and hybrid. Figure 2.5 shows various types of absolute and differential coil

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Figure 2.5: Test coil configurations for eddy current testing of small-diameter tubing

 Absolute  Differential- Self comparisons, external reference  Thru transmission  Reflection (Double)

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Absolute Coils

An absolute coil makes its measurement without direct reference or

comparison to a standard as the measurement is being made (6). Some

applications for absolute coil systems would be measurements of conductivity, permeability, dimensions and hardness.

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Differential coils

Differential coils consist of two or more coils electrically connected to oppose each other. Differential coils can be categorized into two types: (1) self

comparison differential and (2) external reference differential.

a) The self-comparison differential coil compares one area of a test object

to another area on the same test object. A common use is two coils,

connected opposing, so that if both coils are affected by identical test object conditions, the net output is 0 volts or no signal change. The self-comparison arrangement is insensitive to test object variables that occur gradually.

Variables such as slowly changing wall thickness, diameter or conductivity are effectively discriminated against with the selfcomparison differential coil. Only when a different condition affects one or the other test coils will an

output signal be generated. The coils usually being mechanically and electrically similar allows the arrangement to be very stable during

temperature changes. Short discontinuities such as cracks, pits or other

localized discontinuities with abrupt boundaries can be readily detected using the self-comparison differential coli.

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b) The external reference differential coil, uses an external reference to

affect one coil while the other coil is affected by the test object. Figure 2.6 illustrates this concept. This system is used to detect differences between a standard object and test objects. It is particularly useful for comparative

conductivity, permeability and dimensional measurements. Obviously in

Figure 2.6 it is imperative to normalize (or balance) the system with one coil affected by the standard object and the other coil affected by an acceptable test object. The external reference differential coil system is sensitive to all measurable differences between the standard object and test object. For this reason it is often necessary to provide additional discrimination to separate and define variables present in the test object.

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Hybrid Coils

Hybrid coils may be defined as driver/pickup, through transmission, reflection or primary/secdndary coil assemblies. Hybrid coils may or may not be the

same size and are not necessarily adjacent to each other. Figure 2.7 shows one possible hybrid coil arrangement. In the through transmission coil, the excitation coil is on one side of the test object and the sensing coil is on the other. The driver coil induces eddy currents and a secondary magnetic field in the test specimen. Any variation of these secondary events should be

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A hybrid coil arrangement consists of an excitation coil and a sensing coil (reflection coils). In most cases a single probe housing assembly contains

both the driver and the pickup coil(s). The primary magnetic flux interacts with both coils. The voltage developed in the sensing coil is a function of the

current magnitude and frequency applied to the excitation coil, coil parameters of the exciting and sensing coils and the test object characteristics.

Most hybrid coils are designed to improve test sensitivity for a specific application. One example of this is for better detection of subsurface

discontinuities in multilayer structures. The concept of using a smaller pickup coil enhances the ability to detect lower level impedance variations from small volume discontinuities deeper in the test sample. It should be pointed out that if larger volume discontinuities are encountered that a measurable impedance change might be generated by both the exciter and the pick up coil(s).

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Additional Coil Characteristics

Coil configuration is but one of many factors to consider when setting up test conditions. Other coil characteristics of importance are mechanical, thermal and electrical stability; sensitivity, resolution and dimensions. The geometry of the coil is usually dictated by the geometty of the test object. Selection of

smaller probe sizes may affect test sensitivity and/or resolution. The relative importance of the coil characteristics depends on the nature of the test. A blend of theory and experience usually succeeds in selection of proper coil parameters. Coil design and interactions with test objects will be discussed later in this Study Guide.

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Chapter 2

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Q.2.1 Differential coils are usually used in: A. bobbin coils.

B. probe coils.

C. outside diameter coils.

D. any of the above.

Q.2.2 When using a probe coil to scan a test object: A. the object must be dry and polished.

B. the object must be scanned carefully to ensure inspection coverage.

C. the object must be scanned in circular motions at constant speeds. D. the probe must be moving at all times to get a reading.

Q.2.3 A spinning probe would most likely be: A. a bobbin coil

B. an inside diameter coil. C. an outside diameter coil.

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Q.2.4 A feed through coil is:

A. a coil with primary/ secondary windings connected so that the signal is fed through the primaq to the secondary.

B. an encircling coil.

C. an outside diameter coil.

D. both Band C.

Q.2.5 When inspecting a tubular product with an encircling coil, which statement is not true?

A. Outside diameter discontinuities can be found. B. Axial discontinuity locations can be noted.

C. Circumferential discontinuity locations can be noted.

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Q.2.6 An absolute coil measurement is made:

A. by comparing one spot on the test object to another.

B. without reference to or direct comparison with a standard.

C. only with probe coils.

D. by comparative measurement to a known standard.

Q.2.7 When coils in a self-comparison differential arrangement are affected simultaneously with the same test object variables, the output signal:

A is directly proportional to the number of variables.

B. is 0 or near 0.

C. is indirectly proportional to the number of variables. D. is primarily a function of the exciting current.

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Q.2.8 Which coil type inherently has better thermal stability? A bobbin

B. absolute

C. outside diameter

D. self-comparison differential

Q.2.9 A hybrid coil is composed of two or more coils. The coils: A. must be aligned coplanar to the driver axis.

B. may be of widely different dimensions.

C. must be impedance matched as closely as possible. D. are very temperature sensitive.

Q.2.10 Proper selection of test coil arrangement is determined by: A. shape of test object.

B. redolution required. C. sensitivity required. D. stability.

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Chapter 3

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As discussed earlier test coil design and selection is a blend of theory and experience. Many factors must be considered. These important factors are determined by the inspection requirement for resolution, sensitivity,

impedance, size, stability and environmental considerations. To better

understand coil properties and electrical relationships, a short refresher in alternating current theory is necessary. First, the electrical units must be

examined. For example, current and its representative symbol I. Current not only suggests electron flow but also the amount. The amount of electrons flowing past a point in a circuit in 1 second is expressed in amperes:

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Ampere

The ampere (SI unit symbol: A), often shortened to "amp", is the SI unit of electric current (dimension symbol: I) and is one of the seven SI base units. It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics.

The ampere is equivalent to one coulomb (roughly 6.241×1018 _{times the elementary}

charge) per second. Amperes are used to express flow rate of electric charge. For any point experiencing a current, if the number of charged particles passing or the charge on the particles is increased, the amperes of current at that point will proportionately increase.

The ampere should not be confused with the coulomb (also called "ampere-second") or the ampere-hour (A·h). The ampere is a unit of current, the amount of charge

transiting per unit time, and the coulomb is a unit of charge. When SI units are used, constant, instantaneous and average current are expressed in amperes (as in "the charging current is 1.2 A") and the charge accumulated, or passed through a circuit over a period of time is expressed in coulombs (as in "the battery charge is 30000 C"). The relation of the ampere to the coulomb is the same as that of the watt to the joule,

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Demonstration model of a moving iron ammeter. As the current through the coil increases, the plunger is drawn further into the coil and the

pointer deflects to the right.

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Resistance

Resistance is an opposition to the flow of electrons and is measured in ohms. Ohm's Law is stated by the equation:

E = IR Where:

I = Current in Ampere A R = Resistance in Ohm Ω

E = Electrical potential difference in volt V

The resistance of a coil is determined primarily by the length of wire used to wind the coil; its specific resistance is determined by the type of wire (e.g., copper, silver) and the cross-sectional area of the wire.

Resistance = (Specific resistance X Length) / Area Resistance =

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Thus, the resistance of a 10 ft length of 40 gage copper wire with a specific resistance of 10.4 circular-mil-foot at 20ºC would be found as follows:

R = (10.4 X 10) / 9.888 = 10.518 ohm.

In an alternating current circuit containing only resistance, the current and voltage are in phase. In phase means the current and voltage reach their minimum and maximum values, respectively, at the same time. The power dissipated in a resistive circuit appears :in the form of heat. For example, electric toasters are equipped with resistance wires that become hot when current flows through them, providing a heat source for toasting bread.

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Circular-Mill-Foot

A circular-mil-foot (figure 1-3) is a unit of volume. It is a unit conductor 1 foot in length and has a cross-sectional area of 1 circular mil. Because it is a unit conductor, the circular-mil-foot is useful in making comparisons

between wires consisting of different metals.

For example, a basis of comparison of the RESISTIVITY (to be discussed shortly) of various substances may be made by determining the resistance of a circular-mil-foot of each of the substances.

In working with square or rectangular conductors, such as ammeter shunts and bus bars, you may sometimes find it more convenient to use a different unit volume. A bus bar is a heavy copper strap or bar used to connect several circuits together. Bus bars are used when a large current capacity is required.

Unit volume may be measured as the centimeter cube. Specific resistance, therefore, becomes the resistance offered by a cube-shaped conductor 1 centimeter in length and 1 square centimeter in cross-sectional area. The unit of volume to be used is given in tables of specific resistances.

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SPECIFIC RESISTANCE OR RESISTIVITY Specific resistance, or resistivity, is the

resistance in ohms offered by a unit volume (the circular-mil-foot or the centimeter cube) of a substance to the flow of electric current. Resistivity is the reciprocal of conductivity. A substance that has a high resistivity will have a low conductivity, and vice versa. Thus, the specific resistance of a substance is the resistance of a unit volume of that substance. Many tables of specific resistance are based on the resistance in ohms of a volume of a substance 1 foot in length and 1 circular mil in cross-sectional area. The temperature at which the

resistance measurement is made is also specified. If you know the kind of metal a conductor is made of, you can obtain the specific resistance of the metal from a table. The specific resistances of some common

substances are given in table 1-1.

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The resistance of a conductor of a uniform cross section varies directly as the product of the length and the specific resistance of the conductor, and

inversely as the cross-sectional area of the conductor. Therefore, you can

calculate the resistance of a conductor if you know the length, cross-sectional area, and specific resistance of the substance. Expressed as an equation, the "R" (resistance in ohms) of a conductor is