CA1067614A - Pulse-echo method and system for testing wall thicknesses - Google Patents

Abstract

PULSE-ECHO METHOD AND SYSTEM FOR
TESTING WALL THICKNESSES

ABSTRACT OF THE DISCLOSURE

A method and system for improving the pulse-echo type of testing to determine wall thicknesses. It is parti-cularly applicable to pipes. There is a pulse directed transversely to the wall so that reflections from the inner and outer surfaces will indicate the thickness, and another pulse is directed at an angle to the wall with time spacing. The angled pulse path permits penetration so that the presence of an undesirable type of dis-continuity will cause an additional reflection which distinguishes this type of discontinuity.

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Description

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BACKGROUMD OF THE INVENTION

Field of the Invention .
This invention concerns pulse-echo type wall thick-ness measuring in general. More speclfically, it relates to a method and system for improving upon known pulse-echo types of wall thickness measurement.

Descri~tion of the Prior Art In the field of non destructived testing, or measuring of thickness of the walls of elongated bodies, use haæ been made of a longitudinally directed pulse transducer with a 45 degree reflecting surface so as to direct the energy from a parallel direction to one at righ angles that is thus transverse to the walls of the elongated body. Also, in connection with pipe line inspection employing pulse-echo systems, it has been known to make use of a plural transducer with rotator, which employed a scanning or a physical rotation at a rate proportional to the longitudinal travel in the pipe. How-ever, neither of the prior arrangements conceived of slmultaneously introducing another pulse directed at a different angle relative to the surface of the pipe or other wall. This invention makes use of the latter method in con~unction with known elements and arrangements, to provide ~or an improved method and system for deter-mining a characterizat~on of a discontinuity so as to be able to recognize a cavity or thin spot, as distinguished from a welded ~oint or the like.
Thus, it is a object of this invention to provide an improved method or system for measuring wall thickneæs ~067~

with a pulsc-echo type of system wllile employing an additional means for characterizing a discontinuity in sa;d wall thickness that may be observed.
Briefly, the invention provides in a pulse-echo system for measur-ing wall thickness of pipes or the like, the improvement comprising in combination, means for generating a single pulse directed along a path that is longitudinal relative to said pipe wall, means for splitting said single pulse to form first and second named pulses, said splitting means comprising first and second reflecting surfaces, said first reflecting surface having an angle relative to said longitudinal path such that said first named pulse is directed transversely relative to said pipe wall for making said wall thickness measurement, said second reflecting surface having an angle relative to said longitudinal path such that said second named pulse is directed at an angle of incidence relative to said wall that is greater than the critical angle of refraction in order to permit any reflected energy from a discontinuity in said wall to be returned along the .
same path, said second reflecting surface being non-contiguous with said first reflecting surface and having an offset along said longitudinal path .
for causing a time delay of said second pulse relative to said first pulse; :
and means for receiving any reflected pulses from both said first and second ~ : ;
named pulses whereby said wall thickness may be measured and any discontinuity may be characterized.

1067~14 According to another aspect of the invention, there is provided in a pulse-echo system for measuring wall thickness of a pipe, the improve-ment comprising in combination a plurality of pulse generating means located spaced peripherally about the axis and inside of said pipe, said means being oriented for directing the pulses parallel to the axis of said pipe, a first annular reflecting surface having a 45 angle relative to said axis and spaced axially from said plurality of pulse generating means, said first surface extending radially about half the width of said pulses for splitting off a portion of each and directing said portion along a path transverse to the wall of the pipe at that location, a second annular reflecting sur-face having an angle of more than 45 relative to said axis and being spaced axially at a greater distance from said pulse generating means than said first annular surface, said second surface being non-contiguous with said first surface and having an offset axially therefrom to provide said greater distance and to cause a time delay of the remaining portions of said pulses, and said second surface extending radially the other half of the width of said pulses for directing another portion of each pulse along a path having an angle of incidence relative to the wall of said pipe at that location which is greater than the critical angle of refraction in order to permit any reflected energy from a discontinuity in said wall to be returned along said other portion path.

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BRIEF DESCRIPTION OF THE DRAWINGS
The ~oregoing and other objects and bene~its of the invention will be more fully set forth below in connection with the best mode contemplated by the inventor~ for carrying out the invention~ and in connection with which there are illustrations provided in the drawings, wherein:
Figure 1 is a fragmentary schematic showing, partly in elevation and partly in cross section, illustrating one form of apparatus which may be employed with the invention; : ~`
Figure 2 i8 an illustration representing oscillographs which show various echo pulse signals which are the re-flected pulses that are returned from the sur~aces of a wall being measured in the manner indicated by Figure 1, :
Figure 3-4, 5-6, 7-8, and 9-10 are fragmentary ~chematic showings in elevation and crosæ section, and plan views, respectively illustrating variouæ difications as to different configurations of a reflecting surface which may be used in order to control the characteristics of an ultrasonic energ~ beamj Figure 11 is a fragmentary schematic, showlng a longitudinal cross-section taken along the lines 11-11 of Flgure 12. It illustrates another form of the in-vention which is particularly for use in pipe wall thick-nes~ measurement;
Figure 12 is a horizontal cros~-sectional view taken along the lines 12-12 of figure 11; and Figure 13 i8 schematic circuit diagram illustrating an electrical circuit arrangement for use with the com-bination of elements that are illustrated in Figures 11 and 12.

.,-~067614 DESCRIPTION OF THE PRæFERRED EMBODIMENTS

The pul~e-echo technique ls well-known in connection with ultrasonic energy, and has been considerably employed with non-destructive testing or measuring of wall thick-nesses of various types of material including plpes for use in pipelines, and the like. However, in ~aking use of this techni~ue heretofore it has been found that often there is a difficulty in the ability to distinguish be-tween thin or corroded spots and other anomalies such as welded joints and irregular surfaces. This invention provides a method and system which overcomes such ~ ;
difficulties. It makes it possible to have positive identification of thin or corroded spots in the walls of a pipe or the like.
Thus, with reference to Figure 1, there is shown ln longitudinal cross-section, a fragemental portion of a pipe 21. It will be appreciated that the wall 21 of the -pipe mlght also be a wall of a tank or the like. Al~o, it will be understood that inside the pipe 21 ~here is a fluid 22 which will act as a good conductor for acoustlc energy.
In order to measure the thickness of the wall of pipe 21, there is a pulse transducer 25 with a lens 26 for creating a d$rected beam of energy when the crystal is actived. It will be understood that in this type of sy~tem a short time duration, unitary pulse of ultrasonic acoustic energy is created by applylng a short time duration electrical voltage to the crystal in a conventional manner.
The crystal material is preferably lead metaniobate and in 3 the lndicated transducer 25 it will be a flat disc shape (not shown) with silvered faces as its electrodes (not 1067tii~

shown). Since the crystal is piezoelectric in nature, it will deform and thus produce an acou~tic energy pulse.
Because of the orientation of the transducer 25 and the focusing of the lens 26 the pulse will be directed downward along a path 29 until it strikes a flat reflect-ing surface 30 that is located at 45 degrees relative to ~ -the axis of the pipe 21. The energy of that part of the pulse which strikes the surface 30, will then be reflected at right angles to the path 29 and so travel over a path 33 that is transverse to the surface of the pipe wall 21.
The wall 21 of the pipe has a inner surface 34 and an outer surface 35, each of which is a boundary for the material of the pipe 21 such that a reflection sf some of the energy in the acoustic pulse will occur from each of these surfaces. These reflected pulses wlll then travel back along the same path 33 in the other direction and then will be reflected from the surface 30 to travel up-ward along path 29 until they strike the crystal in trans-ducer 25, where an electrical signal corresponding to the acoustic pulse will be generated. The technique for thus first producing an acoustic pulse and after the short time involved, producing the electrical pulse signals created by the returning reflected acou~tic pulses is well-known, as already indicated, and need not be describ-ed in greater detail here.
Figure 2 illustrates four lines of corresponding oscillograph records, showing signal amplitudes as a function of time. The energy is ultrasonic in fre~uen~y so that the time is short and distance 38 on the time scale represents two micro seconds.

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The uppermost trace on the Flgure 2 lllustration, which is designated by the letter "a", shows a pair of reflected pulses 41 and 42. These are the pul~e æignal6 generated by the transduc~r as it responds to the acou~tic energy which would have been reflected from the surfaces 34 and 35 of pipe wall 21 if no cavity or other di~continuity existed.
Referring back to Figure 1 it will be ob~erved that there is another flat reflecting sur~ace 45 spaced ~ome- -what farther away from the transducer 25. This reflecting surface 45 i8 æituated at a angle-of more than 45 degrees relative to the axis of the pipe 21. Con~equently, energy from pulses travelling over half of the path, e.~. a path 46 that is indicated by a dashed line, will travel over another acoustic path 47. Path 47 has an angle of incidence relative to the inner æurface 34 that is greater thQn the critical angle of refraction for the material o~
the pipe wall 21. Consequently, this pulse energy will penetrate the surface 34 of the wall of the pipe 21 and travel upward and out without returnlng unless there is a discontinuity in the pipe wall 21, such as a cavity 51 illustrated. If there i8 such a discontinuity, there will be a reflected pulse, e.g. from the cavity 51, which will return over the same paths 47 and 46 to impinge upon the transducer 25. It will then eenerate a reflected pulse in the same manner as the reflected pulses of the thick-ness measuring paths 33 and 29.
From the foregoing it will be understood that each time the transducer 25 iæ activated to produce an acoustic pulse, such pulse will travel downward in the fluid 22.

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Also, the pulse will have passed through the lenæ
26 so as to focus it to cover an area that is substantially the same size a~ the crystal of the transducer 25. Aæ ~ ~
such pulses travel downward from the transducer 25, the ~ -upper reflecting surface 30 wlll reflect half of each pulse at right angles, so that thiæ portion goes trans-ver~ly to the surfaces 34 and 35 and is reflected from these walls of the pipe 21. The returning reflected pulses thus indicate the thickness of the wall 21.
At a short time thereafter, the reflecting surface 45 will reflect the other half of each pulse so that thiæ
portion goeæ along the path 47 that tranæmits the energy into the wall 21. Such energy travelling in the wall 21 will cause a reflected return pulse only in the event that there iB some discontinuity in itæ path. Such a dis-contlnuity will cauæe a reflected return pulse to appear.
It will be underætood that by having the reflecting æurface 45 located farther away from the transducer 25 than the transverse energy reflecting æurface 30 at a predetermined distance, it may be readily determined how much time delay iæ to be expected between the first pair of re Mected pulses and the single returning pulse~ each along separate paths.
Of particular importance to this invention is the fact that since conditions on the inner and outer wallæ of the pipe 21 may include some discontinuities which will not cause the delayed single reflected pulse to return, e.g.
weld joints or shallow irregularitieæ, such conditions can be distingui6hed from others. In other wordæ some conditions can cause loss of either or both of the first pair of reflected pulæe~ which determlne the thicknesæ

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of the wall 21, i.e. pulses along path 33. ~owever, the other slightly delayed pulse along path 47 will provide an indication as to the presence of a discontinuity which causes the single reflected pulse to return, e.g. a cavity or similar thin spot.
Figure 2 illustrates that part of dif~erent oscillographs, taken under some of the foregoing conditions which shows the returning reflected pulses. Thus, already indicated above, the line "a' of the oscillograph illustrates conditions wherein the reflected returning energy pulses 41 and 4~ are from the inner and outer surfaces of the pipe wall 21 where there is no cavity or other discontinuity. Consequently there is nothing to return any reflected energy pulse along the other pulse path 47. The absence of such later pulse may be noted at a general location 54 on the trace "a".
On the other hand, the trace "b" of Figure 2 illustrates a first pair of reflected pulses 55 and 56 which are closer together in time and so indicate a thinner wall condition of the pipe 210 In additionj there is a third pulse 57 that is one which has returned over the angled path 47 and thus positivity indicates the presence of a discontinuity such as a cavity 51 which is illustrated in Figure 1.
Traces "c" and "d" of ~igure 2 illustrate other conditions similar to trace "bn, but with thinner wall conditions. Consequently, the first pair of re~lected, i.e. transverse pulses are closer together. However, these traces also illustrat the present of a discontinuity such as the cavity 51, since in each case there is a de-layed single reflected pulse 60 and 61 respectively.

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Reîerring to Figures 3-10, there are various modificatiolls shown which relate to the structure of reflectors Ior ultrasonic energy beams~ These are especially applicable to the type of wall thickness measurement under consideration. It is quite feasible to control the shape of the acoustic energy beams in-volved, by means of determining the shape of the reflector surface. mus different types of beam shaping may be carried out. For example, in Figures 3 and 4 the beam may be concentrated vertically while retaining the same width in a horizontal plane.
Figures 3 and 4 illustrate side and plan views of a transducer 65 that transmits pulse beams onto the angled surface of a reflector 66, which has it reflecting surface divided into two halves 67 and 68. These reflecting sur-faces 67 and 68 have a small supplementary angle relative to one another so that the beam of energy reflected will be concentrated in the manner indicated, i e. toward a re-flecting point 71 on a wall 72 of a pipe 73.
On the other hand it may be desilrable to slightly disperse the beam of the acoustic pulse path vertically, and such is illustrated in Figures 5 and 6. There is a transducer 75 that sends its pulses downward toward a reflector 76, which has its reflecting surface divided into two halves making separate surfaces 77 and 78 as indicated. In this case these surfaces have a slight angular difference which creates the dispersal situation.
The transmitted beam of acoustic energy is indicated by a dashed line 81, while the returning path is indicated as a solid line 82. It will be observed that the reflected energy from the wall 72 of the pipe 10~7~

returns farther away from the axis of the transducer 75.
Figures 7 and 8 illustrate a reflector surface structure for spreading the acoustic beam horizontally and not vertically. In this case there is a trQnsducer 85 that has its beam directed onto a reflector 86. The surface o~ reflector 86 has its face divided into two halves 87 and 88. These are flat surfaces angled away from one another in a convex manner a few degree~ in order to spread the beam of acoustic energy horizontally as indicated in figure 8.
Figures 9 and 10 illustrate one more embodiment concerning the reflecting surface structure which may be employed. Since the use of flat surfaces on the reflector will cause a separation into two distinct ~eams, as indicated in Figure 8, it may be preferable to employ a curved convex surface. This i8 illustrated in ~igures 9 and 10 where there i8 illustrated a transducer 91 that is directing its energy onto the reflecting surface of a reflector 92 which like reflector 86 in Figures 7 and 8 is designed to spread the acoustic energy beam horizontally but not vertically. In this case there is a curved sur-face 93 that is continuously curved in a convex manner to form the spreading affect. However, it will be u~derstood that surface 93 is a oylindrical curved surface with a straight center line situated at 45 degrees, as indicated in the elevation view of Figure 9.
Figures 11, 12 and 13 illustrate a system for providlng a multiple tranæducer arrangement that is particularly useful in an instrument for determin~ng wall thickness of a pipe Such an instrument is especially useful for surveying a pipe line.

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As schematically illustrated in Figures 11 and 12, there is an instrument body 101 that has the diameter designed for a proper size to pass through a pipe 102.
In order to make a continuous measurement, or survey of the wall thickness of the pipe 102 there is a plurality of pulse generating transduceræ 105 that are located circumferentially situated on the body 101 of the in- .
strument. These transducers 105 have the axis of each oriented parallel to the axis of the pipe 102.
The pulseæ generated by each of the transdueers 105 are directed down (as illustrated) parallel to the axis of the pipe 102. Consequently, these pulses impinge upon a first annular reflecting surface 106 that has an angle of 45 degrees relative to the pipe axis. This sur-face 106 extends radially about half of the width of the pulse beams which are transmitted from the transducers 105~ so that a portion of each of the pulses are split off and reflected by the surface 106 to travel over a path 109 in each case. It will be understood that these paths are transverse to the surfaces of the pipe wall 102.
Situated axially farther away from the transducers 105, there is another annular reflecting surface 112 that haæ its surface at a angle which is greater than 45 degrees relative to the axis of the pipe 102. This surface 112 extends radially far enough to encompaæs the other half of the pulse ray paths of acoustic energy from transducers 105 so that this portion which is 3~1it off of the original pulæes~ will be reflected along somewhat up-wardly directed paths 113. Each of these paths 113 has an angle of inciden~e relative to the wall of the pipe 102, that is greater than the critical angle of refraction . I .

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so that the pulses of acoustic energy will enter the material of the wall of pipe 102. Then, as explained previously in connection with Figure 1 and 2, the energy from the pulses travelling o~er paths 113 will be reflected back over the paths 113 if they encounter a dlscontinuity such as a cavity 116, illustrated. Such reflected cnergies will be returned to the transducers 105 where electrical signals will be generated.
By having the instrument 101 constructed with a plurality of transducers, in the manner illustrated in Figure 11, 12, and 13 the instrument will be especially suitable for being employed in the survey of a pipeline.
In order to employ the instrument 101 to survey the walls of the pipe 102, the transducers 105 will be pulsed sequently around the circumference of t~e instrument 101. Consequently, as the instrument travels through the pipe 102 there will be a continuous scanning of the walls of the pipe which may, of course, be part of a pipeline. A schematic circuit arrangement for accomplishlng this iæ illustrated in Figure 13, where it will be observed that each of the transducers 105 includes a piezo-electric crystal llg that has electrodes 120 and 121 associated therewith for applying a voltage pulse which will generate the acoustic pulse. mereafter, the crystal 119 and electrodes 120 and 121 will act inversly to generate an electrical signal, in each case, as the reflected return pulse is detected.
Still referring to Figure 13, it will be under-stood that in each case the pulse generating circuit includes a circuit connector 123 that leads from the electrode 120 to a co~trolled electrode 124 of a .

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silicon controlled rectifier 25. The SCR 125 acts to pass a voltage pulse from a charged capacitor 128 to the crystal 119 whenever it is triggered by a signal applied to a circuit 129 to trip the SCR into conduction.
The capacitor 128 is maintain charged by a relatively high DC potenti&l, which is maintained at a terminal 132, with a resistor 133 between the terminal 132 and the high potential plate of capacitor 128.
It will be understood that throughout this specification wherever the abbreviatlon SCR is employed it stands for sillcon controlled rectifier. Such abbreviation is well known to one skilled in the electronic arts.
Referring to the pulse generating circuit of Figure 13 again, it will be noted that there is a common ground circuit 136 that is connected to the other electrode 121 of the crystal 119. Also, the circuit 136 has one side of a variable inductor 137, as well as one end of a resistor 138 connected thereto. In addltion, there is a resistor 141 that is in the control circuit 129. The control circuit goes via a resistor 141 from the output of an amplifier 142. And, the output of a selector circuit 145 goes to the input of the amplifier 142 over a circuit connection 146.
It will be understood that after each acoustic pulse is transmitted, a sufficient period of time is allowed before the next one so as to permit the reflect-ed pulses travelling over the transverse paths 109, ~n addition to those which may return over the angled path 113 to reach the crystal 119 of the transducer.
Also, it will be understood that a circuit connection 10~7~

151, which goes to an amplifier (not shown), will carry the electrical signals generated by the crystal 119 to such amplifier, from which they may go to an oscilloscope (not shown) or otherwise be used to develop oscillograph signals like those illustrated in Figure 2.
It will appreciated that there is an individual control and reflected-pulse amplifier circuit for each of the transducers 105. This is indicated ln Figure 13 where there are rectangles 154 and 155 which represent additional circuits like that described above in connect-ion with the crystal 119. There will, of course, be one such circuit for each of the transducers 105.
It will also be understood that the time elements involved in sending and receiving individual acoustic pulses and reflected return pulses, are relatively short as indicated above in connection with Figure 2. Con quently, a complete scan made be carried out around all of the transducers 105 rapidly enough to provide adequate testing of pipe wall conditions along a pipeline where a normal speed of travel of the instru~lent through the pipeline is maintained.

Method Steps A method according to this invention may be carried out by ~arious and different types of apparatus which are not necessarily equivalent to one another. The method relatesg in general~ to the field of pulse-echo measurement for testing of wall thicknesses, and the following steps should not be considered as limiting the invention nor are they necessarily always carried out in the order recited.

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A first step is that of transmitting a pulse of energy along a path which is transverse to the wall being measured. This is illustrated by the path 33 indicated in Figure 1. The pulse to be transmitted will be one generated by the transducer 2~ half of which is reflected from the surface 30 to change direction by 90 degrees and so travel toward the wall 21 in a direction transverse thereto.
Another step is that of receiving reflected pulses after reflection of the transmitted pulse from the surfaces of the wall being measured, in order to determine the thickness of the wall. mis is illustrated in Figure 1 by the paths 33 and 29 over which the return-ing reflected acoustic energy pulses will go ln travelling back to impinge upon the crystal 25. The crystal generates signals representative of suchrpulses. This is indic~ted in Figure 2 where the time difference between the return-ing or reflected pulses, e.g. pulses 41 and 42, will be a measure of the thickness of the wall 21.
Another step is that of transmitting a separate pul~e of energy along a path having a angle of incidence, relative to the wall being measured, that is greater than the critical angle of refraction for the material of that wall. This is carried out by the splitting of the acou~tic pulse which is transmitted from the transducer 25. The half that is thus split off is re-flected from the surface 45, and then travels over the path 47. This energy pulse will penetrate the wall 21 of the pipe, and if it encounters a discontinuity such as the cavity 51 illustrated, there will be a reflected pulse returned along the same path 47 and back along path ~;7~

46 to the transducer 25.
Then a final step ~s that of receiving the :~
other pulse reflection, if it appears~ ln order to characterize the presence of an anormaly. mis i8 lllustrated by the delayed reflected pulse e.g. pulse 57 of Figure 2, which returns along paths 47 and 46 to the transducer 25. The arrival will be at a time spaced from the first reflected pulses since the length of the paths of travel is greater.
The method may also be described in a more comprehensi~e manner which relates to the multiple transducer arrangement such as that illustrated in Figureæ lla 12 and 13. The steps of such method include the following.
First, the step of generating a plurality of single pulses sequentially, as described in connection with Figure 13. These are directed along paths that are parallel to the axis of the pipe. Thus, in the Figure 11 one of the transducers 105 is shown mounted on the body of an instrument 101. It is oriented so as to direct the pulses in a path parallel,to the axis of the pipe 102. Generation of each pulse may be carried out as described in connection with Figure 13. The triggering of an SCR 125 will create a discharge of the capacitor 128 across a p~th which includes the crystal 119. Consequently, an electrical pulse iæ applied, - `
which creates the piezoelectric af~ect so as to produce an acoustic pulse output.
Then there are the steps of reflecting a portion of each of the single pulses from a 45 degree surface to direct them along paths transverse to the . ~
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pipe wall. Figures 11 and 12 illustrate this step in that the pulse energy travelling down from transducers 105 is reflected by surface 106 and then goes in a direction transverse to the wall iO2 of the pipe, i.e. over the path 109.
Another step Is that of reflecting another portion o~ each single pulse from a surface more than 45 degree relative to said parallel path. This step is illustrated in Figures 11 and 12 by the path 113, which is the path of travel for that portion of the pulse travelling down from transducers 105 that is reflected from the sur-face 112.
Another step is that of receiving reflected pulses which are returned from the transverse directed energy, in order to determine the thickness of the pipe wall. mis is carried out by the conventional circuit controls for having the crystal 119 (of the transducer which has ~ust transmitted its single pulse) connected in a receiver circuit for amplifying generated electrical signals that are caused when the crystal is deformed by the reflected pulse energy returning to it. The earliest reflected pulses returning wlll be those over the trans-verse paths 109 and these provide an indication of the thickness of the pipe wall 102.
Finally there is the step of receiving any reflected pulses which return from the other portion of the split single pulse that was transmitted from the transducer in order to determine whether there is a discontinuity of the pipe of the type which will reflect and return some of this angular energy. This is carried out by maintaining the crystal circuit available for ampli~ying any reflected energy long enough to have a 1C)~7tj14 signal generated if the delayed returned reflected pulse exists.
While the foregoing preferred emboiments the invention have been described above in considerable detail, in accordance with the applieable statutes, this is not to be taken as in any way limiting the invention but merely as being descriptive thereof.

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Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a pulse-echo system for measuring wall thickness of pipes or the like, the improvement comprising in combination, means for generating a single pulse direc-ted along a path that is longitudinal re-lative to said pipe wall, means for splitting said single pulse to form first and second named pulses, said splitting means comprising first and second reflecting surfaces, said first reflecting surface having an angle relative to said longitudinal path such that said first named pulse is directed transversely relative to said pipe wall for making said wall thickness measurement, said second reflecting surface having an angle relative to said longitudinal path such that said second named pulse is directed at an angle of incidence relative to said wall that is greater than the critical angle of refraction in order to permit any reflected energy from a discontinuity in said wall to be returned along the same path, said second reflecting surface being non-contiguous with said first reflecting surface and having an offset along said longitudinal path for causing a time delay of said second pulse relative to said first pulse; and means for receiving any reflected pulses from both said first and second named pulses whereby said wall thickness may be measured and any distontiuity may be characterized.

2. In a pulse-echo system according to Claim 1 wherein said single pulse path direction is paral-lel to said wall, and said splitting means comprises means for reflecting a portion of said single pulse from a 45° angled surface, and means for reflecting another portion from a surface angled at more than 45° relative to said parallel path.

3. In a pulse-echo system for measuring wall thickness of a pipe according to Claim 2, wherein said improvement further comprises a plurality of said single pulse generating means located spaced peripherally about the axis of said pipe, and wherein said splitting means for re-flecting portions of said single pulses is annular and comprises said angled surfaces spaced axially at predetermined different distances from said pulses generated means.

4. In a pulse-echo system for measuring wall thickness of a pipe, the improvement comprising in combination a plurality of pulse generating means located spaced peripherally about the axis and inside of said pipe, said means being oriented for directing the pulses parallel to the axis of said pipe, a first annular reflecting surface having a 45° angle relative to said axis and spaced axially from said plurality of pulse genera-ting means, said first surface extending radially about half the width of said pulses for splitting off a portion of each and directing said portion along a path transverse to the wall of the pipe at that location, a second annular reflecting surface having an angle of more than 45° relative to said axis and being spaced axially at a greater distance from said pulse generating means than said first annular surface, said second surface being non-contiguous with said first surface and having an offset axially therefrom to provide said greater distance and to cause a time delay of the remaining portions of said pulses, and said second surface extending radially the other half of the width of said pulses for directing another portion of each pulse along a path having an angle of incidence relative to the wall of said pipe at that location which is greater than the critical angle of refraction in order to permit any reflected energy from a discontinuity in said wall to be returned along said other portion path.