ASTM E797-95(2001)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:E797–95 (Reapproved 2001)
Standard Practice for
Measuring Thickness by Manual Ultrasonic Pulse-Echo
Contact Method
This standard is issued under the fixed designation E 797; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 4. Summary of Practice
1.1 This practice provides guidelines for measuring the 4.1 Thickness (T), when measured by the pulse-echo ultra-
thickness of materials using the contact pulse-echo method at sonic method, is a product of the velocity of sound in the
temperatures not to exceed 200°F (93°C). material and one half the transit time (round trip) through the
1.2 This practice is applicable to any material in which material.
ultrasonic waves will propagate at a constant velocity through-
Vt
T 5
out the part, and from which back reflections can be obtained
and resolved.
where:
1.3 The values stated in either inch-pound or SI units are to
T = thickness,
be regarded as the standard. The values given in parentheses
V = velocity, and
are for information only.
t = transit time.
1.4 This standard does not purport to address all of the
4.2 The pulse-echo ultrasonic instrument measures the tran-
safety concerns, if any, associated with its use. It is the
sit time of the ultrasonic pulse through the part.
responsibility of the user of this standard to establish appro-
4.3 The velocity in the material being examined is a
priate safety and health practices and determine the applica-
function of the physical properties of the material. It is usually
bility of regulatory limitations prior to use.
assumed to be a constant for a given class of materials. Its
approximate value can be obtained fromTable X3.1 in Practice
2. Referenced Documents
E 494 or from the Nondestructive Testing Handbook,oritcan
2.1 ASTM Standards:
be determined empirically.
E 317 Practice for Evaluating Performance Characteristics
4.4 One or more reference blocks are required having
of Ultrasonic Pulse-Echo Examination Systems Without
3 known velocity, or of the same material to be examined, and
the Use of Electronic Measurement Instruments
having thicknesses accurately measured and in the range of
E 494 Practice for Measuring Ultrasonic Velocity in Mate-
3 thicknesses to be measured. It is generally desirable that the
rials
thicknesses be “round numbers” rather than miscellaneous odd
E 1316 Terminology for Nondestructive Examinations
values. One block should have a thickness value near the
2.2 ASNT Document:
maximum of the range of interest and another block near the
Nondestructive Testing Handbook, 2nd Edition, Vol 7
minimum thickness.
3. Terminology
4.5 The display element (CRT (cathode ray tube), meter, or
digital display) of the instrument must be adjusted to present
3.1 Definitions—For definitions of terms used in this
convenient values of thickness dependent on the range being
practice, refer to Terminology E 1316.
used.Thecontrolforthisfunctionmayhavedifferentnameson
different instruments, including range, sweep, material stan-
This practice is under the jurisdiction of ASTM Committee E07 on Nonde-
dardize,or velocity.
structive Testing and is the direct responsibility of Subcommittee E07.06 on
4.6 The timing circuits in different instruments use various
Ultrasonic Testing Procedure.
conversion schemes. A common method is the so-called
Current edition approved Dec. 10, 1995. Published February 1996. Originally
time/analog conversion in which the time measured by the
published as E 797 – 81. Last previous edition E 797 – 94.
For ASME Boiler and Pressure Vessel Code applications, see related Practice
instrument is converted into a proportional dc voltage which is
SE-797 in Section II of that Code.
then applied to the readout device. Another technique uses a
Annual Book of ASTM Standards, Vol 03.03.
veryhigh-frequencyoscillatorthatismodulatedorgatedbythe
Available from the American Society for Nondestructive Testing, 1711 Arlin-
gate Plaza, Columbus, OH 43228. appropriate echo indications, the output being used either
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E797–95 (2001)
NOTE 1—Slope of velocity conversion line is approximately that of steel.
FIG. 1 Transit Time/Thickness Relationship
directlytosuitabledigitalreadoutsorconvertedtoavoltagefor 6.1.1 Flaw detectors with CRT readouts display time/
other presentation. A relationship of transit time versus thick- amplitude information in an A-scan presentation. Thickness
ness is shown graphically in Fig. 1. determinations are made by reading the distance between the
zero-corrected initial pulse and first-returned echo (back reflec-
5. Significance and Use
tion), or between multiple-back reflection echoes, on a stan-
dardized base line of a CRT. The base line of the CRT should
5.1 The techniques described provide indirect measurement
be adjusted for the desired thickness increments.
of thickness of sections of materials not exceeding tempera-
tures of 200°F (93°C). Measurements are made from one side 6.1.2 Flaw detectors with numeric readout are a combina-
tionpulseultrasoundflawdetectioninstrumentwithaCRTand
of the object, without requiring access to the rear surface.
5.2 Ultrasonic thickness measurements are used extensively additional circuitry that provides digital thickness information.
The material thickness can be electronically measured and
on basic shapes and products of many materials, on precision
machined parts, and to determine wall thinning in process presented on a digital readout. The CRT provides a check on
equipment caused by corrosion and erosion. the validity of the electronic measurement by revealing mea-
5.3 Recommendations for determining the capabilities and surement variables, such as internal discontinuities, or echo-
limitations of ultrasonic thickness gages for specific applica- strength variations, which might result in inaccurate readings.
5,6
tions can be found in the cited references.
6.1.3 Thickness readout instruments are modified versions
of the pulse-echo instrument. The elapsed time between the
6. Apparatus
initial pulse and the first echo or between multiple echoes is
6.1 Instruments—Thickness-measurement instruments are converted into a meter or digital readout. The instruments are
dividedintothreegroups:(1)FlawdetectorswithCRTreadout, designed for measurement and direct numerical readout of
(2) Flaw detectors with CRT and direct thickness readout, and specific ranges of thickness and materials.
(3) Direct thickness readout.
6.2 Search Units—Most pulse-echo type search units
(straight-beam contact, delay line, and dual element) are
applicable if flaw detector instruments are used. If a thickness
readout instrument has the capability to read thin sections, a
Bosselaar, H., and Goosens, J.C.J., “Method to Evaluate Direct-Reading
Ultrasonic Pulse-Echo Thickness Meters,” Materials Evaluation, March 1971, pp.
highly damped, high-frequency search unit is generally used.
45–50.
High-frequency (10 MHz or higher) delay line search units are
Fowler, K.A., Elfbaum, G.M., Husarek, V., and Castel, J., “Applications of
generally required for thicknesses less than about 0.6 mm
Precision Ultrasonic Thickness Gaging,” Proceedings of the Eighth World Confer-
ence on Nondestructive Testing, Cannes, France, Sept. 6–11, 1976, Paper 3F.5. (0.025 in.). Measurements of materials at high temperatures
E797–95 (2001)
(a) Proportional sound path increases with decrease in thickness.
(b) Typical reading error values
FIG. 2 Dual Transducer Nonlinearity
require search units specially designed for the application. 7. Standardization of Apparatus
When dual element search units are used, their inherent
7.1 Case I—Direct Contact, Single-Element Search Unit:
nonlinearity usually requires special corrections for thin sec-
7.1.1 Conditions—The display start is synchronized to the
tions. (See Fig. 2.) For optimum performance, it is often
initial pulse. All display elements are linear. Full thickness is
necessary that the instrument and search units be matched.
displayed on CRT.
6.3 Standardization Blocks—The general requirements for
appropriate standardization blocks are given in 4.4, 7.1.3, 7.1.2 Under these conditions, we can assume that the
velocity conversion line effectively pivots about the origin
7.2.2.1, 7.3.2, and 7.4.3. Multi-step blocks that may be useful
for these standardization procedures are described inAppendix (Fig. 1). It may be necessary to subtract the wear-plate time,
X1 (Figs. X1.1 and X1.2). requiring minor use of delay control. It is recommended that
E797–95 (2001)
standardization blocks providing a minimum of two thick- an inherent error due to the Vee path that the sound beam
nesses that span the thickness range be used to check the travels. The transit time is no longer linearly proportional to
full-range accuracy. thickness, and the condition deteriorates toward the low
thickness end of the range. The variation is also shown
7.1.3 Place the search unit on a standardization block of
schematically in Fig. 2(a). Typical error values are shown in
known thickness with suitable couplant and adjust the instru-
Fig. 2(b).
ment controls (material standardization, range, sweep, or
7.3.2 If measurements are to be made over a very limited
velocity) until the display presents the appropriate thickness
range near the thin end of the scale, it is possible to standardize
reading.
the instrument with the technique in Case II using appropriate
7.1.4 The readings should then be checked and adjusted on
thin standardization blocks. This will produce a correction
standardization blocks with thickness of lesser value to im-
curve that is approximately correct over that limited range.
prove the overall accuracy of the system.
Note that it will be substantially in error at thicker measure-
7.2 Case II—Delay Line Single-Element Search Unit:
ments.
7.2.1 Conditions—When using this search unit, it is neces-
7.3.3 If a wide range of thicknesses is to be measured, it
sary that the equipment be capable of correcting for the time
may be more suitable to standardize as in Case II using
during which the sound passes through the delay line so that
standardization blocks at the high end of the range and perhaps
the end of the delay can be made to coincide with zero
halfway toward the low end. Following this, empirical correc-
thickness. This requires a so-called “delay” control in the
tions can be established for the very thin end of the range.
instrument or automatic electronic sensing of zero thickness.
7.3.4 For a direct-reading panel-type meter display, it is
7.2.2 In most instruments, if the material standardize circuit
convenient to build these corrections into the display as a
was previously adjusted for a given material velocity, the delay
nonlinear function.
control should be adjusted until a correct thickness reading is
7.4 Case IV—Thick Sections:
obtained on the instrument. However, if the instrument must be
7.4.1 Conditions—For use when a high degree of accuracy
completely standardized with the delay line search unit, the
is required for thick sections.
following technique is recommended:
7.4.2 Direct contact search unit and initial pulse synchroni-
7.2.2.1 Use at least two standardization blocks. One should
zation are used. The display start is delayed as described in
have a thickness near the maximum of the range to be
7.4.4. All display elements should be linear. Incremental
measured and the other block near the minimum thickness. For
thickness is displayed on the CRT.
convenience, it is desirable that the thickness should be “round
7.4.3 Basic standardization of the sweep will be made as
numbers” so that the difference between them also has a
described in Case I. The standardization block chosen for this
convenient “round number” value.
standardization should have a thickness that will permit stan-
7.2.2.2 Place the search unit sequentially on one and then
dardizing the full-sweep distance to adequate accuracy, that is,
the other block, and obtain both readings. The difference
about 10 mm (0.4 in.) or 25 mm (1.0 in.) full scale.
between these two readings should be calculated. If the reading
7.4.4 After basic standardization, the sweep must be de-
thickness difference is less than the actual thickness difference,
layed. For instance, if the nominal part thickness is expected to
place the search unit on the thicker specimen, and adjust the
be from 50 to 60 mm (2.0 to 2.4 in.), and the basic standard-
material standardize control to expand the thickness range. If
ization block is 10 mm (0.4 in.), and the incremental thickness
the reading thickness difference is greater than the actual
displayed will also be from 50 to 60 mm (2.0 to 2.4 in.), the
thickness difference, place the search unit on the thicker
following steps are required. Adjust the delay control so that
specimen, and adjust the material standardize control to de-
the fifth back echo of the basic standardization block, equiva-
crease the thickness range.Acertain amount of over correction
lent to 50 mm (2.0 in.), is aligned with the 0 reference on the
is usually recommended. Reposition the search unit sequen-
CRT. The sixth back echo should then occur at the right edge
tially on both blocks, and note the reading differences while
of the standardized sweep.
making additional appropriate corrections. When the reading
7.4.5 Thisstandardizationcanbecheckedonaknownblock
thickness differential equals the actual thickness differential,
of the approximate total thickness.
the material thickness range is correctly adjusted. A single
7.4.6 The reading obtained on the unknown specimen must
adjustment of the delay control should then permit correct
be added to the value delayed off screen. For example, if the
readings at both the high and low end of the thickness range.
reading is 4 mm (0.16 in.), the total thickness will be 54 mm
7.2.3 An alternative technique for delay line search units is
(2.16 in.).
a variation of that described in 7.2.2. A series of sequential
8. Technical Hazards
adjustments are made, using the “delay” control to provide
correct readings on the thinner standardization block and the
8.1 Dual search units may also be used effectively with
“range” control to correct the readings on the thicker block.
rough surface conditions. In this case, only the first returned
Moderate over-correction is sometimes useful. When both
echo, su
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