ASTM E797-95

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: E 797 – 95 An American National Standard
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 material and one half the transit time (round trip) through the
material.
1.1 This practice provides guidelines for measuring the
thickness of materials using the contact pulse-echo method at Vt
T 5
temperatures not to exceed 200°F (93°C).
1.2 This practice is applicable to any material in which
where:
ultrasonic waves will propagate at a constant velocity through-
T 5 thickness,
out the part, and from which back reflections can be obtained
V 5 velocity, and
and resolved.
t 5 transit time.
1.3 The values stated in either inch-pound or SI units are to
4.2 The pulse-echo ultrasonic instrument measures the tran-
be regarded as the standard. The values given in parentheses
sit time of the ultrasonic pulse through the part.
are for information only.
4.3 The velocity in the material under test is a function of
1.4 This standard does not purport to address all of the
the physical properties of the material. It is usually assumed to
safety concerns, if any, associated with its use. It is the
be a constant for a given class of materials. Its approximate
responsibility of the user of this standard to establish appro-
value can be obtained from Table X3.1 in Practice E 494 or
priate safety and health practices and determine the applica-
from the Nondestructive Testing Handbook,oritcan be
bility of regulatory limitations prior to use.
determined empirically.
4.4 One or more reference blocks are required having
2. Referenced Documents
known velocity, or of the same material to be tested, and
2.1 ASTM Standards:
having thicknesses accurately measured and in the range of
E 317 Practice for Evaluating Performance Characteristics
thicknesses to be measured. It is generally desirable that the
of Ultrasonic Pulse-Echo Testing Systems Without the Use
thicknesses be “round numbers” rather than miscellaneous odd
of Electronic Measurement Instruments
values. One block should have a thickness value near the
E 494 Practice for Measuring Ultrasonic Velocity in Mate-
maximum of the range of interest and another block near the
rials
minimum thickness.
E 1316 Terminology for Nondestructive Examinations
4.5 The display element (CRT (cathode ray tube), meter, or
2.2 ASNT Document:
digital display) of the instrument must be adjusted to present
Nondestructive Testing Handbook, 2nd Edition, Vol 7
convenient values of thickness dependent on the range being
used. The control for this function may have different names on
3. Terminology
different instruments, including range, sweep, material cali-
3.1 Definitions—For definitions of terms used in this
brate,or velocity.
practice, refer to Terminology E 1316.
4.6 The timing circuits in different instruments use various
conversion schemes. A common method is the so-called
4. Summary of Practice
time/analog conversion in which the time measured by the
4.1 Thickness (T), when measured by the pulse-echo ultra-
instrument is converted into a proportional dc voltage which is
sonic method, is a product of the velocity of sound in the
then applied to the readout device. Another technique uses a
very high-frequency oscillator that is modulated or gated by the
This practice is under the jurisdiction of ASTM Committee E-7 on Nonde-
appropriate echo indications, the output being used either
structive Testing and is the direct responsibility of Subcommittee E07.06 on
directly to suitable digital readouts or converted to a voltage for
Ultrasonic Testing Procedure.
Current edition approved Dec. 10, 1995. Published February 1996. Originally other presentation. A relationship of transit time versus thick-
published as E 797 – 81. Last previous edition E 797 – 94.
ness is shown graphically in Fig. 1.
For ASME Boiler and Pressure Vessel Code applications, see related Practice
SE-797 in Section II of that Code.
5. Significance and Use
Annual Book of ASTM Standards, Vol 03.03.
Available from the American Society for Nondestructive Testing, 1711 Arlin-
5.1 The techniques described provide indirect measurement
gate Plaza, Columbus, OH 43228.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 797
NOTE 1—Slope of velocity conversion line is approximately that of steel.
FIG. 1 Transit Time/Thickness Relationship
of thickness of sections of materials not exceeding tempera- 6.1.2 Flaw detectors with numeric readout are a combina-
tures of 200°F (93°C). Measurements are made from one side tion pulse ultrasound flaw detection instrument with a CRT and
of the object, without requiring access to the rear surface. additional circuitry that provides digital thickness information.
5.2 Ultrasonic thickness measurements are used extensively The material thickness can be electronically measured and
on basic shapes and products of many materials, on precision
presented on a digital readout. The CRT provides a check on
machined parts, and to determine wall thinning in process the validity of the electronic measurement by revealing mea-
equipment caused by corrosion and erosion.
surement variables, such as internal discontinuities, or echo-
5.3 Recommendations for determining the capabilities and strength variations, which might result in inaccurate readings.
limitations of ultrasonic thickness gages for specific applica-
6.1.3 Thickness readout instruments are modified versions
,
5 6
tions can be found in the cited references.
of the pulse-echo instrument. The elapsed time between the
initial pulse and the first echo or between multiple echoes is
6. Apparatus
converted into a meter or digital readout. The instruments are
6.1 Instruments—Thickness-measurement instruments are
designed for measurement and direct numerical readout of
divided into three groups: (1) Flaw detectors with CRT readout,
specific ranges of thickness and materials.
(2) Flaw detectors with CRT and direct thickness readout, and
6.2 Search Units—Most pulse-echo type search units
(3) Direct thickness readout.
(straight-beam contact, delay line, and dual element) are
6.1.1 Flaw detectors with CRT readouts display time/
applicable if flaw detector instruments are used. If a thickness
amplitude information in an A-scan presentation. Thickness
readout instrument has the capability to read thin sections, a
determinations are made by reading the distance between the
highly damped, high-frequency search unit is generally used.
zero-corrected initial pulse and first-returned echo (back reflec-
High-frequency (10 MHz or higher) delay line search units are
tion), or between multiple-back reflection echoes, on a cali-
generally required for thicknesses less than about 0.6 mm
brated base line of a CRT. The base line of the CRT should be
(0.025 in.). Measurements of materials at high temperatures
adjusted for the desired thickness increments.
require search units specially designed for the application.
When dual element search units are used, their inherent
5 nonlinearity usually requires special corrections for thin sec-
Bosselaar, H., and Goosens, J.C.J., “Method to Evaluate Direct-Reading
Ultrasonic Pulse-Echo Thickness Meters,” Materials Evaluation, March 1971, pp. tions. (See Fig. 2.) For optimum performance, it is often
45–50.
necessary that the instrument and search units be matched.
Fowler, K.A., Elfbaum, G.M., Husarek, V., and Castel, J., “Applications of
6.3 Calibration Blocks—The general requirements for ap-
Precision Ultrasonic Thickness Gaging,” Proceedings of the Eighth World Confer-
ence on Nondestructive Testing, Cannes, France, Sept. 6–11, 1976, Paper 3F.5. propriate calibration blocks are given in 4.4, 7.1.3, 7.2.2.1,
E 797
(a) Proportional sound path increases with decrease in thickness.
(b) Typical reading error values
FIG. 2 Dual Transducer Nonlinearity
7.3.2, and 7.4.3. Multi-step blocks that may be useful for these 7.1.3 Place the search unit on a test block of known
calibration procedures are described in Appendix X1 (Figs. thickness with suitable couplant and adjust the instrument
X1.1 and X1.2).
controls (material calibrate, range, sweep, or velocity) until the
display presents the appropriate thickness reading.
7. Procedure—Calibration and Adjustment of Apparatus
7.1.4 The readings should then be checked and adjusted on
7.1 Case I—Direct Contact, Single-Element Search Unit:
test blocks with thickness of lesser value to improve the overall
7.1.1 Conditions—The display start is synchronized to the
accuracy of the system.
initial pulse. All display elements are linear. Full thickness is
7.2 Case II—Delay Line Single-Element Search Unit:
displayed on CRT.
7.2.1 Conditions—When using this search unit, it is neces-
7.1.2 Under these conditions, we can assume that the
velocity conversion line effectively pivots about the origin sary that the equipment be capable of correcting for the time
(Fig. 1). It may be necessary to subtract the wear-plate time, during which the sound passes through the delay line so that
requiring minor use of delay control. It is recommended that the end of the delay can be made to coincide with zero
test blocks providing a minimum of two thicknesses that span thickness. This requires a so-called “delay” control in the
the thickness range be used to check the full-range accuracy. instrument or automatic electronic sensing of zero thickness.
E 797
7.2.2 In most instruments, if the material calibrate 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 calibrated 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-
zation are used. The display start is delayed as described in
7.2.2.1 Use at least two test blocks. One should have a
7.4.4. All display elements should be linear. Incremental
thickness near the maximum of the range to be measured and
thickness is displayed on the CRT.
the other block near the minimum thickness. For convenience,
7.4.3 Basic calibration of the sweep will be made as
it is desirable that the thickness should be “round numbers” so
described in Case I. The test block chosen for this calibration
that the difference between them also has a convenient “round
should have a thickness that will permit calibrating the
number” value.
full-sweep distance to adequate accuracy, that is, about 10 mm
7.2.2.2 Place the search unit sequentially on one and then
(0.4 in.) or 25 mm (1.0 in.) full scale.
the other block, and obtain both readings. The difference
7.4.4 After basic calibration, the sweep must be delayed.
between these two readings should be calculated. If the reading
For instance, if the nominal part thickness is expected to be
thickness difference is less than the actual thickness difference,
from 50 to 60 mm (2.0 to 2.4 in.), and the basic calibration
place the search unit on the thicker specimen, and adjust the
block is 10 mm (0.4 in.), and the incremental thickness
material calibrate control to expand the thickness range. If the
displayed will also be from 50 to 60 mm (2.0 to 2.4 in.), the
reading thickness difference is greater than the actual thickness
following steps are required. Adjust the delay control so that
difference, place the search unit on the thicker specimen, and
the fifth back echo of the basic calibration block, equivalent to
adjust the material calibrate control to decrease the thickness
50 mm (2.0 in.), is aligned with the 0 reference on the CRT.
range. A certain amount of over correction is usually recom-
The sixth back echo should then occur at the right edge of the
mended. Reposition the search unit sequentially on both
calibrated sweep.
blocks, and note the reading differences while making addi-
7.4.5 This calibration can be checked on a known block of
tional appropriate corrections. When the reading thickness
the approximate total thickness.
differential equals the actual thickness differential, the material
7.4.6 The reading obtained on the unknown specimen must
thickness range is correctly adjusted. A single adjustment of the
be added to the value delayed off screen. For example, if the
delay control should then permit correct readings at both the
reading is 4 mm (0.16 in.), the total thickness will be 54 mm
high and low end of the thickness range.
(2.16 in.).
7.2.3 An alternative technique for delay line search units is
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
8.1 Dual search units may also be used effectively with
correct readings on the thinner test block and the “range”
rough surface conditions. In this case, only the first returned
control to correct the readings on the thicker block. Moderate
echo, such as from the bottom of a pit, is used in the
over-correction is sometimes useful. When both readings are
measurement. Generally, a localized scanning search is made
“correct” the instrument is adjusted properly.
to detect the minimum remaining wall.
7.3 Case III—Dual Search Units:
8.2 Material Properties—The instrument should be cali-
7.3.1 The method described in 7.2 (Case II) is also suitable
brated on a material having the same acoustic velocity and
for equipment using dual search units in the thicker ranges,
attenuation as the material to be measured. Where possible,
above 3 mm (0.125 in.). However, below those values there is
calibration should be confirmed by direct dimensional mea-
an inherent error due to the Vee path that the sound beam
surement of the material to be examined.
travels. The transit time is no longer linearly proportional to
8.3 Scanning—The maximum speed of scanning should be
thickness, and the condition deteriorates toward the low
stated in the procedure. Material conditions, type of equipment,
thickness end of the range. The variation is also shown
and operator capabilities may require slower scanning.
schematically in Fig. 2(a). Typical error valu
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