How a D1.1 Ultrasonic Indication Gets Rated, Classed and Judged
D1.1 rates every ultrasonic indication in decibels rather than by flaw size. The indication rating d equals the indication level a minus the reference level b minus the attenuation factor c, where c is the sound path in inches minus one, doubled. That rating, the probe angle and the weld thickness select a severity class from Table 8.2 or 8.3, and class plus indication length decides acceptance.
Clause 8 of D1.1 does not size flaws. It compares the echo from a discontinuity against a calibration reference and turns the difference into a single number in decibels, then reads that number against weld thickness and probe angle to assign one of four severity classes. Class A is rejected at any length. Class D is accepted at any length. Classes B and C are length-limited, and the limits tighten when the connection is cyclically loaded. The whole system therefore stands on three things being right: the reference level set on the block, the sound path used to compute the attenuation factor, and the correct table for the loading condition. Get the attenuation factor wrong by two decibels and a Class C indication becomes Class B, or a Class B becomes Class A. Nothing downstream catches that error, because the report records only the arithmetic.
Source: Clause text and table values read from AWS D1.1:2000, Section 6 Parts D and F (6.14.6, 6.15, 6.20, 6.21, 6.22, 6.23, 6.25, 6.26, 6.31, 6.32 and Tables 6.2, 6.3 and 6.6), published in full via Public.Resource.Org. Current edition confirmed on pubs.aws.org in August 2026 as AWS D1.1/D1.1M:2025, approved March 2025, comprising Clauses 1 through 11 with eight normative and eleven informative annexes; inspection is Clause 8 and the ultrasonic acceptance tables carry 8.2 and 8.3 designators. Read the dB values off the table in your contract edition before using them to accept or reject.
| Term | What it is | How it is obtained | Where it goes wrong |
|---|---|---|---|
| a, indication level | Instrument reading in dB when the maximum attainable indication from the discontinuity is brought to the horizontal reference line | Adjusted with the calibrated gain control only; the reject, clipping or suppression control stays off throughout | Peaking on the wrong node, or leaving scanning sensitivity in the instrument, inflates a and softens the verdict |
| b, reference level | Zero reference level sensitivity: the gain that puts a maximised trace at reference height using the IIW block | Set by the operator just before testing, at the location of each weld | A corner reflector may not be used for calibration; other portable blocks are allowed only when equivalence to the IIW block is established |
| c, attenuation factor | Correction for the distance the sound travelled | Subtract 1 in. (25 mm) from the sound path, multiply the remainder by 2, round to the nearest dB | Fractions under 1/2 dB round down and 1/2 or more round up; using material thickness in place of sound path is the standard error |
| d, indication rating | a minus b minus c on instruments reading gain in dB; b minus a minus c on instruments reading attenuation | Entered on the ultrasonic test report form | A larger positive d is a weaker reflector; a sign error flips accept and reject |
| Severity class | Class A, B, C or D from Table 8.2 for statically loaded or Table 8.3 for cyclically loaded nontubular connections | Enter the table with d, the weld thickness band and the search unit angle | The same rating can be Class C statically and Class A cyclically; Class A is rejected at any length |
| Length limit | Class A rejected regardless of length; Class B over 3/4 in. (20 mm); Class C over 2 in. (50 mm); Class D accepted regardless of length or location | Length measured by the flaw size evaluation procedure in the same clause | On the cyclically loaded table, Class C drops to 3/4 in. (20 mm) in the top or bottom quarter of weld thickness |
| Scanning level | Sensitivity above zero reference used to search, keyed to sound path | Statically loaded: 14 dB through 2-1/2 in.; 19 dB over 2-1/2 through 5 in.; 29 dB over 5 through 10 in.; 39 dB over 10 through 15 in. | The column is sound path, not material thickness; the cyclically loaded values are 20, 25, 35 and 45 dB |
What the ultrasonic clause is scoped to, and what it excludes
The ultrasonic procedures and standards govern groove welds and their heat-affected zones between 5/16 in. (8 mm) and 8 in. (200 mm) inclusive. Anything thinner or thicker goes to the alternative annex, which is written as an example of an alternative technique rather than as a second set of mandatory rules. The clause states plainly that these procedures are not to be used for testing tube-to-tube T, Y and K connections, which run on separate Class R and Class X criteria with their own figures.
Variations are permitted, and the mechanism matters more than most contractors realise. Variations in testing procedure, equipment and acceptance standards not included in the clause may be used upon agreement with the Engineer, covering other thicknesses, weld geometries, transducer sizes, frequencies, couplants, painted surfaces and testing techniques. Every approved variation shall be recorded in the contract records. An undocumented deviation is not a variation; it is a nonconformance waiting to be found.
Two limitations are worth writing into the procedure. To detect possible piping porosity, radiography is suggested to supplement ultrasonic testing of electroslag and electrogas welds, because the amplitude method reads elongated gas channels poorly. And these procedures are not intended for procurement testing of base metal, though welding-related discontinuities in adjacent base metal, including cracking, lamellar tearing and delamination, are reported to the Engineer for disposition. The wider method comparison sits on RT versus UT for weld inspection.
Zero reference level: the sensitivity everything else is measured from
The zero reference level is attained by adjusting the calibrated gain control so that a maximised horizontal trace deflection, brought to reference line height, results on the display. That number becomes b on the report. Because every later decision is a difference from b, an error there moves every indication rating on the weld by the same amount, in the same direction, invisibly.
The equipment requirements exist to make b repeatable. The instrument is pulse echo, suitable for transducers oscillating between 1 and 6 MHz, with an A-scan rectified video trace. The calibrated gain control or attenuator is adjustable in discrete 1 or 2 dB steps over a range of at least 60 dB, with attenuator accuracy within plus or minus 1 dB. The display's dynamic range must let a 1 dB amplitude difference be detected easily. Internal stabilisation must hold response within 1 dB across a 15% supply voltage change.
The International Institute of Welding block is the standard used for both distance and sensitivity calibration. Other portable blocks may be used provided the reference level sensitivity for the instrument and search unit combination is adjusted to be equivalent to what the IIW block gives. The use of a corner reflector for calibration purposes is prohibited outright. Angle-beam search units must produce a beam within plus or minus 2 degrees of 70, 60 or 45 degrees, with the distance from the leading edge of the unit to the index point not exceeding 1 in. (25 mm).
Scanning level: how far you turn it up to find anything
Scanning happens above reference sensitivity, and the increase is prescribed by sound path rather than by thickness. For statically loaded nontubular connections the increases above zero reference are 14 dB through 2-1/2 in. (65 mm) of sound path, 19 dB over 2-1/2 through 5 in. (65 to 125 mm), 29 dB over 5 through 10 in. (125 to 250 mm), and 39 dB over 10 through 15 in. (250 to 380 mm). Each table footnote says the same thing twice over: this column refers to sound path distance, not material thickness.
The cyclically loaded table raises every one of those figures. Scanning levels become 20, 25, 35 and 45 dB across the same four sound path bands. A cyclically loaded connection is therefore searched harder and judged harder, which is the intended behaviour for a member that will see stress reversals in service.
Once calibrated, the only instrument adjustment permitted during testing is the sensitivity level adjustment with the calibrated gain control. The reject, clipping or suppression control shall be turned off. That prohibition exists because suppression is non-linear on many instruments: it removes low-amplitude grass and, with it, the linearity that the whole decibel comparison depends on. An operator who leaves suppression engaged has invalidated every rating on the report.
The indication rating, worked through end to end
When a discontinuity indication appears, the maximum attainable indication from it is adjusted to produce a horizontal reference level trace deflection. That adjustment is made with the calibrated gain control, and the instrument reading in decibels becomes the indication level a. The attenuation factor c is then computed from the sound path. The indication rating d follows: a minus b minus c on instruments reading gain in dB, or b minus a minus c on instruments reading attenuation in dB.
Take a concrete case. Reference level b is set at 45 dB. An indication is peaked and brought to reference height at 54 dB, so a equals 54. The sound path measures 3.5 in., so c equals 3.5 minus 1, doubled, which is 5 dB. The indication rating d is 54 minus 45 minus 5, or plus 4. The weld is 2 in. thick and the search unit is 60 degrees.
On the statically loaded table, the 60 degree column in the band over 1-1/2 in. through 2-1/2 in. puts plus 4 in Class C, which is accepted unless the indication exceeds 2 in. (50 mm) in length. On the cyclically loaded table, the same column shows Class A at plus 7 and lower, so plus 4 is Class A and the indication is rejected regardless of length. One number, two tables, opposite outcomes. This is the single most common source of dispute on structural jobs, and it is why the loading condition belongs on the report header, not in the inspector's memory.
The attenuation factor is the term most often computed wrong
The rule is short: subtract 1 in. (25 mm) from the sound path distance and multiply the remainder by 2. The factor is then rounded to the nearest dB value, with fractional values less than 1/2 dB reduced to the lower dB level and those of 1/2 or greater increased to the higher level. Two decimal places of sound path therefore collapse into a single integer, and the rounding direction is specified so that two technicians reading the same weld reach the same integer.
The failure mode is substituting depth or material thickness for sound path. On a 45 degree probe, a reflector 2 in. below the surface sits at roughly 2.8 in. of sound path, which is a 1.6 dB difference in c and enough to move a rating across a class boundary on several thickness bands. The 1 in. subtraction represents the near-field and wedge losses already accounted for in the calibration, which is why it is a constant and not a percentage.
Because c grows with distance, the same physical reflector produces a different rating on a first-leg and a second-leg shot. That is intentional, and it is also why the sound path must be recorded alongside the rating. A report showing a, b and d without the sound path cannot be checked by anyone, which makes it undefensible in exactly the way described on what makes an NDT report defensible.
Reading the acceptance tables, and the rules printed underneath them
Both tables are entered on three axes: the indication rating, the weld thickness band and the search unit angle. The thickness bands run 5/16 in. through 3/4 in.; over 3/4 in. through 1-1/2 in.; over 1-1/2 in. through 2-1/2 in.; over 2-1/2 in. through 4 in.; and over 4 in. through 8 in. The first two bands carry only a 70 degree column; the wider bands carry 70, 60 and 45 degree columns. Weld thickness for entry is the nominal thickness of the thinner of the two parts being joined.
The class definitions do the work. Class A, large discontinuities, are rejected regardless of length. Class B, medium, are rejected above 3/4 in. (20 mm) of length. Class C, small, are rejected above 2 in. (50 mm) on the statically loaded table; on the cyclically loaded table the limit is 2 in. in the middle half of the weld thickness but only 3/4 in. (20 mm) in the top or bottom quarter. Class D, minor, are accepted regardless of length or location in the weld.
Four notes sit under both tables and are as binding as the numbers. Class B and C discontinuities shall be separated by at least 2L, L being the length of the longer discontinuity; where they are not, but the combined length plus separation stays within the allowable length for that class, they count as a single acceptable discontinuity. Class B and C discontinuities shall not begin closer than 2L from weld ends carrying primary tensile stress. Discontinuities found at scanning level in the root face area of complete joint penetration double groove welds designated as tension welds are evaluated 4 dB more sensitively, meaning 4 dB is subtracted from d. And on electroslag or electrogas welds, anything over 2 in. (50 mm) long found at scanning level is suspected as piping porosity and evaluated further with radiography.
Scanning patterns, testing angles and coverage
All butt joint welds are tested from each side of the weld axis. Corner and T-joint welds are primarily tested from one side of the weld axis only. The stated intent is that, as a minimum, all welds be tested by passing sound through the entire volume of the weld and the heat-affected zone in two crossing directions wherever practical. Before any of that, the entire base metal through which the sound must travel is scanned for laminar reflectors with a straight-beam unit, sensitivity set so the first back reflection sits at 50 to 75% of full screen height.
For longitudinal discontinuities the pattern combines three movements: rotation of 10 degrees, a scanning distance that covers the section of weld being tested, and a progression of approximately one-half the transducer width per pass. That progression figure is the coverage guarantee. Increase it and you have gaps; the report will not show them.
Transverse discontinuities take different patterns. One pattern is used when welds are ground flush; another, with a scanning angle of 15 degrees maximum, is used when the weld reinforcement is not ground flush. Electroslag and electrogas welds get an additional pattern with search unit rotation between 45 and 60 degrees. The testing angle itself is not free choice: a procedure chart assigns the angle and the pattern by weld type and material thickness. Where base metal laminar content blocks access, the clause allows grinding the weld surface flush, testing from both faces, or using other search unit angles to attain full coverage. Building all of this into an approved written procedure is the work covered by NDT technical procedure development.
Who is allowed to run the test, and what they must demonstrate
Personnel performing nondestructive testing other than visual shall be qualified in accordance with the current edition of ASNT Recommended Practice No. SNT-TC-1A. Only individuals qualified for NDT Level I working under an NDT Level II, or individuals qualified for NDT Level II, may perform the testing. Certification of Level I and Level II individuals is performed by a Level III who has been certified by ASNT, or who has the education, training and experience and has successfully passed the written examination prescribed in SNT-TC-1A.
There is an additional hurdle specific to ultrasonics. Qualification of the ultrasonic testing operator shall include a specific and practical examination based on the requirements of the code, requiring the operator to demonstrate the ability to apply those rules in the accurate detection and disposition of flaws. A generic Level II certificate does not satisfy this on its own; the practical must be against this code's rules.
One exemption often gets misread in the other direction: personnel performing nondestructive tests under this clause need not be qualified and certified under AWS QC1. A certified welding inspector credential is not a substitute for method certification, and method certification is not a substitute for the code-specific practical. The interaction between the two schemes is set out on the SNT-TC-1A compliance page, and the training route to Level II ultrasonics with the required hours is covered under NDT training and in training hours requirements by method.
Where D1.1 acceptance parts company with ASME
ASME construction codes judge an ultrasonic indication by what it is and how long it is. Taking the power piping wording as the verified example, discontinuities evaluated as cracks, lack of fusion or incomplete penetration are unacceptable regardless of length, and all other discontinuities are unacceptable when the indication exceeds the reference level and the length exceeds 1/4 in. for thickness up to 3/4 in., one third of thickness from 3/4 in. to 2-1/4 in., and 3/4 in. above that. Amplitude enters as a single threshold, not as a scale.
D1.1 inverts the emphasis. Amplitude relative to reference, corrected for sound path, is the primary variable, and it is graded across four classes rather than crossed once. Flaw type does not appear in the acceptance tables at all. A planar crack and a rounded inclusion producing the same rating at the same thickness and angle receive the same class, which is why the code carries an explicit warning that users should not consider each method capable of detecting all injurious defects, particularly planar defects at unfavourable orientations.
The reporting philosophy differs too. Only rejectable discontinuities need be recorded, except on welds designated Fracture Critical, where acceptable ratings within 6 dB of the minimum rejectable rating must also appear. ASME codes push the other way, toward recorded data and third-party reanalysis, as covered on the Section V Article 4 page. Fabricators working to both on one project need two procedures, two report forms and one Level III who can defend both; that is the scope of ASNT Level III consulting.
What does the d rating mean in AWS D1.1 ultrasonic testing?
The indication rating d is the indication level a minus the reference level b minus the attenuation factor c, on instruments reading gain in decibels. It expresses how strong the reflector is against the calibration reference, corrected for the distance the sound travelled. A more negative rating is a stronger reflector and a more severe discontinuity; a large positive rating is minor.
How is the attenuation factor calculated?
Subtract 1 in. (25 mm) from the sound path distance and multiply the remainder by 2. Round the result to the nearest whole decibel: fractional values under 1/2 dB reduce to the lower level, and values of 1/2 or greater increase to the higher level. The input is sound path measured along the beam, never material thickness or reflector depth.
Which D1.1 table applies to a cyclically loaded weld?
Table 8.3 governs cyclically loaded nontubular connections and Table 8.2 governs statically loaded ones. The two are not interchangeable. An indication rating of plus four at 2 in. thickness with a 60 degree probe reads as Class C on the statically loaded table and Class A on the cyclically loaded one, which turns a length-limited acceptance into an outright rejection.
What thicknesses is D1.1 ultrasonic testing valid for?
The procedures and standards govern ultrasonic testing of groove welds and heat-affected zones between 5/16 in. (8 mm) and 8 in. (200 mm) inclusive. Below 5/16 in. or above 8 in., testing follows the alternative annex. These procedures are not written for tube-to-tube T, Y and K connections, which carry their own Class R and Class X criteria.
How often must a D1.1 ultrasonic unit be recalibrated?
Recalibration is required after any change of operator, at 30-minute maximum intervals, and whenever the electrical circuitry is disturbed. The clause lists the disturbances explicitly: transducer change, battery change, electrical outlet change, coaxial cable change and power outage. Calibration for sensitivity and horizontal sweep is performed by the operator just before testing, at the location of each weld.
Which indications have to appear on the D1.1 report?
Only rejectable discontinuities need be recorded. The exception is welds designated Fracture Critical in the contract documents: for those, acceptable ratings within 6 dB inclusive of the minimum rejectable rating must also be recorded. Every rejected discontinuity is marked on the weld directly over its entire length, with depth from the surface and indication rating noted on nearby base metal.