Hull thickness and NDTLesson 2 of 5
Ultrasonic thickness measurement and approved firms
You will be able to explain how a thickness reading is made, spot a bad one, work out diminution against the class margin, and check a gauging firm's approval and report.
16 minutes · Open to everyone · Rules checked 20 October 2026
Start here
A thickness report decides how much steel a ship renews in dock, and sometimes whether the ship keeps its class. Thousands of numbers, each one a fraction of a second of sound bouncing inside a plate, end up as tonnes of steel and weeks of yard time. If the gauge read the paint instead of the steel, or the operator chose the one clean spot on a pitted plate, the report is wrong in a way nobody can see from the paperwork.
That is why IACS treats thickness measurement as a regulated service. The firm must be approved by the class society under Unified Requirement (UR) Z17, its operators must hold recognised non-destructive testing (NDT) certificates, and a surveyor must be on board while it works. Click the parts of the gauge set-up below to see what each part does and how readings go wrong.
Try it
Ultrasonic thickness gauge on a coated hull plate
In plain words
An ultrasonic gauge measures steel the way a bat measures distance. It sends a tiny burst of sound into the plate, waits for the echo from the far side, and turns the time into millimetres. Sound travels through carbon steel at roughly 5,900 metres per second (a typical figure; the gauge is set on a block of known thickness, not by trusting the number), so a 15 mm plate returns its echo in about five millionths of a second.
The difficulty is that ships are painted. Paint slows the sound down, and a simple gauge that times from the surface counts the paint as steel. The answer is the multiple-echo gauge. Inside the steel, the sound bounces back and forth several times. The gauge times the gap between two of those internal echoes, which only ever passes through steel. That is why IACS insists on multiple-echo instruments on coated surfaces.
The second difficulty is people. Readings depend on where the probe is placed, how the surface is prepared and whether the operator knows what a ship’s structure looks like. So the firm doing the work must be approved by the class society. Its supervisor must hold an NDT certificate at Level II, its operators at least Level I, and the class surveyor must be on board to direct where the readings are taken and to sign each job.
Finally the numbers have to be judged. Each reading is compared with the original thickness on the drawings. The difference is the diminution. The class rules set how much diminution each member may have. When more than three-quarters of that allowance is used up, the corrosion is called substantial and the area is watched every year from then on.
In detail
What the operator does
- Prepares the surface. Loose scale and dirt are removed so the probe sits flat. On coated steel with a multiple-echo gauge the coating can usually stay; on bare, pitted steel the spot may be ground clean for a single-echo reading.
- Calibrates. On a step block of the same material, across the thickness range expected, before work and at intervals. The surveyor witnesses it.
- Places the probe where the surveyor directs: representative positions for general corrosion, more readings where pitting, grooving or uneven corrosion appears.
- Records each reading against a clear location (frame number, strake letter, plate position) on the reporting form, with the original thickness beside it.
- Reports promptly to the surveyor any excessive or extensive corrosion, significant pitting or grooving, buckling, fractures, holed structure or corroded welds.
Judging a reading: diminution and substantial corrosion
Diminution is simply:
diminution (mm) = original thickness − gauged thickness diminution (%) = diminution ÷ original thickness × 100
The allowable diminution comes from the class society’s rules for that member (UR Z7 paragraph 1.5). Substantial corrosion begins when the diminution exceeds 75% of that allowance. A worked example, using an assumed allowance rather than a real rule value:
| Item | Value |
|---|---|
| Original thickness (from drawings) | 14.0 mm |
| Allowable diminution (example, from class rules) | 3.0 mm |
| Substantial corrosion begins at | 0.75 × 3.0 = 2.25 mm lost, gauged below 11.75 mm |
| Renewal needed at | more than 3.0 mm lost, gauged below 11.0 mm |
| Gauged | 11.5 mm: substantial corrosion, extend gauging, annual re-examination |
For ships built under the IACS Common Structural Rules (CSR), the ESP Code expresses the same idea in net thickness: substantial corrosion is a gauged thickness between t_net + 0.5 mm and t_net. For hatch covers built to UR S21 Rev.6, steel renewal is required below t_net + 0.5 mm for single-skin covers and double-skin plating, and between t_net + 0.5 mm and t_net + 1.0 mm coating or annual gauging may be accepted instead.
Two more terms appear in reports. Excessive corrosion exceeds the allowable limit. Extensive corrosion is hard or loose scale, including pitting, over 70% or more of the area, with evidence of thickness loss.
The approved firm (UR Z17)
| Requirement | Detail |
|---|---|
| Scope | Thickness measurement on all ships except non-ESP ships under 500 GT and fishing vessels |
| Supervisor | Qualified to a recognised industrial NDT standard, e.g. ISO 9712 Level II, and at least 2 years as an operator |
| Operators | Certified to e.g. ISO 9712 Level I, with enough knowledge of ship structures to choose representative positions; at least 1 year tutored on-the-job training |
| Equipment | Multiple-echo pulse-echo instruments on coated surfaces; records of maintenance and calibration |
| Procedures | Preparation, choice and identification of test locations, surface preparation, coating preservation, calibration checks, report content |
| Reporting | To the guidelines in UR Z7, Z7.1, Z7.2 and Z10.x; the report includes a copy of the Certificate of Approval |
| Verification | Each separate job signed by the attending surveyor |
| Certificate renewal | At intervals not exceeding 3 years (5 years for most other service suppliers) |
Approval is not a formality. A society may cancel it where the service was improperly carried out or reported, and for thickness firms the cancellation is reported to the other IACS societies under Procedural Requirement PR 23.
Coming on 1 January 2028: UR Z17 Rev.24 adds that the initial audit of a firm gauging ESP ships must include at least one on-board practical demonstration witnessed by the approving society. Gauging accepted under survey attendance then counts as ongoing demonstration.
The report
The ESP Code requires the report to give the location of measurements, the thickness measured and the original thickness, the dates, the type of equipment, the names and qualifications of the operators, and the operator’s signature. IACS reporting forms (TM1 for deck, bottom and side plating, TM2 for transverse sections, TM3 for longitudinal members, TM4 for transverse bulkheads, TM5 for miscellaneous members, with ship-type versions for bulk carriers and tankers) carry this. Before leaving the ship the surveyor keeps a copy of the preliminary report signed by the operator; once the final report is checked against it, the surveyor countersigns the cover page.
What the rules say
- UR Z7 paragraph 5.3.1 says thickness measurement is “normally to be carried out by means of ultrasonic test equipment” by a firm approved under UR Z17, except for non-ESP ships under 500 GT and fishing vessels.3
- UR Z17 paragraph 4.2 requires firms whose thickness measurements, in-water surveys or ultrasonic tightness tests are used for class decisions to be approved and verified by that society.1
- UR Z17 Annex 1, section 1, requires a supervisor qualified to e.g. ISO 9712 Level II, operators certified to e.g. ISO 9712 Level I with knowledge of ship structures, and multiple-echo pulse-echo instruments on coated surfaces.1
- UR Z17 Annex 1, paragraph 1.7, requires the surveyor’s verification of each separate job, documented by the attending surveyor’s signature on the report.1
- UR Z17 paragraph 6.2 requires the certificate of a thickness measurement firm to be renewed or endorsed at intervals not exceeding 3 years.1
- UR Z17 Rev.24 (in force 1 January 2028) requires the initial audit of a firm gauging ESP ships to include an on-board practical demonstration witnessed by the approving society.2
- IACS PR 19 paragraph 2.2 lets the surveyor decide the final extent and location of readings after the overall survey, and direct extra readings to delimit substantial corrosion.5
- IACS Recommendation 77 paragraph 2.1 asks the surveyor to verify calibration, witness a calibration suited to the material, and check that multiple-echo instruments are used on coated surfaces.4
- The 2011 ESP Code, annex A, part A, paragraph 7.3.1 requires the report to state locations, measured and original thickness, dates, equipment, operators’ names and qualifications, signed by the operator.6
- The 2011 ESP Code, annex 5, paragraph 2.3, makes certification of a thickness measurement company conditional on an on-board demonstration and satisfactory reporting.6
- UR S21 Rev.6 section 7.2 requires steel renewal of single-skin hatch covers where the gauged thickness is less than t_net + 0.5 mm.7
Flag and class notes
- Each IACS society publishes its own list of approved thickness measurement firms, and approval by one society does not automatically cover another. UR Z17 paragraph 5.4 lets a society accept documentary evidence of practical demonstrations already carried out for another society at initial audit.1
- For statutory surveys done by the flag itself, UR Z17 paragraph 4.3 lets the society accept approvals by the flag Administration or organisations acceptable to it. Ask your flag whether it keeps its own list.1
- Acceptance criteria (allowable diminution) differ between societies and by ship type and age; UR Z7 paragraph 1.5 refers to each society’s rules. Ask class for the diminution table for your ship before gauging starts.3
Key numbers
| What | Value |
|---|---|
| Supervisor qualification | ISO 9712 Level II or equivalent1 |
| Operator qualification | ISO 9712 Level I or equivalent1 |
| Supervisor experience | at least 2 years as operator1 |
| Operator on-the-job training | at least 1 year, tutored1 |
| Certificate renewal, thickness firms | every 3 years maximum1 |
| Substantial corrosion | more than 75% of allowable margin used3 |
| Substantial corrosion, CSR ships | gauged between t_net + 0.5 mm and t_net6 |
| Extensive corrosion | scale or pitting over 70% or more of the area4 |
| UR S21 hatch cover renewal, single skin | gauged below t_net + 0.5 mm7 |
What inspectors find
- Single-echo gauges used on coated plating, giving readings inflated by the coating thickness. IACS Recommendation 77 tells surveyors to check the instrument type before gauging starts.4
- Readings taken before the overall survey in places the surveyor later rejects, so the gauging has to be repeated or extended.5
- Reports without the operator’s qualifications, the equipment details or the firm’s certificate of approval, which the ESP Code and UR Z17 require.6
- Firms whose class approval has expired: renewal is due at least every 3 years and the certificate must be attached to each report.1
Questions people ask
Can the crew gauge the hull for class?
Not for class or ESP surveys on ships of 500 GT and above. Crew readings are useful for the owner’s own monitoring, but class credits only measurements by a firm approved under UR Z17 and witnessed by a surveyor.
Do we have to remove the paint?
Usually not, if a multiple-echo gauge is used. UR Z17 requires multiple-echo instruments on coated surfaces; single-echo instruments are allowed only on uncoated surfaces that have been cleaned and ground. Any coating damaged during preparation should be repaired.
Can gauging be done at sea?
Yes. PR 19 confirms that its rules, including surveyor attendance, also apply to thickness measurements taken during voyages. Safe access, ventilation and communication must be arranged as for any tank entry.
What happens if the firm's approval is cancelled?
The society may cancel approval for improper work or reporting, failure to notify changes, or deliberate misrepresentation, and thickness firm cancellations are reported to other IACS members. Readings from a firm without valid approval are not credited.
Check yourself
6 questions. Pass with 60% to mark the lesson as done.
Sources
- 1 IACS Unified Requirement Z17, Procedural Requirements for Service Suppliers, Rev.21 (January 2025, implemented from 1 July 2026), paragraphs 4.1.2, 4.2, 4.3, 5.2.2 to 5.2.5, 5.2.11, 5.4, 6.2, 8.1, 8.2 and Annex 1, section 1 (firms engaged in thickness measurements). https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2023/05/14135618/UR-Z17-Rev.21-Jan-2025-CLN.pdf
- 2 IACS UR Z17 Rev.22, 23 and 24 (Rev.22 in force 1 January 2027; Rev.23 and Rev.24 in force 1 January 2028), Rev.24 paragraph 5.4 and its Note 1. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2023/05/05122440/UR-Z17-Rev.22-23-24-CLN.pdf
- 3 IACS UR Z7, Hull Classification Surveys, Rev.29 Corr.1 (May 2024), paragraphs 1.2.8, 1.4.1, 1.5 and 5.3.1. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2024/05/21130428/UR-Z7Rev.29Corr.1.pdf
- 4 IACS Recommendation No. 77, Guidelines for the Surveyor on how to Control the Thickness Measurement Process, Rev.4 (October 2017), paragraphs 1.6, 1.8, 2.1 to 2.6, 3.5, 3.6 and Notes 2 and 3. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2022/05/20151551/rec-77-rev4-oct-2017-cln.pdf
- 5 IACS Procedural Requirement PR 19, Procedural Requirement for Thickness Measurements, Rev.1 (July 2017), paragraphs 1, 2 (including footnote on voyages), 2.1, 2.2 and 2.3. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2022/02/10150422/pr19rev1.pdf
- 6 IMO Resolution A.1049(27), 2011 ESP Code, as amended, annex A, part A, paragraphs 1.2.9, 7.1 to 7.3, annex 5 (procedures for certification of a company engaged in thickness measurement) and annex 8 (recommended procedures for thickness measurements). https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/AssemblyDocuments/A.1049(27).pdf
- 7 IACS Unified Requirement S21, Evaluation of Scantlings of Hatch Covers and Hatch Coamings and Closing Arrangements of Cargo Holds of Ships, Rev.6 (January 2023, for ships contracted on or after 1 July 2024), section 7.2 (steel renewal). https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2023/08/10144821/ur-s21rev6.pdf
Free awareness training. Al Safwan Marine is not an approved training provider, so no certificate is given, and these courses are not a qualification. They don't replace STCW training, manufacturer training or the certification technicians need for this work. The manufacturer's manual and your flag's rules always come first.
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