What a UT Level I Can Do on the Corpus Christi Ship Channel — and What They Cannot

The scope a Level I is permitted, and why the line is drawn there

Level I is often described as an entry level, which understates it and misleads new technicians. It is a certified level with a defined authority: perform specific calibrations, perform specific examinations described in written instructions, and record results, under the direction of a Level II or III. What it does not include is interpretation and evaluation. The Level I answers the question "what does the instrument show at this location". The Level II answers "is that acceptable".

The line is drawn there for a reason that becomes obvious the first time a thickness reading comes back close to a retirement thickness on a live process line. The number is not the decision. Whether the reading is a genuine backwall or a reflection from the sidewall of a pit, whether the transducer was fully coupled, whether the location matches the TML on the isometric, whether the material is what the procedure assumed, whether the loss is general or a localised pit that a spot reading will miss entirely — those are evaluation questions, and getting one of them wrong changes an operating decision.

This has a practical corollary that new technicians should internalise early: when a Level I sees something the instruction did not anticipate, the correct action is to record it accurately and refer it up, not to characterise it. Writing "no relevant indications" on a scan where something was present but the technician was not sure what it was is the single most damaging habit a Level I can develop, and it is the habit a good supervising Level II will train out in the first month.

Velocity, and the number every new technician gets wrong

An ultrasonic thickness gauge does not measure thickness. It measures the time for a pulse to travel to the far wall and back, and multiplies half of that time by a velocity value stored in the instrument. Everything else follows from that. If the velocity in the instrument does not match the material in front of the probe, the displayed thickness is wrong in direct proportion, and the instrument will display the wrong number confidently and repeatably.

The numbers matter here. Longitudinal velocity in carbon steel sits near 5,900 metres per second; austenitic stainless is close but not identical; aluminium is markedly faster; copper, brass, nickel alloys, cast iron and titanium all differ, and cast materials vary with grain structure in ways that make a nominal value unreliable. A survey calibrated on a carbon steel step wedge and then run on a stainless line produces readings biased by several per cent — enough to matter when the retirement thickness sits a fraction of a millimetre below nominal.

The defence is procedural and is taught at Level I: identify the material before you start, set velocity from the procedure, and verify against a known thickness of the same material where one is available. On a Corpus Christi site that carries carbon steel process piping, stainless in the corrosive services, and nine per cent nickel plate on cryogenic storage, a technician who moves between all three in a shift without changing the setup will generate a data set that has to be thrown away. Velocity is also why shear-wave angle beam work uses a different velocity again — the shear velocity in steel is roughly 3,200 metres per second — and why refracted angle changes with material.

Calibration: what you set, in what order, and what you verify

Calibration is the part of ultrasonics a Level I is explicitly certified to perform, and it is where the practical examination is won or lost. For straight-beam thickness work the sequence is: select the probe and check its condition, set the instrument to the procedure's parameters, set velocity for the material, zero the instrument to remove the transit time through the wearface and couplant, calibrate the range on a step wedge or reference block spanning the expected thickness, and then verify on a second known thickness that was not used to set the calibration. That verification step is skipped by perhaps half of untrained candidates, and it is the step that catches a bad zero.

For angle-beam work the sequence is longer and each element has a purpose. Verify the exit point on the appropriate calibration block. Verify the actual refracted angle rather than trusting the wedge marking, since angle shifts with wedge wear and with material. Calibrate distance or sound path. Set sensitivity by establishing a reference level from a defined reflector. Where the calibration block and the component differ in surface condition or attenuation, apply transfer correction — an adjustment new technicians frequently omit because the block is smooth and the pipe is not.

Two disciplines separate a reliable Level I from an unreliable one. The first is recalibration checks at defined intervals — at the start and end of an examination, on any change of setup, on any suspicion that something moved, and at the interval the procedure requires. The second is recording the calibration: the block used, the settings, the times of the checks. If a calibration check at the end of a shift shows the setup has drifted outside tolerance, the procedure will require re-examining the work done since the last good check. That is a bad afternoon, but a re-scan is recoverable and an undocumented drift on a delivered report is not.

DAC and DGS: two answers to the same question

Ultrasonic amplitude falls with distance because the beam spreads and the material attenuates. That means an identical reflector produces a smaller signal the further away it sits, and a raw amplitude threshold would reject near-surface indications while missing equivalent ones deeper in the part. Both DAC and DGS exist to correct for that, and a Level I should understand the difference even though the Level II sets the reference level and evaluates against it.

A distance-amplitude correction curve is built empirically. The technician takes responses from identical reference reflectors — side-drilled holes, typically — at increasing sound paths in a reference block, peaks each one, marks the screen, and joins the points. The resulting curve is specific to that instrument, probe, cable, wedge and material combination, which is both its strength and its cost: it is real for the setup in hand, and it must be rebuilt when the setup changes. ASME Section V Article 4 practice is predominantly DAC-based, and most refinery and fabrication weld examination in Corpus Christi runs that way.

The distance-gain-size method takes the other route. DGS diagrams are computed from probe frequency, element size and material attenuation, and express a reflector's response as an equivalent flat-bottom-hole diameter at that sound path. Far less block work is needed, and the reference transfers between setups with a correction. European-derived and ISO-referenced specifications commonly call for DGS, which is one of the reasons technicians who intend to work on export-specified projects benefit from understanding both. Neither method makes a Level I an evaluator; both are part of the setup a Level I may be trained and certified to perform.

Thickness measurement, and everything that corrupts the reading

Corrosion thickness work is the highest-volume ultrasonic activity on the ship channel, and it is deceptively easy to do badly. The reading is influenced by surface condition, couplant, temperature, probe wear, the geometry of the loss, and the operator's technique, and each one fails in a characteristic way that a well-trained Level I learns to recognise on the screen rather than in the number.

Rough or scaled surfaces scatter the signal and reduce the backwall echo; heavy pitting on the far wall produces a jagged, unstable backwall rather than a clean one, and a single spot reading through a pit field will report the sound wall between pits and miss the deepest point entirely. Coating thickness adds to the reading unless the instrument and mode account for it. Elevated temperature changes both velocity and probe behaviour and needs high-temperature couplant and a correction. Doubling — where the instrument locks onto a second backwall echo and displays twice the true thickness — is a specific and dangerous failure mode on thin wall, and it is why procedures set a minimum thickness for a given probe and why a Level I is taught to sanity-check readings against nominal wall.

The disciplined counters are all learnable at Level I: prepare the surface as the procedure requires, use the specified couplant and enough of it, take multiple readings within a defined area rather than one, watch the A-scan rather than the digital readout, record the minimum found and the area covered rather than a single number, and mark and photograph the location. On a refinery or terminal circuit that data will be entered against a TML and trended over years. A reading taken 300 millimetres from where the last one was taken corrupts a trend line more thoroughly than a slightly inaccurate reading taken in the right place.

Couplant, surface condition and the practical realities of the channel

Ultrasound does not cross an air gap. Everything about coupling exists to eliminate that gap, and coupling is where field conditions on the Corpus Christi channel intrude on textbook technique. Standard glycerin or cellulose gel works at ambient. High-temperature couplant is required on hot lines and behaves differently, degrading with dwell time on the surface. On stainless, on nickel alloys and on any component destined for cryogenic or oxygen service, couplant chemistry matters — halogen and sulphur content are restricted because residues promote stress corrosion cracking, and using a general-purpose couplant on austenitic stainless in a plant with a halide restriction is a genuine incident, not a technicality.

Surface condition is the other half. Mill scale, loose rust, thick or blistered coating, weld spatter and heavy paint all attenuate the signal or produce a false interface response. The procedure will state the required preparation, and the temptation on a large survey with hundreds of points is to skip it where the surface looks acceptable. It shows up as scattered readings the Level II cannot reconcile. Marine and barge work on the channel adds antifouling coatings and immersion-related surface films; fabrication shops add scale from hot work.

Coupling quality is visible on the A-scan long before it is visible in the number. A Level I who watches the waveform sees an unstable, low-amplitude or noisy backwall and re-couples. A Level I who watches only the digital display records whatever appears. This is the single clearest behavioural difference between technicians who progress quickly and technicians who plateau, and it is the reason a good practical examination puts the candidate on a real surface rather than a polished block.

The examinations: general, specific and practical

Certification at Level I under an employer's written practice rests on three examinations. The general examination covers ultrasonic principles: wave modes and velocity, frequency and wavelength, attenuation and beam spread, near and far field, acoustic impedance and reflection at interfaces, refraction and mode conversion, transducer construction and damping, and the basic instrumentation. The topical outline is normally taken from ANSI/ASNT CP-105, which is what makes a training record checkable against a syllabus rather than a course title.

The specific examination covers the employer's own equipment, procedures, techniques and acceptance criteria. If the employer runs a particular thickness procedure with a named calibration block and a defined data-recording format, that belongs in the specific examination. Buying a generic bank and administering it as the specific examination is a common shortcut and a straightforward audit finding, because the certificate then asserts competence in procedures the candidate was never tested on.

The practical examination puts equipment in the candidate's hands. Set up, calibrate, verify, examine specimens to a written instruction, record results. Most written practices set a minimum of 70 per cent on each part with a composite of at least 80 per cent, which means a candidate can pass every part individually and still fail overall. The practical is where candidates fail, and the reasons are consistent: skipping verification, poor couplant discipline, working from the digital readout rather than the A-scan, and recording data that cannot be traced to a location on the specimen.

Corpus Christi: who employs UT Level I, and what they inspect

The industrial character of Corpus Christi is a deepwater ship channel and the export economy built along it. Crude and refined product export terminals and dock facilities, refining capacity on the channel, LNG liquefaction and cryogenic storage on the bay side, midstream terminals and tankage fed by pipelines out of the Eagle Ford, heavy plate and structural fabrication including wind-tower and module work, barge and vessel repair yards, and — distinctively for a Gulf port — rotary-wing depot maintenance at the naval air installation. That last item makes Corpus Christi one of the few markets in Texas where refinery ultrasonics and aerospace ultrasonics are both hiring in the same city.

Each of those buys a different ultrasonic product. Terminals and refineries buy corrosion thickness monitoring and turnaround weld examination. LNG and cryogenic storage buy weld volumetric examination on nine per cent nickel plate and austenitic materials, where velocity, attenuation and grain structure behave differently from carbon steel and where couplant chemistry is restricted. Fabrication shops buy weld examination to ASME Section VIII, ASME B31.3 and AWS D1.1. Marine repair buys hull and plate thickness work and lamination checking. Aerospace depot work buys bond, bolt-hole and component inspection under NAS 410 personnel rules with their own hour tables and their own requirement for documented on-the-job training.

For a Level I entering this market, the practical consequence is that generic ultrasonic training is only the start. The instrument is the same everywhere; the procedure, the calibration standard, the acceptance code and the written practice are not. Choosing an employer determines which of those five worlds a technician learns first, and the technicians who move fastest to Level II are the ones who understand from the beginning that the specific examination — the one written against their employer's own procedures — is the part of their certification that actually describes what they can do.

From Level I to Level II, and how the hours are counted

Progression to Level II is bounded by documented experience hours in the method, not by classroom time. Under a written practice based on SNT-TC-1A, a Level II candidate needs additional classroom instruction and a substantially larger block of experience in ultrasonics, logged by date, job, technique and hours, and attested by someone qualified to attest to it. The bottleneck is never the course. It is whether an employer logs the hours properly, which is a reason to ask about record-keeping before accepting a first NDT job.

The other bounded quantity is breadth. A technician whose entire Level I experience is spot thickness readings on one terminal has hours but not range, and it shows in a Level II practical the first time an angle-beam setup is required. Deliberately seeking exposure to angle-beam weld scanning, to encoded scanning, to different materials and to different calibration standards during the Level I period is what makes the Level II examination a formality rather than an obstacle.

One recurring situation is worth planning for. Technicians who move between employers lose employer-based certification and must be certified again under the new employer's written practice, which may credit prior training and experience but owns the examination decision. Keeping personal copies of training records, experience logs and examination results — not just the certificate — makes that transition weeks rather than months. Technicians who expect to work on export-specified or international projects should also consider ISO 9712 certification, which is held by the individual and travels. Atlantis NDT delivers UT Level I and Level II preparation to both SNT-TC-1A and ISO 9712, in classroom, on-site corporate and blended formats, built around your own procedures and calibration standards; contact info@atlantisndt.com for a consultation and a quote.

Request a consultation