What is the difference between RT and UT?

Radiography passes radiation through the weld and images what it absorbs, so it reads volume well — porosity, slag, incomplete penetration. Ultrasonics sends sound in and times the echoes, so it finds tight planar flaws radiography misses and reports depth. RT leaves a permanent image; UT gives position without one.

RT vs UT: Radiographic vs Ultrasonic Testing Compared

Radiography and ultrasonics compared on what each finds, what each misses, access, safety, records, codes and what drives the cost of each.

Radiographic testing (RT) and ultrasonic testing (UT) are the two volumetric NDT methods used to find flaws inside welds, castings and plate. Both are recognised by the main construction codes — ASME Section V (Article 2 for radiography, Article 4 for ultrasonic examination of welds), AWS D1.1 and API 1104 — but they work on different physics, so each finds some flaws the other misses.

Once the method is chosen, the weld still has to meet the code: see AWS D1.1 acceptance criteria for structural steel, and NDT inspection cost by method for how each method is quoted.

The short answer: ultrasonics finds tight planar flaws such as cracks and sidewall lack of fusion and tells you how deep they are; radiography images volumetric flaws such as porosity and slag and leaves a permanent image anyone can re-read. UT needs access to one side; most RT techniques need both. Which to specify depends on the flaw you expect, the access you have, whether a radiation exclusion zone is workable, and whether the contract wants an image on file.

RT vs UT: key differences at a glance

RT records how much radiation the material absorbs along the beam path; UT times and measures echoes from reflectors in the sound path.
UT is the stronger method for cracks and lack of fusion; RT is the stronger method for porosity, slag and incomplete penetration.
UT reports through-wall depth; a radiograph is a two-dimensional projection and carries no depth information.
RT needs a licensed source, a radiation safety programme and an exclusion zone; UT carries no radiation hazard.
Both need qualified personnel: ASNT Level II or ISO 9712 certification in the method, plus radiation safety certification for radiographers in the United States.

Radiographic vs ultrasonic testing: side-by-side comparison

Radiographic testing (RT) and ultrasonic testing (UT) compared
Criterion	Radiographic testing (RT)	Ultrasonic testing (UT)
Physics	Radiation absorbed along the beam path, recorded on film or a digital detector	Sound reflected from interfaces, timed and measured on screen
Strongest on	Volumetric flaws: porosity, slag, incomplete penetration	Planar flaws: cracks, sidewall lack of fusion, laminations
Weakest on	Tight planar flaws that do not lie close to the beam direction	Characterising scattered porosity and the shape of slag
Flaw depth	Not available: the image is a projection	Available from echo time and probe angle
Wall thickness	Approximate only, by profile technique	Direct pulse-echo measurement
Access	Source on one side, film or detector on the other for most techniques	One surface
Record	Film or digital image that can be re-read years later	Digital data when encoded and archived; encoded phased array gives a reviewable record
Safety and permits	Licensed source, dosimetry, area control and an exclusion zone	No radiation; couplant and surface preparation
Materials	Most materials; contrast depends on thickness and density	Fine-grained metals; coarse-grained austenitic and cast material scatters the beam
Interpretation	Image interpretation by a qualified Level II	Signal interpretation by a qualified Level II; more dependent on the operator
What drives the cost	Crew of at least two at a temporary jobsite, source logistics, barricade time, film or detectors	Technician time for manual work; equipment, scan planning and analysis time for phased array

When to specify radiography

The flaws you expect are volumetric: porosity, slag, or incomplete penetration at the root.
The contract or the client's specification requires a permanent image as a deliverable.
The part is a casting or a thin-section weld where an image of the whole volume is the clearest evidence.
Both sides of the joint are accessible and the work can be scheduled around an exclusion zone.

Radiography's limits follow from the same physics: a tight crack removes almost no material along the beam, so it registers only when the beam runs nearly parallel to the crack face, and the image carries no depth.

When to specify ultrasonics

Cracks, lack of fusion or other planar flaws are the main concern, including on fatigue-loaded joints.
Flaw depth or through-wall height is needed, for example for an engineering assessment by the owner's engineer.
Only one side is accessible, or an exclusion zone would stop other work on an operating unit.
Wall thickness or corrosion has to be measured as well as weld quality checked.
Results are needed while the scan is under way.

Ultrasonics has its own limits: it depends more on the operator, needs couplant and a usable surface, and characterises scattered porosity less clearly than a radiograph.

When phased array replaces radiography

Encoded phased array, often paired with TOFD, is now accepted in place of radiography on many welds. The substitution is a code and contract question rather than a preference: The applicable construction code and adopted Code Case revision determine whether ultrasonic examination may replace radiography, together with material thickness, data acquisition, qualification and acceptance conditions. Confirm the permitted route in the controlled code before selecting the technique. Either way it needs a qualified procedure, qualified personnel and the owner's or Engineer's agreement before the first weld is scanned. Our guide to phased array in lieu of radiography sets out what each code permits.

Using both methods on the same weld

Where the expected flaw population is mixed and the consequence of a miss is high, the two methods are combined: ultrasonics screens every weld and catches planar flaws, and radiography confirms and characterises the welds that show indications and supplies the image some contracts require. Fatigue-loaded and fracture-critical joints are the usual candidates. For the code clauses behind that choice on a single weld, see RT or UT on a weld, decided by flaw orientation.

What the codes require

ASME Section V sets radiographic technique in Article 2 and ultrasonic examination of welds in Article 4. Acceptance criteria come from the construction code that invokes them, such as Section VIII or B31.3.
AWS D1.1 requires visual inspection on every weld. Radiography and ultrasonics apply when the contract documents call for them, each with its own acceptance criteria in Clause 8.
API 1104 covers radiographic and ultrasonic examination of pipeline girth welds, with separate acceptance criteria for each method.
Because the criteria differ by method, a weld can pass one and fail the other. Record the governing code and edition with every result.

Personnel and radiation safety

Both methods need personnel qualified to the employer's written practice under ASNT SNT-TC-1A or certified to ISO 9712. In the United States an industrial radiographer also needs radiation safety certification through a certifying entity recognised by the Nuclear Regulatory Commission or an Agreement State, and works under a licensee's programme per 10 CFR Part 34, which requires at least two qualified individuals at a temporary jobsite. Ultrasonic testing carries no equivalent licensing burden.

Frequently asked questions

What is the difference between RT and UT?

Radiography passes radiation through the weld and images what it absorbs, so it reads volumetric flaws such as porosity, slag and incomplete penetration well and leaves a permanent image. Ultrasonics sends sound into the weld and times the echoes, so it finds tight planar flaws that radiography misses and reports their depth.

Which is better for weld inspection, RT or UT?

Neither in general. Ultrasonics is the better choice when cracks or lack of fusion are the concern, when only one side is accessible or when an exclusion zone is not workable. Radiography is the better choice when porosity or slag drive acceptance or when the contract requires an image on file. Critical joints often get both.

Is ultrasonic testing safer than radiography?

Yes, in the sense that ultrasonics involves no ionising radiation and can run beside other trades. Radiography is safe when it is run under a licensed radiation safety programme, but that programme requires area control and an exclusion zone, which restricts when and where the work can be done.

Can UT and RT find the same defects?

They overlap on incomplete penetration and on larger inclusions and voids, and they diverge at the extremes: ultrasonics finds tight planar cracks and gives depth, while radiography shows the pattern of porosity and the shape of slag. That complementarity is why the two are combined on critical work.

What drives the cost of RT and UT?

There is no single price for either. Radiography carries a floor set by regulation, with a crew of at least two at a temporary jobsite, source logistics and barricade time that stops nearby work. Manual ultrasonics is driven mainly by technician time, and phased array by equipment, scan planning and analysis. Compare quotes per completed and reported weld; Atlantis quotes on your scope rather than publishing rates.

Do RT and UT technicians need different certification?

Each needs certification in its own method, at Level II to interpret and sign results. In the United States a radiographer also needs radiation safety certification and works under a licensee's radiation safety programme; an ultrasonic technician does not.

Updated 2026-10-12.

Where the results from this method end up

A method is only as useful as the record it leaves behind. Inspection companies running this method at scale need the result tied to the asset, the technician’s certification state and the instrument’s calibration status at the time of test — that bundle is what a client audit asks for. The NDT inspection software buyer’s guide and inspection management software cover how that record is held as structured data instead of filed PDFs.

Need ultrasonic testing (UT) done, or learned?

Atlantis provides ultrasonic testing (UT) as an inspection service, trains technicians in it, and lets teams practise it on the Practical NDT simulator.

Get certified in the method you need

ASNT SNT-TC-1A training in ultrasonic and radiographic testing, led by an ASNT Level III — self-paced online or on-site at your facility. Build hands-on skill first on Practical NDT, an online practical NDT simulation portal.

UT training · RT training · Try Practical NDT · Ask which method fits

Speak to an ASNT NDT Level III

Discuss your inspection requirements with Atlantis NDT. Include the asset, method, location and documents you need reviewed.

Ultrasonic testing finds tight planar flaws — cracks, sidewall lack of fusion — that radiography misses when they sit more than a few degrees off the beam axis, and it reports through-wall depth. Radiography images volumetric flaws such as porosity, slag and incomplete penetration, and leaves a permanent film or digital record. UT needs one-sided access; RT needs two.

Radiographic and ultrasonic testing are the two volumetric methods recognized by ASME Boiler and Pressure Vessel Code Section V — radiography in Article 2, ultrasonic examination of welds in Article 4 — and by AWS D1.1 Clause 8 and by API 1104 for pipeline girth welds. The physics sets the split. Radiography measures how much radiation the material absorbs, so a flaw shows only if it removes enough material along the beam path; a tight crack must lie within roughly a few degrees of the beam to register at all. Ultrasonics measures echo time and amplitude, so a planar flaw normal to the sound path is the easiest thing it sees, and time of flight yields a depth a radiograph cannot supply. Radiography also carries regulatory overhead in the United States: industrial radiographers work under 10 CFR Part 34 or an Agreement State equivalent, with dosimetry, area control and a Radiation Safety Officer.

Source: ASME BPVC Section V, Article 2 (Radiographic Examination) and Article 4 (Ultrasonic Examination Methods for Welds); AWS D1.1, Clause 8; API 1104 acceptance standards for nondestructive testing; US industrial radiography licensing under 10 CFR Part 34 and Agreement State equivalents.

Technically reviewed by Anoop Rayavarapu — ASNT NDT Level III (UT, RT, MT, PT, VT)

RT vs UT by flaw type — which method actually finds it
Flaw type	Radiography (RT)	Ultrasonics (UT)	Preferred
Tight planar crack, transverse to the weld	Poor — detected only when the beam lies within a few degrees of the crack face	Strong — the crack face reflects the beam and returns depth	UT
Sidewall lack of fusion in a bevel	Poor — almost no change in absorbed radiation along the beam	Strong when the probe angle is matched to the bevel angle	UT (angle beam or phased array)
Incomplete penetration at the root	Strong — clear straight-line density change on the image	Strong — distinct root geometry echo	Either
Slag inclusion	Strong — shape, length and distribution are directly readable	Detected, but shape and type are harder to characterize	RT
Scattered or cluster porosity	Strong — the pattern itself is the evidence	Scattered low-amplitude echoes, poorly characterized	RT
Lamination in plate or in the scan path	Poor — the flaw lies parallel to the beam	Strong with a 0° straight beam	UT
General wall loss and corrosion thinning	Only approximate, via a profile technique	Direct pulse-echo thickness measurement	UT
Through-wall flaw height for fitness-for-service	Not obtainable from a projection image	Obtainable by time-of-flight diffraction or phased array tip diffraction	UT

Detection capability is one axis; contract and code are the other. Some projects specify RT because a permanent image is a required deliverable rather than because RT detects better. ASME Section VIII Division 1 permits ultrasonic examination in place of radiography for certain welds under UW-11, subject to procedure and personnel requirements.

Can ultrasonic testing replace radiography for weld inspection?

Often, yes. ASME Section VIII Division 1 permits ultrasonic examination in place of radiography for certain welds under UW-11, and modern API 1104 practice accepts automated ultrasonics or phased array on pipeline girth welds. The substitution is never automatic: it needs a qualified procedure, qualified personnel, and agreement from the owner or Engineer before the first weld is scanned.

Which is faster for production welds, UT or RT?

Ultrasonics. A technician scans a typical weld in roughly 5 to 15 minutes and reads indications on the spot, while radiography needs source and detector setup, exposure, processing and interpretation, commonly 45 to 120 minutes per weld. Radiography also halts other trades inside the exclusion boundary during exposure, so schedule loss exceeds the inspection time itself.

Why does radiography miss cracks?

A radiograph records how much radiation the material absorbs along the beam path. A tight crack removes almost no material in that direction unless the beam runs nearly parallel to the crack face — practically, within a few degrees. Sidewall lack of fusion fails for the same reason. Ultrasonics carries the opposite bias: a planar flaw facing the beam is the strongest reflector it can meet.

Which method tells you how deep a flaw is?

Ultrasonics. Echo transit time converts directly to sound path and, with the known probe angle, to depth below the surface. That is why through-wall height for a fitness-for-service assessment under API 579-1/ASME FFS-1 comes from UT — usually time-of-flight diffraction or phased array tip diffraction. A radiograph is a two-dimensional projection and carries no depth information.

What extra certification does an industrial radiographer need in the United States?

Beyond ASNT Level II in radiographic testing, a US industrial radiographer needs radiation safety certification through a certifying entity recognized by the Nuclear Regulatory Commission or an Agreement State, and must work under a licensee's radiation safety program per 10 CFR Part 34 — personal dosimetry, calibrated survey instruments, area posting and a designated Radiation Safety Officer. Ultrasonic testing carries no equivalent licensing burden.

When is it worth running both RT and UT on the same weld?

When the expected flaw population is mixed and the consequence of a miss is high. Ultrasonics screens every weld quickly and catches planar flaws; radiography then confirms and characterizes the small percentage showing indications, and supplies the permanent image some contracts require as a deliverable. Fatigue-loaded and fracture-critical joints are the usual candidates for both methods.