20.2 Technique Selection and Essential Variables
Key Takeaways
- Method is the NDT family; technique is the specific way the method is applied; the procedure is the approved document that freezes method, technique, essential variables, and acceptance.
- Magnetic-particle essential variables include current type and direction, magnetizing method, and media; swapping AC for DC without allowance changes depth of examination and the required lift test.
- Penetrant essential variables include type, method, sensitivity, dwell, removal, developer, and part temperature; shortening dwell or changing Method C to a hose wash is a procedure breach, not a field improvement.
- Radiographic essential variables include energy, source-to-object distance, IQI demonstration, and density; ultrasonic essential variables include angle, frequency, and the last legal calibration.
- Visual essential variables include lighting, viewing distance and angle, and direct versus remote aids; changing any essential variable without procedure allowance stops production until a Level III addresses it.
Official ASNT specific-exam topic Techniques sits beside Applications and Codes on both the General Industry and Pressure Equipment papers. Applications asked which method can be run here. Techniques asks which way that method is run, and what happens when a Level II changes one of the knobs the procedure froze. The general exam already taught current types, penetrant families, radiographic geometry, ultrasonic angles, and visual lighting. This section is the specific-exam skill: name the essential variable, know the effect of changing it, and refuse to freelance.
Method, technique, and procedure are not synonyms
The specific exam punishes candidates who treat these three words as one instruction.
| Term | What it names | Examples | Who freezes it |
|---|---|---|---|
| Method | The NDT family | Magnetic particle testing (MT), liquid penetrant testing (PT), radiographic testing (RT), ultrasonic testing (UT), visual testing (VT) | The code, the purchaser, or the procedure writer choosing a family |
| Technique | The specific way the method is applied on this job | AC yoke versus prod DC; Type I Method C versus Type II Method A; Ir-192 double-wall versus X-ray single-wall; 70° shear versus 0° straight beam; direct VT versus remote video | The qualified procedure |
| Procedure | The approved written document | Revision, scope, essential-variable list, technique sheet, calibration, process-control checks, report form, and the acceptance standard | Level III (or the organization the written practice names) |
A method can be right and the technique still wrong. Magnetic particle testing is the right method for a carbon-steel toe crack; a residual DC coil shot with no field across the toe is the wrong technique. Ultrasonic testing is the right method for a clad bond; a 70° shear-wave weld scan that never interrogates the interface is the wrong technique. The procedure is how the organization proves the technique was the one that was qualified.
If a stem asks whether the Level II may "improve" a technique because the shop is behind, the answer is no. Improvement is a procedure revision, not a booth decision.
What an essential variable is
An essential variable is a parameter whose change can change the examination's capability — sensitivity, coverage, or the meaning of the record — enough that the organization requires requalification or Level III approval before production continues. A nonessential variable can change inside the procedure's stated window without a new qualification (a different lot of the same penetrant family, a cable of equal length when the procedure allows it). The written procedure, not the inspector's confidence, draws that line.
Industry documents that list essential variables for technique qualification include ASME Section V (Articles 2, 4, 5, 6, 7, and 9), ASTM E1444 / E709 (MT), ASTM E1417 (PT), ASTM E94 (RT), and ASTM E164 (UT). Attribute the list to the document the procedure invokes. Do not invent an unpublished ASNT essential-variable table.
Changing an essential variable without allowance has one legal outcome: the examination is not the examination the procedure describes. Indications collected after the change do not automatically inherit the old acceptance table. Stop. Restore the variable, or get the procedure amended.
Magnetic particle testing — current type and direction
MT technique items cluster on four knobs.
| Essential (or treated as essential in most procedures) | Why it matters | What a silent change does |
|---|---|---|
| Current type — AC, HWDC, FWDC, residual | AC concentrates flux at the surface (skin effect); DC families drive flux deeper | An AC-yoke toe-crack procedure turned into a DC shot is a different depth of examination. The daily lift test also changes: industry practice in ASTM E1444, ASTM E709, and ASME Section V Article 7 commonly uses 10 lb (4.5 kg) for AC yokes and 40 lb (18 kg) for DC yokes |
| Field direction — circular versus longitudinal; yoke orientation | A crack parallel to the field produces little leakage | One-direction coverage misses the crack the other direction would have shown |
| Magnetizing method — yoke, coil, prods, central conductor | Each method produces a different field shape and a different hazard (prod arcing, coil L/D) | Prods on a finished pressure face can arc-pit a surface the procedure never authorized |
| Media and lighting — wet fluorescent versus dry visible; UV-A versus white light | Sensitivity and the viewing rules change | Fluorescent particles viewed in shop sunlight are not the technique that was qualified |
Worked picture. The procedure says AC electromagnetic yoke, two orientations, wet fluorescent, UV-A at the surface. The Level II grabs a DC yoke "because it sees deeper" and dry powder because the booth hose is down. That is two essential-variable changes. Deeper is not better for a surface fatigue crack — AC was chosen because the target is at the surface — and the lift-test weight and the lighting rules no longer match. Production stops.
Second picture. A coil shot is set for longitudinal magnetization of a bar to find transverse cracks. The Level II, in a hurry, does only that shot and signs for "full coverage." Longitudinal magnetization is the wrong direction for longitudinal seams. Direction is not a courtesy; it is the technique.
Liquid penetrant testing — type, method, and dwell
PT technique is a process window, not a brand of red dye.
| Variable | Typical technique choice | Effect of an unauthorized change |
|---|---|---|
| Type | Type I fluorescent; Type II visible dye | Sensitivity and lighting rules change (UV-A and darkened booth versus white-light intensity) |
| Method | A water-washable; B post-emulsifiable lipophilic; C solvent-removable; D hydrophilic post-emulsifiable | Field welds lose their portable Method C process if someone "washes" them like Method A |
| Sensitivity class (Type I) | The class the procedure qualified | A lower class can miss the tight crack the qualification used |
| Dwell | A minimum (many procedures use about 10 minutes) and a maximum before the film dries | A two-minute dwell on a tight fatigue crack is a missed indication, not a time-saver |
| Removal | Water wash inside the ASTM E1417-style temperature and pressure window (commonly taught as about 50–100 °F and ≤40 psi); Method C is a solvent-damp wipe, never a solvent hose | Overwash strips penetrant from the discontinuity; a Method C flood dissolves the indication |
| Developer and part temperature | Form and time the procedure names; part inside the stated band | Developer skipped, or a 200 °F part, is not the qualified process |
Worked picture. A field nozzle on 316L is a Type I Method C job: solvent-removable fluorescent penetrant, portable, no rinse station. The helper arrives with a water hose and a water-washable can "because it is faster." Method is an essential variable. There is no rinse-water control on the platform, and a hose on Method C chemistry is not Method C. Stop. Use the named method or get a revised procedure.
Second picture. The Level II shortens dwell to three minutes because the night shift wants the vessel back. Tight cracks have not filled. The record that says "no relevant indications" is a record of an incomplete process, not of a clean part.
Radiographic testing — energy, SOD, and the IQI
RT technique is geometry plus proof that the image could have shown what the procedure cares about.
| Variable | What the procedure freezes | Unauthorized change |
|---|---|---|
| Energy (kV, or isotope: Ir-192, Co-60, Se-75) | Contrast and penetration for that thickness | Energy too high washes contrast; energy too low never penetrates; a different isotope is a different technique |
| Source-to-object / source-to-film distance (SOD / SFD) | Geometric unsharpness (Ug) and intensity (inverse square) | Closing SOD to finish the shot before lunch increases Ug and can destroy IQI visibility |
| IQI type, size, and placement | Hole-type (ASTM E1025) or wire (ASTM E747); the essential hole or wire must be visible | A radiograph that does not demonstrate the required IQI is not a valid examination, even if the weld "looks pretty" |
| Density (film) or the digital quality metric | Many ASME Section V Article 2 film techniques use about 1.8–4.0 optical density through the area of interest (1.8 X-ray / 2.0 gamma minima are commonly printed; procedure wins if tighter) | Density outside the window means you do not interpret — you reshoot |
| Screens, film class, geometry (SWSI, DWSI, DWDI) | Definition, speed, and which wall is imaged | A double-wall technique used where the procedure required single-wall is a different examination |
Worked picture. The technique sheet says Ir-192, a stated SOD, a 2-2T hole-type IQI on the source side, density 1.8–4.0 through the weld. The Level II walks the source in to cut time. The film is dark enough in the center, but the 2T hole is gone and Ug is above the procedure limit. That is not a close-enough radiograph. Energy and SOD were essential; the IQI is the proof. Reshoot at the qualified geometry.
Ultrasonic testing — angle, frequency, and calibration
UT technique items fail candidates who treat the probe kit as a box of interchangeable sticks.
| Variable | Why it is essential | Unauthorized change |
|---|---|---|
| Angle | 0° (straight beam) for laminations, remaining wall, and many bonds; 45° / 60° / 70° shear for weld bevels and specified depths | A 70° first-leg scan of a shallow toe is not the same coverage as a 45° scan that reaches the root on a thicker weld. Skip distance and intercept change |
| Frequency | Higher frequency → shorter wavelength → better resolution, more scatter; lower frequency → more penetration | Raising frequency on a coarse austenitic weld can erase the back-wall in scatter; lowering frequency on a thin clad can fail to resolve the interface |
| Calibration — range, zero/delay, velocity, sensitivity, DAC/TCG | Every amplitude and depth number inherits the last legal cal | An expired calibration, a worn wedge that moved the exit point, or a 6 dB drop on the reference hole is not a license to add gain and keep scanning |
| Probe type and couplant | Dual-element vs single; contact vs delay; shear vs refracted longitudinal | A dual-element clad probe swapped for a single-element weld probe is a different technique |
Worked picture. The weld procedure is built on a 70° shear wave, IIW calibration, and a DAC from side-drilled holes. The Level II cannot get a back-wall on the IIW block with that wedge, so they switch to a 45° wedge they like and scan anyway, using yesterday's range. Angle and calibration are both essential. The skip and the first-leg intercept are no longer the ones the technique sheet drew. Recalibrate on the named angle or stop.
Clad-bond flavor: the technique is straight-beam pulse-echo, frequency high enough to resolve the clad thickness, gain set so a known unbond or the substrate back-wall behaves as the procedure shows. Substituting a weld-angle probe "to be more sensitive to cracks" abandons the bond examination.
Visual testing — lighting and distance
VT has essential variables even though there is no current meter.
| Variable | Typical control | Unauthorized change |
|---|---|---|
| Lighting type and intensity | White-light minimum at the surface (industry VT procedures commonly cite about 100 fc / 1000 lux for direct VT; the procedure wins); fluorescent-aid lighting if the procedure uses it | A dim corner examination is not the qualified VT technique |
| Viewing distance and angle | The procedure's maximum distance and minimum angle to the surface | "I could see it from the grating" is not the technique |
| Direct versus remote | Eyes and mirrors versus video, boroscope, or drone as qualified | A phone video shot down a nozzle is not automatically a remote-VT technique |
| Magnification and measuring aids | What the procedure allows for the acceptance table's units | Inventing a crack depth from "how dark it looks" at 10× is not a measurement |
ASME Section V Article 9 treats VT as a controlled method. Lighting and distance are how that control is real. Changing them without allowance is the same class of error as changing radiographic SOD.
What you do when a variable will not hold
The specific exam's favorite wrong answer is the helpful inspector.
- The yoke will not lift 10 lb, so the Level II holds it closer and adds extra powder.
- The wash water is 120 °F, so the Level II shortens the rinse "to compensate."
- The IQI is not visible, so the Level II interprets the pretty weld anyway.
- The IIW calibration expired at midnight, so the Level II uses yesterday's DAC because "the instrument looks the same."
- The tank farm lights are below the VT minimum, so the Level II accepts the weld by flashlight memory.
Each of those is a technique noncompliance. The correct sequence is:
- Stop production examinations that depend on the broken variable.
- Restore the variable (repair the yoke, wait for legal wash temperature, restore SOD, recalibrate, add light) or obtain a Level III procedure change.
- Record the stoppage and the restoration. Do not back-date a lift test you did not perform.
Techniques items reward the candidate who can say which variable moved and what capability moved with it. They do not reward the candidate who recites equipment brand names.
Putting technique selection on a stem
Read the stem for the method already chosen, then for the target discontinuity, then for the variable the inspector changed. If the change is on the essential list above, the examination is invalid until the procedure allows it. If the stem asks you to choose a technique, pick the one whose essential variables match the target:
- Surface toe crack, carbon steel → AC yoke, two directions — not a residual DC coil as the only shot.
- Portable stainless field weld → Type I or Type II Method C — not a Method A hose.
- Shop butt weld, two-side access, porosity map → RT at the qualified energy, SOD, and IQI — not a walk-in source.
- Clad bond → 0° pulse-echo at a resolving frequency — not a 70° weld scan.
- Direct VT of a weld face → procedure lighting and distance — not a glance from the next landing.
That is Techniques on the specific exam: the knob, the effect, and the refusal to turn it without paper.
A qualified magnetic-particle procedure specifies an alternating-current electromagnetic yoke for surface-breaking weld-toe cracks. The Level II substitutes a DC yoke because it "sees deeper" and continues production without a procedure change. What is the correct evaluation?
Which statement correctly distinguishes method, technique, and procedure on the ASNT NDT Level II specific exam?
A film-radiography procedure fixes energy, source-to-object distance, and the IQI. The Level II shortens SOD to raise intensity and finish before a shift change, and the required IQI holes are no longer visible. What is true?