How to Choose an Ultrasonic Thickness Gauge: A Practical Guide for Pipes, Tanks and Corrosion Checks
Choose an ultrasonic thickness gauge in this order: confirm that ultrasound can reach and reflect from the back wall, define the real thickness and geometry, select the probe, then compare gauge range and resolution. For ordinary steel plate and pipe, the standard 5 MHz dual-element probe is the normal starting point. Small contact areas may need a 7 MHz micro-diameter probe; coarse-grained materials may need the 2 MHz coarse-crystal probe; hot surfaces require the specified high-temperature probe, couplant and work procedure. If paint must remain in place, verify the coating and substrate limits of the gauge’s coating-penetration mode rather than assuming that the full headline range applies through paint.
| Inspection situation | Start with | Verify before buying |
|---|---|---|
| General steel plate, tank or pipe wall | Standard 5 MHz probe | Expected wall range, pipe geometry, access and surface condition |
| Small contact patch or relatively thin component | 7 MHz micro-diameter probe | Minimum measuring area and published pipe lower limit |
| Cast iron or another coarse-grained material | 2 MHz coarse-crystal probe | Stable back-wall echo on a representative sample |
| Hot surface | GT-12 high-temperature probe | Surface temperature, brief-contact method, temperature-rated couplant and host protection |
| Painted steel where the metal wall is required | Standard probe in supported coating-penetration mode | Both coating thickness and substrate thickness fall inside the mode limits |
The important phrase in that table is “start with.” A catalogue range cannot prove that a rough, curved, coated or internally corroded component will return a usable echo.
First confirm that ultrasonic thickness gauging fits the component
ISO 16809:2025 describes ultrasonic thickness determination using contact or immersion techniques based on ultrasonic pulse time of flight.1 ASTM E797/E797M-21 covers manual contact pulse-echo thickness measurement and notes that the method is used from one accessible side, including for wall thinning caused by corrosion and erosion.2
That does not mean every solid object is an easy target. Before comparing instruments, answer three questions.
1. Can the material transmit useful ultrasound?
Steel, aluminium, copper, many plastics, glass and ceramics can be measured when ultrasound propagates through them and a back-wall reflection can be resolved. Highly attenuating, coarse, porous or inhomogeneous materials can weaken or scatter the signal. Cast iron is a familiar example: the Linshang manuals specifically direct the user to the coarse-crystal probe for coarse-grained materials.456
For an unfamiliar casting, composite or lined component, ask for a demonstration on a representative sample. “Metal” is not a complete ultrasonic specification.
2. Is there a usable back wall?
The simple pulse-echo relationship is:
Thickness = sound velocity × round-trip time ÷ 2
The pulse must travel through the material, reflect from the opposite surface and return. ASTM E797 says the method is primarily applicable to flat components with parallel surfaces and has limited application to non-parallel or concentric components unless a resolvable back-wall reflection can be obtained.2 The Linshang manuals similarly state that the opposite surface should be parallel or coaxial with the measured surface.456
Internal corrosion makes this harder because a rough or angled back wall may scatter energy away from the probe. A displayed number should therefore be repeatable after re-coupling and should make sense beside nearby readings.
3. Can the probe sit correctly on the surface?
The probe face must fit within the available contact area. On curved surfaces, the split line of the dual-element probe should be oriented perpendicular to the pipe axis, following the manufacturer’s instruction.456 Edges, weld profiles, heavy scale and tight elbows may prevent full seating even when the nominal pipe diameter looks adequate.
Sound velocity is part of the measurement
The gauge converts time into thickness using the selected sound velocity. If the velocity setting is wrong, the thickness result is wrong in the same proportion, assuming the instrument is timing the intended echo.
For example, a velocity setting that is 2% above the actual value produces a reading approximately 2% high. A true 10.00 mm wall would appear near 10.20 mm. This is a direct consequence of the time-of-flight equation, not a statement of instrument accuracy.
The current Linshang manuals recommend setting velocity with a known-thickness piece made from the same material as the test object.456 That is stronger practice than selecting a generic material name when alloy, heat treatment, temperature or microstructure is uncertain.
Keep three ideas separate:
- Resolution is the smallest displayed increment.
- Accuracy is how closely a result is expected to agree with a reference under stated conditions.
- Repeatability is how closely repeated readings agree under the same setup.
The LS211 and LS212 display 0.001 mm increments below 10 mm, but their published standard-probe accuracy in that range is ±0.05 mm.45 The extra digits can help show movement or variation; they do not turn the gauge into a one-micrometre measurement system.
Choose the probe before the gauge model
The latest LS211 V2.44, LS212 V2.44 and LS213 V1.44 manuals specify the same four probe options. The values below are manufacturer specifications on 45# steel, not universal performance guarantees.456
| Probe | Frequency and minimum measuring area | Published 45# steel range | Published pipe lower limit | Intended starting application |
|---|---|---|---|---|
| Standard | 5 MHz; Ø10 mm | LS211: 0.8–600 mm; LS212/LS213: 0.8–350 mm | Ø20 × 3 mm | Conventional plate and wall-thickness work |
| Micro-diameter PT-06 | 7 MHz; Ø6 mm | 0.75–80 mm | Ø20 × 2 mm | Smaller contact areas and thinner workpieces |
| Coarse-crystal ZT-12 | 2 MHz; Ø12 mm | 3–200 mm | Ø30 × 4 mm | Cast iron and other coarse-grained material |
| High-temperature GT-12 | 5 MHz; Ø12 mm | 3–200 mm | Ø30 × 4 mm | Elevated-temperature workpieces |
Pipe limits in the manuals are expressed as outside diameter × wall thickness. Treat them as published lower limits under the named steel condition, not as proof that every corroded pipe at that geometry is measurable.
Standard probe: the default for conventional steel work
Use the standard probe first when the surface is accessible, the Ø10 mm minimum measuring area fits, and the expected wall lies within the applicable model range. It is also the probe specified for the coating-penetration function on these three gauges.
Micro-diameter probe: for smaller contact areas
The PT-06 reduces the published minimum measuring area to Ø6 mm and the pipe lower limit to Ø20 × 2 mm on 45# steel. Its 0.75–80 mm range is narrower than the standard probe’s range. Select it because the geometry needs it, not because 7 MHz automatically means “more accurate.”
Coarse-crystal probe: for scattering materials
The ZT-12 uses 2 MHz and is the manufacturer’s specified option for cast iron and other materials with large crystal particles. The latest manuals publish a 3–200 mm range on 45# steel and a different accuracy expression from the other probes: ±0.1 mm below 10 mm and ±1%H at or above 10 mm.456
Current Linshang product webpages show a different coarse-probe accuracy value above 10 mm. This guide follows the newer downloadable manuals and recommends confirming the manual revision supplied with the probe. That discrepancy is exactly why a purchasing decision should retain a dated specification record.
High-temperature probe: a special procedure, not a heat shield for the gauge
The GT-12 probe table gives an exposure range up to 500 °C, while the same manuals list the gauge host operating range as 0–50 °C.456 Do not read “500 °C probe” as “500 °C complete instrument.”
ASTM E797/E797M-21 is scoped to contact measurements not exceeding 93 °C.2 Work above that temperature needs a procedure appropriate to the equipment, site and governing standard. Independent guidance also notes that hot measurements require brief contact, a suitable couplant, cooling between readings and calibration at the measurement temperature when maximum accuracy is required.7
Measuring the metal beneath paint
An ordinary pulse-to-first-back-wall measurement can include coating delay and make the metal appear thicker than it is. The Linshang coating-penetration function uses two consecutive substrate back-wall echoes so the surface-coating delay is excluded.456
The latest manuals give the same conditions for LS211, LS212 and LS213:
- standard probe required;
- coating thickness: 0.2–2.5 mm;
- 45# steel substrate thickness: 4–60 mm.
These limits are much narrower than the normal standard-probe range. The mode reports the substrate wall beneath the coating; it does not report paint thickness. If the inspection task is to measure the coating itself, use the appropriate coating-thickness method instead: How to Choose a Coating Thickness Gauge: Automotive vs Industrial Use.
Do not assume that rust scale, multilayer linings or an unknown coating behave like the stated 0.2–2.5 mm paint condition. Verify the actual system.
LS211 vs LS212 vs LS213
AIMITEK currently presents these three models as one product family.8 Their main documented selection difference is the standard-probe range and display resolution.
| Model | Standard-probe range on 45# steel | Display resolution | Sensible reason to choose it |
|---|---|---|---|
| LS211 | 0.8–600 mm | 0.001 mm from 0.8–10 mm; 0.01 mm from 10–100 mm; 0.1 mm from 100–600 mm | The application genuinely needs the range above 350 mm |
| LS212 | 0.8–350 mm | 0.001 mm from 0.8–10 mm; 0.01 mm from 10–100 mm; 0.1 mm from 100–350 mm | The finer display increment below 10 mm is useful and 350 mm is sufficient |
| LS213 | 0.8–350 mm | 0.01 mm from 0.8–100 mm; 0.1 mm from 100–350 mm | Routine wall checks where a 0.001 mm display step is unnecessary |
The latest three manuals publish the same standard-probe accuracy expression: ±0.05 mm below 10 mm and ±0.5%H at or above 10 mm, where H is the reference thickness.456 At 20 mm, ±0.5%H equals ±0.10 mm. This is a calculation from the specification, not a prediction for every surface and material.
The manuals also list coating-penetration mode, automatic gain, QC and statistics modes, storage for up to 999 readings, probe recognition, Bluetooth/app functions, PC software and a sound-velocity setting range of 1000–19999 m/s.456 These functions help organise measurement; they do not establish an allowable minimum wall.
Plan the inspection, not just the purchase
A corrosion survey needs repeatable locations. One reading tells you the wall beneath one probe position; it does not describe an entire pipe spool or tank shell.
A practical field plan should define:
- the component and inspection boundaries;
- material, nominal wall and engineering acceptance basis;
- measurement grid or named locations;
- surface-preparation limits;
- probe, mode, sound velocity and verification blocks;
- how often zero/reference checks are repeated;
- how suspect readings are re-coupled and investigated;
- how values, locations and instrument settings are recorded.
ISO 16831:2025 addresses performance verification of pulse-echo equipment dedicated to thickness determination, including numerical-display and A-scan instruments using single- or dual-transducer probes.3 The exact verification and competency requirements still come from the applicable inspection procedure, contract and governing code.
Surface condition and couplant
Contact measurement needs couplant to displace air between the probe and test surface. The Linshang manuals instruct the user to apply coupling fluid, hold the probe still and confirm the coupling indication.456
Loose scale, heavy rust, weld spatter and roughness can prevent stable seating or scatter the sound. Prepare the surface only as permitted by the approved procedure. The manuals warn that sliding the probe over a rough surface can scratch and wear its face.
Repeat an unexpected reading
When a value changes sharply, lift and re-couple the probe, check the surface, verify the selected material velocity and mode, and take nearby readings. An isolated minimum that cannot be repeated should not silently become an engineering decision.
A corrosion thickness gauge is not designed to detect cracks or characterise discontinuities. Evident’s 27MG manual states that unexplained corrosion-gauge readings may require further testing with an ultrasonic flaw detector operated by a properly trained person.7
Common selection mistakes
Buying maximum range before checking the minimum wall
A 600 mm upper range says little about performance on a small-diameter pipe with local wall loss. Confirm the probe geometry, lower range and representative sample first.
Treating the third decimal place as accuracy
LS211 and LS212 can show 0.001 mm below 10 mm, while the standard-probe accuracy specification there is ±0.05 mm. Resolution and accuracy answer different questions.
Assuming through-paint mode uses the normal range
For this product family, the published coating-penetration limits are 0.2–2.5 mm coating and 4–60 mm 45# steel substrate. Check both layers.
Using the 500 °C probe figure as a system rating
The 500 °C value belongs to the GT-12 probe exposure row. It does not apply to the host, ordinary couplant, cable handling or operator safety.
Letting QC mode define the acceptance limit
QC mode compares a reading with limits entered by the user. The limits must come from a competent engineering assessment or applicable code; the instrument does not generate them.
Treating thickness gauging as flaw detection
Thickness gauges provide local wall values. Crack detection and discontinuity evaluation require appropriate flaw-detection equipment, setup and training.
Pre-purchase checklist
Send the supplier the following information rather than asking only for “an ultrasonic thickness gauge”:
- material and grade;
- nominal wall and lowest expected remaining wall;
- component type, pipe outside diameter and available contact area;
- surface condition and coating thickness, if known;
- surface temperature during measurement;
- whether readings are spot checks or a recorded survey;
- applicable inspection standard or internal procedure;
- required probe, data export and documentation.
If the application involves a difficult casting, hot surface, small pipe or heavily corroded back wall, request confirmation on a representative sample before purchase.
Frequently asked questions
Can an ultrasonic thickness gauge measure from one side?
Yes, when ultrasound can propagate through the material and a resolvable back-wall reflection returns to the probe. One-sided access is a central advantage of manual pulse-echo thickness measurement.2
Can it measure through paint?
Only within the supported coating-penetration conditions. The current LS211, LS212 and LS213 manuals specify the standard probe, 0.2–2.5 mm coating and 4–60 mm 45# steel substrate.456
Which probe is intended for cast iron?
The 2 MHz ZT-12 coarse-crystal probe. Performance should still be confirmed on the actual casting because grain structure and attenuation vary.
Does the gauge measure corrosion depth?
It determines remaining local wall thickness along the sound path. Corrosion loss is inferred by comparison with verified nominal or previous thickness data.
Which model suits routine steel pipe checks?
If the standard probe fits and 0.8–350 mm covers the required range, LS213 is the straightforward option. LS212 adds a 0.001 mm display step below 10 mm; LS211 extends the standard-probe upper range to 600 mm. The inspection need, not the largest number, should decide.
Make the sound path your first specification
The most defensible selection is the gauge-and-probe combination that produces a stable, repeatable echo on the actual material and geometry. Start with the component, choose the probe, verify coating and temperature conditions, and only then decide how much range and display resolution you need.
View the AIMITEK ultrasonic thickness gauge range or contact AIMITEK with the material, wall range, pipe diameter, coating and temperature. Ask for the exact probe and current manual revision to be confirmed in the quotation.
References
- ISO 16809:2025 — Non-destructive testing — Ultrasonic thickness determination, ISO, Edition 3, June 2025; abstract and status accessed 16 September 2026.
- ASTM E797/E797M-21 — Standard Practice for Measuring Thickness by Manual Ultrasonic Pulse-Echo Contact Method, ASTM International; scope and significance/use accessed 16 September 2026.
- ISO 16831:2025 — Characterization and verification of ultrasonic equipment for the determination of thickness, ISO, Edition 2, May 2025; abstract and status accessed 16 September 2026.
- LS211 Ultrasonic Thickness Gauge User Manual V2.44, Shenzhen Linshang Technology Co., Ltd.; downloaded and checked 16 September 2026.
- LS212 Ultrasonic Thickness Gauge User Manual V2.44, Shenzhen Linshang Technology Co., Ltd.; downloaded and checked 16 September 2026.
- LS213 Ultrasonic Thickness Gauge User Manual V1.44, Shenzhen Linshang Technology Co., Ltd.; downloaded and checked 16 September 2026.
- 27MG Ultrasonic Thickness Gauge User’s Manual, Revision E, Evident, September 2022; high-temperature guidance and flaw-detection limitation.
- Linshang LS211/LS212/LS213 Ultrasonic Thickness Gauges, AIMITEK; current product family and internal destination accessed 16 September 2026.