For mechanical-strength assessment on in-service plant components, the ultrasonic thickness gauge is the workhorse, with bulk-material wall-thickness measurement down to typical 0.1 mm resolution on carbon-steel pipework being its standard role in corrosion surveys and fitness-for-service work.
The eddy current tester does not measure wall thickness; it characterises surface and near-surface conductivity and lift-off, with a demonstrated measuring range of 1 nm to 100,000 nm (100 µm) on the EddyCus TF lab 2020MT platform, which makes it a coating and surface-defect tool, not a bulk-strength tool [S1].
What Each Probe Actually Measures
Ultrasonic thickness gauges emit a pulsed acoustic wave, typically 2–10 MHz for general metals, time the back-wall echo, and derive thickness from the known sound velocity of the alloy, with sound-velocity values around 5,900 m/s for steel and 6,320 m/s for stainless being standard entries in the instrument menu [S2]. The method requires couplant, a clean acoustic path, and a measurable back-wall echo; on corroded or heavily pitted pipework, surface preparation and the use of dual-element (D-Mode) probes is common practice to bypass near-surface noise.
Eddy current testers drive a coil at frequencies from roughly 100 kHz to a few MHz, sense the impedance change caused by induced eddy currents in the conductive target, and infer either lift-off, conductivity, or a defect signature [S1]. The EddyCus TF lab 2020MT operates in a non-contact, single-point configuration with manual calibration, reading real-time sheet resistance and layer thickness on metal, glass, material, and foil substrates, with 1 nm minimum and 100,000 nm maximum thickness range stated on the OEM spec sheet [S1]. The method needs no couplant and no optical access, which is why it dominates thin-foil and coated-conductor QA, including inline cells.
Decision Matrix: Pick the Probe by Failure Mode
Use the following four-criterion table to shortlist the instrument for a given inspection task; the choice is driven by the failure mode, not by which instrument is cheaper on the day. [S5]
On layer/coating integrity (1 nm to ~100 µm), eddy current wins because the ultrasonic pulse cannot resolve a sub-micron coating over a thick substrate; the EddyCus TF lab 2020MT spec of 1 nm to 100,000 nm is the controlling number [S1]. On bulk wall loss (1 mm to 100 mm), ultrasonic wins because the eddy current skin depth at typical test frequencies penetrates only the top millimetre or so of carbon steel, well short of a 6 mm or 12 mm pipe wall. On conductivity verification of non-ferrous alloys, eddy current is the only practical bench method; conductivity meters in the same SURAGUS family, such as the EddyCus TF map 2525RM, are explicitly catalogued for this duty [S1]. On through-thickness defect sizing in welds, ultrasonic flaw detection (a sister category to ultrasonic thickness gauges, listed in the catalogues of multiple Chinese OEM manufacturers) is the matched technology, while eddy current is limited to surface-breaking cracks.
Mechanical Strength: Where the Two Actually Overlap
Wall-thickness loss is a primary driver of burst pressure in corroded pipework, and that is the variable the ultrasonic thickness gauge returns to the engineer as a quantitative input. The other variables that drive mechanical strength, including residual stress, hardness, and surface fatigue, are handled by separate NDT methods, with hardness testers, both Leeb and ultrasonic-contact-impedance types, being co-supplied by the same Chinese OEM tier (e.g. Shenyang Yushi, founded 1985 on a 15,000 m² site, and Beijing Dragon Electronics) that markets the UT thickness gauges [S5][S7].
Eddy current enters the mechanical-strength picture indirectly: it verifies the integrity of conductive coatings and platings, which protect the underlying alloy from corrosion-driven wall loss, and it detects surface-breaking fatigue cracks in non-ferrous components such as aluminium alloy aerospace skins and copper-alloy heat-exchanger tubes, where crack initiation is the precursor to a strength-limiting flaw [S3]. Eddy current is therefore an upstream quality gate (is the protective system intact, is the surface cracked?) while ultrasonic is the downstream residual-strength gate (how much wall is left).
Standards, Calibration, and Operator Reality
Both probe families are commonly delivered under ISO 9001 factory quality systems on the Chinese OEM side, with thickness gauge, hardness tester, and flaw detector lines from manufacturers such as the supplier listed on made-in-china.com and Beijing Dragon Electronics all sharing the same certification frame [S4][S7]. The benchtop eddy current thickness platform from SURAGUS uses manual calibration and offers software-assisted manual mapping plus data export, which is consistent with what a QA cell needs for traceability of a coating-thickness survey [S1].
Coating-thickness gauges using a hybrid eddy current and magnetic principle, such as the HC series, are sold at roughly US$300 per piece (MOQ 1 piece) and a stated production capacity of 1,000 pieces per month from one Chinese supplier, which gives a useful order-of-magnitude reference for the price band of an entry-level eddy-current-based coating gauge in 2026 [S6]. This price band is consistent with the role of eddy current coating gauges as a high-volume QA consumable, rather than a one-off lab instrument.
Limitations That Drive the Wrong-Result Risk
Eddy current thickness readings on ferrous substrates can be confused between coating thickness and substrate contribution unless the substrate is non-conductive or the coating is non-magnetic; dual-mode (eddy current + magnetic induction) gauges exist precisely to disambiguate this [S6]. For an eddy current tester used on a ferromagnetic steel substrate to characterise a non-conductive coating, the operator must verify that the substrate contribution is properly nulled, otherwise the reading reflects a mix of coating thickness and substrate permeability variation rather than pure coating thickness.
Ultrasonic thickness readings fail on rough, scaled, or heavily pitted surfaces without grinding, and on components where the back-wall echo is lost, for example on austenitic stainless welds with coarse grain or on cast iron with flake graphite, the reading can be off by tens of percent or simply absent [S2]. In those cases, an alternative NDT route such as phased-array UT for austenitic welds or eddy current for surface defects is required, and the engineer should treat the UT thickness value as a screening number, not a final mechanical-strength input.
Selection Recommendation by Use Case
For corrosion-survey wall-thickness mapping on carbon-steel pressure equipment and process pipework, the ultrasonic thickness gauge is the correct primary tool, with D-Mode probes preferred on corroded external surfaces [S2]. For thin-metal-foil and conductive-coating QA in a bench or inline cell, the eddy current platform such as the EddyCus TF lab 2020MT with 1 nm to 100,000 nm range is the correct tool [S1]. For mixed-mode cells where a single instrument must handle both coating thickness on aluminium and wall thickness on steel, the practical move is to specify two instruments rather than one compromise unit, and to budget accordingly; the entry-level hybrid coating gauge is in the US$300 range while a bench ultrasonic thickness gauge with full velocity menu and through-coating capability sits in a higher price band [S2][S6].
Inter-Article Context
Conductivity-resolved eddy current work connects directly to bench QA infrastructure; the spec criteria for benchtop conductivity meters in laboratory QA benches overlap with the calibration side of an eddy current thickness setup. Where mechanical strength is governed by a coating or plating layer rather than the substrate, the comparison between these two probe families feeds into the broader NDT selection logic that drives a torque wrench tester vs force gauge corrosion-resistance decision map at the fastening end of the assembly. [S1]
A trackable near-term signal is the 2026-07 activity on the Shenyang Yushi (registered 2014) and Beijing Dragon Electronics supplier fronts, which are both maintaining ultrasonic thickness gauge, hardness tester, and flaw detector lines as a single bundled capability, suggesting that the Chinese OEM tier is positioning UT as the mechanical-strength axis of a multi-technique QA cell rather than a standalone product [S5][S7]. Watch for eddy current coating-gauge price points staying near the US$300 band through the next sourcing cycle as a leading indicator of where the high-volume NDT procurement is heading [S6].
For component-level specifications, see thickness gauge.