A roundness tester measures geometric form (circularity, cylindricity, runout) on a turned, ground or bearing component, while a thickness gauge measures how thick a wall, sheet, film or coating is — the two answer different questions on the same shop floor and are not interchangeable in a dimensional QA loop.
Specifying the wrong one wastes calibration budget and audit time: a roundness tester cannot tell you if a 2 mm steel pipe wall is below tolerance, and a thickness gauge cannot flag a 12 µm out-of-round on a journal. The decision comes down to the dimension you are controlling, the resolution you need, and the surface you are probing.
Roundness Tester: Scope, Output and When to Use It
A roundness tester (also called a circularity tester) rotates a workpiece on a precision spindle while a displacement probe traces radial deviation, then resolves the result against a least-squares or minimum-zone reference circle [S1]. Resolution is sub-micrometre on benchtop models; typical full-scale range sits at ±25 µm to ±500 µm depending on the probe and amplifier gain.
Use it when the controlling spec is geometric: bearing raceways, sealing lips, piston bores, shaft journals, valve stems, and any turned component called out on a print with a roundness symbol (⌭) or a cylindricity callout. Cylindricity and concentricity measurements require a dedicated tester with axial travel or dual-axis probing — a thickness gauge has no equivalent output.
Thickness Gauge: Three Sub-Families, Three Different Targets
The "thickness gauge" label hides three very different sensing technologies, and confusing them is the most common spec error in incoming-inspection labs. Coating thickness gauges (eddy current/magnetic induction) target paint, powder, zinc and plating layers on metal substrates; one ATO-listed digital paint coating thickness gauge covers 0-1300 µm with ±3%+2 µm accuracy and 0.1 µm resolution at $140.76 [S6]. Ultrasonic thickness gauges (UT) measure wall thickness through-coating on steel, plastic, aluminium and glass; entry models span 1.0-245 mm with 0.1 mm resolution, mid-range UT hits 1-225 mm on metal/PVC [S3], and floor/rock units push to 40-800 mm at $1,150. Mechanical/dial gauges, such as Cometech's SM-series portable rubber/plastic gauge, cover soft-material sheets with a large analog dial and no electronics [S1].
Selection rule: pick coating gauges for non-destructive film measurement on metal, pick UT gauges for through-thickness on pipes/tanks without cutting a coupon, and pick mechanical dial gauges for soft elastomer/plastic sheet where the probe pressure of a UT transducer would compress the sample. For pipe-wall and pressure-vessel work, see the ultrasonic thickness gauge reference page.
Roundness vs Thickness: A Four-Criterion Decision Frame
Lining the two up against the same decision criteria makes the call mechanical for most spec engineers. (1) Measurement target: roundness tester = geometric form; thickness gauge = wall/film/coating thickness. (2) Typical resolution: roundness testers reach 0.01 µm on the radial axis; entry coating gauges are 0.1 µm, entry UT gauges are 0.1 mm. (3) Probe interaction: roundness uses a non-contact or low-force displacement probe on a rotating part; UT uses gel-coupled acoustic pulse; coating gauges use a spring-loaded tip that must rest flat on the part. (4) Output unit: roundness is reported in µm (form error); thickness is reported in µm or mm (absolute dimension). [S1]
Concrete pairing: if a print says "12.000 ±0.005 mm wall, surface finish 0.4 Ra", you need a thickness gauge for the wall and a surface roughness tester for the finish — a roundness tester is irrelevant. If a print says "bore ⌭ 0.008 mm", you need a roundness tester, and a thickness gauge is the wrong tool. The coating thickness gauge page covers the film-side spec; the roundness tester page covers the form-side spec.
Who Should Specify Which Instrument
Specify a roundness tester if you are a bearing manufacturer, hydraulic-pump shop, automotive component supplier, or any QA lab that releases parts against ISO 1101 geometric-tolerance symbols. Benchtop units from makers such as Tokyo Seimitsu, Taylor Hobson, and Mitutoyo dominate this niche; the typical price band for a production-floor unit sits in the tens of thousands of USD and depends on centring stage, column height and column travel. [S2]
Specify a thickness gauge if you are a galvanizer, powder-coater, shipyard, pressure-vessel fabricator, pipeline operator, rubber goods maker, or maintenance crew doing wall-thickness surveys. Coatings shops buy eddy-current/magnetic units in the $100-$500 range [S6]; pipeline NDT teams buy UT gauges in the $100-$1,200 range [S3]; rubber/plastic processors buy analog dial units under $200 [S1]. Beijing TIME High Technology Ltd. lists thickness gauges, ultrasonic flaw detectors, and coating thickness gauges alongside hardness and roughness testers in its main product range [S4].
Limitations, Failure Modes and Cross-Checks
Roundness testers are sensitive to part cleanliness, datum centring, and spindle runout — a dirty journal or a bent part will read as a form error. Always run a master ball or ring artefact at the start of a shift; ISO 1101 and the relevant roundness-test method govern centring strategy. UT gauges fail on highly attenuative materials (cast iron, fibreglass, very coarse-grain austenitic stainless), on rough or painted surfaces without couplant, and on geometries where the back-wall echo is masked by a competing interface. When in doubt, cut a coupon for a force gauge verified micrometer read, or hand the part to a deadweight tester equipped lab for a higher-traceability path. [S3]
Standards, Sourcing and Audit Trail
Roundness specs trace to ISO 1101 (geometrical product specifications) and the roundness/cylindricity measurement chain in ISO 12780 / ISO 12781; instrument calibration uses ring and ball artefacts traceable to a national length standard. Thickness-gauge specs trace to ISO 19840 (coating thickness on rough substrates), SSPC-PA2 (coating thickness measurement), and ASME Section V Article 5 (UT thickness gauging); calibration uses coated thickness standards and step wedges traceable to a national metrology institute. On the procurement side, made-in-china supplier Far Asia Co. of Beijing markets TT210/TT220/TT230/TT260 coating thickness gauges under ISO 9001 [S5], and the broader Beijing TIME catalogue sits behind an ISO 9001 management system [S4]. Keep the certificate PDF, the probe serial, the artefact log, and the operator's initials on the same record so the next auditor can rebuild the measurement in minutes.
Next node: pull this article, the thickness gauge reference page, and the roundness tester reference page into a single training packet for new inspectors, and track two trackable signals — (1) the print symbols on the next 20 released parts, and (2) the calibration dates on the ring/ball artefacts and step wedges currently sitting in the metrology cabinet.