Tube wall thickness is never uniform. Drawn tube walls vary around the circumference; welded tube varies at the seam; machined tube varies with setup. Specs are written around minimum wall — so the job is finding the thinnest point, not taking one reading and moving on.
Why one reading isn’t enough
Wall eccentricity means the thin side can be anywhere. The reliable method is a scanning measurement: a gage that rides the tube while you rotate it and slide it, with the dial showing the dip at the thin spot. Mechanical thickness gages are built for exactly this motion — no couplant, no material calibration, works on metal or plastic tube alike.
Matching gage reach to tube length
- Near the tube end: a standard 301 Series min-wall gage covers the first few inches.
- Deep into the tube: the 334 Series long reach gages put a contact far down the bore while you scan.
- Digital + data logging: the 656 Series electronic longer-reach gages add a digital readout and SPC output to the same deep-reach measurement.
Contact tips for round parts
Use a ball contact inside the tube and a flat or ball outside — the ball finds the true minimum on the curved inner wall instead of bridging it. All Dyer thickness gages offer multiple standard tip sets, with custom carbide contacts available in days.
Common questions
What accuracy can I expect? Direct reading gages hold about ±0.0005″; electronic versions resolve finer and feed data collectors.
Ultrasonic instead? If both surfaces are reachable, mechanical is faster and material-independent. UT earns its keep when you can only access the outside — full comparison here.
Checking tube against a minimum-wall spec? contact Dyer Gage or call the gage hotline at 1-800-631-3333 with the tube ID, length, and tolerance — reach and contacts get spec’d to the part.