Technical Foundation
What Is a Thickness Gage?
A thickness gage measures material thickness or coating thickness using one of several distinct methods, and the right method depends on what's being measured and from how many sides it can be accessed. Ultrasonic gages send a sound pulse through the material and measure the time for it to echo off the back wall, allowing wall thickness measurement from one side only — essential for inspecting pipe, tank, and pressure vessel wall thickness for corrosion loss without cutting the part open. Dial and digital mechanical gages use a spring-loaded contact point against a fixed anvil, requiring access to both sides of the material, common for sheet stock and foil.
Coating thickness gages use magnetic induction (for coatings on ferrous steel) or eddy current (for non-magnetic coatings on non-ferrous metal) to measure a coating layer's thickness non-destructively, without damaging the coating — a method distinct from and not interchangeable with base material thickness gaging. Accuracy on ultrasonic gages depends heavily on proper transducer coupling (a liquid couplant between the probe and surface) and accurate sound-velocity calibration for the specific material being tested, since different metals and plastics transmit sound at different speeds.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Measurement Method | Ultrasonic (one-side access), mechanical dial/digital (two-side), magnetic/eddy current (coating) | Method choice depends on material access and whether base metal or coating thickness is being measured |
| Range | Varies by method — ultrasonic often 0.02"-8"+, coating gages typically mils/microns | Range must cover the expected thickness with margin, especially for corrosion-loss monitoring near minimum wall |
| Material Calibration | Sound velocity setting (ultrasonic) or substrate type (coating gages) | Gage must be calibrated to the specific material's sound velocity or magnetic properties for accurate readings |
| Coating Gage Type | Magnetic induction (ferrous substrate) vs. eddy current (non-ferrous substrate) | Wrong gage type for the substrate metal produces inaccurate or no reading |
| Resolution | Typically 0.001" mechanical, finer on digital ultrasonic units | Corrosion trending and thin-wall inspection need finer resolution than general stock measurement |
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Applications
Where Thickness Gages Get Used
Thickness gages are specified based on whether the job requires base material measurement, coating measurement, or non-destructive one-side wall thickness inspection.
Pipeline & Pressure Vessel Inspection
Ultrasonic thickness gaging to monitor corrosion and erosion wall loss without shutting down or cutting into equipment.
Metal Fabrication & Stock Verification
Dial and digital thickness gages verifying incoming sheet and plate stock meets specified gauge.
Automotive Paint & Coating Inspection
Magnetic and eddy current coating thickness gages verifying paint and primer film build on vehicle panels.
Industrial Coatings & Powder Coating QC
Coating thickness verification ensuring specified dry film thickness on protective and decorative coatings.
Tank & Storage Vessel Inspection
Ultrasonic gaging programs tracking wall thickness trends over the service life of storage tanks.
Aerospace & Precision Manufacturing
Precision thickness verification on formed and machined thin-wall components.
FAQ
Thickness Gages Questions, Answered
When do I need an ultrasonic gage instead of a mechanical dial gage?
Ultrasonic gages measure thickness through the material from a single accessible side, essential for pipe, tank, and vessel wall inspection where the inside surface can't be reached. Mechanical dial or digital gages require access to both sides of the material (the part fits between the anvil and the contact point), making them suitable for sheet stock and foil but unusable for closed vessels or one-side-access inspection.
Why do I need to set the sound velocity on an ultrasonic thickness gage?
Different materials transmit ultrasonic sound at different speeds — steel, aluminum, and plastics all have distinct sound velocities — and the gage calculates thickness from the round-trip echo time using that velocity value. Using the wrong material's velocity setting produces a thickness reading that's proportionally wrong, so the gage must be calibrated for the specific material being measured, ideally verified against a known-thickness sample of that same material.
What's the difference between magnetic and eddy current coating thickness gages?
Magnetic induction gages measure non-magnetic coatings (paint, powder coat, galvanizing) on a magnetic (ferrous steel) substrate. Eddy current gages measure non-conductive coatings on non-ferrous metal substrates like aluminum. Using the wrong gage type for your substrate metal either won't read at all or will give an inaccurate result — always match the gage to the base metal, not just the coating type.
What is couplant and why does my ultrasonic gage need it?
Couplant is a liquid gel applied between the ultrasonic transducer and the test surface to eliminate the air gap, which would otherwise reflect nearly all the sound energy and prevent a reading. Without proper couplant application, an ultrasonic gage will give erratic or no readings even on a perfectly good transducer.
How do I use a thickness gage to monitor corrosion over time?
Establish a grid of fixed measurement points (often marked or documented by location) on the vessel or pipe, take an initial baseline ultrasonic reading at each point, then re-measure the same points on a set inspection interval to track wall loss trends over time. Consistent point location and gage calibration between readings is critical for the trend data to be meaningful.
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