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How to Use an Ultrasonic Thickness Gage
Guide · Updated October 28, 2026
An ultrasonic thickness gage sends a sound pulse into the part from one side and times the echo off the back wall. Thickness = velocity × time ÷ 2. It measures pipes, tanks and castings where you cannot reach both sides with a micrometer.
Make calibration blocks
Use gage blocks or measured steps as calibration standards. The free Gage Block Calculator builds the stack.
Step by step
- Set the material: Pick the material or enter its sound velocity.
- Zero the probe: Do the probe zero on the gage's built-in disk if it has one.
- Calibrate: Calibrate on a block of the same material and similar thickness.
- Clean the surface: Remove scale and loose paint.
- Apply couplant: Gel or glycerin fills the air gap; sound will not cross air.
- Measure: Press the probe flat; read when steady.
- Map: Take several readings; record the minimum.
Worked example: steel wall
Evident (Olympus) lists longitudinal velocity for 1020 steel as 0.232 in/µs (5,890 m/s) and 4340 steel as 0.230 in/µs (5,850 m/s). A round-trip echo of 2.15 µs in 1020 gives 0.232 × 2.15 ÷ 2 = 0.2494 in. If the gage velocity were set 2% low (0.2274 in/µs), it would read 0.2444 in: velocity error goes straight into the reading, which is why you calibrate on the same material.
Two-point calibration
Measure a thin and a thick standard, for example 0.100 in and 0.500 in steel gage blocks. The gage solves for velocity and zero offset. Readings between the two are most trustworthy.
Coatings
Paint is slower than steel and can add to the reading. Use a gage with echo-to-echo mode to read through coatings, or remove the paint.
Curved surfaces
On small pipes use a small dual-element probe and align the split across the pipe axis per the maker's instructions.
Common problems
No reading: no couplant or rough surface. Reads double: thin part beyond the probe's range. Unstable: corrosion pitting on the back wall; take more readings and use the minimum.
Worked example: finding the velocity yourself
No table value for your alloy? Measure a sample with a micrometer: 0.4000 in. The gage reads 0.4120 in at a 1020 steel velocity of 0.232 in/µs. True velocity = 0.232 × 0.4000 ÷ 0.4120 = 0.2252 in/µs. Enter that, re-check the sample, and it should read 0.4000 in.
Tips
- Gage blocks: build a stack.
- Tube mic: tube micrometer.
- Temperature: temperature compensation.
FAQ
How does it work?
Thickness = velocity × echo time ÷ 2.
Sound velocity in steel?
About 5,890 m/s (0.232 in/µs) for 1020 steel per Evident; calibrate on your material.
Why couplant?
Sound will not cross an air gap.
How do I calibrate?
On known blocks of the same material, thin and thick.
Can it read through paint?
Only in echo-to-echo mode.
Related guides and calculators
- How to Build a Gage Block Stack
- How to Use a Tube Micrometer for Wall Thickness
- Gage Block Temperature Compensation in the Shop
- How to Check a Micrometer with Gage Blocks
- How to Use a Blade Micrometer
- How to Use Box Parallels
- All free machinist calculators
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