Ultrasonic Measurement

The bottom line

An ultrasonic thickness gauge measures “the time of the echo”, and no echo means no reading. Every coupling action serves one purpose: send the sound into the part effectively, and bring the echo back effectively.

Choosing the couplant

Condition	Recommended type	Note
Room temp, horizontal, part facing up	Water- or oil-based couplant, glycerin	Good flow, easy to clean
Vertical and overhead surfaces	High-viscosity couplant	Prevents running off, keeps contact steady
Hot workpiece	High-temperature couplant	Room-temp couplant fails or even vaporises
Rough surface	High-viscosity couplant	Fills micro peaks/valleys, improves acoustic contact
Surface that must stay clean	Choose per process; dry coupling or low-frequency probe if needed	Confirm the permitted medium first

Probe technique

Clean: before measuring, remove rust, scale, mud and loose coating — these scatter the sound;
Apply: put couplant on the spot, spread it evenly, avoid trapping bubbles;
Seat: keep the probe face perpendicular to the surface and flat overall, do not press only one side;
Pressure: keep steady, consistent force. Too little gives poor coupling; too much may thin the couplant layer and shift the reading;
Hold still: keep the probe still while reading; avoid sliding on the surface;
Verify: take 3 readings at one point and watch the spread. A large spread means a coupling or surface problem.

Troubleshooting order when readings misbehave

When the reading jumps, reads low or fails to appear, work through this order — the order matters because it follows probability:

Coupling: re-apply couplant, clean the probe face, adjust pressure. This is the most common cause;
Surface: rust, scale, excessive roughness, uneven surface;
Workpiece: internal scaling, corrosion pitting, internal defects that scatter the echo;
Settings: wrong sound speed, unsuitable measuring range;
Instrument: worn probe, loose cable contact, low battery.

Handling different surfaces

Scale and rust: clean with sandpaper or a wire brush down to bright metal, but do not grind the part itself thin;
Paint and coating: remove if grinding is allowed; if not, switch to through-coat mode;
Hot surface: use a high-temperature probe and couplant, and respect the probe temperature limit;
Curved surface: use a small-diameter probe, increase contact pressure and keep it perpendicular.

Common misconceptions

❌ “A jumping reading means the gauge is broken” — in the vast majority of cases it is coupling; re-applying usually restores it at once.
❌ “Any couplant will do” — viscosity and temperature rating differ; the wrong one on an overhead or hot job fails outright.
❌ “More pressure is always better” — too much pressure makes the couplant layer too thin or squeezes it out, lowering coupling quality.

Frequently asked questions

What causes most ultrasonic thickness errors?

Sound-velocity setting and coupling quality account for the vast majority of errors; actual gauge faults are relatively rare. Rule these two out before suspecting the instrument.

Why is a small velocity error worse on thick parts?

Because velocity error is proportional: the thicker the reading, the larger the absolute deviation. A 2% velocity error on a 100 mm part puts you off by about 2 mm.

How do I correct the sound velocity setting?

Back-calculate it from a same-material block of known thickness. Measure the block and adjust the velocity until the reading matches the known value, then use that setting on the part.

My readings jump or won’t read. What is the fastest check?

Re-apply the couplant. Poor coupling is the prime cause of jumping or unreadable readings, and re-applying couplant often pinpoints the problem immediately.

How do I tell if the problem is the part or the gauge on site?

Measure a same-material block of known thickness once. If the block reads normally but the part is abnormal, the problem is in the part (rough surface, internal rust, defects, or thickness below the lower limit), not the gauge.