How to choose an ultrasonic thickness gauge
By Grant, Inspection Tool Specialists. Reviewed November 2026.
Three things decide which ultrasonic thickness gauge you need: whether you need only a wall reading or also need to see the signal behind it, the material and thickness range you measure, and the condition of the surface at the test point. Answer those and most of the range falls away.
This guide covers thickness gauges for wall and corrosion checks on metal, the probes and calibration blocks that go with them, and when a flaw detector is the right instrument instead.
How does an ultrasonic thickness gauge work?
The gauge sends a short sound pulse into the material through a probe and times the echo from the far wall. Thickness is the speed of sound in the material multiplied by the time, divided by two. Steel carries sound at about 5.9 mm per microsecond, so the gauge must be set to the speed for the material you are testing. A wrong speed gives a wrong thickness, and it is a common cause of unreliable readings.
A couplant, a thin gel or grease, is needed between the probe and the surface so the sound passes into the metal. Only one side of the part needs to be accessible.
Thickness reading, waveform or flaw detector?
| Type | What it shows | Use it for |
|---|---|---|
| Digital thickness gauge | A number only | Quick wall checks and corrosion surveys on clean, accessible surfaces |
| Thickness gauge with A-scan | The number plus the echo waveform | Confirming the reading comes from the back wall, on corroded, pitted or laminated material |
| Thickness gauge with A/B-scan and logging | The number, the waveform and a cross-section view, with stored readings | Mapping wall loss across a plate or pipe and keeping a record |
| Flaw detector | The waveform, used to locate and size flaws | Cracks, inclusions and weld inspection; needs training and a written procedure |
A digital gauge is enough when the metal is clean and the back wall is clear. An A-scan earns its cost when the metal is pitted or corroded, when you suspect laminations, or when a reading looks wrong and you need to see why. For remaining-wall checks on blasted steel, where the rust and coating are removed before testing, the waveform adds little, so a digital gauge is usually the right choice.
Probes matter as much as the gauge
- Straight-beam probe N05 (R3,220) for general thickness measurement on steel.
- Narrow-pulse probe P5EE (R5,200): a 5 MHz probe that the manufacturer lists as standard for the MT180, MT190, MT600 and MT660. Its stated range in steel starts at 2 mm.
- Micro-diameter probe N07 (R1,800) for local pitting, small areas and restricted access.
- Coarse-grain probe N02 (R4,080) for materials such as cast iron, which scatter the signal.
- High-temperature probe HT5 (R5,520) for hot surfaces. Check the maximum surface temperature on the datasheet before use.
Calibrate before you measure
- Set the speed of sound by measuring a block of the same material with a known thickness, rather than relying on a handbook value alone.
- Check the gauge on a step block before each batch, after changing probes, after a long break, and after a large change in temperature. Keep the record.
- Choose blocks that cover your thickness range. The 4-step B-101 blocks (R2,995) and 5-step B-101 blocks (R3,995) cover general work, and the thin-wall DB-101 and B-101 blocks (R5,100) suit thin walls.
- V1 and V2 blocks are used for angle probes and flaw detector work, and cost from about R4,100 to R9,100.
What limits a reading?
- Surface condition: rough, scaled or pitted surfaces give unstable readings. Clean the test point to bare metal, apply couplant, and use a small probe where pits are local.
- Temperature: hot surfaces need a high-temperature probe, and the gauge must be used within the probe's limits.
- Thin walls: every probe has a lower limit in a given material. Check the datasheet before you accept a reading near the bottom of the range.
- Coatings: some gauges state a through-coating function, such as the MT600. Test on a sample and confirm the coating limit before you rely on readings through paint.
- Laminations and pits: a single number can mislead. An A-scan shows whether the echo you are reading comes from the back wall or from something inside the metal.
- Bonded layers: an assembly such as steel bonded to rubber or ceramic is not a routine thickness job. The gauge may not separate the layers, so confirm that it can before you quote, and ask which face can be measured.
What our recent enquiries show
Our recent ultrasonic enquiries fall into three groups. The first is remaining-wall checks on steel that has been blasted and cleaned. Those usually need a gauge with fine resolution, a scan or minimum-thickness mode, a small probe for pitting and a steel block for calibration. The second is multi-layer assemblies, where the first question is whether the layers can be told apart. The third is cracks in castings and weld inspection, which is a flaw detection job and needs the material, the thickness range and the size of the defect before a useful quote is possible.
Which gauge should I buy?
| Application | Suggested starting point |
|---|---|
| Quick wall checks, tight budget | M100+ digital thickness gauge |
| Remaining-wall checks on blasted or corroded steel, field use | MT160 thickness gauge, with 0.01 mm resolution and a scan (minimum-thickness) mode |
| Several probe types, with the standard 5 MHz probe | MT190 multi-model gauge |
| Colour display, Bluetooth and a through-coating function | MT600 multi-model gauge |
| Thickness reading plus an echo waveform | TUM-4DL with A-scan |
| A/B-scan waveform with data logging | UM-5DL with A/B-scan and logging |
| Cracks, inclusions and weld inspection | TFD320 Plus flaw detector or MFD350B flaw detector |
As a guide to our current range, including VAT: digital thickness gauges from about R3,700; multi-model gauges from about R14,000 to R18,500; thickness gauges with A-scan from about R31,000; A/B-scan gauges with logging from about R50,000 to R73,000; flaw detectors from about R46,000 to R102,000. Calibration blocks start at about R3,000. Prices change, so check the product pages.
Questions we will ask you
Tell us the material and components to be tested, the minimum and maximum thickness, the defects you need to find if any, the surface condition and any coating, the temperature, whether you can reach only one side, and the standard or specification you must follow. Tell us too whether you need probes, calibration blocks and reporting software. If the work is for a client or under a procedure, send us the procedure, because it sets the calibration and reporting requirements. With that we can recommend a gauge and provide pricing.