Sound Velocity in Materials: A Table for Thickness Gauging

Sound velocity for a thickness gauge is the longitudinal ultrasonic velocity in the part material. Per the Evident table it is 5890 m/s in 1020 steel, 5740 m/s in 302 stainless steel, 6320 m/s in aluminum and 4660 m/s in copper. Table values are approximate: for accurate readings, calibrate velocity on a sample of the same material.

Why a thickness gauge needs sound velocity

An ultrasonic thickness gauge measures time, not thickness. It times how many microseconds a pulse takes to cross the wall and come back, then converts the time to thickness using the formula from the Evident tutorial:

T = V × t / 2, where T is thickness, V is the sound velocity in the material and t is the measured round-trip transit time.

Thickness is directly proportional to velocity. If the gauge is set 2 % above the actual velocity, every reading will be 2 % too high. That is why Evident stresses that a measurement can be no more accurate than the calibration. How the gauge itself works is covered in "Ultrasonic thickness gauge: how it works and its limitations".

Longitudinal sound velocity table

Values are taken from the Evident "Material Sound Velocities" table (appendix to the thickness gauging tutorial). They are typical longitudinal velocities, suitable as a starting setup.

Metals and alloys

Material	m/s	in/µs
Steel 1020 (carbon)	5890	0.2320
Steel 4340 (alloy)	5850	0.2300
Steel 302 (austenitic stainless)	5740	0.2260
Aluminum	6320	0.2490
Copper	4660	0.1830
Brass	4430	0.1740
Cast iron (soft)	3500	0.1380
Cast iron (hard)	5600	0.2200
Nickel (pure)	5630	0.2220
Inconel	5820	0.2290
Titanium	6100	0.2400
Molybdenum	6250	0.2460
Tungsten	5180	0.2040
Zinc	4170	0.1640
Tin	3320	0.1310
Lead	2160	0.0850
Zirconium	4650	0.1830
Beryllium	12900	0.5080
Iron oxide (magnetite)	5890	0.2320

Plastics, composites and other materials

Material	m/s	in/µs
Acrylic (Perspex)	2730	0.1070
Lucite	2680	0.1060
High density polyethylene (HDPE)	2460	0.0970
Low density polyethylene (LDPE)	2080	0.0820
Polyvinylchloride (PVC)	2395	0.0940
Polystyrene	2340	0.0920
Nylon	2600	0.1020
Polyamide	2200	0.0870
Fiberglass	2740	0.1080
Graphite/epoxy composite	3070	0.1200
Rubber (polybutadiene)	1610	0.0630
Silicone	1485	0.0580
Silicon	9620	0.3790
Diamond	18000	0.7090

Liquids

Material	m/s	in/µs
Water (20 °C)	1480	0.0580
Glycerin	1920	0.0760
Motor oil	1740	0.0690

Velocity in liquids matters, for example, with immersion transducers and delay lines, and glycerin and water are common couplants.

Why velocity is calibrated on a sample

Evident explicitly calls the table a general guide. The actual velocity in a given part can differ noticeably from the table because of:

• chemical composition and microstructure;
• grain or fiber orientation;
• porosity;
• temperature.

The spread is largest in cast metals, fiberglass, plastics and composites. In castings, velocity can change from point to point because of varied cooling rates and grain anisotropy; in fiberglass, because of a varying resin/fiber ratio. In these materials a setup "from the table" will almost certainly be off.

Two-point calibration per Evident

1. Take two samples of the same material as the part, with precisely known thicknesses. They need not be commercial test blocks, but the material and, ideally, the surface finish must match.
2. The thickness ratio should be at least 2:1, optimally 5:1 or more. Both thicknesses should lie within the range you will measure.
3. On the thick sample, run velocity calibration and enter its actual thickness.
4. On the thin sample, run zero calibration and enter its thickness.
5. Check readings on a sample of intermediate thickness.

How to spot the error: if the error grows with thickness, the velocity is wrong; if readings are off by a fixed amount on every step, the zero is wrong. In either case repeat the calibration. A step wedge is convenient for this, but for a non-standard material it is better to make samples from the material itself.

Recalibrate when the material or transducer changes, or when the part temperature changes significantly.

How temperature changes sound velocity

Per Evident, sound velocity in all materials depends on temperature:

• it normally decreases as the material gets hotter and increases as it gets colder;
• it changes abruptly at freezing and melting points;
• the effect is much greater in plastics and rubber than in metals and ceramics.

The practical rule: calibrate velocity at the same temperature as the measurement. Measuring a hot pipe with a room-temperature setup often leads to significant error. For plastics and rubber, Evident says calibration at the measurement temperature is required.

On hot surfaces: standard contact transducers are damaged above approximately 50 °C, and above approximately 100 °C special high-temperature couplants are needed. More in the article "How to measure pipe wall thickness with ultrasound".

Which gauge to choose

Every ultrasonic thickness gauge takes a velocity setting; the difference is which transducers and modes it supports.

• Corrosion on steel pipes and vessels — 45MG: dual element transducers as standard, velocity range 0.508–18.699 mm/µs, single element transducers with the SE option.
• Precision measurement with single element transducers, multilayer materials, measurement through coatings — 39DL PLUS.

Model comparison is in "Which Olympus thickness gauge to choose"; to match a model to your task, use the thickness gauge selector. KEG TRK supplies Evident (Olympus) thickness gauges in Kazakhstan. Price on request.

Sources

• Evident. Material Sound Velocities — longitudinal velocity table and the variation caveat.
• Evident. Theory of Operation — formula T = V × t / 2, temperature effect.
• Evident. Velocity and Zero Calibration — two-point calibration.
• Evident. Factors Affecting Measurement — velocity variation in cast metals, fiberglass, plastics.
• Evident. How to Use Ultrasonic Thickness Gauges in Special Conditions — temperature and sound velocity.