The speed of sound is the parameter that links the time of flight of the pulse to the thickness: knowing it accurately is essential for ultrasonic measurement, and measuring it also makes it possible to characterise materials.
- Key parameter: links time of flight and thickness
- Material dependent: steel ~5920 m/s, aluminium ~6320 m/s
- Calibration: on a known thickness to determine v
- Velocity mode: measurement of v on a known thickness
- Characterisation: hardness/nodularity of cast iron
- Influenced by: temperature and microstructure
- Measurement accuracy: requires the correct v
Ultrasonic thickness measurement is based on the relation s = v × t / 2, where t is the time of flight measured by the instrument and v is the speed of sound in the material. An error in the velocity translates directly into an error in the thickness: this is why the velocity must be set correctly for every material.
The speed of sound is not merely a figure to be entered: measuring it provides information about the material itself, because it depends on density, elasticity and microstructure.
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Why the velocity is decisive
Every material transmits ultrasound at a characteristic velocity: about 5920 m/s in steel, 6320 m/s in aluminium, and lower values in plastics. Since the instrument measures a time, the conversion into thickness requires the correct velocity: using the wrong value produces a systematically incorrect measurement.
For this reason thickness gauges allow the material to be selected or the velocity to be entered manually, and they provide calibration on a known thickness (a reference block) in order to determine exactly the actual velocity of the batch under examination.
Measuring the speed of sound
The reverse procedure is equally useful: by measuring the time of flight on a part of known thickness, the instrument calculates the speed of sound in the material. Some instruments have a dedicated velocity mode (VX), which expresses the measurement directly in m/s.
The velocity obtained in this way is an indicator of the properties of the material: in ductile cast iron, for example, the speed of sound is correlated with the degree of nodularity of the graphite, and therefore with the mechanical properties; variations in velocity point to differences in microstructure or heat treatment.
Factors that influence the velocity
- Type of material and alloy: every material has a characteristic velocity.
- Temperature: the velocity varies with the temperature of the part, and has to be compensated for in high-temperature measurements.
- Microstructure and heat treatments: they influence density and elasticity.
- Anisotropy: in some materials the velocity depends on the direction.
Instruments and good practice
For accurate measurements it is advisable always to calibrate the instrument on a sample of the same material and at the same temperature as the part to be measured. The Dakota NDT thickness gauges distributed by RODER allow the velocity to be set, one- or two-point calibration, and, in the advanced models, direct measurement of the velocity. You will find the models on the linked catalogue pages.
Related products
RODER instruments for this topic. Go to the catalogue pages for models, technical data and documentation.
Dakota ultrasonic thickness gauges
UT thickness gauges for corrosion and precision (ZX, CMX, MX, PMX, CMX10-DL series).
Frequently asked questions
Because the thickness gauge measures a time of flight and converts it into thickness using the velocity: an incorrect value produces an incorrect measurement.
About 5920 m/s; in aluminium about 6320 m/s. Every material has its own value.
By measuring the time of flight on a part of known thickness: the instrument calculates the corresponding velocity.
Yes: it is correlated with density, elasticity and microstructure; in ductile cast iron it indicates the degree of nodularity.
Contacts & Information
Contact for general information : info@roder.it
Machine Vision Division of RODER : www.visiotronix.com
RODER Instruments Division : www.innovacheck.com
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