Ultrasonic thickness measurement (UTM) is a method of performing non-destructive measurement

Nondestructive testing

Nondestructive testing or Non-destructive testing is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage....

 (gauging) of the local thickness of a solid element (typically made of metal, if using ultrasound testing for industrial purposes) basing on the time taken by the ultrasound wave to return to the surface. This type of measurement is typically performed with an ultrasonic thickness gauge

Ultrasonic thickness gauge

An Ultrasonic thickness gauge is a measuring instrument for the non-destructive investigation of a material's thickness using ultrasonic waves....

.

Ultrasonic waves have been observed to travel through metals at a constant speed characteristic to a given alloy with minor variations due to other factors like temperature. Thus, given this information, called celerity, one can calculate the length of the path traversed by the wave using this simple formula:



where

is the thickness of the sample

is the celerity of sound in the given sample

is the traverse time

The formula features division by two because usually the instrumentation emits and records the ultrasound wave on the same side of the sample using the fact that it is reflected on the boundary of the element. Thus, the time corresponds to traversing the sample twice.

The wave is usually emitted by a piezoelectric cell built into the measurement sensor

Ultrasonic thickness gauge

An Ultrasonic thickness gauge is a measuring instrument for the non-destructive investigation of a material's thickness using ultrasonic waves....

 head and the same cell is used to record the reflected wave. The sound wave has a spherical pattern of propagation and will undergo different phenomena like multipath reflection or diffraction. The measurement does not need to be affected by these since the first recorded return will normally be the head of the emitted wave traveling at the shortest distance which is equivalent to the thickness of the sample. All other returns can be discarded or might be processed using more complicated strategies.

Advantages

• Non-destructive technique
• Does not require access to both sides of the sample
• Can be engineered to cope with coatings, linings, etc
• Good accuracy (0.1 mm and less) can be achieved using standard timing techniques
• Can be easily deployed, does not require laboratory conditions
• Relatively cheap equipment

Disadvantages

• Usually requires calibration for each material
• Requires good contact with the material
• Cannot take measurement over rust
• Requires coupling material between the measured surface and the probe.

Typical usage

UTM is frequently used to monitor metal thickness or weld quality in industrial settings. NDE Technicians equipped with portable UTM probes reach steel plating in sides, tanks, decks and the superstructure. They can read its thickness by simply touching the steel with the measurement head (transducer). Contact is usually assured by first removing visible corrosion scale and then applying petroleum jelly or another couplant before pressing the probe against metal. This easy procedure is used to inspect metal to determine quality and safety without destroying or compromising its integrity. It is a requirement of many classification societies

Classification society

A classification society is a non-governmental organization that establishes and maintains technical standards for the construction and operation of ships and offshore structures...

Ultrasound thickness measurement is also widely used in medicine as a diagnostic imaging tool. Ultrasound is used to monitor a gestating fetus, organs, muscles, veins, and other human and animal tissues.

Classification Requirements for UTM Hull Surveys

Classification societies have detailed requirements for the thickness measurement of hull structures. These requirement depend greatly on vessels type, age and length. All IACS member classifications have similar requirements since they need to comply with IACS guidelines . The allowable diminution thickness depends on the building rules of each classification. Also depending on type the societies request 1 or 2 operators. The UTM operators need to be Level II certified according with SNT-TC-1A or similar standard. Also the company that performs the ultrasonic thickness measurement survey must be approved by the classification that the vessel is registered with. The classification society review the documented procedures of the UTM company and audit them on board in order to issue an approval certificate. Finally the equipment used need to be type approved by the classifications.