These second, third and fourth echoes occur as a result of the sound
reflecting backwards and forwards between the front and back walls of
the material under test.
This is extremely important as it means the time delay being measured represents the thickness of the material only and does not include the thickness of the paint or coating.
This UT measurement technique is so successful and accurate it has been accepted worldwide by the marine industry and adopted by other industries such as oil and gas.
The diagram above shows which echoes are measured and that the time delays refer to the metal thickness only – eliminating the material coating completely.
The Classification Societies (DNV, Lloyds and others) also recognise that the results provided by Multiple Echo thickness gages are valid for ships surveys to prove the integrity of the ship for insurance purposes and to prove to a new owner that the ship is sound. These gages indicate where the thickness of steel has been reduced by corrosion so that the surveyor can determine if the steel needs to be replaced.
So how does Ultrasonic Thickness measurement work...?
The Ultrasonic Thickness Gage works by sending a short pulse of high frequency sound through the material being measured. The gage then measures the time taken for an echo to be received from the back wall. With this data, the thickness of the material is determined.
With MULTIPLE ECHO, measurements of the time delay between any three consecutive back wall echoes are taken. This means the coating thickness – up to 6 mm (and up to 20 mm using our 'Deep Coat' mode) – is completely ignored. As these time delays must match before a measurement is displayed, the gage has automatically verified that the reading given is a TRUE MEASUREMENT.
An Ultrasonic Thickness Gage must be calibrated on a sample of a known thickness or to the velocity of sound of the material it is measuring.
Coatings such as paint, epoxy or bitumen have a velocity of sound which is around one third that of steel.