Technology

The diagram above shows which echoes are measured and that the time delays refer to the metal thickness only – eliminating the material coating completely.

Instead of measuring the time taken for an ultrasonic pulse to travel through a piece of material and to be reflected back to the source, a method was developed by Cygnus Instruments to measure the time between the second, third and fourth echoes to be received.

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 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 and the process industry – especially when measuring pipe thickness.

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 gauges 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 gauges also 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 it work…?

The Ultrasonic Thickness Gauge works by sending a short pulse of high frequency sound through the material being measured. The gauge 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.

Single echo technology only reads the time taken for one back wall echo to pass through both the material under test AND its surface coating. This gives an apparent material thickness which is wrong, because the sound has passed through the surface coating as well. Coatings such as paint, epoxy or bitumen have a velocity of sound which is around one third that of steel. This gives rise to a vastly greater material thickness reading than is really the case, because the surface coating is included.

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. The gauge does not need to be zeroed or calibrated at switch on. And because these time delays must match before a measurement is displayed, the gauge has automatically verified that the reading given is a TRUE MEASUREMENT.

An Ultrasonic Thickness Gauge must be calibrated 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.