The Basics of Ultrasonic Measurement

Taken from our most recent eBook entitled ‘Ultrasonic Thickness Gauges in Potentially Explosive Environments’, we take a look at the basics of ultrasonic measurement. 

An ultrasonic device operates by combining the creation of a pulse, or series of pulses of sound by measuring the return of those pulses from the target back to the device. These pulses and echoes are sent and received by a piezoelectric transducer.

Equipment that employs the pulse/echo to operate use transducers to:

• Convert electricity into sound = pulse

• Convert sound into electricity = echo

Simple thickness measurement devices are used to measure the time of flight of a sound wave between the pulse and the echo. The time between the pulse and the echo equates to two times the material thickness when multiplied by the known material velocity value.

Fig 1.0 Schematic representation of ultrasonic test device.

Fig 1.0 shows the schematic of a pulse/echo ultrasonic thickness measurement device. The pulser/receiver in this example excites the transducer, which fires a sound wave into the material and measures the time it takes for the rear surface echo to return to the transducer.

Since the time required for the wave to return is actually the time it takes for the wave to travel from the transducer to the rear surface and back to the transducer, this actually represents twice the thickness of the pipe wall.

Fig 1.1 shows the thickness measurement equation when using ultrasound

Fig 1.1 shows the thickness measurement equation when using ultrasound.

Measurement accuracy

Measurement accuracy ultimately depends on the accuracy of the time measurement, but more importantly, it depends on the velocity of sound in the material being measured.

Also known as the “material velocity,” this value changes from material to material and must be known if an accurate measurement is to be derived by the device.

Figure 1.2 shows a selection of approximate material velocities for typical industrial materials.

Fig 1.2 Material velocity examples.

These numbers can be difficult to remember, especially for NDT inspectors who move from one material to another. Higher-end ultrasonic gauges allow users to select material velocities from a drop-down list and can even store material information in a database that links the testing location, parameters and measured value for later analysis.

This is important, because the more accurate the material velocity data, the more accurate the UT device reading will be.

Thickness Measurement

Ultrasound is perfect for measuring thickness, because it requires access to only one side of the material being measured. In the case of a pipe, the pipe can even be filled with fluid and in operation, which means an outage is not required to conduct the test.

A simple thickness measurement can be enhanced by using predefined max/min alarms to help deskill the task of identifying problem areas.

Typical thickness measurement applications include:

• Thick cast metal parts

• Thick rubber tires and belts

• Most thick or sound-attenuating materials

• Fiberglass storage tanks

• Composite panels

Check out our UT5000 Ultrasonic Thickness Gauge video to see why you should use ours.

For more information on the UT5000 Ultrasonic Thickness Gauge and how it can prevent accidents in hazardous or potentially explosive environments, download our FREE eBook.

Ultrasonic Thickness Gauges in Potentially Explosive Environments.

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