How Does An Ultrasonic Thickness Gauge Work?

Are you looking for more information about how exactly your small, handheld device can accurately and reliably ascertain the thickness of a given material? If that is the case, then continue reading and you will gain a basic insight into the principles and methods used. This may help you understand your device and utilize the brilliant technology within more effectively.

What is an ultrasonic thickness gauge?

An ultrasonic thickness gauge is any electronic device that uses the emission and reception of sound waves to measure the thickness of a given material. They can come in different shapes and sizes, as well as having vastly different capabilities and usability. Depending on your industry role and requirements, you may find one device is a lot more useful than another because different scenarios may have need of different or multiple functions.

A thickness gauge has two main components; the body and the probe. The probe is connected to the body by an electrical cable, which sends and receives the required data from the body. The body itself contains sensitive and intricately organized computational equipment that is preprogrammed with everything you need to for comprehensive thickness measurement.

How does the gauge work?

The first step is knowing what you need to measure, which sounds simple enough but can be easy to get wrong. Every material has a different internal sound velocity, which is to say that sound travels through different materials at different speeds. This is a vital part of calculating the thickness of a material, because thickness is equal to the echo cycle time divided by the sound velocity. It uses the principles of the basic distance, speed, and time triangle in a much more in depth way.

In order to accurately calculate the thickness, the device needs to know the velocity of the sound within the material you’re measuring. Because this will be unique to the material itself, it is important to calibrate the device you are using to the material you wish to measure. For information about calibration, you can take a look at our YouTube channel that contains videos covering a wide variety of topics. Below, you can see a table outlining a few of the most common engineering materials and their associated internal sound velocities.

Material Velocity (m/s)
Aluminium 3040 – 6420
Brick 3600 – 4200
Concrete 3200 – 3700
Copper 3560 – 3900
Glass 3950 – 5000
Iron 3850 – 5130
Lead 1160 – 1320
Steel 4880 – 5050
Wood 3300 – 5000

 

After calibrating, your device is ready to measure the specimen with accuracy. The second step is to determine whether or not the surface you need to obtain the thickness of is obscured by a coating of some kind. A coating is considered any material that covers the surface and prevents direct contact between the probe and the surface itself. Examples of coatings can include, but aren’t limited to:

  • Dirt
  • Dust
  • Rust
  • Paint
  • Lacquer
  • Lichen, Moss, and plant life
  • Seaweed (for underwater/marine measurements)
  • And Many Others

If you are using a device equipped with Single Echo, then these layers covering the surface will cause a discrepancy in the measurements and give you inaccurate results. In this scenario, it is important to ensure the surface is scrubbed clean and insulating gel is used to form a clean connection between the probe and the surface.

For devices that can use Echo-Echo and Multiple Echo methods, then the coating is less of a problem and can be worked around. Echo-Echo allows your ultrasonic thickness gauge to ignore a coating up to 1mm thick. Our pioneering Multiple Echo method can ignore up to 6mm thick coatings and with Deep Coat mode enabled, coatings up to 20mm can be ignored.

The measurement process includes the device producing an ultrasonic pulse, which is sent through the material you are measuring. The timing starts when the pulse is initially emitted from the probe and ends when the return echo, which reverberates back through the material after bouncing off the back wall, is detected. See the image (below) for a visual explanation.

As you can see in the image, the sound wave effectively travels a distance equal to twice the thickness of the material. This ends up giving the basic equation for the thickness as Lm = ct/2 where Lm is the thickness of the sample, c is the celerity of the sound specific to the sample, and t is the traversal time through the sample.

How do the different measurement methods work?

Single Echo is the most straightforward, and as the name suggests, it uses a single timing to measure the thickness. This requires a clean sample of the material, which can be hard to come by in many industries outside of controlled testing environments.

Echo-Echo and Multiple Echo use two and three timings respectively to eliminate the thickness of the coating and gain an accurate result for the sample thickness.

As shown in the image above, the second sound wave cycle starts at the border between the coating and the sample. This is the key factor in eliminating the coating from the measurement, because it means that timing one and timing two will be slightly different. Multiple Echo has a third timing to use, which increases the accuracy of the measurement when the coating is thicker. For this, timings two and three are usually the same or infinitesimally different, such that the computational hardware and software can eliminate the coating and log the thickness of the sample only.

Would you like further information or assistance?

If you are in search of information about your specific model of ultrasonic thickness gauge or would like to know more, get in touch with our team at Cygnus Instruments. You are welcome to give us a call at your earliest convenience on +1 410 267 9771 or alternatively fill in the online form to contact us in writing.