2. Principle of Operation
Ultrasonic Thickness Gauges consist of the gauge, probe and connecting cable. The probe houses a piezo electric crystal which vibrates and produces ultrasound when an electronic pulse is applied. This ultrasonic pulse is transmitted into the material being measured and the time is recorded for the ultrasound to travel through the material and return back to the probe. The returned ultrasound is detected by the crystal which sends an electronic signal back to the gauge. The gauge then calculates the metal thickness using this information as well as the pre programmed velocity of sound of the material being measured.
It is therefore important that the gauge can be calibrated with varying velocities of sound. Just being able to set a gauge to ‘mild steel’ is not sufficient as there are varying grades of mild steel. All have different velocity of sound properties which vary in the region of 5890 m/s and 5960 m/s.
The basic principle of ultrasound, known as single echo, works well on steel with no coating. But if a coating is present then this has a different velocity of sound to that of the steel, generally in the region of 2000 m/s. Therefore, if a gauge is calibrated for steel, and the ultrasound also travels through the coating, it will take approximately three times longer to travel through the coating than it will an equivalent thickness of steel, resulting in a false reading.
There are two options to overcome this problem; the first is to remove the coating, but this is costly, time consuming and means that the coating has to be re-applied wherever a reading has been taken. In some cases, where there is an excess of corrosion, it is common to take more
Multigauge 6000 Drone Gauge Kit (Gel Dispenser)
measurements in that area. This would inevitably mean that more of the coating has to be removed to achieve accurate measurements.
A much simpler and cheaper solution is to use a gauge with multiple echo capabilities. Multiple echo gauges completely ignore coatings, as long as they are solidly adhered to the surface, and measure just the metal substrate. Coatings up to 20mm, depending on the type of coating, can be ignored. Multiple echo also works well on corroded metal.
Tritex NDT have developed a range of gauges, all utilizing the multiple echo technique, specifically for applications such as marine surveying, underwater inspection by divers and ROV’s. The next natural progression is to use this expertise to develop a lightweight gauge specifically designed to be mounted onto drones.
3. Mounting a Thickness Gauge onto a Drone
Multiple echo gauges are therefore ideally suited for use on a drone, to take high level inspections without the need to use rope access, scaffolding or rafting techniques. They can also be used within dockyards to inspect cranes, bridges and other difficult to reach places.
The Multigauge 6000 Drone Thickness Gauge, developed by Tritex NDT, transmits real time measurements wirelessly up to a distance of 500 metres using its integrated RF transmitter. The gauge takes approximately three measurements per second, even though coatings, which is ample time to take an accurate measurement of the structure being inspected. The readings are displayed and stored on dedicated Communicator software, within templates in a grid or string format. The software has an
‘autologging’ feature to ensure all measurements are captured and none are missed. Measurement
data can also be transmitted using RS232, in parallel with the wireless output, so that measurements can be overlaid onto the drone’s video output.
As ultrasound doesn’t travel in air, it is important to use ultrasonic gel between the face of the probe and the surface being measured. Tritex NDT have addressed this problem with the dedicated Gel Dispenser. This is operated wirelessly from the ground station and dispenses just the right amount of couplant onto the face of the probe before each measurement. A lightweight probe holder, which has movement in all directions, helps align the probe with the surface being measured. It also has a compression spring fitted to absorb any unwanted
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