Something new from Tritex:
The
Multigauge 6000 Drone
Thickness Gauge 1. INTRODUCTION
Inspecting steel ships for corrosion has always been necessary and over the years, various methods have been utilised for this purpose. Also, inspection times are becoming shorter because time is money. It is obviously important to prevent corrosion occurring in the first place and therefore essential that good coatings are used. However, the problem then arises when there is a requirement to measure the metal thickness or to check for corrosion on the vessel without disturbing or damaging the protective coatings.
Tritex NDT are manufacturers of ultrasonic thickness gauges used to measure metal thickness to determine corrosion rates over time, without removing any coatings as long as they are solidly adhered to the surface. Thickness gauges are a useful tool in aiding surveyors to assess the condition of ships and small craft quickly and efficiently with minimum disruption. The metal thickness can be measured from one side only and corrosion levels checked to build up a good understanding of the overall
condition of the vessel. In some cases, on large ships, measurements are required in in high or difficult to reach places and so either rope access or scaffolding is used.
However, the Tritex Multigauge 6000 Drone Thickness Gauge means that measurements can now be taken in these places, and in some cases, negating the need for rope access or scaffolding. Tritex NDT are pleased to be the first company offering a dedicated commercially available drone thickness gauge to overcome these problems.
2. HOW ULTRASONIC THICKNESS GAUGES WORK
Ultrasonic Thickness Gauges transmit an ultrasonic pulse 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 gauge then calculates the metal thickness using this information, as well as the pre-programmed velocity of sound of the material being measured.
64 | The Report • September 2018 • Issue 85
It is therefore important that the gauge can be calibrated to account for different velocities of sound. For example, mild steel can vary from 5890 m/s to 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. The slower velocity of the coating will affect the accuracy of the thickness reading giving a false measurement.
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 additional measurements in that area. This would inevitably mean that more of the coating has to be removed to achieve accurate measurements. Removing coatings is impossible to do with a drone.
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