Difficult inspection environments or the preference for non-destructive testing make ultrasonic depth testing equipment incredibly useful. Determining the thickness of water tanks, pipes, or the corrosion of either, are just a couple of the various applications.
Very thick, or sound attenuating materials (such as cast metals, rubber and some plastics) can pose a bit of a challenge, but feature sets on equipment can often accommodate this.
However, non-uniform, attenuating materials such as fibreglass can be frustrating, even with the appropriate gear. The inconsistent nature of gel coatings that often accompany fibreglass adds another dimension of difficulty. Determining the thickness of a tank made of such material was the task faced by TechRentals. The customer had tried various methods (with various technicians), but was having significant trouble getting any meaningful data for the wall depth.
Fibreglass tanks are coated with a roll-on gel finish. This gel coating leaves behind large waves and bubbles which isn’t ideal for ultrasonic thickness testing. The resulting fluctuation can be as much as 5mm, making calibration of equipment difficult. However, it was determined that the best piece of gear to use in this situation was the Olympus 38DL ultrasonic thickness gauge.
Unfortunately, during the site visit, the difficult nature of the material meant that the unit was unable to register the fibreglass, as there was no usable signal. Experimenting with different transducers and velocity settings could only manage a distorted signal, resulting in highly erratic, meaningless data.
A TechRentals team member then proceeded to procure off-cut samples (various thicknesses) of the material, and brought them back to the Perth lab to try and devise a solution for the customer.
After experimentation with different transducers and various couplants, a combination that yielded promising results was discovered. The M2008 transducer tuned towards 2740m/s (the approximate sound speed of fibreglass) formed part of the solution, as well as a thicker couplant to exclude outside noise and air interference for a usable contact signal. All that was left to do was calibrate measurements to take into account the gel coating.
Calibration requires two measurements with the Olympus 38DL, one from a ‘thick’ sample and one from a ‘thin’. The ‘thick’ measurement was taken on a dense section of fibreglass with NO gel coating. This allowed an appropriate frame of reference for the unit to base its readings. One that ideally removed as many variables as possible. The ‘thin’ calibration measurement can then be taken from any section of the material (with or without gel coat), provided it is thinner than the first.
What resulted was the ability to measure any and all sections of that type of fibreglass for the selected range (in this case 5mm-22mm, coated or uncoated) with a discrepancy of 0.1-0.2mm. This discrepancy is the minimum quoted by the Olympus 38DL specifications, so the customer was more than satisfied and able to continue work on their original application.