Ultrasonic weld testing

I have recently had to replace the tyres on my car and, on the sidewall, I saw the initials TWI. My first thought was that The Welding Institute (TWI) was probably involved in the steel used in the manufacture of the tyres and, being of a curious nature, I carried out a search on the Internet and found that my assumption was incorrect. TWI actually stands for ‘tread wear indicator’, referring to the humps of rubber that are present in the tread pattern to 
indicate the maximum wear permitted. I decided to go for the more expensive tyre option, which is supposed to give ‘A-rated’ performance in many of the characteristics as well as noticeably more grip and less noise.

I am in contact with an apprentice who is very eager to perform all NDT inspections as required. His manager offered to help with training on ultrasonic weld testing in his own time and at his house between Christmas and New Year. Sadly, the other person due to attend had been partying too hard and did not make full attendance. However, this did allow the manager, who used to work for an Authorised Training Organisation (ATO) as a trainer, to spend more time with the other apprentice. 
When I next saw this apprentice he was very eager to discuss ultrasonic transducers, having gained experience in wrought, forged and cast components but little in weld testing. He understood the difference between single crystal and dual crystal, operating frequency and angle of incidence, but with the additional instruction on weld testing he appreciated the need for a suitable ultrasonic set that could give the best response to the probes being used. He mentioned the range of transducers that his manager had and how important it was to use the correct one, as well as the importance of where the lead connector is situated so that access is not compromised when testing in confined areas. This took me back to the decision-making I had employed when choosing which replacement tyres to opt for. I used the Internet to compare prices for the tyres, which also gives access to an international marketplace for NDT equipment; it is very easy to spend time looking at different ultrasonic sets and their features, along with transducers. Do all transducers work well with the chosen ultrasonic set or do only certain ones provide optimum performance? There appears to be no meaningful guidance, unlike with my tyre purchase, where characteristics such as wet weather performance and noise are graded in a similar way so that an assessment can be made. Should 
transducers be re-shoed? Remould tyres for cars would probably no longer exist due to performance issues, so do the same performance issues apply to transducers?

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