It's complex.
Within industry, inspectors come up through the ranks as it were. You don't just leave school and become an inspector. They will had a broad experience with the components and materials already. It's really important that when inspect you have an appreciation of the type of materials, the manufacturing process and what type of defects apply to each.
Generally in dive shops you don't have that depth of experience, thus sometimes people can't tell the difference between a steel and Ali tank, they may not appreciate fully factors like tooling marks vs natural cracks, or different types of corrosion.
For visual inspection you need to have the right candidates and give them in depth training - 2 days IMO doesn't do this
With regard to NDT, the current equipment and training (for use on one type of Ali cylinder) is substandard (and that's me being polite
I've said that you could use both Eddy Currents (EDD) or Ultrasonics (ULT)to inspect, which is true but over simplified.
EDD is more complex especially with regard to steel tanks, since different alloys and heat treatments during manufacture will cause differences in conductivity to varying degrees. This of course can impact teh test, needing something as simple as a slight setting change, all the way to needing a different procedure. You are reliant on the inspector being correctly able to identify the type of tank and the manufacture in order to apply the correct procedure
ULT's is simpler from a materials standpoint - effectively is it steel or Ali, but the inspection procedure would be technically challenging because of reasons I highlighted earlier
Costs:
For NDT inspections, you need a written procedure applicable to that component. It takes upwards of 200hrs (longer for ULTS) to develop write and validate that procedure - in the case of Steel maybe multiple procedures, which is then circulated to all approved facilities. The procedure dictates what component(s) its valid for, the equipment type, settings to be applied, and inspection method and likely defects and artifacts and actions upon finding them.
I've been out of the business for a while so take the following with a pinch of salt. A suitable Ult tester (not thickness gauge from Ebay) will be approx $5000, For Eddy current including high speed guns for rotary inspection $10,000
Training - in order to both test and make a decision (we call it sentencing) you need to be a Level II inspector
The American Society of NDT (ASNT) requires a minimum of 80hr training for Level II in each discipline $5000 + Travel, hotel , and subsistence costs
Lets just round that up to a $20,000- $30,000 ball park figure for each shop wanting to carry out NDT, Oh and the inspector needs to re certify every 2 years.
This is why visual inspection, despite its pitfalls is the "go to" method. It's cheap.
My question though, is why do divers get so het up over tanks failing inspection? Looking at DGX a new HP 120 with valve is only $400. A home washing machine, TV, Laptop or mobile phone all cost considerably more and yet are considered disposable items to be replaced frequently. A 20-40yr old tank needing replacement and there's a massive outcry. I don't get it.
For dive centres adding a total of $3 per week would cover the purchase cost of an Ali tank - effectively new tanks and valves each year (Obv I understand why they don't)
Anyway just my POV I'm sure many will disagree
