Subex do have steel tanks which is nice. However 28% is nitrox. Its about 21% therefore its enriched air. And ALL the training agencies class it as such and require a certification to use. There is no such thing as "air 28". What they're doing is giving a lean nitrox mix to unqualified people.
Thistlegorm is easily doable on air but you'll get less bottom time and/or more stops then with a nitrox mix.
Thistlegorm is easily doable on air but you'll get less bottom time and/or more stops then with a nitrox mix.