John_B
Grasshopper
I don't know how universally accepted the DIRF card is or isn't for Nitrox fills, but if the card says Nitrox and the shop doing the fills won't accept it then the OP probably needs to either educate them or else find a new shop. I've only experienced this once with my IANTD Nitrox card, and the problem was that the DM-in-training had never seen an IANTD card before.I would argue against the GUE card because it is not universally accepted.
Really, if you can show then you can analyze and mark a tank with the correct MOD (which is pretty easy for EAN 32 ) then what's the problem?