The full scale business is mechanics. The guts can work over whatever range you like, but the visual only needs to go to 300bar.
I wouldn't use the psi gauge unless I was in the us. Like I said - very limited market. It's a risk to be aware of though. If the yank has lots more gas then you to start with and you both divide by three. He breathes more and then has a problem - there is insufficient gas in your tanks to get you both out.