Honestly, a computer(much less a backup one) is not necessary........There are tables for diving, just use the old brain & you'll make it.......But the DIR's won't believe that, just ask them........Remember, though, that's J'sDT(Joe's Diving Theory), not many read that book.....lol........A computer is just another trick to put in your bag, & also, @ what point do you quite carrying backups for backups.......I'm thinking @ 16 redundant tanks & regulators you'll be 100% safe(assuming your buddy is equally equiped,lol).......