I have never had a computer fail while dive in progress but have always used a plain old watch as back up on dives beyond 50-60. With main concern being saftey stops, due to constant air level checks. I use my trusty Invecta or rolex divemasters both are automatics not batteries, and I make sure I set the outer ring so I know the time limit I have. I have also after 6 months finally started fully trusting my Air integrated comp, but I still make sure I take a watch down on any advanced type dive no matter what, as well as a 19 cu pony.