Backup Dive Computer

[arcFlash]

So I like my Nitek Duo but need to get a back up for some deeper dives in the future. It's unlikely I'll see trimix but 2 gasses is a must. I'm planning on a max of 150' on air with deco.

Is it common to get two the same so the algorithms match or is there a reason to have two different?

A depth/timer is as much as my Nitek so should I just put one of the nitek in gage mode and use my tables as backup? I was planning on 2 computers and my tables as a last resort.

[john j]

I've heard it suggested many times over the years that you should dive with two different computers (having different algorithms) and follow the most conservative one during the dive.  On the surface, this sounds like a great idea, but it isn't practical on a real decompression dive.  I considered doing this myself, but after studying the issue, I concluded that the people making that suggestion have never really tried it themselves or else they wouldn't suggest it.  I have never seen anyone actually doing it.

I have done many dives where some people in the group had "computer A" and others had "computer B".  Trying to stay together for all the deco stops is impractical and not necessarily less risky, so when we get put into that kind of situation, we usually allow the group to separate on the ascent.

It would be confusing and a significant task load to constantly be comparing two computers that had radically different algorithms and trying to decide what to do on the fly.  You would likely end up "bending" one computer and then for subsequent dives, that computer might be locked or deco calculations might be inaccurate.  Another problem is that if one computer asked for deep stops and the other didn't, you would be "pleasing" one of the computers and the other might be penalizing you heavily for your deep stops.  VR3 versus HS Explorer comes to mind.  When I was on the VR3 and I hung around for the deep stops that the HS Explorer folks did, my VR3 kept tacking on more and more deco time, while the HS Explorer folks' deco time was going down.  That was maddening and I can't imagine doing that to yourself intentionally by running two computers.

It's not necessarily the total deco time that keeps you from getting bent anyway.  The depths of the stops are very important.  So, running two computers and following the one with the longest deco time might make you feel safer, but you could be fooling yourself.

I use one computer plus one additional depth/timer instrument.  My decompression schedule backups are:

1.  My buddy's computer
2.  Tables
3.  Memory (Memorize generally what the stops and times will be).  If you do enough deco dives, you will become familiar with what kinds of stops are required for certain dives.  Deco is not a precise science, so if I lost all computers and tables, I'd put something together that would get me out of the water.  Spend an evening with some dive planning software and model some dives that you want to do.  Print out the deco schedules and compare them.  Some patterns and ratios should be evident.  That kind of knowledge could be very helpful if you have a problem.

[arcFlash]

Excellent, It hadn't occurred to me that the models would differ enough to lock out one computer but I guess. My concern about different computers was more like, if one fails, I can't just pick up on the other as, like you said, one may have had a deeper stop.

I was trained to write the next deeper and next longer stops on my slate in the event I needed to do it old school. And that my buddy may be below me and that I can't count on anyone but myself.

I'll go the same and I already know how to read it so it's one less night on the couch reading the book over and over.

Thanks a bunch.