Advice needed about backup computers

I dive an IQ 700 computer and so far it is great; however, I often worry about a failed computer ruining a day of diving. I am headed to Hatteras for some wreck diving. Is this a realistic concern and if so what are some relatively inexpensive, (and smaller than another IQ 700) solutions.

Computer failures are rare... but they do happen. One thing you can do to save yourself from having to purchase a second computer is to immediatly record all of your dive info after your dives on a slate or in to a log book. If the computer fails you can abort the single dive and recalculate everything using tables as a backup plan.

If you are dead set on getting a second computer, any basic puck style computer would do. Borrow one from a friend, rent one from a dive shop or buy a cheap used on off eBay/Craigslist or similar.

I'm not trying to sway you from getting a backup by any means but I know that every LDS wants to push "backup this", "backup that" to you just incase and this is one of those cases where you might ask yourself, is it really worth another $200? Could reverting to tables do an equal job as a backup? Sure they might cut your dives shorter but it really is just a "what if".

Another suggestion I've seen here is that if your computer fails and you've been doing multi-level dives so that you are "off the tables", it is still not so bad. If you assume you are in the highest group (Z on the PADI table), either before or after the dive with the problem, you can still dive again. The SI requirements are not too bad.

You don't have to calculate the dives as multi-level, you can use a square profile. That being, take your deepest point and run the tables that way for the duration of your dive as if you went from the surface to the deepest part... stayed at that depth for the duration and then surfaced . In most cases this will enter you in to calss "Z" on the PADI tables as vondo indicated but there are times that it will not automaticaly put you there.

Sorry I wasn't clear. I wasn't suggesting calculating ML dives, rather that doing ML dives and treating them as square could put out of the range of the tables. Here's what I would do:

1) Calculate each dive as square. If you are still on the table, use that PG. If not proceed to 2)
2) If you abort a dive shortly after starting, assume you are Z at the end of the previous dive, then calculate the SI and bottom time of the aborted dive (as a square profile)
3) If you abort a dive near then normal end, assume you are Z at the end of the aborted dive.

Most of what Splash-X said. If you are diving out of an LDS renting a second computer from them is probably the best bet. I bought a used puck off of Ebay for $50 a couple years ago just for this safety feature. Be sure to get one that you can change the battery yourself and one in which the battery won't break the bank. My main computer is a used Mares Tutor which takes a AAA rechargeable battery. After almost four years and close to 200 dives it's still cooking.

Right now I have a Mares M2, when I can afford something better (possibly air integrated wrist unit), that will become my backup. I think it was $170 on Scubatoys, about the cheapest you'll find a new Nitrox computer. It's not small, but should fit in my BCD pocket fine when it becomes a bacup.

what about a bottom timer, scuba pro, if you do have a fail, it should keep you accurate, and cheaper than new computer, scubapro has a decent one,, and if memory serves it even turns itself on when it gets wet, so you dont have to remember to do anything..

UWATEC DIGITAL DEPTH GAUGE - SCUBAPRO-UWATEC

The PADI wheel is a decent back up tool. It will allow you to calculate multi level dives and it does not rely on batteries or any mechanical/electrical functioning, other than the rivet in the middle. You'd have to use equivalent air depths for nitrox diving.

Common sense can be a big help as well. If my computer fritzed while diving and I was in a group diving computers and the same gas, I'd have no problem following my buddy's computer AS LONG AS the profiles were well within NDL, not pushing anything, and I would certainly do an extended safety stop. At recreational time/depths, single tank use, with reasonable surface intervals, a 10-12 minute stop at 10-15ft does an impressive amount of off-gassing. You just have to plan for the gas needed at the stop.

The point here is that if you are reasonably aware and educated about profile management and dive behavior that is linked to DCS, you can really minimize your risks with or without a computer by making intelligent decisions about your profiles. The extended stop and very slow ascent from the stop are two easy ways to very efficiently lower your pressure gradient on the surface.

Computers as any other equipment can fail; so a little redundancy won't hurt... I won't recommend you buying another model of dive computer though. Different models have different algorithms so distinct NDL and/or deco profiles, so if you use two different dive computers, which one should you follow? Which one is right? So, if you think the right way to go is with two dive computers, buy two of the same.
I personally like better the idea of having bottom-timer (like Uwatec ones or even a dive watch) and use it with a water-proof dive table as some of the others have mentioned here as a backup.
Nevertheless, even if you're diving with two computers, always plan your dives before getting in the water with tables; understand what you're going to face and your expected limits... You should be in control of your dive, not a computer (or two).