Back up dive computer

[Jdillardtiger]

First of all, I would like people's opinions on when a second dive computer is absolutely necessary and is it a good idea for most situations (within rec limits)? I know you would have to wear it on every dive for it to correctly calculate everything. What about two different brands? They would most likely use different algorithms and therefore have different no stop times among other things. Would this be ok? Or is this not recommended. I am new to diving and am trying to learn as much as I can. So I am looking at computer failures and what to do.

 

[Searcaigh]

Use a watch and depth gauge, should your computer fail, you should be able to calculate your end pressure group and continue diving using tables / PADI Wheel ...... if you have them.

 

[vladimir]

First of all, I would like people's opinions on when a second dive computer is absolutely necessary and is it a good idea for most situations (within rec limits)?

Since a dive computer is never "absolutely necessary," a second one must certainly be less so. When would it be most useful? When you have a series of repetitive dive days planned, as on a dive vacation (particularly on a liveaboard), and the failure of your primary computer would require skipping dives to desaturate enough to start with a fresh computer (rental, for example) or with tables.

I know you would have to wear it on every dive for it to correctly calculate everything.

Yes.

What about two different brands? They would most likely use different algorithms and therefore have different no stop times among other things. Would this be ok?

No problem, just don't bend either one of them.

If your diving is going to be two dives each Saturday, then I would say a second computer is a complete waste of money. If they are going to be squarish profiles, the first computer might be a waste of money too. To the extent that you do multi-level dives or repetitive dives a computer will afford you more dive time (or, at least, make the extra dive time more readily apparent).

 

[Charlie99]

My plan for computer failure is pretty simple. IF I am quite sure that I ended the dive within NDL, then I simply assume that I am at the highest repetitive dive group and do my later dives on tables.

Most of my dives are multilevel, so many of my dives are way off the tables. Typical multilevel dive at Molokini will be 80' or perhaps 120' max depth, with a dive time of 60 or 70 minutes. So I just assume PADI pressure group Z and go from there. I know that for my air consumption of 0.4 to 0.45cfm and AL80 tanks I cannot run into the problems related to the ad hoc YZ rule of 3 hour minimum SI, and ignore it. I can marginally get into the 1 hour minimum SI requirement of the WX rules, so do combine the assumption of PG Z with a 1 hour minimum SI rule.

So far, after about 850 dives I have never had a computer failure and so therefore have never had to actually use my plan. My simple, reliable 12 year old Oceanic hockey puck just keeps on ticking.

I've noticed that divers with air integrated computers are more prone to having computer failures. Many of the "failures" are simply low batteries in either the wrist unit or the transmitter on the regulator.

 

[g1138]

The most important thing to remember is you have to always dive with your backup computer, if you want to make immediate use of it possible. If you're primary computer dies on you after your dive, or even mid dive, you'll then have to rely on your backup for later dives. In order to be safe your backup has to be up to date on what your primary computer was recording. Or else you should wait 24hrs before diving again, whether it be on tables or backup computer.

Just a preference. If you're going to be using different dive computers you better use the most conservative one as you're primary. If your less conservative dive computer dies, you may get in trouble with your more conservative backup (ie. computer lockout).

In reality though, I don't see the real use of an extra dive computer for recreational dives. I only know 3 divers who carry back up computers. And even then they only carry them during the class dives, in case a student's rental computer dies on them mid dive.

However, if you do a lot dive trips with repetitive dives, and would rather not deal with rental computers etc. Then you better have a:
- analog pressure gauge
- dive timer
- depth guage
You can combine the latter two into a wrist dive computer and carry that with you on every dive.

 

[Petert71]

I'm very much a newbie diver, but I'm a little concerned that you could relying on your computer to to the point where you'd consider a backup. For an OW diver you should be able to plan the dive to the point where you need instruments to know three things - your remaining air, your depth and your dive time. Simple gauges will tell you all of this, a computer is an easy way to manage the dive.

If any of this equipment fails during the dive (be it in a computer or gauges), the dive is over and you assend in a controlled manner, doing your safety stop if possible. Obviously it's more complicated for multi level dives and muliple same day dives, but best to play it safe.

A computer is a convinience and I love mine because it makes diving easy, but you shouldn't have to rely on it 100% if you're diving OW safely.

 

[scubamarketing]

I plan my basic dive profile with tables (although around here, cold then air supply tend to run out before NDL's), but make my dives with two computers, both the same brand. I really only do this because I have two computers (well, more than two, but I think that would be a bit much) due to gear accumulation over the years. Both watch sized, so it's easy to have one on each wrist. It was never a concern or plan "growing up", but it's easy to do now, so I do. Never had a computer fail on me.

 

[Bubbletrubble]

+1 to the comments made by vladimir. I also like what Charlie99 had to say (as usual).

Just a preference. If you're going to be using different dive computers you better use the most conservative one as you're primary. If your less conservative dive computer dies, you may get in trouble with your more conservative backup (ie. computer lockout).

@g1138: Eh? Computers only lock-out (if lock-out is incorporated into the design) when the user surfaces with an unfulfilled deco obligation.
If one computer dies, then you use the other one. No conflict at all.
Diving two computers which use different algorithms only "works" if the user ensures that the dive profile doesn't violate the more conservative computer.

 

[Rascally Rabbit]

I'm very much a newbie diver, but I'm a little concerned that you could relying on your computer to to the point where you'd consider a backup. For an OW diver you should be able to plan the dive to the point where you need instruments to know three things - your remaining air, your depth and your dive time. Simple gauges will tell you all of this, a computer is an easy way to manage the dive.

If any of this equipment fails during the dive (be it in a computer or gauges), the dive is over and you assend in a controlled manner, doing your safety stop if possible. Obviously it's more complicated for multi level dives and muliple same day dives, but best to play it safe.

A computer is a convinience and I love mine because it makes diving easy, but you shouldn't have to rely on it 100% if you're diving OW safely.

You are correct in that you shouldn't need to rely on it 100%. That said if a computer fails that wouldn't stop you from making a safe ascent. If you had been making multiple multi level dives during that day on your computer and you don't have a reliable backup way to track your NDLs or PO2 then the safest course it to refrain from further diving until you can be sure you are back to a clean slate. This can really be a bummer on an expensive live aboard vacation.
This is the reason that people consider backup computers. If you dive a second computer on all the dives that day and one computer fails you can continue diving without the need to surface from that dive and because the second computer was tracking your gas uptake from the first dive of the day you can continue diving the rest of the day as planned.
Because computers credit your NDLs and PO2 in real time it may not be possible to convert from the computer to tables and continue diving. It is possible that an early dive that was not in violation on the computer may when converted to tables be in violation. So this could end your day of diving even if you had the depth and time figures from all your dives. If fact it often will if you do your deepest dives early and max out the bottom time on your computer.

 

[aquaregia]

I dive with two computers because I own two computers. I keep them together in the bag, and my arm's plenty long enough for two. If my buddy has a dead battery, it's fixable pre-dive. If a friend (not my buddy -- I'd've caught his during the buddycheck ) nearby has a dead battery, I can hand it over mid-dive (as a BT, naturally).

The funny part is that I usually dive tables. Sometimes I do pretty steep profiles, repetitively, and then I choose to dive a computer. If I'd paid thousands of dollars for a dive trip you can be damn sure I'd wear two computers. The one time I had a computer failure was on a dive trip, and I was glad to have a watch and a 30 year old depth gauge.

If I was converting from a computer to tables after a computer failure, I wouldn't do much guessing on multilevels for repetitive dives. Stepping outside the model is effectively working out your own deco strategy, and I don't trust myself to do that ex post facto. There would be the temptation to add "just a little more" risk for a "big" payoff in dive time.