Am I the only one that dives with 2 computers?

[tbone1004]

my Petrel is set at GF 40/70. Means I get out of the water with 70% tissue loading assuming I'm on deco. If on a liveaboard, Petrel fails, dive is over. Come to the surface, worst case I'm at 70% tissue loading, so calculate your TNT. NAUI says my max at 70ft is 45 minutes, 45*.7=31.5, so I come out of that dive at worst a G diver, and then continue on with my liveaboard diving tables. PADI says 40 minutes max, so 40*.7=28, come out as an N diver, and continue on with liveaboard diving tables. Not rocket science. If you have a computer that runs GF's or where you know you are coming out with a certain theoretical tissue loading, multiplying that loading by the NDL times on the dive tables will give you a fairly conservative number. Next dive is going to stink as far NDL's, but better than waiting 24 hours which is idiotic, or having a more conservative computer as your backup and then you realize you bent it by accident and then it locks you out for 24 hours.

Only time I'd ever recommend a backup is if it was running IDENTICAL algorithms as your primary, otherwise you can run into all sorts of problems. Backup can be set on a less conservative setting than the primary as a sanity check if you have to blow a few mins of deco/safety stop whatever, but they should always be running identical algorithms. Alternatively, just keep a set of tables in your pocket, and call it good. Alternatively again, you can also run on the rule of 130 where 130-depth *rounded deeper to nearest 10ft=time. You should know about how deep you are, and should always have a backup timing device, so problem solved again there with simple math in your head or slate.

 

[Ceco]

Speaking about buddy system, I imagine I could consider my buddy as a backup computer at the same time (at least to get certain level of estimation), provided we strictly stick to buddy system that makes our profiles "nearly identical". Chance of loosing both, your computer and your buddy, is less than minimal.

Disclaimer: novice in diving (Please, don't beat me )

 

[Adobo]

Speaking about buddy system, I imagine I could consider my buddy as a backup computer at the same time (at least to get certain level of estimation), provided we strictly stick to buddy system that makes our profiles "nearly identical". Chance of loosing both, your computer and your buddy, is less than minimal.

Disclaimer: novice in diving (Please, don't beat me )

That's what I do. I bring a computer and so does my buddy. If computer fails, dive is over. I tell my buddy and he/she runs the ascent.

This works for diving at home (no more than 4 dives/weekend). This works for my vacation diving (no more than 3 dives/day).

I'm always extremely surprised how quickly people will plunk money down on the table for not only super fancy computers (hoseless air integrated, multi gas, so on and so forth) but also a backup for the fancy computer. And, in this case, for recreational diving. For me, I think the better use of funds is better training. And by better, I mean something better than the garden variety open water/advanced open water that a lot of divers seem to stop their training at when they decide to buy all manner of dive computers.

Another option to spend funds on is to have an optimal BC for diving at home (cold water/drysuit) and an optimal BC for vacation diving (tropical water/thin wetsuit).

 

[wedivebc]

Only time I'd ever recommend a backup is if it was running IDENTICAL algorithms as your primary, otherwise you can run into all sorts of problems.

Please describe what problems that would cause?

 

[mikeycanuk]

Always dive 2 computers. Cobalt primary, BUD secondary. We did have zoops but switching Nitrox is (was) a pain in the a**. The BUD is super quick and easy to switch.

Never had to use the BUD yet.

 

[redacted]

Please describe what problems that would cause?

The only problem I have ever seen is when the dive gets a bit aggressive and a diver with 2 computers decide to ignore the conservative computer and follow the liberal computer, locking out the conservative computer.

 

[abscuba]

+1 for tbone. If going to run 2 computers it is a good idea that they are on the same algorithm. Not all computers are the same and do not produce the same NDL numbers. Some are very conservative and others are less and the relationship is not linear. So going on blind faith that you are good means you could be hoping in the water with a higher tissue loading than you think and have reduced your margin for error. This is less of an issue for recreational divers as you are staying out of deco. When I dive tech I always have at least 2 computers and are both identical as I go into deco and you want them all on the same profile. Best way to stay safe is to plan your dive that lost art no one does anymore. I use my computers are my back up bother recreational diving and technical diving. I use my tables for rec, and vplanner for tech and when I hop in the water I have the plan and back up plans on my wrist. If I screw up all those plans then my parachute is my computers. So if I have a computer failure I know where I stand as of tissue loading and there is no ambiguity as to how I should plan my next dive. Technology is a great tool, but it is a tool the diver should always be in control.

 

[tbone1004]

Please describe what problems that would cause?

So the point of a backup computer is not to have to look at it unless the primary fails. So if you have different algorithms and they are calculating your NDL's or decompression differently, then you either have to wait for both of them to clear, or dive the most conservative one. Often times that isn't your primary. I'm willing to bet that the OP with the Luna and the Zoop will have to be most careful with making sure the Zoop isn't violated. Now, you dive the Luna as your primary and it says you have 35 mins NDL but the Zoop only says 28 mins. Zoop will likely lock you out *I say likely because Suunto won't publish their algorithms, so you have no way of knowing what it is going to do*. If the Zoop locks you out for 24 hours, you've now lost your backup computer. If you then have to make sure that you aren't violating the Zoop, now your Luna is really your backup because you have to obey the Zoop to prevent lockout.

Identical algorithms or computers where the conservatism can actually be set removes that issue. I.e. diving two Petrels. One set at 30/70, the other set at 40/85. 30/70 is primary, 40/85 is set more liberal in case you have to blow deco for whatever reason. The 40/85 will always clear before the 30/70 so no problems there, and if you feel like you're going to hurl and you have 5 minutes left on your stop but the 40/85 is cleared, then you can surface and while the 30/70 will be mad at you, it won't ever lock you out. Problem is the Petrel is $750, and that's far too expensive for a backup computer for recreational diving at 3x the cost of the Zoop, but the Zoop is a terrible backup computer for the reasons listed above, it's one of the most conservative computers out there that has no problems locking you out for 24 hours. I'll stick with a watch and tables in my pocket. Petrel plus printed tables at the same GF's that I'm diving work well enough for me thank you.

Edit: One of the two examples of why the same algorithm is important that you can actually see. Look at the NAUI dive tables vs. PADI. You'll notice at first glance that NAUI seems a lot more liberal with NDL's, often going significantly longer than PADI. Then you go and start looking at repetitive diving and see that PADI is much more liberal for shorter SIT's. If you plan consecutive dives with NAUI vs. PADI you'll see that the relationship is anything but linear.

Now you can also look at the chart published by AUP for their dual algorithm.
Oceanic Dual Algorithm | Simply Scuba Blog

Here you can see that the computers running DSAT actually follow each other, but the computers running Z+ are all different. Uwatec is using ZHL-8, Suunto/oMares are running RGBM, Tusa is running ZHL-16C, etc and you can see the lines crossing. Suunto starts more liberal, but then becomes one of the more conservative computers.

The examples above are visual representations of why it is really bad to mix algorithms on backup computers, it means instead of running backup, which implies that you don't use it until you need it, you now have to monitor both of them independently and dive whichever is most conservative which is a colossal PITA.

 

[Steve_C]

All these absolute statements. It depends a lot on the diving one is doing. In my case I really do not want to trash the second dive of a two dive trip. First dive is usually 80-100 ft. Usually multilvel. If I turn the dive, then except in a dire emergency I am not going up. I am heading back to the anchor line and do a safe ascent. So I could be down for several more minutes.

I have had a computer flood on me though that was on a reef dive in the Keys so NDL exceeds dive time.

I do have a second computer. It only has to save me a couple dives and it pays for itself. It also saves me a lot of what if concerns.

My backup is a zoop. It does not lock you out if you go into deco unless you are skipping steps coming up.

I can use tables. But I get more diving and more relaxed diving with two computers. On a given dive depending on water and other conditions the profile can vary greatly.

 

[fisheater]

Yes.

I have one on my wrist and another (running the same algorithms) on my scooter. I like being able to see my depth constantly during scooter runs and like having a backup, in case my main computer fails.