Necessity of a back up computer/watch for NDL diving

[TMHeimer]

Someone earlier said they were a commercial diver and uses tables. I may be absolutely and completely wrong, BUT I would think that commercial divers would dive more of the classic square profile, and therefore the tables are very accurate. I have looked at many of the graphs of my rec dives. I don't want to use multi-level dives because to me that implies a modified square profile, i.e. you go to a depth for a given amount of time and then you ascend to a specific depth for a given amount of time (maybe more than once) and then you surface. The graphs of my dives indicate "variable" depths. The computer keeps track of all of the depth changes and times them and then calculates the loading and unloading of N2 in my tissues. For my type of diving, a computer allows me to dive MY profiles safely. I don't push NDLs, but I have been close. I don't carry enough gas to deco sooooooo I watch my NDL and pay attention to other factors as well. So far, I have not had DCS in over 1200 dives. That doesn't mean that I won't get DCS on my next dive. Just be safe and dive smart.

Cheers -

Before I got my first computer (2008?) I did a number of charter 2 tank dives (to 100', 80', etc.--normal stuff--hour long SI) with just watch, depth gauge and tables. But they were also very square profiles as I was almost always on the bottom hunting shells. As mentioned, I use both computer and analog stuff on my "deep" dives. It's nice to see your actual bottom time on the DC, taking into consideration small changes in diving depth ("now guess I'll swim up 10' and over the wreck").
Something I like to mention (and have done so to students in the past) is the benefit of "rounding" you get with tables.
With tables of course you use your deepest depth as if you were always down there. This is of course impractical if you are doing any real kind of a multi-level dive-- you lose bottom time or find yourself in pressure group "ZZZ minus 1" on the table. But if your profile is square, chances are you are nose to the bottom several times, but usually 3-4 feet above it. So, your real max depth is closer to 77' than 80'. You don't get that extra margin with a DC. The bottom time left is exactly what the DC reads--according the the model of course, not exactly when each individual is at his/her limit since they don't have that technology yet.
Before DCs became popular, I imagine divers knew all this and may or may not have considered the "extra" minutes they may have because of table max depths being used. I think it is important for divers to know the DC means what it says.

 

[broncobowsher]

It is amazing how people have twisted this thread to fit there personal use.
The question was standard recreational NDL diving, with someone else with them (who probably also has a computer).
There was never question about pushing the NDL limits
There was never question about doing every possible dive on a vacation.
There was never question about doing tables.
This is just an average Joe going out for a dive. I'll take that as if there is a problem the dive gets thumbed and average Joe goes home.

Does that average Joe (not the super hero diver Joe that you think you are) actually need a second computer?
And the poorly written question asking if we (not if he) dives with a second computer. But not asking if it is needed.
Any post here that implies pushing NDL limits is derailing this thread from the original question into something that it isn't.
Basic open water class is still (barely) taught how to dive without a computer at all. Technically you don't need any at all. But it sure is nicer to have one.

Now if the question was brought up of a dive holiday where there will be dive after dive, that would be a different discussion. About 13 of the prior pages will answer the question that was never asked.

 

[Boiler_81]

The OP does say no decompression and no overhead. He did not indicate the planned number of dives a day or week. He did state "versus missing one or two dives of the day, which is fairly unlikely". This may be interpreted that he is concerned about missing dives on a dive vacation. in which case my answer still stands. I dive with a backup due to the experience of a buddy having to sit out a day at Chuuk due to a DC failure.

 

[caruso]

But if your profile is square, chances are you are nose to the bottom several times, but usually 3-4 feet above it. So, your real max depth is closer to 77' than 80'. You don't get that extra margin with a DC. The bottom time left is exactly what the DC reads--according the the model of course, not exactly when each individual is at his/her limit since they don't have that technology yet..

I read this part several times about how you don't have that extra margin with a DC that you do with tables on a square profile with your nose to the bottom and each time I read it, it makes less sense to me.

 

[Storker]

I read this part several times about how you don't have that extra margin with a DC that you do with tables on a square profile with your nose to the bottom and each time I read it, it makes less sense to me.

Literally none of my dives have had a true square profile. My PDC tracks that not-square profile and calculates less N2 loading when I'm shallower. If I follow my computer, and even more if I deliberately ride the NDL gradually shallower, I will have a higher N2 at the end of my dive loading than what I'd had if I were diving tables.

I don't believe im the only diver who does quite a lot more not-square-profile dives that square profile dives, and for us table diving is quite a bit more conservative than computer diving.

And in @TMHeimer 's example, the way I understand him is that even on a square profile, it's more likely than not that some of your bottom time isn't at max depth. The computer takes that into account, the tables don't. That's the extra margin he gets by diving tables instead of a computer.

 

[dmaziuk]

Now if the question was brought up of a dive holiday where there will be dive after dive, that would be a different discussion. About 13 of the prior pages will answer the question that was never asked.

OP did mention trips to GBR. AFAIK most tend to be weekend trips, and we take our long weekends seriously down under. I don't remember there ever being a five day-long one though, back when I lived there. So 4 days is not out of the question.

Still, by base Haldanean models you're pure as driven snow after a night's sleep. If your computer packs it on any given no-stop dive, you're at most in pressure group Z. It's really not that hard to make guesstimates -- as long as you don't try to continuously run the Schreiner's equation in your head: that's impractical if at all possible.

 

[caruso]

If I follow my computer, and even more if I deliberately ride the NDL gradually shallower, I will have a higher N2 at the end of my dive loading than what I'd had if I were diving tables.

table diving is quite a bit more conservative than computer diving..

Well sure. Dive computers almost always provide a more generous amount of bottom time than tables and a timer. With the longer bottom times it naturally follows that the safety margins are lower.

 

[scubadada]

I don't disagree that diving tables to the NDL limit vs. diving your computer to the NDL limit may have different risk for DCS. However, what if you are diving tables and end up spending time a little below the max depth instead of above and/or overstay your time by a little? The risk would end up being equal or greater to riding your computer to the NDL. So, it depends.

I haven't dived tables for a long time. However, in my first 10 years of diving in Southern California, I used USN tables, an analog watch, and a capillary depth gauge. It was not uncommon to have some of the dive below the planned depth or to overstay a few minutes using these aggressive tables. I am thankful that I had no problems, I was young, and perhaps lucky.

 

[TMHeimer]

I don't disagree that diving tables to the NDL limit vs. diving your computer to the NDL limit may have different risk for DCS. However, what if you are diving tables and end up spending time a little below the max depth instead of above and/or overstay your time by a little? The risk would end up being equal or greater to riding your computer to the NDL. So, it depends.

I haven't dived tables for a long time. However, in my first 10 years of diving in Southern California, I used USN tables, an analog watch, and a capillary depth gauge. It was not uncommon to have some of the dive below the planned depth or to overstay a few minutes using these aggressive tables. I am thankful that I had no problems, I was young, and perhaps lucky.

I agree. Of course you have to follow the rules no matter which you use. For tables, you can't go even one foot below your planned max depth--then you'll have that small rounding to benefit from (as Storker re- explained). With DCs you can't exceed the remaining bottom time it gives you. You have to do things right in both cases.
broncobowsher, Yeah, there is extra discussion on the thread besides the OP's question--that often happens on SB, or any forum. That at times irritates me too, especially when people give unwanted opinions. See--one of my first posted threads in 2007 asking if anyone knows of places you can collect live (sea)shells. The responses often went way beyond just extra info. and discussion.

 

[Neilwood]

It is amazing how people have twisted this thread to fit there personal use.
The question was standard recreational NDL diving, with someone else with them (who probably also has a computer).
There was never question about pushing the NDL limits
There was never question about doing every possible dive on a vacation.
There was never question about doing tables.
This is just an average Joe going out for a dive. I'll take that as if there is a problem the dive gets thumbed and average Joe goes home.

Does that average Joe (not the super hero diver Joe that you think you are) actually need a second computer?
And the poorly written question asking if we (not if he) dives with a second computer. But not asking if it is needed.
Any post here that implies pushing NDL limits is derailing this thread from the original question into something that it isn't.
Basic open water class is still (barely) taught how to dive without a computer at all. Technically you don't need any at all. But it sure is nicer to have one.

Now if the question was brought up of a dive holiday where there will be dive after dive, that would be a different discussion. About 13 of the prior pages will answer the question that was never asked.

There is a good reason for the thread drift - it is the point in bold where the OP mentions the possibility of missing one or two dives of the day (which becomes progressively worse on multi day, multi dive trips)

I am currently trying to decide whether I should wear a back up in the form of a second dive computer (full redundancy as I need an AI feature) or watch (limited redundancy as I don't have an SPG) for rec diving strictly within NDL and am wondering what other SB members are doing and what their rationale is.

At the moment, I lean towards the conclusion that a backup device being unnecessary because in the event of a total dive computer failure or prolonged loss of AI signal, I would generally be aware of my air and NDL remaining, with the ability to shoot my DSMB up as an ascend line even in the event of buddy separation, with a little knot tied into the line at safety stop depth that would allow me to count 3 minutes in my head whilst remaining at the target depth. Should I lose my DSMB, the old slow conservative bubble guided direct ascend to the surface, with an optionally guessed safety stop in the shallows is still a backup-backup solution. The other point is that my dive computer would have to fail many times over for a second device and the hassle that comes with it to be practically and financially justified versus missing one or two dives of the day, which is fairly unlikely.

I'd like to know how you feel about the issue, whether your backup device has proven useful in the past (remember, let's assume no deco/overhead/solo diving for this scenario), whether you wear a dive watch out of old habit, or whether you put up with a second computer, just in case?