Necessity of a back up computer/watch for NDL diving

[Divectionist]

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?

 

[BlueTrin]

I don’t use one, mostly because I don’t own a second device.

In case of failure I would start to go shallower and abort the dive.

 

[JohnnyC]

From a “safely finish the dive” standpoint, not really. You can estimate depth well enough and you’re diving within the NDL’s so realistically it doesn’t matter if you surfaced immediately. From a repetitive dive standpoint, you may be screwed unless it’s your first dive, you know your profile, have a set of backup tables, and add a little conservativism.

The issue otherwise is you have no concrete way of tracking your residual nitrogen loading from previous dives accurately unless you immediately logged everyone and assumed they were square profiles. Most likely it would not bode well.

 

[markmud]

Hi Divectionist,

Do you vacation dive or liveaboard dive?

I do. And on one trip my HP hose failed. I had my second computer. My primary was a hose mounted AI computer.

We scrounged up an SPG from people on the boat and I was diving again in 15 minutes. All of my nitrogen and O2 data was on my back-up computer.

Recovering from the dive you are on is easy. Your buddy has a depth guage/computer; use it for your safety stop. Safety stops are not mandatory; without an accurate depth meter just guess your depth while on SS. Just stop at what seems to be 20-25 fsw for a minute, then 15-20 fsw for a minute and finish with a minute at what seems to be 10-15 fsw.

YMMV,
markm

 

[Caveeagle]

I usually carry two computers, although there are exceptions. For example a dive where I have done the same profile many times, and I know a typical depths and times based on location. And buddy separation is highly unlikely.

When diving new or different location, or routes, or when with a new buddy, I am a little more committed to being totally self reliant.

Sure you can carry tables, and when planning significant deco, I will cut basic table and have it on a slate. But even then it’s just as easy to have a backup computer as to carry a bottom timer.

 

[In2theabyss]

I always have my watch and have a small spg as a backup. I only do limitied tech at the moment, however once i get deeper into the tech stuff I will invest in another computer. For no overhead, NDL I am comfortable with just my watch as you can always surface if the situation warranted such

 

[RyanT]

I voted watch, but my backup is actually a puck computer in gauge mode, so it gives me both depth and time. This is useful if you've pre-planned your dives. If you're just diving your computer, @JohnnyC pretty much summed it up.

 

[BDSC]

I dive an AI Sherwood Wisdom. I’ve never carried a backup computer on a dive. I do carry a spare hose and computer on a dive trip just in case. My spare is also a Wisdom.

I have well over 1000 dives and have never had a computer failure.

 

[dlofting]

If you know that you'll have no residual nitrogen from one dive to the next you can use a single computer. This would apply to someone doing one dive per day, or even two if you don't mind pushing things a bit. Once you get to three dives a day and beyond you really want to have two computers

 

[rongoodman]

If you know that you'll have no residual nitrogen from one dive to the next you can use a single computer. This would apply to someone doing one dive per day, or even two if you don't mind pushing things a bit. Once you get to three dives a day and beyond you really want to have two computers

Even doing five dives daily on liveaboard depth and time schedules my Shearwaters don't show any significant loading in the slow compartments over the course of a week(using 32%). The fast and medium compartments clear during the surface intervals or, at most, overnight. Deeper or longer dives are obviously a different ball game.