Using a computer alongside pragmatic deco plans

[huwporter]

After nearly 10 years of deco diving using only a bottom timer, I seem to have picked up a Perdix. I don't believe this permits any skimping on dive planning, but it seems pointlessly puritanical to run it as a glorified bottom timer only.

So those of you that use a computer together with 'traditional' GUE-style pragmatic deco plans, how do you do it? Follow the pragmatic plan, with the computer acting as a backup 'brain', or follow the computer with the pragmatic plan as backup? The former seems easier to incorporate in to team diving practices, unless the entire team is also running computers (as seems to happen more frequently these days). And what gradient factors do you set?

 

[PfcAJ]

I plan the dive but generally do what the box says. The two are usually close. If I expect 30mins deco the tts will read 30mins at the first switch. That's kind of my mental check to see how much deco there is vs what I expect. If they diverge wildly it's probably because I'm a dummy and didn't set the right gas or something. More of an issue with using travel gases for t2 and up dives. The smaller differences are usually because what I planned to do didn't exactly match what I did. Say I planned for 200' but really did more like 180', the difference will be apparent with less deco called for by the computer. If it makes sense I'll follow the computer.

Linear vs exponential stop distribution is kinda "meh" to me. It's easier to do a linear ascent and I don't seem to think there's any real deco advantage or disadvantage to doing one method or the other. At 70' I might wait let the 60' stop clear (could go to 50' according to the box) and then do the same thing at 60'. This smooths it out a bit and minimizes the time I spend at 30' compared to an exponential "follow the computer exactly" ascent.

 

[huwporter]

Yeah, that makes sense. For deco times past 75 minutes and depths past 75m or so it gets increasingly convoluted to come up with pragmatic plans that cover every possible contingency - particularly on a site with a wide range of possible depths - so it makes sense to maybe have a plan for what you 'expect' to do and a deeper/longer plan as a 'get me out of dodge' backup in the back pocket. Then if you end up doing a shorter bottom time and/or shallower average and the computer's times are reasonable then you can take advantage to get out of the water faster than the 'expected' plan would get you.

For linear vs exponential, yeah, I agree up to a point. It doesn't seem totally optimal to, example, do 8s (or longer) all the way through the intermediates, something like (eg) 4,4,8,8,16 seems to work well and is still easy enough to follow as a team. But your 'follow the ceiling staying 1 stop away' sounds like it'll work well in the real world.

 

[Jack Hammer]

For tech dives I too have found the computer to be fairly close to our written plan. Some cave or recreational dives, though, our overall plan is a bit vague and we may wing it for a bit after we've finished in the area we planned to see. The computer is very nice for this.

 

[elmo]

As a relatively recent GUE convert (currently T1), almost all of my deco diving is pre-GUE training. But the issue is the same: you have agreed a plan with your team, and you have a dive computer that might say something a little different. We'll follow our plan through the deco profile, but if we're at our final stop and all our computers clear a few minutes ahead of plan, we might ascend if we all feel like it. I see no reason not to do the same now I'm T1.
As for gradient factors, I suggest having GF high the same as used for planning, and GF low greater than used for planning. Why? Well if you're adjusting your plan and end up shifting a few minutes from deeper than shallower, then you could break the ceiling defined by gf low (but still be quite safe). Shearwaters aren't horrible and beep at you (or your buddies) when you break the ceiling, but it's still nicer to avoid the warning. I have my Petrel set to gf 60/85.
One trap for new players is that it's very easy to forget to switch gas in the dive computer when you switch gas in reality, especially if you're used to diving with bottom timers only. But it's ok, if you follow your plan like you would with a bottom timer you're ok, even if the dive computer doesn't think so.

 

[beester]

I have a petrel 2 as main and a depth gauge as backup (in my right pocket). Generally I set the petrel to gauge for rec or T1 dives because I've done enough dives in that range to know the deco profile and I don't need the backup brain that the DC is.

For deeper or longer dives I set it to computer (OC/TEC). We still plan the dive using a planner and pragmatically align the stops (specially the 50% and 35/30 stops) and the computer is backup. It's good to have in my opinion because although it adds a small bit of taskloading (not forgetting the gas switches) it's a very very good backup brain. Next the computer result is in the same ball park (5 min more or less on a 2 h deco dive is the same in my book).

I am however a bit afraid that we but also future generations of GUE divers might lose the deco planning skill. There is a risk that teams will start bypassing the decoplanning part and rely totally on the DC.

Some anecdotes that illustrate the advantages and disadvantages.

I have a good RB80 friend who is now diving his petrel 2 always in computer mode as backup. This after getting an underwater DCS hit. He was diving on a project week in Italy. The dive they planned was 40' BT at 120m. Because of a freeflow on his bottom gas (side) they had to abort the dive after 15-18 min BT. He switched to his backgas and they started the ascend. They had planned for minutes less and more than 40' and also for an abort dive at the bottom up to 10 minutes but not for this scenario. He had his petrel 2 on gauge and they guestimated their deco profile. moving up from the 12 to 9m stop he got hit quite hard in his ankle. (they moved back down and restarted the 50% deco). In any case after that dive he looked up the deco and their guestimation had ommited a significant part of deco.

I was diving a 70m wreck. Petrel 2 was on OC/TEC (computer using GF 30/85). About 4 minutes on the bottom I realise that my computer is flashing red (it had turned back on my arm on the bungee so didn't see it and was busy securing the anchor line for other teams going to drop down a few minutes later). Apparently I hadn't switched gas on the computer after last dive and it was still on O², so with a PPO² of 8 bar. Of course I switch back to bottom gas on the computer but the deco calculated by the computer is now totally skewed because of my O² episode

(just for clarification before anyone misreads this I was breathing bottom gas, it was just the computer not switched correctly)

 

[huwporter]

Thanks guys, all good food for thought. I agree, I don't expect to find much/any value at the T1 level - doubt I'll even bother to programme the gasses for those - nor really at light T2 level either. And strongly agree, there is a definite risk of becoming personally and institutionally lazy at planning to be vigilant against.

We're currently running a project on a sea mount off Sydney with depth ranges from 65-100+m (trailer below!) so average depth could be anything - we could plan for 90m and end up doing a 70 average - so using the computer as above could add value.

 

[beester]

Of topic but this is very cool Huwporter... I need to visit Australia

 

[Compressor]

My apologies if error is on me but I cannot view the video: No link in black box.

 

[Dave Bevan]

I kinda feel the same. I did my fundamentals and ran my petrel in BT mode for a while. But now I kinda feel that having such a powerful tool on your wrist and not using it is kind of a waste. I mean if I have a real emergency I want to know my exact profile to get out when perhaps my mind is too task loaded to work out my ratio. But this is skewed as despite doing the training there just aren't any GUE divers in my area.

I think as long as you are able to work out your ratio and compare to ensure you and your computer are on the same page then really you are covered on both fronts. Its like another layer of safety.

However I understand the concerns as I am only just starting to get into deco diving so I'm very much in danger of being too lazy for Ratio and just using it as a backup.