Need advice about doubles equipment

[TSandM]

During one of the dives for my "deep" specialty, we did a simulated deco -- 8 minutes breathing off a hang tank. Maybe this is what is planned.

 

[matt_unique]

The instructor said it would be a real deco dive, but his friend (presumably also a NAUI instructor) who lead some of the dives said the same thing as you about not exceeding rec limits... He said we would use EAN27, but program our computers for air, push them into deco and then do some deco stops.

The idea was to simulate a situation in which for whatever reason you unintentionally end up in deco, so you have to do deco stops on the fly. Our plan was to go to about 38m and stay there until our computers were in deco, form a daisy chain and ascend directly in open water to 30m for 1min stop, then to 20m for 2min, 10m for 3min and finally 5m for 5min. At this point we were to break the chain and instead grab a hold of my gf from back of her bc while she would attach a spool to an smb and deploy it, followed by the final ascent.

In general the idea would be to make these increasing stops at every 10m depending on the relative air supply - remember that this was to be an unintended deco, so the plan would be dictated by remaining air.

What happened though was that the resort lost electricity over night, so they couldn't fill the tanks with nitrox so we went down with air. Also, I only got to about 36m, my gf just hit 38m when the instructor already wanted to head up (whereas we were supposed to stay there for 10-20min depending how long it took for our computers to hit deco).

He then ascended to 30m and started looking for a nurse shark that had been sighted there earlier but didn't find it, and I asked about what happened to the plan, so we then did the daisy chain ascent in open water and smb deployment, which was pretty cool. In his defense, the guy was wearing a Suunto so he was already in deco whereas my Versa Pro was in the green the whole time and my gf's versa pro peaked at the first yellow. It was a pretty cool exercise though and in hindsight I'm not sure much would have been gained anything by staying at depth for another 15min - plus my gf said she was narced anyway.

I suspect you now know this plan is a disaster. There would never be a need to assume 'real' deco risks before someone had the proper training, equipment, gas, and plans. A recreational computer may be cleared from deco obligations while a diver may still require deco time. Using only air, for a 20 min dive to 38 meters (125'), you would be looking at the following profile (this assumes an excellent SAC rate):

>>Dec to 125ft (2) Air 50ft/min descent.
Level 125ft 17:30 (20) Air 1.00 ppO2, 125ft ead
Asc to 60ft (22) Air -30ft/min ascent.
Stop at 60ft 0:50 (23) Air 0.59 ppO2, 60ft ead
Stop at 50ft 2:00 (25) Air 0.53 ppO2, 50ft ead
Stop at 40ft 2:00 (27) Air 0.46 ppO2, 40ft ead
Stop at 30ft 4:00 (31) Air 0.40 ppO2, 30ft ead
Stop at 20ft 24:00 (55) Air 0.34 ppO2, 20ft ead
Surface (55) Air -30ft/min ascent.

Off gassing starts at 86.2ft

OTU's this dive: 21
CNS Total: 7.2%

70.6 cu ft Air
70.6 cu ft TOTAL<<

The plan this instructor proposed included 11 minutes of deco when you should in fact have 32 minutes. This plan was a disaster but fortunately you did not experience any negative consequences.

--Matt

 

[PerroneFord]

Man, those deco times look UGLY when you don't carry 50%! Matt, I'm guessing the instructor wasn't running his profile on V-Planner

 

[matt_unique]

Man, those deco times look UGLY when you don't carry 50%! Matt, I'm guessing the instructor wasn't running his profile on V-Planner

I hear ya - and even less ugly with 70%.

Literally the first topic we discussed in Deco Procedures was recreational computers and how they offer zero real decompression information....

--Matt

 

[*Floater*]

yeah, the deco schedule was far from optimal, but the idea wasn't to go for anything ideal because this was unintended deco, but rather to spread out the remaining air to maximize the deco time. Even with this constraint it was far from optimal, but this was not a tech course and in a situation like this something is better than nothing. In time I plan to work my way up to adv nitrox and deco procedures.

About formal training for the doubles, my first dive in them was more or less under the instructor's eye. He picked an easy site for that purpose. In fact he didn't want me to use them on our deep dive, and changed the schedule for the current dive too so I'd have an easy first dive site with them. He also spent about 20min on a buoyancy check. Turned out my correct weight with a SS backplate in warm water was no extra weights - I dumped 8 lbs from my waist and from around the tanks during that check and felt great diving afterwards.

What kind of things would an instructor teach me about diving doubles?

For independents I was going to breath one down to 2/3rds, about 140 bar, then switch to the second one using my primary second stage and use that down to 1/3rd, or about 70bar, then switch back to the other tank. With everyone else on a single I probably wouldn't need to make more than these 2 switches, maybe only 1. I would rather use manifolded doubles, but I just felt so good in doubles that there's no way I want to use singles again unless I have to...

 

[PerroneFord]

Instructor can take you through the various failures and teach you the proper responses to each. Primary failure, secondary failure, tank o-ring failure, valve failure, roll-off, isolator failure.

I agree with you about diving the doubles. I dove them for a MONTH and had to go back to singles. Man was it painful putting that weghtbelt back on! So much so, I splurged and bought a set of doubles.. but I can't dive them yet dangit! Soon...

I was able to shed my weightbelt for my single though... so I feel good about that at least.