Info Helium is expensive and deep air is not my thing. -Let's talk about "Big" dives.

[Angelo Farina]

How can I plan these two dives theoretically speaking?

This info you have graciously provided is your personal experience? Are there technical instructors who have dived twice a day to these depths?

I am a recreational CMAS instructor. Not tech.
But, as said, these dives at 40m with some deco are fully recreational here. So yes, I planned and made such double dives at least 30 or 40 times in my life. At the time no computers, everything was planned using the goid old US Navy tables, with the CMAS modifications (ascent at 10 m/min instead of 18, ascent time added to bottom time).
In Cmas courses the usage of these tables for repetitive deco dives was taught already in the very first course.
We were also trained that "every dive is a deco dive", and that, for a given max depth, a dive planned just beyonds the NDL limit is inherently safer than a dive planned to stay just within the limit of NDL.
So, when doing two dives per day, it was common practice to plan both of them deep and with deco.
Possibly the morning one was even deeper, say 50m, and the afternoon one was just 40m for limiting the deco time.

 

[Learner Diver]

Possibly the morning one was even deeper, say 50m, and the afternoon one was just 40m for limiting the deco time.

How long did you stay at the bottom when diving both depths? What was your minimum surface interval?

 

[lermontov]

How long did you stay at the bottom when diving both depths? What was your minimum surface interval?

sounds to me like your going to use this information to do a dive that your not trained or ready for ?

 

[Dave Whitlow]

I agree with everything you are saying except "A giant mess of cylinders" 4 tanks are not a giant mess.

I apologize if my original post did not state some assumptions about the level of divemanship (is that a word?)
The assumption is that you are already qualified and trained to dive Trimix to 80 meters and beyond. This implies that you have had "video or mentor feedback", your "bottle rotations actually are clean" and that you can "deal with an emergency midwater etc."

I think that 80-100 meters is still OK for OC but too expensive. (Every bubble from the regulator is like $3)
The jump to CCR is needed for anyone doing more than 10 helium dives a year.

IMHO 4 tanks are not a giant mess of cylinders if they are diluent, O2, deep bailout and deco mix. Adding an extra O2 and converting the suit gas into another 3l of 32% isn't hard either. Once you start talking about bottle rotations you're heading in a bad direction and a rethink is needed. Although that does depend on location. As my diving is UK in tidal waters a lot of bottle might lead to an interesting day!

You talk of the 'jump' to CCR and, having made that logical transition 14 years ago (although I still blow bubbles for shallow stuff) I feel it is often overstated and seen as a bigger barrier than it really is.

Sure, there is a significant outlay on a CCR (a CO2 scrubber with a computer controlled solenoid) but in the UK they can be bought (second hand) for around £3k (how much OC helium will that buy?) and after a 5-6 day training course you are deemed competent to dive CCR to a moderate depth (40m). I always emphasise to prospective CCR divers the importance of getting that 'recreational trimix' CCR ticket as part of the first course as, IMHO, helium beyond 30m is necessary, and not expensive when you're only using 100 litres of diluent per dive.

The biggest change is managing buoyancy, the acceptance that lungs are not a useful supplement to neutral buoyancy, and the reality that you really need to be properly weighted and correctly manage wing and drysuit gas. On CCR, taking a deep breath to rise over an obstruction is a disappointment you soon learn and you go round and maintain constant depth, and avoid wasting precious diluent.

Once you are diving CCR (and have bought the extra stuff nobody told you you'd need) the cost of deeper dives is similar to shallow dives and the idea of deep air or 'managing' narcosis become ridiculous. For deeper dives CCR is the future!

 

[Learner Diver]

sounds to me like your going to use this information to do a dive that your not trained or ready for ?

Sounds like you're jumping to conclusions. Did you read the sign?

 

[rx7diver]

How can I plan these two dives theoretically speaking? ...

These two dives (i.e., a 130 fsw air dive with BT = 40 min, followed by a generous surface interval and a repetitive dive having similar dive plan) are not particularly difficult to plan. Like others above, I learned to plan these kinds of dives (using the U.S. Navy Standard Air Deco Table) in my first open water course (a YMCA/NAUI course in1986).

In real life, I would plan also for a delay at depth, a slightly deeper depth, or both (so, worst case, a 140 fsw air dive with BT = 50 min). So, my gas supply (including my emergency gas reserves) would allow for this worst case scenario.

My old Navy tables give an ascent time of 76:20 min:sec for this air dive with air deco. This is quite a long time if you're diving in open ocean where the weather might change, or sharks might appear, or ship traffic might become an issue. And this is quite a long time if the water is very cold, so your exposure suit must keep you warm enough during this long period of time when you're relatively inactive. And if you're diving dry, your exposure suit must keep you warm enough even if it floods!

Some OC divers will switch to 100% oxygen when they arrive at the 20 fsw deco stop, and deco out at that depth. Doing this will significantly reduce that 68 (= 24 + 44) minutes they would have needed to spend at the 20 fsw and 10 fsw deco stops on air.

Also, you can shave a bit more time off of your ascent time if you use a nitrox bottom gas (e.g. EAN24) and if your first deco gas is a stronger nitrox (e.g. EAN50) that you will begin breathing at an appropriate, safe depth as you ascend, before you switch to 100% oxygen when you arrive at your 20 fsw deco stop.

Note: Introducing nitrox and/or oxygen into this can significantly complicate things: You're carrying more than one gas, so you must ensure that you don't switch to the wrong gas at the wrong time. You're dealing with high octane breathing mixes, so you must know how to account for this in order to avoid oxygen toxicity, including on your repetitive dive and if you're diving over multiple days.

I hope this helps.

rx7diver

 

[Angelo Farina]

How long did you stay at the bottom when diving both depths? What was your minimum surface interval?

Bottom time was typically 25 to 35 min at 40m. Less at 50m, of course, say 20 to 30.
But when planning, we always planned a possible extra time to the next line in the table, so gas calculations were done respectively for 30 to 40 min at 40m.

 

[J-Vo]

Would doing two of those dives you have described above in a single day be considered a big dive? Could I ask how long of a surface interval would be appropriate for you?

My group often does "shallow" second dives to 40 meters to wrap up a dive day. Once you have been trained, planning it isn't a big deal.

 

[admikar]

Sounds like you're jumping to conclusions. Did you read the sign?

I would suggest you stop asking total strangers on a public forum and present those same questions to the instructor you are doing your course with.
I have stumbled upon you in several different threads here. You seem to talk a lot of specifics for someone that is not a diver yet. That's what probably made lermontov make that conclusion.

 

[SelfDiver]

Shrugs fine then you were awesome and A+ coming out of class (unlikely but ok) how do you know you're actually maintaining proficiency not getting sloppy?

Maybe its time to skip the 60m+ dives.

I am not sure I understand this reply.
Are you saying that most people's skills are not that polished right after tech certification?
Why is it time to skip the 60m+ dives?