Been an interesting discussion though.Wow this thread really went off the rails.
That sounds like gas planning. It doesn't take much to do, especially with MultiDeco or suchlike. After a few dives, you've got a template in your head as what the plan is.Sometimes there are instructors I know who want to do deep recreational dives when not teaching and we also do not gas plan the dives but do have a dive plan and no pony bottles. After all we do want to boat captions to know where we expect to be picked up and our expected dive time.
Lets be honest here, the gas plan is very simple. You know the depth; you know the max bottom time (from the NDL tables/planning software); you know your SAC (or use a default assumption); you know the gas you'll be using; you know your ascent profile (3 mins at 5m
As it's an NDL dive, it won't be that long a dive, especially if using air.
Taking a pony cylinder is simple enough to plan for: there's no adjustment to the main gas plan, all you need to do is be able to add up 3 mins at the bottom given your SAC and depth, plus the ascent including
There's not many dive profiles to remember. 20m/60', 25m/75', 30m/100', 35m/120' and 40m/132'.
Code:
Example, for a 30m dive with air to NDL. From PADI RDP it's 20m bottom time with 3 min 5m safety stop. SAC assumed at 25 litres/min (that's high, but cautious)
Gas required = ∑ of:
20 x 25 x 4(ATA) = 2000 litres (bottom time)
1 x 25 x 4 (ATA) = 100 litres (ascent from 30m to 20m, use worst case)
1 x 25 x 3 (ATA) = 75 litres (20m to 10m)
1 x 25 x 2 (ATA) = 50 litres (10m to 5m)
3 x 25 x 1.5 (ATA) = 115 litres (safety stop)
1 x 25 x 1.5 (ATA) = 40 litres (surface)
--------
total: 2380 litres.
Plus reserve (=50 bar x cylinder size)
A 12 litre tank would be 12 x 220 bar = 2640 litres.
This means that someone with that SAC would need to restruct their bottom time to their minimum gas limit:
380 litres (the amount to surface)
600 litres (the reserve of 50 bar x 12 litre tank)
approx 1000 litres required to surface
Using a 12 litre tank, this is 1000/12 = 85 bar minimum gas
Do you need a pony? It's up to you to decide using your personal risk assessment and assessing the dive profile.
Maybe here's the difference between purely recreational trained and experienced divers and technical divers: the above is pretty trivial to calculate and risk assess.
For a benign, simple, shallow, time-limited dive in warm clear water on a reef you probably don't need a pony.
For a deeper dive to the NDL dive in a poor visibility cold water wreck dive in current, then you probably do need to take a pony named Prudence. Or not; it's your decision.
By definition a deco dive has an overhead and that's generally considered to be a technical dive (yes, some agencies such as BSAC allow for light deco within a recreational dive).Some recreational dive operations don't have divers doing deco dives even if deco certified that is the way they run their operations as their guides are often not deco certified.
This is because many divers get bored of being constantly limited to minuscule bottom times and shallow depths given the expense of doing the dive in the first place. This is why they move through the levels of technical training: sort your core skills (e.g. Fundies, etc), learn about decompression diving (Advanced Nitrox & Deco Procedures - ANDP, etc.); deeper diving with trimix (45m on helitrox); even deeper diving with multiple stages (normoxic trimix to 60m); even advanced trimix (100m+). But nearly all of them will move to CCR to constrain costs of 'mix diving.Most of the divers on this thread seem to be certified for Deco and mixed gas diving... I guess we can include Nitrox certified as mixed gas.. In my last decade of recreational diving I have yet to see anyone gas plan a recreational non deco multilevel dive.
NDL dives are trivial to plan, see above.
AIUI, the Shearwater / Suunto planners only give you the dive profile, they don't calculate your gas requirements.Sometimes I do gas plans before a dive on my Shearwater, but that really is for the fun of gas planning and seeing if that matches up with gas consumption for the actual dive.
Then you use a rule of thumb for your SAC: 25 litres / min (as in the example above).Not many of the recreational divers I meet know their SAC rates, they don't know about GF or Surf GF, CNS PPO2 etc as they just do dives and not go into deco.
Surf GF is simply the NDL limits, CNS is irrelevant, PPO2 is part of Nitrox training. NDL = No Deco Limits, therefore no deco (or in the case of BSAC, use their tables and planning you'd have been taught in your Sports Diver course)
Yes, definitely. But it's up to the diver to decide.Should dive operators be teaching recreational divers to bring a pony? I would say they would not be interested in that as then they need to provide pony bottles. I've yet to see a dive operation offer pony bottles for rent they seem to be an individual diver who brings their own.