Planning a dive trip in a few months and wondering what I should do, bear with me. Twinsets or Stage/Pony Cylinder

[HenryUK]

Why not learn side mounted twinsets? Easy to learn and easy on our aging physiques. This also translates to other scuba and CCR modes if the mood strikes you. Enjoy!

I’m still young (fit and strong apparently), but this certainly something I’ll be considering in future… I have dived cenotes in Mexico and would love to go further, so side mount and CCR are definitely things I’d like to look into…. But one step at a time for now, maybe in 10 years…

Thanks for the reply

 

[inquis]

how do I workout my average depth?

You don't need the average depth, just do it separately for each various depth level and add up. Yes, this is a pain for multi-level dives, which is why dive planning apps exist. Put in your profile and it does all the calculations for you. Subsurface (desktop version) is one that will give you the gas requirements, including info about the ascent on your pony (the computation of which is equivalent to the "minimum-gas" concept).

 

[HenryUK]

You don't need the average depth, just do it separately for each various depth level and add up. Yes, this is a pain for multi-level dives, which is why dive planning apps exist. Put in your profile and it does all the calculations for you. Subsurface (desktop version) is one that will give you the gas requirements, including info about the ascent on your pony (the computation of which is equivalent to the "minimum-gas" concept).

Thanks, I’ll look into dive planning apps (I only have a work laptop and unfortunately can’t download useful software )….

 

[HenryUK]

You don't need the average depth, just do it separately for each various depth level and add up. Yes, this is a pain for multi-level dives, which is why dive planning apps exist. Put in your profile and it does all the calculations for you. Subsurface (desktop version) is one that will give you the gas requirements, including info about the ascent on your pony (the computation of which is equivalent to the "minimum-gas" concept).

They are also on the app store so now download - many thanks

 

[BoltSnap]

‘fag box’

You keep them in boxes?

 

[Wibble]

You keep them in boxes?

An educational thing; certain words have very different meanings in different countries. Us Brits laugh at the American "fanny". Same for our common word for a cigarette box!

 

[BoltSnap]

An educational thing; certain words have very different meanings in different countries. Us Brits laugh at the American "fanny". Same for our common word for a cigarette box!

You smoke "fags" and put them in boxes?

 

[Wibble]

@HenryUK -- the great thing about this thread/discussion is it's getting you to think about the issues you're trying to mitigate. Like most things, they're hard at first, then become easier once you've done it a few times.


For your average depths... If you're planning ahead of time then you have to assume the worst case. For your wreck dive, you'll assume the 40m/132ft as a square profile for planning purposes. The reality will be different as you'll mooch around above the seabed, often around the deck or wheelhouse which could easily be 10m/33ft higher.

Groan; digs out the PADI RDP cards rather than using MultiDeco....

• From the RDP, a 30m/100ft dive on air is 20 mins NDL (No-Decompression Limit).
• A 40m/132ft dive on air is 9 mins NDL.

Thus if you go for depth, you've 11 mins less on the bottom phase than if you mooch around the top of the wreck.

28% Nitrox will give you a few more mins of bottom time -- The equivalent air depth EAD is 36m, so maybe 3 more mins - 12 mins at 40m.

Now your problem is this; your PLAN that you've (possibly) written on your slate is for a bottom time of 12 mins, but this assumes a square profile at 40m. You COULD have two plans, one for 40m and another for 25 mins at 30m. Or another for 35m. Where does it stop!

Therefore you'll use your computer. These are good at keeping a tally on your NDL. You know your bottom time will be somewhere between 12 mins and 25 mins; the computer will tell you when time's up.

BUT (this is Basic Scuba); you MUST keep an eye on your remaining gas pressure. Using the gas calculations further up this thread, you will know how much gas you'll need for the ascent from 40m to the stops and to the surface. Add to that the 50bar/750psi safety margin (you need to be on the boat with that left in the cylinder or suffer the wrath of the DiveMasters!


If this were me and I had to stick to the NDLs, I would more accurately work out the gas required for the ascent + stops + ascent. This would be a number X. To that I'll add 50bar/750psi and the result is my Minimum Gas. The dive would then be on the computer where the ascent is triggered at hitting EITHER Minimum Gas OR hitting the NDL on the computer.

You also need to factor in your buddies who will have worked out their figures in advance. With a reasonable 16 litre SAC you'll possibly find the others will be heading up before you -- and, as they're buddies, you must also leave with them.


As a complete aside and well beyond the scope of Basic Scuba... With Accelerated Decompression using oxygen rich gasses and the correct equipment (twinset + stage(s); rebreather) you could do an hour on the bottom with around 50 mins of decompression. We've a lot of dives like that around the UK. Lots more training required. And much more money!

 

[inquis]

They are also on the app store

The mobile version of Subsurface doesn't have as much functionality as the desktop, including the lack of a planner. There are, of course, other mobile apps with planning capabilities. That might be another thread to find one that meets your needs (includes minimum gas and variable ascent rates).

FWIW, I would not use an 11 liter cylinder for an NDL/OW bailout from 40 m. I would use 10 m/min to the safety stop and then 3 m/min from there. Assuming 3 minutes on the bottom and your emergency SAC of 25 l/min, that takes 900 liters. (I doubt you'd maintain that rate throughout the entire ascent, but let's stay with the 900 liters.) An AL40 (5.5 l) has 1150 liters (27% more than you need). Your call, obviously, but you should try carrying whatever you think you need first.

A pony should be unobtrusive so you won't be tempted to leave it behind.

 

[NYCrecdiver]

Con’s:

1. Buoyancy and trim issues

I dive with a 13 cf pony bottle using a shark mount to attach to my primary tank and have no trim issues.
I have trim pockets on my top cam strap and put 2 lbs in the one on the side of pony and 4 lbs on the other side.
That being said, I might want to get a larger pony. 13 CF is not enough to comfortably ascend and do a safety stop. In an air out situation, there's a possibility I'd have to skip the safety stop.

Have you considered sidemount diving? You also get redundancy.