Switching to doubles - Am I in a rush?

[kylehorn]

"

At 120 feet I'm likely going to hoover a single tank before I get close to my NDL on N02."​

At 120 feet, you will be no richer than EANx30 and lower would be better. 30% gives you an EAD of 103 feet. Based on your OW training you will round 103 to 110. The PADI Tables give you a 15 minute NDL, many computers would be less than that. A single tank will let you get close to that and even exceed it. Doubles give air reserve, your NDL's don't change and the need for selecting the correct O2 mix for the depth does not change either.

I didn't take the time to look at the NDL for 120 on EANx30 (which is typically the mix I breath). You're definitely right that a single tank would definitely suffice in that situation. Maybe a better example would be at a shallower depth, where I might have a NDL of say 45 or 50 minutes. I actually never heard about EAD in any of my training - which is kind of scary because of how practical it is when you're diving mixes they don't have tables for. Thanks for sharing that and helping me learn something.

 

[Sevenrider860]

"

I actually never heard about EAD in any of my training"

I read that statement and started thinking you must have not payed any attention during class. Then I looked at the PADI Enriched Air Diver manual and the only location I find Equivalent Air Depth Tables mentioned is as Option 1 when using an Air-Only computer. The last sentence is "If you are interested in learning to do this, your instructor can show you how to use these tables." I guess knowing how to use EAD tables is now considered outdated by PADI.

I keep a waterproof EAD Table and the US Navy Air Table in my pocket as a backup.

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[kylehorn]

I'm glad you mentioned it, it's definitely useful regardless of whether or not PADI thinks it's outdated. Good back up for computer failure or some unforeseen mishap.

 

[p1p]

You'll need to learn EAD/END for mixed gas training.

 

[Mustard Dave]

I set a personal limit of 30 metres max. on a single cylinder in the UK. Cold water makes freeflows very likely and poor visibility means the risk of buddy separation is increased. I think the deepest I've ever been on a single in the UK is about 26 metres. Any time I have been deeper I have had a pony or twins.

When I did the PADI deep speciality, I had to do the three dives required on different dates, with quite a gap between them due to reasons I won't bore you with. Dive one was in standard jacket style BCD and side slung pony, which I'd used before. By dive two, I'd made a stainless steel backplate and was using it with a single cylinder, small wing and one piece harness. I used this with a 7 litre steel side-slung stage for redundancy on dive 2. Before dive 3, I was booked on a trip to the sound of Mull and knew some of the dives would be in the 30-40 metre range. I knew I didn't like ponies and had always had to borrow one previously, so I went out and bought a larger wing and a set of twinning bands. I had two identical 12 litre 232 bar steel cylinders (roughly the same as a HP100) so I bolted them together and strapped them onto my BP as independent twins.

When I got back from Mull, my cylinders were getting close to an inspection so I had them stick a manifold on and got somebody from the club to check I was doing shut-downs correctly. When I did dive 3 of the deep spec, we were joied by a DM I had not met before. He came up to us at the car to see if we needed any help with our kit. When I hauled the twinset out of the boot, his expression was priceless. He'd only ever dived singles and when I pulled out the 7' hose and asked where it went he had to call the instructor over, who creased up laughing and told me to stop taking the piss out of his DM.

Since then all my UK diving has been on the twinset. If I'm doing an easy quarry dive, I have to put up with the weight to walk across the car park but at the end of the first dive, I just dump it at the side, grab something to eat, and grab it again for the second dive. On a hard boat I find them easier. All the kit is going to the boat anyway so I'm not carrying extra weight. Once on the boat, I get two dives out of it so I don't have to mess about swapping cylinders, while everybody else is fighting for space and struggling to slip a cam-band over a cylinder on a pitching boat.. I have to admit, they are a pain in the arse on a RHIB though.

I do not agree with this theory that they encourage you to push your NDLs. If you need to rely on cylinder volume to stop you exceeding NDLs, you should think about taking up golf. You are taught to plan dives and monitor your instruments on the OW course - if you cannot manage that, you are not competent enough to be diving. The main reason for twins is redundancy. This is essential for techies but there's certainly no harm in having that benefit on recreational dives. If you go down the tech route later, the kit will not feel alien when you do your first course.

 

[rongoodman]

If one is doing a 2-dive (no-deco) charter, is it typical to switch out tanks between dives? It occurs to me that unless the diver switches out tanks, he is starting the second dive without the ample reserve that presumably prompted him to use doubles in the first place. Switching out tanks on a doubles rig is obviously not a quick thing to do like it is with a single tank. I imagine it requires tools.

If I thought that I would't have an adequate reserve on the second dive, I would bring the doubles and a single stage of bottom gas. The first dive would consume the stage and maybe a little of the back gas, leaving almost a full set of doubles for the second. (About the only time you would take apart a set of manifolded doubles would be for an inspection or hydro.)

 

[Colliam7]

Appreciating that this is slightly off topic:

The last sentence is "If you are interested in learning to do this, your instructor can show you how to use these tables." I guess knowing how to use EAD tables is now considered outdated by PADI.​

No, I don't think that is necessarily the case, although I can see why the thought might come to mind. Rather, I think PADI is reflecting the mindset of a number of 'modern' divers, who would prefer to just follow their computer. And, there is probably an element of instructor preference at work - some instructors prefer to emphasize computer use, while others still have students do 'problems'.

For better or worse, I fall in the latter category. I teach 2-3 group nitrox classes each year, and still go over EAD calculations and concepts, and have students work RDP problems based on use of nitrox mixes and Air RDP values, so they can see how it is done, and deepen their understanding of the principles of nitrox use.
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[Buoyant1]

Do YOU think you are in a rush? If you feel you NEED doubles for the type of diving you are doing, then do it, if there is no NEED, then why bother.... are the other people you are diving with using them? If they are then maybe, you should....are you doing dives where doubles are necessary?

I switched because most of the guys I dive with were diving them, and did some advanced courses... I've been having some medical issues, so I put the advanced courses on hold until we figure out what the problem is, (should I be diving? my doctor didn't say to stop) but I already had the doubles set up and ready to go, so I just kept them, and have found them to be worth it.

Most of this is set up for YOUR speed...do things at YOUR pace, and YOUR comfort level. If you have dive buddies that can mentor you, then by all means go with it!

 

[fdarden]

I haven't seen very many people on a charter boat with doubles unless there is a planned tech dive, however there have been a few occasions, and usually the biggest issue is the tank holders, and how to stow the tanks. If they are using the "cup" style holders, it can be an issue, but you simply work around these things. Im interested in what charter operators have to say about this. I doubt too many charters are going to flat out turn you away because of your doubles. In my limited experience with this they will ask you to limit your dive time if they see you with lots of gas, and/or ask you not to go into DECO, and that is all reasonable and understandable. It would be nice for some actual charter operators to chime in on this.

 

[Lorenzoid]

fdarden, some posts over on another thread are currently addressing this.