Single Cylinder vs. Doubles

[The Iceni]

JamesK once bubbled...
I will dive as deep on singles as I will on doubles. Around 100feet. That is as deep as I go. I am not Trimix Certified yet.

Hi James,

I do not know what sort of diving you do but suspect you find many interesting dives at 100 ft (33 meters) to make it worthwhile staying to look around for more than 25 minutes.

Even with 32% Nitrox this is at the margins of a dive requiring decompression stops and as you know these penaltes rapidly clock up the longer you stay on the bottom.

Most British divers use 12 or 15 litre cylinders, which really only contain enough gas for a no-stop dive at that depth.

Granted, the pony will give you enough gas for an ascent and perhaps a short decompression schedule but you will not be carrying enough redundancy, with a pony of bottom gas, safely to extend your bottom times.

I could remain fascinated exploring a new wreck for over an hour so need obviously more redundancy than provided by a pony. A free flow or first stage failure would not be good news at the start of an ascent from such a dive.

Perhaps our concepts of "recreational limits" differ?

Decompression dives are almost becoming mainstream this side of the pond.

 

[JamesK]

Paul,

My main type of diving is cave diving. It is what I enjoy the most. Out of my last 30 dives, 5 of them are open water. Going into deco on some of my cave dives, ecspecially the deeper caves, is a norm. I stay away from any type of deco diving when diving singles.

I am confused as to what you are asking though. Are you saying that I should not be diving to 100ft on singles, or that I should not be doing dives to 100ft on doubles in an effort to extend my bottom time because of deco obligations?

 

[The Iceni]

JamesK once bubbled...
I am confused as to what you are asking though.

HI James,

Just highlighting the fact that it is not as simple as the original question on depth.

As Roakey has already stated

Depth is not the issue, overhead is. And overhead can either be physical, as in a cave or physiological, as in requiring decompression.

I'm probably being pedantic, sorry.

 

[JamesK]

I understand what you are saying now. However, I basically took the question at face value and did not feel like analyzing anything. I will dive a single to 100ft without hesitation if the dive plan and the conditions warrant it. In other cases, such as my cave diving, doubles are the only way to go. Yet in others, doubles, stage bottles, and deco bottles are necassary. Yet, the one common factor is that all of the dives are to 100ft.

 

[flyant]

I am a New Jersey Wreck Diver and I have just recently switched from diving singles with pony bottles to Isolated Doubles (no manifold between tanks). I use the Ultimate velcro double Bands from Aquaexplorer. http://www.paradisedive.com/aquaelink.htm After speaking with another diver with the same setup a couple of years ago, and after careful consideration I've come to the conclusion that this is the most versatile one for me. This setup allows a diver to switch from single tank to double in a matter of minutes.

I do not want my personal configuration of this gear to influence anyones decision of whether they use isolated double or not. I do think that everyone would be interested how I'm using them and I would welcome any comments or suggestions:

I am a diver of limited experience and dive no-decompression dives in the range of 80-130 ft. Safety then cost are my primary concerns. The ownly gear I do not own are tanks and I have made the decision to go isolated doubles because I prefer the margin of safety. Instead of renting three tanks (2 singles & 1 pony) for a two dive trip I'm renting three full size Al 80's.

Dive Profile: Using To Primary tanks and keeping it simple:
I use my left tank exclusively at the bottom (shortly after reaching the anchor I switch to my left tank) My Right tank I use for ascents, descents, water entry and exit (using only 1/3 of the tank). After my first dive I switch out my left tank and dive with a full left tank and a partially used right one. Note: I always leave the bottom with a minimum of 1000 psi in both tanks. I am quite comfortable with switching my regulators and I Keep both regulators secured to my front with breakaway attachments.

This is just the way I use this setup as you can see it is very flexible. You can modify your usage to suit you needs including different sized tanks

 

[AquaTec]

well thats the most interesting rig i have read about on this board to date.

do you have the same gas in both tanks

about ten years ago i would have a single 80 tank on back and then two 80's hanging on the side....but i evolved and learned better ways

I would politely suggest that you look at getting some training in diving with doubles...theres nothing wrong with bringing with you as bunch of extra tanks, which is what you seem to be doing, but you can imporove on your system

 

[flyant]

Yes I am diving air in both tanks.

To give you more background many NJ dive boats require an independent alternative air source on all dives (especially on dives @ or below 90ft). Also diving in NJ is mainly all about the wreck diving there are hundreds of wrecks off of the coast. The non-penetrating diving environment has its own set of hazards, primarily entanglement on fishing line or other debris on the wreck and the possibility of severely limited to zero visibility. I got certified in 1986 O.W. P.A.D.I , my certification dive as well as almost all of my dives since then have been off of dive boats down to wrecks. As I have stated my diving experience is limited and I plan to evolve my diving methodology. Further training as you have stated is a key to that growth and next season (I know you dry suit divers are diving all year round but I only have a 7mm wetsuit) I plan on taking the Advanced open water course and eventually the wreck diver and deep diver courses.

Diving with double as I see it add only two primary concerns. 1st and foremost is the change in buoyancy that must be accounted for and 2nd is the added weight you need to lug around while entering and exiting a dive boat. Before diving in the Ocean I got a hold of two AL 80's and tested my rig in the Atlantic City Bay by emptying both tanks to below 500 psi and adjusting my weight for neutral buoyancy at the surface. As far as the weight is concerned I am a big guy so the workload isn’t increased that much for me. Additionally isolated doubles increase the workload necessary for diving because air management of two primary air sources has to be effortless. So I checked my access to my gauges and my regulators and made adjustments. Both regulators when unused are routed to lie on my chest. Both pressure gauges are located slightly below them at waist level. This setup insures that I never have to reach or find any of my gauges or regulators. As far as the added work load I have alleviated any confusion by insisting that for the present time I will only use my left tank when I am at the bottom of the ocean and my right tank for all ascents, descents, entries and exits. Before I dove this configuration I discussed it with more experienced divers at my local dive shops as well as the divemaster of the boat. While it is not the most common configuration (1/3 to 1/2 of the divers I've dove with use manifolded doubles) it is safe and effective.

I must point out that I am not adding an additional tank for the use of any advanced or extended type of diving for which I am not trained. I am making what I think is a practical adjustment to my rig that replaces the required pony bottle with a full sized tank. As I have stated I plan to take more advanced training but I want the configuration that I dive with today to be very similar to that which I dive tomorrow. This configuration I believe will give me a chance to acclimate to the change in buoyancy and the increased air source management in a controlled environment before I deal with the more complicated matters such as wreck penetration and decompression diving. After further training I hope to take advantage of extended dives on wrecks such as the Varanger ("28 mile wreck", 140 ft) and U-853 (130ft). The following sites will give you an idea of what I'm talking about: http://www.njscuba.net/dive_sites/index2.html , http://www.diversion2.com/shipwrecks/index.html

Enjoying some of the better wrecks off of the Jersey Coast is what inspires me.

Again I would appreciate any comments, criticisms, or recommendations from those of you with more experience and knowledge.

 

[padiscubapro]

ok lets play devils adocate..

You say you need 3 80s for a two dive trip to have some safety margin.. I say probably not.. Many people I know do multiple no deco dives to that depth (and deeper) with a single set of twin 95s(oms 98s ect) with an Isolator.. Let me show you why..

if you get twin 95s first off youll reduce your weight requirement and will use less gas for you inflation.

Lets assume you have an RMV of .75 cft/min at the surface thats 3 cft per min at 100 feet.. at 20 mins (Padi and many other's NDL not Navy) you only used 60 cft of gas... (you should actually have more since decent time is counted as part of bottom time - and this should cover you 15ft safety stop)
on a non overfilled tank you have 190-60=130 cft of gas left
starting pressure is 2640, ending pressure approx 1805 psi
applying the rule of thirds you want a reserve of 130/3 = 43 cft of gas in reserve or approx 600psi. you have appox 90 cft of gas to do another 20 minutes of bottom time which is 50% more than you used on the first dive and will still have 43cft in reserve.

If your store overfills you have even more gas available..

IF you decided to dive nitrox @36%, then your NDL would be 30 mins each dive and twin 95 wouldn't be enough You'd have to use larger tanks or get them extremely overfilled to maintain adequate reserve on the second dive.

The task loading is simplified, and chance of making a mistake (like forgetting to turn on the bottom tank) is reduced. Once you become confortable with steel doubles (and drysuit otherwise you need redundant buoyancy)your ready to progress..

 

[SM Diver]

Dr Paul Thomas once bubbled...
With a manifolded twin set a diver has almost guaranteed access to all of his back gas, even with a first stage failure.

Not so even with a pony, which is why I always dive with my twin set.

Perhaps I am being ultra-cautious as I am no cave diver, but I always wear my seat belt when driving.

In addition, although possibly heavier a twin set is less cumbersome, more stable and better balanced.

At least it makes sense to me!

I agree with that logic 100%, and it has lead me to diving doubles. 95% of my diving is spearfishing, where self reliancy is of utmost importance.

To date, I have been diving single HP100s down to 110 fsw. I am setting up two sets of doubles, twin HP65s, and twin HP80s.

As far as getting multiple dives out of a single set of doubles, I'll get up to three dives off of those 80s, unless the ledge is loaded with fish. I'll probably average at least two dives, and I anticipate two dives off of the 65s.

The cost for me was about $725 dollars for the manifolds with isolators, bands, bolt kits, etc. I already had two 65s that I would use for shallow excursion (40-50 fsw), and I was able to trade three used AL80s for a single PST HP80, with the extra HP80 costing about $289. That put's me just over a $1000 for the conversion. Wait, the BCD is costing $600 with the backplate. ****, that puts me at $1,600. Wait, the cave diving course to perfect using the gear will cost $300. All right, $2,000. Oh yea, turtle fins ........

Damn, that was an expensive conversion!

 

[flyant]

You said

You say you need 3 80s for a two dive trip to have some safety margin.. I say probably not.. Many people I know do multiple no deco dives to that depth (and deeper) with a single set of twin 95s(oms 98s ect) with an Isolator..

No that is not exactly the point. To recap previously a typical two dive trip would consist of an al 80 with a pony bottle on the first dive then swapping for a fresh Al 80 for the second dive. Using isolated doubles in my case serves two purposes: 1. To Replace the Pony Bottle as an independent Air source and 2. To increase my available volume of gas (currently air). Manifolded doubles are not considered an independent alternate air source for New Jersey wreck dives and would still require pony bottles.

I do not need 3 Al 80's to have some safety of margin but it certainly increases it while allowing for growth as my diving skills evolve. Note an Al 80 rents for $9.00 and a pony bottle for $7.00. A difference of a mere $2.00 (plus the $135 for the bands) a dive increases my margin of safety while opening up new realms of possibility for my diving.

Did I hear you say steel doubles with an isolator and a dry suit? ...I'm a wetsuit diver who rents tanks. The point I'm trying to make is that I am easing into my skills and evolving my gear configuration. I don't intend to approach that "ultimate gear configuration" until I complete my training. As I mentioned Safety first then cost are priorities to me. At any were from $1500 to over $2000 for such equipment that equates to between 15 and 20 dives for me (for the cost of a typical $75 dive trip + $28 rental tanks) that’s a lot of dive time, which is more important to me. Did I mention I am an old recycled student going to school and have limited resources?

This configuration is what I believe is right for me at this point in my diving life. I believe at this stage of my diving skills this is an appropriate setup that I will continue to evolve with personal experiences and from input from other more seasoned divers. Eventually I intend to dive a dry suit but that is not in my immediate future.

You also said

IF you decided to dive nitrox @36%, then your NDL would be 30 mins each dive and twin 95 wouldn't be enough You'd have to use larger tanks or get them extremely overfilled to maintain adequate reserve on the second dive.

This is exactly the type of flexibility and growth potential I envision with this setup. While twin 95's wouldn't be enough, certainly swapping out a third tank would be (My velcro double bands can hold any size/type tanks I wish to place in them).

This setup has added advantages such as being able to hand off an entire tank to your buddy in an emergency since each tank is strapped in separately. Plus how many of you out there want to travel by air with your tanks and have to go through the hassle of transporting them and removing their valves. You don't have to do that with this setup because you have the ability of simply renting tanks at your destination.

Oh Yeah I almost forgot I probably didn't make it clear both of my tanks are turned on and checked before I leave the boat since I consider both primary sources of gas (air in my case). At this point the only thing I have to remember is to switch regulators every time I reach the anchor on my way up and down the anchor line.