Question Rec cold water diver: Single, Double or Sidemount? (focus on safety)

[Tracy]

Why is SM the boat captain's problem?

Everything is the captain's problem.

 

[Angelo Farina]

This forum provides so much useful information, it's incredible. But I couldn't find anything in regards to different setups for recreational diving. I'm not sure if the question is stupid Let's try. This will be a lot of text from my side and I'm wondering how experienced divers would rate the different setups.

My scuba background:

• I'm an AOWD with about 60 dives - most of them the typical holiday/tropical dives. I did and like more advanced dives such as diving in currents, night dives, entries of a cavern (no caves). Though I enjoy diving in the "deeper" region for rec divers (let's say 20-30 meters), this is not something I pursue to dive deeper, just to be deeper as anytime before. I like taking some pictures or videos of the dives, so I usually have some additional equipment with me.

General intention:

• I want to dive (more or less) full year in local rivers and lakes. Water temperatures varies from 4°C to >20°C, of which May - October (including) are above 10°C. Only January and February are below 5°C. I have no intention to dive at those temperatures.
• Pure recreational diving. I have no intention to do extra deco stops, use mixed gases, go below 40 meters, dive into narrow caves, or start using rebreathers. I do think that all of that is spectacular and would do that in another life but diving should be fun and easy for me. This does not mean I don't want to use technical stuff for diving - in fact I love technical stuff and I'm willing to invest training, money and time just to use the technics, though not at the limit. It's like to have a decent sports car: You can accelerate and brake faster than the others and the cornering speed is higher - but you don't have to drive at the limit to have fun (i.e. 1 km/h faster and you crash). Yes, waste of money for the potential of the car, but still so much fun.
• I don't have any issues with back, knees etc., I'm tall and strong, so moving dive equipment around, regardless the configuration, is very easy.
• Safety is priority number 1. Only diving with a buddy (or in a small group), never single.

Questions:

• I wonder which are the opinions of experienced divers which setup would be "best" or which have negative sides which could be eliminated with an "upgrade":
  • Single tank.Let's say 12 liter tank. 2x valves, 2x 1st stages etc.
    • Most simple set up. If free flow occurs, closing the primary valve should give enough air to either ascend normally or maybe use a little bit of buddy's air while ascending (depending how fast the valve is closed and how much air is left in the cylinder).
    • Dives will usually be limited by air supply, not by NDL.
  • Double tank. Let's say 2x8.5 liters or 2x10 liters. 2x valves, extra valve in between (manifold), 2x 1st stages etc.
    • More "advanced" setup. May give more dive time.
    • I'm not sure if this setup will increase safety at a free flow compared to a single tank? What could happen that this configuration is superior, i.e. the extra valve between the tanks?
      If I close (let's say) the right valve, is just the exit to the 1st stage blocked but air from the right cylinder can go to the left first stage? Or is the whole right tank shut off?
  • Sidemount. Let's say 2x7 liters or 2x 8.5 liters.
    • Most "technical" setup. May give that little "extra" to diving by just using it, though not really necessary. It's like using a dedicated espresso machine + milk frother instead of a fully automatic coffee machine. The espresso machine will give the same espresso/cappuccino, but the subjective taste is superior :-D
    • This setup should result in increased safety at a free flow: Just use the other regulator. Even if the easy-to-reach valve of the affected tank is not closed - the other should provide enough air to safely reach the surface.

I think the real questions come down to:
Is a double tank configuration in any case better than a single tank configuration for rec diving?
Or will the only question be: Go single tank or sidemount. A single tank is as safe as a double tank (my assumption) but sidemount adds the technical side and improves safety.

Looking forward to your opinions and any downsides/benefits I did not mention in above text

My experience in very cold water (under 1m of ice, in alpine lakes) is very limited.
Say a total of 5 dives.
I used either an old-style twin tank (2x10 liters, two valves, two fully independ regs, no manifold valve, mechanical reserve lever) or a similarly- configured single tank (15 liters, again two independent valves and regs, and reserve).
I found them substantially equivalent and "safe enough" for rec diving (well, I am not sure if exploring a lake under 1m of ice, entering through a small hole, is considered rec nowadays).
Any tank smaller than 15 liters is too small in my opinion.
Diving in cold water has already its own complexity due to a bulk suit, thick gloves and perhaps a full face mask. I would not add the further complexity of two independent side-mounted tanks to this.

 

[Wildsparrow]

I really appreciate all the inputs and good discussion. Especially the various experiences is very helpful to me.
Just the discussion about SM on boats is a little bit off-topic, as I will not use a boat at my local dive sites. And even if: I'm sure that SM would be an option. It's just lakes, even tiny ones compared to the Great Lakes (I've been there once and it's like the ocean)

Some suggested to talk with a local instructor. This is certainly something I will do in the near future, but keep in mind: If I talk with one or two local instructors, they will have their opinion and experience, just as you people have your opinion and experience. If there was the best and only configuration for the type of diving I want to pursue, there would be no discussion required at all :-D

In the meantime I did more research about pony bottles and H-valves. It seems that the concept of pony bottles is widely used in America, whereas the H-valve is a preferred option in Europe/especially Germany.

I do not pursue diving below ice and full face masks. Let's keep it above 5°C in the water and >0°C outside the water

Here is my current thinking against a pony bottle:
I assume that the only valid configuration would be two regulators on the main tank and one regulator on the pony bottle. Reason: If my buddy gets into an OOA situation, he would use my second regulator. If the first stage fails (two people breathing from one stage in cold water), everyone is OOA. So the failure of my buddy would bring me into a life-threatening situation. I would switch to my pony bottle. That would save me, but not him, as there might not be enough air in the pony bottle for both of us. And if so, the pony bottle would be quite large, almost the size of a regular tank. And in that case, I could just carry two tanks with two individual 1st stages and two 2nd stages. Even if he needs my second regulator, he would breathe from an individual 1st stage. And even if one of my two 1st stages fails there should be enough air in a single tank to safely ascent with both divers breathing from the same tank.
---- Just looking from a price perspective, with the pony bottle configuration I would have two tanks and three regulators, each of them requiring maintenance. But personally I can only use the air from one tank. In comparison, with a doublemount configuration, I would also have two tanks, but this time more air for diving, and only two regulators. Instead of maintaining a pony bottle and third regulator without actually using those items, I would "only" have to maintain for two tanks and two regulators, which would give me more usable air/joy.
--- I want to highlight that I do find the concept of the pony bottle great, especially if you already have a nice one tank backmount setup and you want to increase your safety. I doubt that this is the perfect solution for me when I need to buy the whole equipment for 30-50 dives/year. And: Always within the limits of no deco stops, no overhead environment etc.

My current thinking for the H-Valve on 1 tank:
I would need to further understand the risk of getting a free flow towards the end of the dive (cold water, freezing 2nd stage): If there is water in my tank/hoses, which could cause a free flowing 2nd stage, would that happen more likely during descent? With enough air in the tank, I would have enough time to close the valve of the primary 1st stage, switch to my second 1st stage, and abort the dive safely. If the free flow tend to happen at the end of the dive (less air in the tank? Does this have an influence of the water vapor in the air, reaching the 2nd stage, building up ice?), I would need to be really quick to notice the issue & react accordingly. (On a side note: Yes, practice makes perfect, so it is clear for me to regularly practice valve drills etc.)

My current thinking for 2 tanks (backmount or sidemount):
I would most probably have more than enough air with me for my dive sites. Regardless of the time during the dive, if there is a free flow, I have enought time to handle the situation.

My current thinking of a double BM:
I do not like the bulky wing/donut, as I would probably go for a smaller tank size (7 / 8.5 liters), whereas the double wings are designed for larger tanks (12 / 15 liters). I also do not like the space I need to handle/store the tanks. Separate tanks are easier to handle compared to tanks which are mounted to a pack of tanks. And yes, the trunk of my car is quite small. Overall I think the double BM is great for redundancy and safety, but it might be too much for my rec diving.
(approximately 11 liters = 80 cuft)

My current thinking of a SM:
Great for safety/redundancy, especially valve handling. Separate tanks can be transported more easily. I could go for steel as I do not need to handle the tanks in caverns (buoyancy, penetration), which would reduce the amount of separate weights. Would add some tec aspects to my diving (though not needed), which is an aspect I would certainly enjoy.

If there are any flaws in my thinking, if you agree or disagree, I'd be happy to read your opinions. I haven't decided yet and it might be that I would initially buy the basic parts (such as the drysuit) and rent the remaining parts to try stuff out. Currently I slightly prefer the sidemount route.

 

[Angelo Farina]

I really appreciate all the inputs and good discussion. Especially the various experiences is very helpful to me.
Just the discussion about SM on boats is a little bit off-topic, as I will not use a boat at my local dive sites. And even if: I'm sure that SM would be an option. It's just lakes, even tiny ones compared to the Great Lakes (I've been there once and it's like the ocean)

Some suggested to talk with a local instructor. This is certainly something I will do in the near future, but keep in mind: If I talk with one or two local instructors, they will have their opinion and experience, just as you people have your opinion and experience. If there was the best and only configuration for the type of diving I want to pursue, there would be no discussion required at all :-D

In the meantime I did more research about pony bottles and H-valves. It seems that the concept of pony bottles is widely used in America, whereas the H-valve is a preferred option in Europe/especially Germany.

I do not pursue diving below ice and full face masks. Let's keep it above 5°C in the water and >0°C outside the water

Here is my current thinking against a pony bottle:
I assume that the only valid configuration would be two regulators on the main tank and one regulator on the pony bottle. Reason: If my buddy gets into an OOA situation, he would use my second regulator. If the first stage fails (two people breathing from one stage in cold water), everyone is OOA. So the failure of my buddy would bring me into a life-threatening situation. I would switch to my pony bottle. That would save me, but not him, as there might not be enough air in the pony bottle for both of us. And if so, the pony bottle would be quite large, almost the size of a regular tank. And in that case, I could just carry two tanks with two individual 1st stages and two 2nd stages. Even if he needs my second regulator, he would breathe from an individual 1st stage. And even if one of my two 1st stages fails there should be enough air in a single tank to safely ascent with both divers breathing from the same tank.
---- Just looking from a price perspective, with the pony bottle configuration I would have two tanks and three regulators, each of them requiring maintenance. But personally I can only use the air from one tank. In comparison, with a doublemount configuration, I would also have two tanks, but this time more air for diving, and only two regulators. Instead of maintaining a pony bottle and third regulator without actually using those items, I would "only" have to maintain for two tanks and two regulators, which would give me more usable air/joy.
--- I want to highlight that I do find the concept of the pony bottle great, especially if you already have a nice one tank backmount setup and you want to increase your safety. I doubt that this is the perfect solution for me when I need to buy the whole equipment for 30-50 dives/year. And: Always within the limits of no deco stops, no overhead environment etc.

My current thinking for the H-Valve on 1 tank:
I would need to further understand the risk of getting a free flow towards the end of the dive (cold water, freezing 2nd stage): If there is water in my tank/hoses, which could cause a free flowing 2nd stage, would that happen more likely during descent? With enough air in the tank, I would have enough time to close the valve of the primary 1st stage, switch to my second 1st stage, and abort the dive safely. If the free flow tend to happen at the end of the dive (less air in the tank? Does this have an influence of the water vapor in the air, reaching the 2nd stage, building up ice?), I would need to be really quick to notice the issue & react accordingly. (On a side note: Yes, practice makes perfect, so it is clear for me to regularly practice valve drills etc.)

My current thinking for 2 tanks (backmount or sidemount):
I would most probably have more than enough air with me for my dive sites. Regardless of the time during the dive, if there is a free flow, I have enought time to handle the situation.

My current thinking of a double BM:
I do not like the bulky wing/donut, as I would probably go for a smaller tank size (7 / 8.5 liters), whereas the double wings are designed for larger tanks (12 / 15 liters). I also do not like the space I need to handle/store the tanks. Separate tanks are easier to handle compared to tanks which are mounted to a pack of tanks. And yes, the trunk of my car is quite small. Overall I think the double BM is great for redundancy and safety, but it might be too much for my rec diving.
(approximately 11 liters = 80 cuft)

My current thinking of a SM:
Great for safety/redundancy, especially valve handling. Separate tanks can be transported more easily. I could go for steel as I do not need to handle the tanks in caverns (buoyancy, penetration), which would reduce the amount of separate weights. Would add some tec aspects to my diving (though not needed), which is an aspect I would certainly enjoy.

If there are any flaws in my thinking, if you agree or disagree, I'd be happy to read your opinions. I haven't decided yet and it might be that I would initially buy the basic parts (such as the drysuit) and rent the remaining parts to try stuff out. Currently I slightly prefer the sidemount route.

You nailed it. I would keep it simple: a single tank "large enough" with double valves and reserve. Possibly a Beuchat one, which has two fully independent air paths down into the tank, ensuring that if one freezes the other works.
In my experience, free flow is caused by the second stage, no air flow is due to ice in the first stage, locking the piston.
Above 5 °C you will not encounter any of these two problems. If you want to avoid them also below 5°C, use sealed diaphragm fist stages (Scubapro MK17 or their clones) and a full metal 2nd stage (nothing beats my SP 109-156 for this).
A manifolded twin tank does not really provide more safety in case of free flow: the time necessary for closing the manifold valve is the same as closing the valve of the free flowing reg.
So, as said, keep it simple.
I really do not understand why this single tank-double valve setup is so rare in US: It is the standard here in Europe, with DIN attachment of course (which can be converted to yoke if required by screwing in an insert).
Do not ignore the additional safety of the reserve valve.
In case one reg free flows and you are slow closing the valve, it avoids your tank empties completely, After eventually closing the valve, you pull the reserve and you magically get 50 bars remaining. Better than a pony tank, in my opinion. Which is also a big object to carry around and to get entangled with.
All old tanks here have two valves AND a reserve...
Here my 15-liters Cressi steel tank, with double valve (not H) with reserve:

 

[SlugLife]

Here is my current thinking against a pony bottle:
I assume that the only valid configuration would be two regulators on the main tank and one regulator on the pony bottle. Reason: If my buddy gets into an OOA situation, he would use my second regulator. If the first stage fails (two people breathing from one stage in cold water), everyone is OOA. So the failure of my buddy would bring me into a life-threatening situation.

If I'm following, you're suggesting the increased rate of air through the first stage of two divers actually CAUSES a freeze? That makes some kind of sense. I suppose diving in very-cold water might be where your needs vastly differ from mine.

A pony is a smaller, lighter-weight, convenient way of adding redundant air. But if you're going doubles with 2-regs or sidemount, then it doesn't serve a purpose.

A little "secret" about pony-bottles, is a lot of people skip the VIP, Hydro, and Fill costs, and simply transfill the bottle. Maybe keep an eye on the used-market to see if one pops up really cheap. I nabbed an AL19 for $50, after it had been sitting unsold for some time & sent the seller a low-ball offer.

Would add some tec aspects to my diving (though not needed), which is an aspect I would certainly enjoy.

I see SideMount as a sort of "challenge." Some will find it fun, while others find it frustrating. But if you're the kind of person who enjoys a challenge of sorts, then I think you'll enjoy sidemount.

 

[SlugLife]

I am far from being the best SM diver from a boat or the shore. However, in both cases I mentioned, I was good enough that there were backmount divers that were less "seamless." In both cases, when I called ahead, the operators seemed happy to take my business and there was no grumbling I notice while I was on the boat. The biggest issue seemed to be ensuring there were tanks with DIN valves available.

I do think the courtesy of calling ahead made a difference. And, I would not be upset at an operation for saying no since in a group of sufficiently experienced backmount divers I would have stood out and been a pain to everyone, crew and divers alike.

I always keep in my save-a-dive kit or regulator bag 2x yoke/din adapters, and 2x "pro valve" inserts. If I happen to need to use someone else's tank, or they need to use mine, or I happen to have a tank I haven't swapped the valve yet, then come in handy.

 

[Celt]

want to dive (more or less) full year in local rivers and lakes.

I'd start in the lakes shallow with a simply rig, 12ltr back mounted. Gradually work towards rivers in fast water and poorer vis, the last thing you want there is gear hanging off you.

 

[Wildsparrow]

I'd start in the lakes shallow with a simply rig, 12ltr back mounted. Gradually work towards rivers in fast water and poorer vis, the last thing you want there is gear hanging off you.

There is no poor vis in Switzerland

 

[Celt]

There is no poor vis in Switzerland

Fabulous,

 

[rx7diver]

If I'm following, you're suggesting the increased rate of air through the first stage of two divers actually CAUSES a freeze? That makes some kind of sense. I suppose diving in very-cold water might be where your needs vastly differ from mine. ...

Moreover, cold water divers know to NOT simultaneously inflate a BC, inflate a drysuit, and inhale using a single first stage (because of the increased risk of inducing a free-flow).

rx7diver