Question Sidemount or backmount doubles?

[tbone1004]

As a matter of fact, UTD is the only agency using that system, as far as I know.

GUE certainly does not use it, and I don't think ISE does either (assuming it is still active).

ISE is still around but they use an entirely different approach to sidemount to either UTD or GUE

 

[mdwalter]

I am an AOW diver that wants more bottom time and thus more breathing gas. For single-tank dives, I really like my backplate & wing setup. No issues with that for those kinds of dives. But I would really love to get some more bottom time and expand my reach/range. I will definitely seek proper training to go beyond my current certification in terms of maximum depth and no-deco limits. But with which setup?

I have scoured these forums and the internet for objective pros and cons for sidemount and backmount setups. (‘Objective’ being the operative word here. Most comments seem to be very much in favour of either sidemount or backmount diving.)

Even though I will not be cave diving or doing penetrations into particularly tight spaces - for which sidemount was originally developed - there are some benefits to sidemount that look very appealing to me. Here’s what I found, or at least the major differences between the two setups.

Backmount:

• One stable unit, preferred on boats (?)
• Quite heavy to log around on dry land (I don’t have the strongest back in the world)
• Not as streamlined as sidemount (?), but adding stage bottles doesn’t hurt streamlining further

Sidemount:

• Two units, may not be as stable on boats if tanks are donned above water
• Tanks can be hauled separately
• Sleeker in the water (?), but adding stage bottles negates this advantage somewhat
• Valves are easier to reach, and visible

A few questions regarding these two lists:

• Anything I missed on either list? There are some question marks in there, any comment on those?
• Is a set of backmount doubles harder or easier to find than sidemount tanks at dive centers/liveaboards (provided I bring my own BC and regulator set)? If itineraries and schedules allow for double tank dives, is one of these configurations easier to get when traveling and renting tanks?

Obviously, there is proper training and setting-up the configuration involved in both. Investment in special gear, too (another, bigger, wing for doubles or a sidemount BC, regulator sets) and maybe in tanks. I already have two steel 12L tanks, though. I just need to put new valves on them and sidemount rigging if I would get into sidemount, or try to build a double set out of them…

I realize your question is from 6 months ago and you have already received 13 pages of responses, so I'll keep this simple. Since you already are comfortable with BP/W, doubles would be an easy next step from a comfort and familiarity perspective. Many people actually find it easier to maintain trim and buoyancy in doubles than a single BM tank. On the other hand, SM would allow you to switch back and forth easily from single BM to SM with the tanks you already have, whereas once you band your 2 tanks together you won't be using either of them for single tank dives anytime soon.
As for your pro/con lists:
I find SM very easy to use from a boat or shore, and easier to walk to the water with tanks on than doubles.
Stages or deco bottles with doubles do indeed reduce streamlining, especially if worn in the traditional hanging down style (less if worn like sidemount tanks).
Stages or deco bottles with SM do also reduce streamlining, but no more than they do with doubles.

Please ignore the posters who tell you to just get a bigger single tank, that's just foolish. It adds even more drag and no redundancy for maybe 30% more gas instead of the 100% more gas and full redundancy of a 2 tank system.

 

[mdwalter]

Thanks for all the opinions so far. In terms of availability abroad (when not wanting to fly in my own tanks), any big differences there?

Yes, absolutely. Sidemount is your friend when traveling. The harness is much smaller and rolls up nicely due to no BP needed. Then all you need is 2 cam bands, some string and a few clips to use the rental AL80s available pretty much everywhere as SM tanks. Finding doubles to rent is hit or miss except in N FL, Riviera Maya, and a few other cave-centric destinations. Otherwise you have to bring your own bands and manifold (in addition to your BP/wing), drain and disassemble your rental tanks (if they let you), put your bands and manifold on them (if they let you), and pay to have the doubles filled. A little less problematic if you do independent doubles (SM on your back) but then you still need to bring bands and BP (just don't need to deal with swapping out valves and refilling).

 

[mdwalter]

I numbered above.

1) for many years I thought sm was way more stable. If you are properly setup in doubles, you will be just as stable. It took me a long time to realize I was wrong in my thinking

2) You still need to do a valve drill and even an s-drill. They're just different. Sure you can see your valves, but you still need to know how to address the potential issues as well as how to exit on a bad valve via feathering

3) I don't know why this buy a rebreather phallacy so that you don't need all the tanks exist. Before I was ccr certified I thought the same thing because I heard it so much. It is very false. In reality, you need to plan your gas so that you can surface/exit if you lose your rebreather at the furthest point of penetration or depth. That typically means carrying similar volumes and types of gases as in oc. Having to carry just as much gas was the biggest shocker I learned in ccr training

You're wrong on point 3. You do need less gas on the rebreather. Let's take as an example a dive where I would take double 80s and 1 bottom stage, and let's say it's in a big wreck or a cave where I'm doing a long penetration where I really need to come all the way back the way I went in. So for OC this is rule of thirds, so I turn after using the stage (80 cf) and have the other 160cf in the doubles for the exit and emergencies. Now if I do this same dive with my chest mount RB, I don't need the stage because I will not need that 80cf for the way in (yeah, I will use like 3 or 4 cf for the RB, but negligible) and I still have the 160cf of back gas for bailout even if the RB fails at the max turn point. So I'm dropping 1 tank and replacing it with the RB. The trick here is to know where that point is that the first 80cf would be used up, so I don't run the risk of diving the RB past that point where I have enough bailout for the exit.
For deco gas you are correct, more or less, because if I've bailed out at max pen I will need just about as much as if the whole dive was OC (maybe a bit less since I will have done part of the dive at constant PO2 rather than constant FO2, but for planning purposes I just take the same as for OC).

Second example, for a dive where I am at depth and have gone a max distance from the boat or shore and am just about to turn and head back when my RB fails, but there is a direct path to surface, I only really need enough bailout to get me to me to the surface (<NDL) or my first deco stop, and less deco bailout gas because I've only done half a dive so deco will be shorter. Because the RB could also fail all the way at the end of the dive I will still carry full deco bailout gas equal to what I would for the full dive on OC, so again I concede the RB does not save me on deco gas I need to carry. But for this dive I only need a significantly smaller amount of back gas for bailout than I would need for the full dive on OC. For example, if it would require 160cf on OC and I would carry an extra 80cf for emergencies, but surfacing upon RB failure only requires 20 or 30cf, I would just take the 80cf stage and leave the double behind. Now I've dropped 2/3 of my OC tank load in place of the RB.

Third scenario, I need less gas on the boat or in my truck in most any scenarios, because unless I actually have a RB failure I will only be using a few cf per dive so I will use the same tanks for the whole day or maybe the whole week without needing to refill or switch to new tanks for repetitive dives.

 

[grantctobin]

You're wrong on point 3. You do need less gas on the rebreather.

Who taught your cave ccr class? Correct me if I'm wrong, didn't you get your first CCR/Choptima two months ago? @rddvet point three is largely valid, and the lack of understanding of said point is why so many people run around cave country with insufficient bailout.

 

[Rayk]

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[ginti]

Who taught your cave ccr class? Correct me if I'm wrong, didn't you get your first CCR/Choptima two months ago? @rddvet point three is largely valid, and the lack of understanding of said point is why so many people run around cave country with insufficient bailout.

I am not CCR certified, but the reasoning of @mdwalter seems acceptable to me. Could you please explain me where he is wrong? Thanks a lot!

 

[tbone1004]

Please ignore the posters who tell you to just get a bigger single tank, that's just foolish. It adds even more drag and no redundancy for maybe 30% more gas instead of the 100% more gas and full redundancy of a 2 tank system.

While a larger single tank does have a larger front profile and as such does obviously increase drag, it is not only insignificant but it is imperceptible to the diver. Doubles are definitely something you can notice from a drag profile.

On your less gas comment. The CCR replaces your "in" gas, but does not replace the out gas. You have to be going a LONG way "in" before it makes an appreciable difference.
Your comments about shorter bottom times requiring less gas is very wrong though because it does not account for hypercapnia where you need to plan for at least 5 minutes at 2cfm SAC rate to deal with hypercapnia which should be a fixed amount for any CCR bailout planning. Basically 10cf/ATA, PLUS the rest of the gas for return/ascent.

The comments about direct ascent imply that you are actually in an area that you can make a safe direct ascent. I have not encountered many of those in the real world, the vast majority are infinitely safer to carry the appropriate bailout gas to get back to the anchor line and come up as planned. "No one died from having too much gas"

 

[rddvet]

Who taught your cave ccr class? Correct me if I'm wrong, didn't you get your first CCR/Choptima two months ago? @rddvet point three is largely valid, and the lack of understanding of said point is why so many people run around cave country with insufficient bailout.

Thanks. I just saw his comment. I definitely stand behind my comment. Any instructor who teaches you need less bailout than you’d need gas in oc scares me. Going into ccr I heard the old wives tales that it’s true. But after taking classes I learned it’s completely inaccurate
Edit: my comment was also biased some because almost all of my dives on ccr are cave dives but even if I was ow diving it would likely still be a deco dive which creates an artificial overhead.

 

[SlugLife]

Second example, for a dive where I am at depth and have gone a max distance from the boat or shore and am just about to turn and head back when my RB fails, but there is a direct path to surface, I only really need enough bailout to get me to me to the surface (<NDL) or my first deco stop, and less deco bailout gas because I've only done half a dive so deco will be shorter. Because the RB could also fail all the way at the end of the dive I will still carry full deco bailout gas equal to what I would for the full dive on OC, so again I concede the RB does not save me on deco gas I need to carry. But for this dive I only need a significantly smaller amount of back gas for bailout than I would need for the full dive on OC. For example, if it would require 160cf on OC and I would carry an extra 80cf for emergencies, but surfacing upon RB failure only requires 20 or 30cf, I would just take the 80cf stage and leave the double behind. Now I've dropped 2/3 of my OC tank load in place of the RB.

Third scenario, I need less gas on the boat or in my truck in most any scenarios, because unless I actually have a RB failure I will only be using a few cf per dive so I will use the same tanks for the whole day or maybe the whole week without needing to refill or switch to new tanks for repetitive dives.

Who taught your cave ccr class? Correct me if I'm wrong, didn't you get your first CCR/Choptima two months ago? @rddvet point three is largely valid, and the lack of understanding of said point is why so many people run around cave country with insufficient bailout.

(No rebreather or cave experience here, so apologies if I say something uninformed.)

Context matters, and I'd say mdwalter is at least correct on scenario 2 and 3. For open-water, you want enough redundant gas to safely surface or staged-air (plus a little). A rebreather failure in open water would suggest you immediately proceed to deco & surfacing. So long as the rebreather works, you have a significantly extended bottom time. It's the same reason a 19 to 30cu redundant is good enough for most open-water-recreational; in an emergency, you're not retracing your steps back, just proceeding to the surface.

Cave and penetration diving would be a different animal, mostly because your penetration and return trip is typically the same distance.