Question CCR, SM diving, and proper weighting?

[stuartv]

I have had several events recently that culminated with listening to a podcast last night. The podcast was episode 82 of the "Speaking Sidemount" podcast, where the guest is Jake Bulman, from ProTec in Mexico. During that podcast, there is a portion where Jake talks about proper weighting for diving in a sidemount config using a CCR.

This highlighted other recent experience and my observations.

I have been taught 2 different approaches to weighting for CCR diving.

Classic: Weight yourself so that you can be neutral when all primary sidemount cylinders are (near) empty. With cave-filled LP85s, this means being at least 16 or so pounds negative (with an empty wing), at the start of the dive. Maybe a bit more.

New: Weight yourself including your SM cylinders' gas as ballast. I.e. Weight yourself to be neutral when primary SM cylinders are full.

Podcast is here:
Discussion of weighting starts around 19:00.

I think the Classic method is pretty well understood and does not need further explanation.

The New method is explained as follows (as best I understand it):

99% of the time, you are not bailed out. In all that time, being neutral with an empty wing makes buoyancy VASTLY easier on a CCR.

But, you DO have to plan things so that you CAN bail out and use all your BO gas without losing control of your buoyancy.

The thinking for that, in the New method of weighting, is that, if you do have to bail and you cannot maintain your buoyancy, you flood your CCR on purpose. It was noted that, if you do have to bail, there is a high likelihood that your CCR is already flooded anyway. It was not stated in the podcast, but I will add: If you have other BO cylinders beyond your primary SM cylinders, and they become positively buoyant, and that makes you unable to stay down, you ditch them. When it's empty, let it go.

I think this means, in reality, you don't weight yourself to be perfectly neutral (with an empty wing) when all your cylinders are full. You have to be at least enough negative to offset the O2 and Dil that you know you will use during the dive. If carrying deco cylinders in addition to the primary SM cylinders, you might need to be additionally negative by the amount of gas in one of those - so that you can fully drain it without becoming positive.

But, to be clear, I do not claim to be fully expert in the New method. Which is why I'm posting this.

Thoughts on the full details of how to employ the "New" method?

Thoughts on actually using the New method versus the Classic method? Is this a method that really only works for Mexico cave diving, where they are not using thick exposure protection and probably not carrying much or any deco gas?

 

[JonG1]

I listened to it too and it is intriguing, I'd be nervous in ow but in the ohe where a handy bit of ballast is likely to be around maybe...

Probably or possibly rebreather dependent in terms of how easy it would be to flood it without too many air spaces.

Pretty sure the Sidewinder is partially flooded most of the time anyway from what I gather
; )

 

[tbone1004]

I think a lot of it is situational AND highly gear dependent.
Disclaimer I haven't listened to the podcast yet but I will on the drive today so may edit the post based on what he said, but this is in response to your post.

Assumptions are that deco/stage bottles don't count, so it's only the 2 primary sidemount bottles.

A "common" configuration for me right now is diving my Kisskat with LP120's as bailout. If I'm diving in a nitrox depth cave that is roughly 24lbs of gas. I dive with a kydex backplate and no extra lead and am quite comfortable in a drysuit with that configuration. If divers are diving with steel tanks it is highly unlikely they are diving with any appreciable amount of lead that they can dispose of, if any at all. Even if I'm diving those tanks with 10/70 for something crazy deep it's still 7lbs and I just don't have that to get rid of.

You said he's in Mx so it is most likely he's diving AL80's with air/nitrox so call it 12lbs. If you're in a drysuit, in Mexico, with a rebreather, and AL80's you're probably wearing lead, so let's say you do have 12lbs of lead and the question is whether to wear the lead or not.
First question is whether the CCR has the loop capacity to fully flood and offset that amount of buoyancy, and that is going to depend on the unit. The Sidewinder certainly does not have enough dead space to flood to offset 24lbs of steel tanks, but probably has 4-6lbs to offset between the loop hoses and the scrubber. Remember that counterlung volume doesn't count because it's compressible so it's just like flooding a wing which has 0 effect on buoyancy. I'm not a big fan of intentionally flooding the loop, so this concept applies where you are diving with a very small amount of gas to counteract.

I'm of the opinion that if you are diving sidemount then you should try to use AL80's whenever possible because it does make the whole sidemount thing much easier than diving with steel tanks and the 12lbs is really inconsequential. This may be skewed by over a decade diving in Florida with big steel tanks where you are constantly egregiously overweight, especially since the vast majority of my diving is done in a 7mm wetsuit vs. a drysuit. Get enough hours in and you shouldn't be actively noticing difference in buoyancy like that, your body should be reacting subconsciously to those changes.

I would not in a million years go with this "new" way if I was in open water, the risk of an uncontrolled ascent is too high for a really negligible benefit in convenience IMO. If I was diving in Mexico, with AL80's, and in a drysuit, I still probably wouldn't bother because I'm not going to notice whether I'm 12lbs overweighted or not because I'm used to dealing with that amount of weight. Your mileage may vary but I would not recommend trying this with big steel tanks, not to mention that it would also require you to change your weighting every time you change your mix which would be a PITA if you are regularly diving with different trimix gases.


Edit: listened while Supercharging. The way he described it, it sounds like the same argument when you hear people teaching to use drysuit only for buoyancy control. This works fine if you are using a relatively small single tank where your lungs can realistically counteract the mass of the gas but it is only applicable with an extremely narrow range of configurations. Of course it will make things "easier" when teaching and of course it makes it "easier" when you are at shallow depths, but to slightly adjust the infamous quote from @cerich , this is an equipment solution to a skills problem compounded with a patience problem. Do I struggle with buoyancy if I have a pair of LP121's pumped up with nitrox when in a drysuit to hold a 10ft stop while on a rebreather, of course I do, anyone who says they don't is full of it, especially if you're on deco with your drysuit pumped up, but this is not something that we should be broadcasting as "best practice" or something that most people should even consider.

 

[rjack321]

Thoughts on actually using the New method versus the Classic method? Is this a method that really only works for Mexico cave diving, where they are not using thick exposure protection and probably not carrying much or any deco gas?

"Works" is a slippery term - although many cave divers have been underweighted on OC for decades too. Either way, I prefer to think of this as a normalization of deviance to make up for poor lazy buoyancy.

 

[rjack321]

My own personal MX weighting is a little bit on the light side, perhaps 2-3lbs. I can hold a motionless stop with near empty 80s without flooding my CCR at 20ft. Mostly because there is some wetsuit compression there. Could I hold the same stop at 2m? Probably not, I'd float up. If I have to traverse a super shallow section of cave 2m deep I will be swimming and it's not like that amount of buoyancy will pin me to the ceiling without flooding my CCR. The threat of deco in MX is trivial and the consequences aren't too bad either with chambers in the area.

I am much heavier in cold water (2 to 8C) because if I am on a BO exit I will be shedding heat on OC like crazy (compared to CCR) and probably with a deco obligation. So I am weighted for empty BOs and a non-flooded CCR. Even modest DCS when you have to dry cave out of the system or hike kilometers to get back to the truck is super serious. Additionally its 12+ hours to the only chamber in British Columbia taking divers in Vancouver. So absolutely no way will I dive underweighted, shiver on deco because I would float up with gas in my suit, or rely on flooding my unit to get out etc.

 

[justinthedeeps]

I don't dive caves, but in open water I plan to ditch empty bailouts as necessary, rather than severely overweight myself on every dive. Thought process: if you are emptying bailouts it's probably a serious enough situation to warrant jettisoning gear. If you're under control then pass it off to a buddy or send it up your already-deployed SMB line.

If you are weighted for empty bailouts (several more kg?), then you are diving with an inflated wing with far less safety margin for a flood. You are also carrying around a lot more weight for entry/exit that you rarely (if ever) will need, and have to find places to pack it on. I suppose the extra weight could be droppable, but that's one more missable step during an emergency.

 

[mr_v]

Here are some napkin calculations give my situation: open water diving, 3L steel tanks + AL72 BO. BO gas is used for suit, SMB inflation. [Yeah, I know...]

Faber 3L cylinders are -2.64# when full and -0.66# empty. You gain 2 buoyancy pounds per tank, so 4 in total. However, you don't typically run these cylinders 'til they are empty and if you bailout in the deepest part of the dive, they're probably have a considerable amount of gas. So, let's say that two 3L tanks gain only 2#s when things hit the fan.

My AL72 is +2.8#s at 500PSI. So, in theory, if I keep this configuration and continue a BO ascend, I am going to be 2(3L tanks)+2.8(BO tank)=4.8 or ~5lbs positive.

In the configuration above, I can't ditch any of the tanks. My BO gas is my suit/SMB inflation and 3Ls are in the back...

The best bet is to be slightly overweight at the start of the dive. While not ideal, with several sources of buoyancy control and an SMB, I can figure out how to come up on the line while being slightly overweight. Yes, buoyancy control may get funky, but it beats floating to the top.

Realistically, if I calculate my BO gas properly, I should have more than 500PSI in the BO tank once I am on the surface, so with the config above, I am okay with being ~3# overweight going in.

 

[Garth]

When I dive my sidewinder I don’t use any weight in a drysuit with arctic undergarments. I also don’t use my wing at all. I would be comfortably diving that way in open water but in open water I would likely be on my hammerhead which I also don’t use any weight and rarely use my BC.

I think it’s more of a personal preference and personal body type. In a pool without any gear on I sink like a rock unless I’m treading water.

 

[tbone1004]

@mr_v you are using the mass of the gas not the inherent buoyancy of the tank. 4.8lbs at 500psi not 2.8. They are also talking about cave diving where you would need 2x bailouts. Depending on how lean you are with gas planning and how much **** hits the fan will depend on how much gas is used so somewhere between 5-10lbs at the end of the dive when you're trying to hold a deco stop.

The most concerning thing for me is how much they are relying on flooding a Sidewinder which has very little floodable volume in it, essentially it's barely more than the loop hoses themselves and to flood the inhale hose is going to take some acrobatics. The sorb is put directly in the canisters so there is not a lot of airspace inside the canisters themselves.

 

[stuartv]

@mr_v you are using the mass of the gas not the inherent buoyancy of the tank. 4.8lbs at 500psi not 2.8.

I think you said that backwards, but yeah. I was going to give him that problem to figure out next time I see him. But, I think he's pretty busy and out of town for the next week or so.