Sidemount Question

[SuPrBuGmAn]

1/3 of an LP 108 at 3600 psi is 49 cu ft of gas - a swing weight of about 3 lbs. 1/6th of an LP 108 is obviously half as much, or 25 cu ft and about a pound and a half.

In comparison 1/3rd of an AL 80 is 25.6 cu ft - basically the same weight and swing weight as 1/6th of an AL 80. So regardless of tank size weight etc, I'm willing to be to my next paycheck that breathing 25 cu ft of gas from an LP 108 results in the same net change in buoyancy as breathing 25 cu ft of gas from an AL 80.

If anything, it makes more sense to use 1/6ths on an LP 108 than it does on an AL 80, given the 3 lb versus 1.5 lb swing weights.

Considering I actually dive larger steels(LP95s and HP130s previous to owning the LP95s) and smaller tanks(AL80s and LP72s) on the regular basis, I can guarantee you that there is a much more noticable swing in bouyancy/trim with the smaller tanks as they become neutral(or even positive) compared to the heavier steels which are negative throughout the dive. Play with the math all you want, I'll stick with what I actually feel happening as I dive. Diving thirds off an AL80 can give you one tank sitting negative and the other floating positively, believe me, you can feel it. Heavier tanks, that stay negative throughout the dive, not so noticable. I still swap out at 1/6ths for other reasons.

The same thing applies to cave diving. Handling the (minimal) additional task loading is not a problem, but why do it when there is basically no need? There is always the risk that something else will go wrong during the dive creating additonal task loading or distraction. Why voluntarily put more on your plate when you can avoid it?

Never did like it when people used plane analogies, or car analogies, or whatever compared to cave diving(or much of anything else). Its not the same. IMO the minimal addition of task loading is beneficial in having tanks closer to evened out in the case of needing to donate. It also serves as check for your regulators, that they are operational throughout the dive(push through a restriction mid-dive and I'd rather find a problem with my regulator 1/6 later than 1/3 later). Its also a muscle memory check to keep you comfortable with swapping regs. I don't really even consider it a hassle to swap regulators, its natural at this point. I'm guessing most people diving sidemount regularly find it pretty natural as well. *shrugs* We're talking about two additional swaps. I stand by my previous statement that if swapping regulators is too complicated, the diver needs to reevaluate why he's there in the first place.

 

[DA Aquamaster]

My impression is that you and karstdvr are not in agreement either. Is it a problem or isn't it? My vote is on the "is not" side.

I agree an AL80 as either a stage or sidemounted is a PITA because it gets positive and very light in the tail end, but that happens over the course of the dive regardless of the psi differential.

I find divers swimming around with near empty AL80's in near vertical positions banging into the ceiling and/or generally being very unstreamlined to be annoying made more so by the number of divers with pictures of themselves with a suitably cool number of badly streamlined bottles. Makes you wonder sometimes. But the problem is not the swing weight of the Al 80, it is the traits of the tank itself. An AL72 works a lot better in that regard if you want a stage or sidemount in the 80 cu ft range - and it has a smaller diameter.

I agree to a very limited extent about airplane analogies. In a plane, you continue to move forward. In the above example I mentioned the pilot still going to be on approach at 90 kts or more and there is no time to stop and think about things - the situation demands an immediate response and any delay in response or failure to act just puts the pilot further and further behind the airplane facing an ever larger problem. That kind of time pressure combined with extreme levels of multi tasking is commnon in instrument flying but is rarely encountered in cave diving, where stop, breathe and think are usually workable options. So the analogies are not perfect as the flying examples are usually a lot more demanding in terms of task loading and time pressure.

But that also makes them excellent fodder for discussion as if a philosophy or idea works in the extreme, everything below that level is gravy. All things being equal, when things seriously go to **** in a cave and time pressure exists to the point where stopping and thinking are not really valid options, I'll put my money on the diver who can think and act under serious time pressure and is configured in a manner that reduces task loading to maximize the resources available to deal with the unexpected. If you train and configure for it, you'll be able to deal with it if it arises. You can tell the difference between divers under pressure who are on the ball and "ahead of the airplane" so to speak and those who just sort of hesitate or look dazed and confused. We have way too many of the latter, even in cave diving.

And again, I just don't see an upside to adding more gas switches when there is zero benefit. You cite switching frequently as an advantage to ensure the regs work. If your regs are prone to stopping, 500 psi versus 1000 psi will not make a difference and the issue is better resolved/prevented with adequate maintence. Even if the unexpected does happen, by design with only two switches you have enough gas in either tank to exit at any time in the event a reg fails. Any perceived advantage with more frequent switches is just an illusion.

In some areas of a cave, the distraction caused by a gas switch or even something really easy like unclipping and reading an SPG can be the difference between perfection and raising an errant puff of silt with momentary less than perfect trim. Sidemount is not a license to treat a cave any differently or create a disturbance when it can be avoided. In that regard, 4 switches rather than 2 just doubles the opportunities to to leave a mark or rasie some silt when it did not need to happen. So again why do it?

 

[SuPrBuGmAn]

My impression is that you and karstdvr are not in agreement either. Is it a problem or isn't it? My vote is on the "is not" side.

Thats fine, I'm perfectly OK with disagreeing with him as well. Its nothing personal against you, I've just found my experiences by actually diving smaller tanks that it DOES affect your trim.

I agree an AL80 as either a stage or sidemounted is a PITA because it gets positive and very light in the tail end, but that happens over the course of the dive regardless of the psi differential.

It happens, but its a hellovalot easier to deal with when they are both floating at closer to the same position. The additional swaps keep that difference at a minimum.

I find divers swimming around with near empty AL80's in near vertical positions banging into the ceiling and/or generally being very unstreamlined to be annoying made more so by the number of divers with pictures of themselves with a suitably cool number of badly streamlined bottles. Makes you wonder sometimes. But the problem is not the swing weight of the Al 80, it is the traits of the tank itself. An AL72 works a lot better in that regard if you want a stage or sidemount in the 80 cu ft range - and it has a smaller diameter.

In some places, an AL80 is all you can get ahold of to dive. In some circumstances, smaller tanks are better. I'll dive my LP72s rather than my LP95s when I'm going through cave that requires some smaller restrictions to be negotiated or if I've got to pull a tank or two off to pass an area. I like the LP72s better than the AL80s(which I had used at first), they don't get positive, but the butt ends still get extremely light(which is why I clip them to my waist D-ring instead of the doorhandles on the back of the Nomad - keeps the back ends from rising so high).

But that also makes them excellent fodder for discussion as if a philosophy or idea works in the extreme, everything below that level is gravy. All things being equal, when things seriously go to **** in a cave and time pressure exists to the point where stopping and thinking are not really valid options, I'll put my money on the diver who can think and act under serious time pressure and is configured in a manner that reduces task loading to maximize the resources available to deal with the unexpected. If you train and configure for it, you'll be able to deal with it if it arises. You can tell the difference between divers under pressure who are on the ball and "ahead of the airplane" so to speak and those who just sort of hesitate or look dazed and confused. We have way too many of the latter, even in cave diving.

I agree with all the above, I think the disagreement is more on whether anyone really needs to stop and think to swap a regulator? I look down at my SPGs(no unclipping necessary, they are visible throughout the dive) and if there's enough of a difference, I deal with one clip and swap regulators. If the **** is hitting the fan, swapping regulators every 500-600psi isn't my main concern, but its a bit comforting to know that both my available tanks are close if I had been swapping at 1/6ths up to that point...especially in a situation where if I have to donate a tank, or for whatever reason I lose gas in one.

And again, I just don't see an upside to adding more gas switches when there is zero benefit. You cite switching frequently as an advantage to ensure the regs work. If your regs are prone to stopping, 500 psi versus 1000 psi will not make a difference and the issue is better resolved/prevented with adequate maintence. Even if the unexpected does happen, by design with only two switches you have enough gas in either tank to exit at any time in the event a reg fails. Any perceived advantage with more frequent switches is just an illusion.

A perfectly working regulator may not be working at the end of the dive. I've had a regulator working at 100%, up until I pushed through a sandy restriction. When I made the next swap, it would stop freeflowing. It happens, stack the cards in your favor. I'd rather start my exit 500psi closer to the entrance than not, even when I know my other tank will have enough for me to exit on... Never know when something ELSE might come up.

In some areas of a cave, the distraction caused by a gas switch or even something really easy like unclipping and reading an SPG can be the difference between perfection and raising an errant puff of silt with momentary less than perfect trim. Sidemount is not a license to treat a cave any differently or create a disturbance when it can be avoided. In that regard, 4 switches rather than 2 just doubles the opportunities to to leave a mark or rasie some silt when it did not need to happen. So again why do it?

Thats just it, with a little practice, regulator swapping is second nature. Its no longer a distraction. It shouldn't cause any momentary lapse in trim or bouyancy. I stand by my previous statement that if swapping regulators is too complicated, the diver needs to reevaluate why he's there in the first place.



An honest question, do you dive sidemount regularly? I'll readily admit that I know nothing about flying an airplane. I'm simply curious if our experiences with different tanks are that different, or if your getting your information from something other than experience?

 

[ae3753]

Ah! That's my set-up (learned from Steve Bogaert). Someone had asked how I routed my hoses for sidemount tanks, and I wrote something up.

 

[ae3753]

Kelly, I'm curious ab out this statement. I've tried every position possible & have found having them point down to be the lowest profile position.

Here's a video at the end of a dive (4th dive after class). Since my first stages are up, the SPG rests on top of the tank, between the tank and my body.


Here's a picture of my front. Need to retie the bolt snap on the 2nd stage to bring up my primary regulator, but still not bad from a profile perspective.

 

[sskasser]

In some areas of a cave, the distraction caused by a gas switch or even something really easy like unclipping and reading an SPG can be the difference between perfection and raising an errant puff of silt with momentary less than perfect trim. Sidemount is not a license to treat a cave any differently or create a disturbance when it can be avoided. In that regard, 4 switches rather than 2 just doubles the opportunities to to leave a mark or rasie some silt when it did not need to happen. So again why do it?

If switching regs or checking my pressure causes so much of a distraction that I lose control of my bouyancy/trim...then I need to work on my skills a little more outside the cave....whether I'm switching twice, or four times, or fifty. Because you are right, sidemount, or any gear configuration, is not an excuse to cause damage.

 

[karstdvr]

Because you are right, sidemount, or any gear configuration, is not an excuse to cause damage.

Sometimes I think people treat off the shelf sidemount rigs as you do the "quick start card" that comes with electronics,and software. People are anxious to use it,but not willing to master it before taking it in a cave environment.

 

[Tegg]

Sometimes I think people treat off the shelf sidemount rigs as you do the "quick start card" that comes with electronics,and software. People are anxious to use it,but not willing to master it before taking it in a cave environment.

Same could be said for any dive gear.

 

[Jill Heinerth]

Whoops

 

[Jill Heinerth]

YouTube - RebreatherPro.com Tip: Mounting Streamlined Bailout Bottles