Gas Management With Sidemount

[Jack Hammer]

He who questions training only trains himself at asking questions

 

[PhatD1ver]

You are misunderstanding what I am saying @PhatD1ver.

Any rigid system can 'break', not having one cannot.

For example: any routine you might have developed is broken on decent as soon as you add a third tank.

There is no real reason why you should not switch regs several times in any interval you might set, but a lot of reasons to not be able to do a comfortable switch at any one certain point in time (forcing you to postpone it).

Not using rigid intervals reduces the need for exact calculation and the resulting taskload.

I'm a little lost here. Why would I make a dive with a third tank if it wasn't factored into a gas management PLAN, one that included switches that help me monitor my working SAC to be sure that it's consistent with a dive plan that would require a 'third tank'.

I don't disagree that the switches shouldn't involve monitoring my SPGs, but there is also TIME which is a critical factor in any dive requiring a stage or deco tank. Which is part of the task loading and why any good instructor will force you to do both continuously during a dive and your training. And will also create scenarios to distract you from making a switch or checking your time and pressure according to the plan.

But thinking that you can just 'wing it' is not gas management, whether you check your SPG or not. They train learning a switch sequence for a reason.

 

[Jax]

I breathe 300 from the right (long hose), then 600,then 600 . . .

If I am off by 400, I tilt. My tanks, combined, weigh as much as me and my gear.

Let's face it - use what works for you.

 

[Razorista]

If I am off by 400, I tilt. My tanks, combined, weigh as much as me and my gear.

You must have giant tanks if that is true
To be off by 400psi with that tank size would be close to impossible with frequent switches.
I would perhaps do 6 switches (with my 19liters I switch more than once every 10 or 15 bar, less than half a pound of maximum weight change) for every switch you do, so I could never be off by more than a few hundred grams of gas weight.

But thinking that you can just 'wing it' is not gas management, ...

What I am proposing is not the 'easy way' PhatD1ver.

What I have been saying is that gas management is the priority, comfort isn't an issue at all, so it should not be a factor in the calculation.
Take the calculation NetDoc published for example, he mentioned most of the important things to remember (if I remember the article correctly).
And the way he proposed is of course mathematically correct, but diving simply is not a precise science.
Gas management is save enough if you are off in the number of switshes by a very large amount and can be safely calculated, as the margins of the proposed minumum reg switch methods show.
Since I do not remember NetDoc's numbers anymore, I will use Jax's now:

Within those 300psi I might switch regs two or three times, but once at least, in roughly equal intervals during each part of the dive.
With more switches singular timing differences balance themselves out, after ten switches you stayed on each tank for a comparable amount of time.
If you exert yourself a lot you switch afterwards, if you can see a problem coming you switch before it.

Other things might influence the reg you breathe too, gas management is not influenced by that if calculated correctly:
I switch to the longhose when entering restrictions.
In a wreck that can mean 'at every bulkhead or hatch', sometimes 2 switches for every 10 meters, if I do not know the location and have less than optimal visibility.
If you know the location it can mean staying with the longhose reg for longer than normal to reduce switches, so you stay longer on the neck bungeed reg on descent and approach, breathe the longhose during penetration, then go back to prioritizing the neck bungeed reg for most of the rest of the dive.

Mostly I have reasons to prioritize the neck bungee side, however:
I am not a cave diver, but I do a lot of simple overhead dives, wrecks mostly, ice rarely, submerged objects a lot, things like that and I use an smb or a few lift bags occasionally.
Personally I do not like to breath the longhose when handling reels and spools in bad visibility, because of the boltsnap dangling from the hose, I like to have that one clipped off when handling line or tools close to my face.
If I have to do that inverted or in a current however, I would not dream of trying to remember to catch the moving clipped regulator before it does something I do not like, so I prefer breathing the longhose then again.

On long dives with large groups I like to 'overbreathe' the 'non long hose' side to be prepared for repeated air sharing 'episodes' in the last quater of the dive.

I always try to remember to switch to the neck bungee before surfacing, to be able to drop the regulator for talking without thinking about clipping it of then - that can sometimes mean quarter hours (accompanying divers on their first dive, or feeling that way when expecting experienced buddys, or things like that).
So during those 'phases' I mostly breath that reg and it is the likely one to occasionally spit or freeflow when hitting the water the wrong way.

And will also create scenarios to distract you from making a switch or checking your time and also tehpressure according to the plan.

Not a problem if your plan is 'switch after he has distracted you and before every planned waypoint of the dive'.

...with a dive plan that would require a 'third tank'.

A third tank breaks the normal routine.
Most of the time you might just breath that third one down until empty and use your sidemount tanks for redundancy.
But if you use large steels, for example, your stages are likely to not cover a third of your gas, let alone two thirds.

So at some point of the dive you will either have to stop switching (to a deco-gas stage most likely), or start switching.
Easy to forget, more likely when entering deco after a long dive than anywhere.
Switching out of a 'drowsily remembered routine' could prove fatal at that point.
Forgetting all your training and panicking when realizing at the first deco stop that you will have to admit to be ooa in a minute to your dive buddy or even instructor when an underused tank or reg fails completely after missing to switch to it for a while - similar things happened to a lot of overconfident tec divers.

That's the examples for advanced divers, now the one more likely to happen to the less advanced ones:
Those are only examples however, I could name dozens more, serveral of which happened to myself or close associates, some few might be hearsay but sound realistic.

Getting 'dreamy' on a long decent can happen to anyone.
Forgetting about one or two switches after you had trouble stabilizing and hit bottom (or nearly did so).
Getting so concentrated on the screen of a GoPro or adjusting the lights on a large camera while dropping down a spectacular wall.
And getting round eyed when checking for the next switch and realizing to have missed two - can turn into a real emergency then, without a reason other than 'shock' (and bad but normal training).
Dropping equipment and retrieving it just a bit to deep, etc...

Inexperienced divers might even underestimate the amount of gas they will use when getting stuck in a downward current or just thinking too little before looking below that nice looking overhang down by their feet.

The main problem in all cases would be the stress resulting from having to calculate.
If you just don't, your tanks will most likely be equalized, because switching is second nature to you, or they will be off by the amount they will be off, no way around it anyway.
Either you start frantically checking the spg now for a time, or you just dive the underused tank for a bit longer and always switch back immediately if you forget for a second.
Knowing psi or bar will not help in most situations - gas calculation works best with a dry head before the dive.
You can start looking at your spgs again when you think you should have your tanks equalized again - minutes later at least, with the problem out of the way and a clear head.

Now, regarding an ooa situation, I personally prefer the scenario to have forgotten about a lot of switches when reaching the point my eyes start to bulge out trying to get air from my current tank.
What I do not want is having to recalculate gas volume in a real ooa emergency.
I want to know I have about halve the gas I carry for myself with a positive probability margin.
Most likely I will not have time to spare to read or calculate any value I might read anyway, before surfacing or solving the problem on the other diver.
Different 'in caves' but even any cave and long deco dive will be as short as remotely possible afterwards.
To be blunt: You do not really need the information when you will die if you cannot solve the problem without that information.

Most people acknowledge by now that ooa is a very remote risk in a sidemount config.
It will most likely happen to backmount single tank divers around the sidemounter, not himself.
You have to do very high risk dive profiles to have a statistical chance of ever being out off air before the first random single tank backmounter you encounter solo diving in a place nobody should know about.

 

[Jax]

You must have giant tanks if that is true
To be off by 400psi with that tank size would be close to impossible with frequent switches.
I would perhaps do 6 switches (with my 19liters I switch more than once every 10 or 15 bar, less than half a pound of maximum weight change) for every switch you do, so I could never be off by more than a few hundred grams of gas weight.

Okay, maybe I exaggerated a little, but my dive buddies notice it. LP85s or HP100s.

 

[lermontov]

This is important!

Reading this thread, it's less about gas management and mostly about gas balancing. I'm with Bob. What ever you decide on, make it consistent.

I like to keep tanks within 300 PSI. I breathe one tank down 300 psi or to when it's a multiple of 300 and then start the back and forth. Say my tanks are at 3500. The left gets hit first down to 3300, then I breathe the right to 3000, the left then down to 2700 and so on. I use two Hollis DG03 with transmitters. Each has the transmitter on that side as the primary gas and the one on the other side as the second gas. I use rechargeable batteries in the transmitters and they get changed out every other dive. The only errors that I've encountered have been human induced.

what do you do when you get down to a third in each tank? do you keep going on alternate tanks or keep ⅓ aside for OOA emergency and stay on one tank ?

 

[The Chairman]

what do you do when you get down to a third in each tank? do you keep going on alternate tanks or keep ⅓ aside for OOA emergency and stay on one tank ?

Since I've turned the dive by then, and I rarely go all the way to thirds, I keep switching. Don't put all your eggs in one basket!

 

[MXGratefulDiver]

what do you do when you get down to a third in each tank? do you keep going on alternate tanks or keep ⅓ aside for OOA emergency and stay on one tank ?

In open water, thirds isn't a requisite gas reserve since your bailout strategy is to go up, and the last thing you'd want to do is run one tank beyond the point of a useful bailout since that means you've lost your redundancy. If I'm inside a cave or other overhead then I shouldn't get below thirds, and if I have then I've screwed up big-time. But I'm still going to alternate, for the same reason. But I'm going to make sure my buddy knows I'm below thirds, and that he's effectively solo diving at that point, since he can't count on me for reserve gas. Redundancy is the priority at this point ... but recognize that this is a broken dive, and shouldn't be allowed to happen ...

... Bob (Grateful Diver)

 

[Colliam7]

what do you do when you get down to a third in each tank? do you keep going on alternate tanks or keep ⅓ aside for OOA emergency and stay on one tank ?

Good question. If I am down to a third in each tank, I am already on my ascent (perhaps I am at the tie-in on the wreck we are diving, beginning the ascent, but usually I am already on the way up). And, at that point, on a recreational dive, I do not continue switching. I breath from one cylinder until I am on the surface.

 

[lermontov]

In open water, thirds isn't a requisite gas reserve since your bailout strategy is to go up, and the last thing you'd want to do is run one tank beyond the point of a useful bailout since that means you've lost your redundancy. If I'm inside a cave or other overhead then I shouldn't get below thirds, and if I have then I've screwed up big-time. But I'm still going to alternate, for the same reason. But I'm going to make sure my buddy knows I'm below thirds, and that he's effectively solo diving at that point, since he can't count on me for reserve gas. Redundancy is the priority at this point ... but recognize that this is a broken dive, and shouldn't be allowed to happen ...

... Bob (Grateful Diver)

"he's effectively solo diving at that point,'--other than buddy sharing you mean? youve still got a third left its just spread across 2 cylinders? the was a post I read (cant remember where) about what constitutes an overhead e.g. a wreck with windows 4m away is different to a cave 100m in- i suppose its a situational decision