Gas Management With Sidemount

[wetb4igetinthewater]

SM noob here.

I'm reading a bit on SM. One of the things that popped up was when to switch 2nd stages. One suggestion was to breath a 1/3 of one tank, then 1/3 of the other. Another one was to breath 30 bar/500 psi from one tank, and then 30 bar / 500 psi from the other. Now, I was thinking was why not first breath 30 bar / 500 psi from the one tank, and then be breathing 60 bar / 1000 psi to reduce the number of switches.

Now I'm sure there is a reason why that isn't discussed, so I'm here asking the experts. What's the issue with starting out with 30 bar / 500 psi, and then all other switched after 60 bar / 1000 psi?

 

[kr2y5]

I'm also an SM noob, so I may be missing something, but I never really learned to appreciate the benefit of the fixed systems. I would just check gas often, and switch regulators as needed to make sure I'm breathing from the tank that is more full. After all, the whole point is to keep the difference between the tanks as small as possible: if you lose gas in the tank that's more full, you will have less than half of it remaining. The smaller the difference between the tanks, the closer to 50% will be left in such a scenario. When regulators are draped around the neck, like in backmount, switching them seems simple enough that even a noob like me does not really feel the need to reduce the number of switches.

 

[rjack321]

1000psi is kinda a big differential. Depending on the tank they will be riding quite different. And if they are really large tanks your balance will be off and you'll be swimming lopsided. Use good habits from the get go and switch every 300 to 500 psi or so.
Using 300 in one then 600 in the other before switching back to #1 is fine.

 

[Kevrumbo]

On Z-System Sidemount, I initially breath the first 40 bar off the Right Tank on descent, and then alternate 70 bar between the two SM tanks (AL80's) afterwards at operational depths of 45 to 60m; using a delta of only 30 bar at such depths, you would be frequently switching between tanks about every two minutes -->I'm already switching every 5 to 8 minutes with a delta of 70 bar depending on depth & workload. . .

 

[Reku]

If you breath too much air from one tank and then switch and then breath that down - if you run into a problem where your buddy needs air you may not have enough air for both of you (this is from a cave diving standpoint) and you both will die trying to exit the cave. That is why you switch more often, at turn around time (1/3) you want to have enough left to get out and still be able to buddy breath in the event of an emergency.

Here's some pictures from an awesome book I own.

Do not do this:

Do this instead:

 

[Kevrumbo]

If you breath too much air from one tank and then switch and then breath that down - if you run into a problem where your buddy needs air you may not have enough air for both of you (this is from a cave diving standpoint) and you both will die trying to exit the cave. That is why you switch more often, at turn around time (1/3) you want to have enough left to get out and still be able to buddy breath in the event of an emergency.

Here's some pictures from an awesome book I own.

Do not do this:
View attachment 370666
Do this instead:
View attachment 370667

For classic Independent Doubles Sidemount . . .yes the above is a valid point.

 

[DA Aquamaster]

Actually the OP has it exactly right. Unfortunately most SM divers don't think it through.

Let's say you have a pair of cave filled 85s with 3600 psi in each tank.

You start the dive on the left tank and breath 1/3rd of it (switching at 2400 psi). This leaves you a 1200 psi differential, which is about 38 cu ft, which weighs about 2.8 pounds, which isn't much of a weight differential. If you find that it is, you might want to work on your configuration a bit.

You continue the dive breathing off the right tank until you reach 2400 psi - which is your turn pressure if you're in a no flow system doing normal rule of thirds gas planning.

(If you're in siphon or leaving more reserve for a slower exit, then turn at what ever pressure you normally would 2300, 2200, 2100, it doesn't matter, 2400 is the critical presses as it's the 'worst case" pressure if you lose all the gas in one tank at max penetration.)

At this point in the dive (turn pressure in both tanks and at max penetration), you've used 1/3rd of the gas in each tank and you have 2/3rd (2400 psi) left in each tank. So if you suddenly lost all the gas in each tank, you've still got twice as much gas as you used for the penetration left in the remaining tank.

You stay on the right tank until you get down to 1200 psi, and then you switch back to the left tank and continue the exit, and you should be out at or before 1200 psi in the left tank.

In practice, most cave fills today seem to be in the 3800-4000 psi range, but I still dive 1200 psi thirds if I;ve diving in small, tight passage, so it adds even more gas reserve for a slower, silty exit.

Over time as you become a more experienced diver you'll become very aware of depth, work load and gas consumption and you'll get very good at estimating remaining gas. You'll find yourself checking the SPG for the first time about 200-300 psi short of your turn pressure, because that's when your sophisticated diving computer (brain) is telling you you're getting close to turn pressure. You'll check again at the switch at turn pressure, and you'll check a third time just before you make the second switch, which is also keyed by being about half way out of the cave/back to the boat or each, etc, where I'll normally switch anyway, above 1200 psi to balance the gas at exit, and to split the excess reserve gas between each tank.

So...two switches, minimal complexity, no excessive gas differential side to side, and a great deal of flexibility in balancing the reserve on the way out by varying the last switch slightly.

To me it's a total no brainer argument, but you'd be surprised how many people disagree with this approach.

The opposing view is to just switch every few hundred PSI to keep both tanks within a few hundred psi of each other. There's nothing wrong with that I suppose, and no one seems to be dying from it. But it adds a number of gas switches and leaves the diver overly involved with gas switching when their attention might be better directed to other aspects of the dive. Worse, then their at depth, they run the risk of being more concerned with balancing the tanks than actually recognizing they are in fact at turn pressure and turning the dive.

I prefer to keep it simple by just needing to remember a single number - the turn pressure for the dive.

 

[tbone1004]

what he said. assuming 3000psi fills breathe the first 500psi, then breathe 1000psi out of the other tank, then when you hit 2000psi on the first tank, turn, and continue breathing that to 1500psi, then switch. Cuts your gas switches in half and you are never more than 500psi different between the tanks. This the most common gas protocol I have heard of at least among the florida sidemount community.

My gauges are in bar, and with HP steels they usually start out somewhere around 250bar, so I breathe to 200bar which is roughly 700psi, then breathe the other until 150bar which is obviously past turn pressure, then I'll stay on the first tank until I'm all the way out. I usually come out with around 150bar in each tank. This is roughly a 3lb difference in my tanks when at their maximum swing which is not pleasant, but tolerable. On AL80's it's around 1lb which isn't really noticeable. The important thing for me is to do it around where your gauges have easy to read marks. For recreational diving, I use 50bar increments because they're the "big numbers" same with imperial, but in 500psi increments. In cave diving it's a bit annoying because none of the "Easy" pressures coincide with thirds or sixths, so that's a bit annoying, but workable. Just means you have to look at the gauges a bit more closely

 

[MXGratefulDiver]

There are several different approaches that will work. Some are going to be more beneficial than others, depending on the type of dive you're doing. For most diving, I personally find using time to be a reasonable approach (for most dive profiles) ... for example, switch on 10-minute intervals. Why? Well, because we're more conditioned to look at our dive computers (particularly those of us who wear them on our wrist) than we are our SPG's ... and 10-minute intervals are easy to remember. Even if you are a little bit under or over it tends to even out over the course of the dive. And you always have the option to check your gauges if you're not sure ... the important thing is to keep the tanks somewhat even ... I prefer keeping mine within 200-300 psi of each other.

What system you use is less important than that you have a system, understand its limitations and applicability to the dive profile, and use it consistently. I personally find that easier to do when the "trigger" for switching depends on something I do out of habit anyway ... which is to look at my dive computer.

... Bob (Grateful Diver)

 

[decompression]

I like what Bob eludes to, the why and what factors influence gas management.
- tank type, size and pressure
- dive type, overhead, solo, depth
- human factors

All these things come into play when looking at SM gas management.