Gas Management With Sidemount

[joshk]

I'm missing something that concerns you. Please explicitly explain your concern.

Does one do an actual exit while feathering a valve or just open and close it a few times in open water? How was the concept of "consumption doesn't change" (when clearly you're doing more work than before) explained or proven?

 

[lowviz]

OK, got it.

First, breathing isn't something that you calculate to four decimal places. You breathe at a rate that keeps you comfortable. Unfortunately, there is quite a bit of variability in that. Deal with it. Learn what is comfortable for you in benign and threatening conditions. Expect a big difference in the two. Know these values. Add in work effort and you now a have real-world gas consumption range.

...//... How was the concept of "consumption doesn't change" (when clearly you're doing more work than before) explained or proven?

Explained: If you are calm, you aren't doing all that much more work. So it doesn't change much.

Proven: Forget it. This is physiology not physics.

 

[lowviz]

Overlooked one of your questions in my last post.

Does one do an actual exit while feathering a valve or just open and close it a few times in open water? ...//...

One does an actual exit for a fair distance in Intro to Cave. One repeats it until the instructor is satisfied.

 

[joshk]

Overlooked one of your questions in my last post.One does an actual exit for a fair distance in Intro to Cave. One repeats it until the instructor is satisfied.

That's what I was curious about. Thanks for clearing that up!

 

[Dhboner]

I do a valve feather "drill" on pretty much every dive (to stay sharp with the skill). It is very easy and I doubt my gas consumption increases at all. In a real emergency it might, but I suspect not very much, given that I practice the procedure so often.

 

[LFMarm]

Now that Shearwater computers allow to set a tank switch reminder, how many PSI/bar delta would you recommend switching at? I am thinking 46 bar which would mean switching 3 times to get to the 1/3 reserve (assuming a starting pressure of 207 bar) but it would be easy to set it up to lower pressures, e.g., 34 bar for 4 switches, 27 bar for 5 switches, 23 bar for 6 switches.

Keen to know if you would make more frequent switches now that it’s easy to track consumption without checking the SPGs every few minutes.

 

[tbone1004]

Now that Shearwater computers allow to set a switch tank reminder, how many PSI/bar would you recommend using? I am thinking 46 bar which would mean switching 3 times to get to the 1/3 reserve (assuming a starting pressure of 207 bar) but it would be easy to set it up to lower pressures, e.g., 34 bar for 4 switches, 27 bar for 5 switches, 23 bar for 6 switches.

Keen to know if you would make more frequent switches now that it’s easy to track consumption without checking the SPGs every few minutes.

I wouldn't bother setting the switch tank notice. It's going to change on every fill anyway, but more importantly I just switch when the tanks are noticeably off in terms of their buoyancy. I was working with a cave class about a month ago and I was diving LP120's/19L bottles and the students were diving HP100's/13L bottles with everything filled to 250bar. There was exactly a 0% chance that I was going to have to call the dive on pressure so I never checked once we were in the water. I checked before we got in to make sure that the tanks were where I thought they were, and I checked them at the end of the dive to see if I needed fills. The most that they were "off" from each other when we came out was about 300psi/20bar. There is strategic gas switching when you are cave diving where you always start with the long hose, turn with the long hose, and end with the long hose to make sure it is most easily available to donate during the "prime times" to have to share gas, and I was switching whenever one bottle felt heavy enough that it was just starting to be annoying.
All of that to say, if you pay attention to your body and your gear, you will feel the discrepancy in buoyancy of each tank and that will prompt you when to change to the other bottle, and it may not be when a computer wants it to switch because of what's going on in the dive.

 

[Subcooled]

I'm reading a bit on SM. One of the things that popped up was when to switch 2nd stages.

Do not overthink.
Short answer: often

The more often you change the cylinder you breathe from, the more balanced you will be. It sucks when e.g. your right side is more buoyant than your left side. The torque caused by this will strain your leg muscles and give you muscle cramps or otherwise **** time.

If you do overhead diving (deco, ice, cave, ...) then you really really want to keep the cylinder pressures close to each other and in addition adhere to strict thirds or even quarters. Murphy's law dictates that whenever your cylinder or first stage fails, it is always the cylinder with more gas in it that is affected. Hence, keep the pressures as close to each other as is possible. This minimizes your loss.

One suggestion was to breath a 1/3 of one tank, then 1/3 of the other.

Just make sure you have 2/3 left in each cylinder when you turn back. You used 1/3 of each cylinder on the "inward" journey and thus you will need 2/3 of ONE cylinder to get out again (would one cylinder become unusable).

If you change regulators more often, then you might have more breathing gas reserves when disaster strikes.

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.

That is how I do it.

Or you could change every 5 bar if you prefer. Changing regs only takes three seconds if you wear a neck bungee instead of those clip-to-D-ring things.

What's the issue with starting out with 30 bar / 500 psi, and then all other switched after 60 bar / 1000 psi?

This way the pressure difference is max 30 bar (from +30 to -30) and hence the weight difference (and torque) is minimal and also if the fuller tank fails, then you have almost as much gas in the other tank. What would happen if you would breathe one cylinder empty and then the other would fail? You get the idea.

 

[Subcooled]

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.

Sure, valid logic, but more switches = more buffer.
Work hard, live long.

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

Me! Am I the first?

But it adds a number of gas switches and leaves the diver overly involved with gas switching

Sir, are you using the APEKS side mount system? Clipping regs to D-rings is hard work and is better avoided. We have had a better solution since 1980's - the neck bungee.

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.

That is very true of anyone who is more obedient than fearful of death. Fear is a healthy thing indeed.

 

[Subcooled]

his cave fill is 3600

I do not understand this "cave fill" thing at all.

Cylinders do have their rated fill pressures for a reason. Stick to them. A fill is a fill is a fill.
Open water or overhead should make no difference. The cylinder remains the same.

You can do a cave dive as well with 3600 psi as with 1800 psi. Just turn around when one third of the gas (or less) is consumed.