Gas Management With Sidemount

[Marie13]

I change regs every 500 psi.

 

[Marie13]

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.

You’re in Europe. Not something you have to deal with.

 

[Subcooled]

You’re in Europe. Not something you have to deal with.

Great

 

[Cdncoldwater]

I dive strictly rec so my view may be different. I was taught from OW to dive the rule of 1/3rds and when I went to SM I learned the rule of 1/6ths and was switching every 200-300 psi which built muscle memory, helped with trim and enforced SPG awareness. I've kept my method close to this but now that I'm diving solo more often I will extend my switches to a max of 500psi; one thing I always do is balance my tanks at my turn pressure.

 

[Subcooled]

when I went to SM I learned the rule of 1/6ths

What do you mean by the rule of 1/6ths? It sounds good but it is an unfamiliar term to me.
Does it refer to gas switches?

 

[inquis]

What do you mean by the rule of 1/6ths?

To my thinking, it would be to have first and last usages based on 1/6. Meaning: R1/6, L2/6, R2/6 (with the turn half-way through that), L2/6, finishing with R1/6.

This puts the long hose in use at the start, turn, and end, arguably the more risky points of the dive. (Switches based on thirds only has the long hose in use at start & end.) It also cuts the buoyancy differences in half (compared to thirds), which is perhaps only useful for larger gas quantities.

Note, this is still "thirds" as far as planned consumption & reserve is concerned. It's just twice as many switches, yielding the above mentioned benefits. One could apply the same approach to a more conservative total usage plan if desired.

 

[lermontov]

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.

here is an article you might find useful- see later post

 

[Subcooled]

To my thinking, it would be to have first and last usages based on 1/6. Meaning: R1/6, L2/6, R2/6 (with the turn half-way through that), L2/6, finishing with R1/6.

OK, now I understand. Sounds familiar. 1/3 with max 35 bar difference in pressure (-35, -70, -35 and turn around and 35, -70...). This is ideal if you have 210 bar in your cylinders. You could change more often though, if you want.

This puts the long hose in use at the start, turn, and end, arguably the more risky points of the dive.

In the case that your configuration includes a long hose, and if your dive buddy really needs it, you can give which ever hose you prefer. You do not need to have that reg in your mouth. Some extreme emergencies can happen, but in those cases you can switch from short hose to long hose later. Note: a long hose is not always used in sump diving.

It also cuts the buoyancy differences in half (compared to thirds.

Thirds refer to your turn around pressures, not to the regulator switching pressures. To consume 1/3 of a cylinder without switching regs would be utterly stupid - at least in overhead diving. Switch often. Have more buffer.

 

[tbone1004]

@Subcooled we will have to agree to disagree on frequent gas switches being beneficial in cave diving. It is not.

Assuming 3600psi fills with a 2400psi turn pressure.

Start on long hose because the start of the dive is highly risky to having to share gas due to equipment failure.
Breathe to 3000psi and switch to short hose.
Breathe short hose from 3600psi down to 2400psi and switch to long hose
Long hose to 2400psi and turn around, this puts long hose in your mouth at farthest point of penetration, again, just in case.
If in high flow cave, just stay on long hose for the rest of the exit. You'll typically come out somewhere around 1800psi in long hose, 2400psi in short hose. You stay on the long hose and even though it ends up with less gas than the short hose, it's in your mouth the entire time.
If in low flow cave, stay on long hose until around 1800psi then switch to short hose for the rest of the exit and you'll be somewhere around 1800 long, 1200 short. This doesn't have long hose in your mouth at the end, but it has more gas.

You are never more than 600psi "lopsided". In "big boy" bottles, i.e. my LP120's/19L this equates to a 2.5lb differential between left and right at the gas switch. In "normal" bottles, LP85/13L, that equates to about 1.5lbs. Hardly enough to make any significant difference. If you're doing it with AL80's, then it's even smaller.

 

[Cdncoldwater]

What do you mean by the rule of 1/6ths? It sounds good but it is an unfamiliar term to me.
Does it refer to gas switches?

Because it was SM, you divide your gas in 1/6ths to maintain balance/trim and to improve awareness of two Independent SPGs. I imagine this would be the same with Independent BM doubles. As I dive rec I don't know if it is borrowed from tec or something my instructors recommended on their own.