Standard gasses

[211ratsbud]

They were a response to the "best mix" nonsense of the 1990s and early 2000s. (which lingers today but most of us appreciate there's a better way)

I agree and in many ways owning a dive shop the light shines brighter with the method of which they are created. I don't Bank a trimix but I do Bank 32.

That's a huge Plus for time particularly because I am a destination people don't drop their tanks off for a couple days or a day it's usually needed by the next morning.

 

[Maxco]

I don't have standard gasses.......... I have a set of rules based on the loop density, dil flush, END, mod rules -

I have several Diluent in the shed and if i want to go diving i see what I've got vs what I need. I tend to get a heavy mix of around 15/50, then air top it after a days diving, that then becomes 18/30 ish.

If the gas in the shed doesn't match what i want, I go to the dive shop and adjust it with HE on top till it fits my criteria.

I don't believe in only diving with approved gasses, one of the massive benefits with a rebreather is that you don't have to be massively fussy with gasses as long as it fits your criteria defined in step 1.

 

[Tassi Devil Diver]

Standard gases for bailout so all divers in a team are on the same page and bailout schedule, dill specific to the dive and personal preferences.

 

[LFMarm]

I don't have standard gasses.......... I have a set of rules based on the loop density, dil flush, END, mod rules -

I have several Diluent in the shed and if i want to go diving i see what I've got vs what I need. I tend to get a heavy mix of around 15/50, then air top it after a days diving, that then becomes 18/30 ish.

If the gas in the shed doesn't match what i want, I go to the dive shop and adjust it with HE on top till it fits my criteria.

I don't believe in only diving with approved gasses, one of the massive benefits with a rebreather is that you don't have to be massively fussy with gasses as long as it fits your criteria defined in step 1.

If you do the math, standard gases are quite close to optimal if you look at the latest recommendations on density (5.2 g/L), END (30m), pO2. The added benefit is team diving with same gases and the fact that some shops stock them for quicker refills. Of course if you top off you will deviate from optimal/standard gases and that may be a very good reason to have your own mixes.

 

[Maxco]

my dive shop doesnt stock any mix, its all blended whilst you wait, usually slow too. If they had a bank of pre mixed gas i would probably use that to save time / faff

 

[ginti]

I don't know where that "you can carry less gas" with a ccr notion came from. Before getting a rebreather I thought it was completely accurate. It really isn't. I'm pretty much never bringing less bottles than if I was OC.

For cave diving too? Assuming you don't use the stages and they are for bailout only, and assuming no reserve to solve problems, you should be able to double the distance compared to an OC dive... Although, sure, depending on the cave, how much reserve for problems you want, etc.

But overall it seems to me in caves you actually bring less gas for the same dive. Am I missing something?

 

[tbone1004]

For cave diving too? Assuming you don't use the stages and they are for bailout only, and assuming no reserve to solve problems, you should be able to double the distance compared to an OC dive... Although, sure, depending on the cave, how much reserve for problems you want, etc.

But overall it seems to me in caves you actually bring less gas for the same dive. Am I missing something?

I don't see how you can double the distance, at absolute most you get 50% farther on the same bottles but that doesn't leave any room to deal with hypercapnia. I've found that for <2000ft I end up with more total gear, and it's not until >5000ft of penetration that I end up with slightly less gear, but in that 2000-5000ft range I end up with about the same amount of "stuff" coming with me since the CCR is replacing the penetration gas.

 

[ginti]

I don't see how you can double the distance, at absolute most you get 50% farther on the same bottles but that doesn't leave any room to deal with hypercapnia. I've found that for <2000ft I end up with more total gear, and it's not until >5000ft of penetration that I end up with slightly less gear, but in that 2000-5000ft range I end up with about the same amount of "stuff" coming with me since the CCR is replacing the penetration gas.

What I have in mind is that you do not touch the stages except if you bailout, so you need OC gas only for the exit + problem management (like hypercapnia, but of course there is more depending on the cave: flow, eventual silt out in highly silty caves, restrictions, etc.) when diving in CCR.

For example, imagine a dive with 2 stages in CCR. You can leave the first stage at a distance that you can cover breathing it (it is full when you leave it, since you are breathing from the CCR). Then you can continue the dive based on the amount of gas in your second stage bailout (or backgas if your configuration is GUE style); then you need to come back. Of course, you want to add some safety to deal with problems. And, of course, oxygen and diluent adequate for the dive, but I consider them part of the CCR.

So you cover with 2 stages (or one stage + backgas) + CCR the distance you can cover in OC with the two stages (or one stage + backgas) minus the gas you want to save to solve eventual problems.

Assume open circuit, same dive. You would need for the same dive 4 tanks minus the amount of gas you want to keep for reserve (which is at least one third of backgas).

So basically you are right in this range, because you would bring, assuming doubles, two stages plus the backmounted doubles - which is almost equivalent to a CCR set except if you go for something ultracompact, right?

But if you add another stage in CCR, you need to add two stages in OC. However, assuming shallow caves, and depending on your SAC, we are in the range you mentioned (5000ft+) - and I am rather sure if you add calculation for reserve gas we end up to your result.

Again, I am overly-simplifying the management of problems (we all know, thirds or stricter rules in OC, account for hypercapnia or other problems in CCR, eventual morphology of the cave, flow, etc.), I would never plan a cave dive without taking into account more factors and writing down all I need, but I see an advantage in long penetrations, which is what I meant...

 

[inquis]

@ginti - For a fair comparison, I think it's useful to maintain at least the same reserve factor for whatever problems arise, since silt, navigation errors, and busted hoses don't care about OC/CC. For OC, diving thirds of total gas, that safety factor is 2. At the turn, you have 2x the gas needed to exit -- hopefully the exit + problems consume no more than that. Using 2x AL80 stages + LP85s is about 360 cuft, so you could penetrate for 120 cuft. On CC -- still turning when you have 2x the gas needed for exit -- you would be 180 cuft from the exit. Relative to the OC penetration, you're 50% further in [=180/120 - 1].

The only way to go more than 50% further on CC is to reduce your safety margin. But if you're OK doing that, you could also go further on OC, and you're right back where you started at 50% further. On the flip side, many will increase the safety factor on CC because of the potential severity of hypercapnia -- an additional risk that is specific to CC. Bumping up the CC safety factor by 50% takes it to 3, which would limit the penetration to a 120 cuft distance. Same tanks needed as OC, same penetration as OC.

 

[ginti]

@ginti - For a fair comparison, I think it's useful to maintain at least the same reserve factor for whatever problems arise, since silt, navigation errors, and busted hoses don't care about OC/CC. For OC, diving thirds of total gas, that safety factor is 2. At the turn, you have 2x the gas needed to exit -- hopefully the exit + problems consume no more than that. Using 2x AL80 stages + LP85s is about 360 cuft, so you could penetrate for 120 cuft. On CC -- still turning when you have 2x the gas needed for exit -- you would be 180 cuft from the exit. Relative to the OC penetration, you're 50% further in [=180/120 - 1].

The only way to go more than 50% further on CC is to reduce your safety margin. But if you're OK doing that, you could also go further on OC, and you're right back where you started at 50% further. On the flip side, many will increase the safety factor on CC because of the potential severity of hypercapnia -- an additional risk that is specific to CC. Bumping up the CC safety factor by 50% takes it to 3, which would limit the penetration to a 120 cuft distance. Same tanks needed as OC, same penetration as OC.

yes assuming you also breath 1/3 of the stages...

anyway, what I meant at the beginning was in case of several stages, not just one or two - which is in agreement with what @tbone1004 wrote, although of course if one factors hypercapnia and the reserve increases the advantage is not anymore that big... all good