Standard gasses

[rjack321]

To go straight to helitrox I believe you need OC AN/DP not everyone has it. Some people go CCR for no bubbles.

AN/DP was a requirement for my MOD1 and I am appalled it's not universally required for CCR. You're using 100% and mixing on your back and its actually quite possible to dive a nitrogen generating machine (I don't know word for a worse than air EAD).

 

[rjack321]

Wouldn't 12/65 be the better choice, then? Especially considering you're using it on dives as shallow as 170 ft? [edit: and it gives the target 1.2 at 90m]

The few 90m dives I have done I appreciated the lower O2 in 10/70 and shallower END too - with some decent dil flushing capability. But I don't do those enough to really have a standard.

 

[LFMarm]

Wouldn't 12/65 be the better choice, then? Especially considering you're using it on dives as shallow as 170 ft? [edit: and it gives the target 1.2 at 90m]

For all practical purposes, 12/65 and 10/70 are very similar (same as any 2 successive std gases like 21/35 and 18/45). I would definitely use 12/65 for a 90m dive although I would rather have 1.0 at the bottom vs 1.3 for the slight advantage on dil flush (I dive a SP of 1.3 at the bottom). Not a big difference for me but when I need to bank a mix for a season I go for the one that I prefer for my deepest planned dives.

The reason I set a MOD of 90m/300ft for the 10/70 is linked to density. I always try to max out at 5.2 g/L.

 

[LFMarm]

Wouldn't 12/65 be the better choice, then? Especially considering you're using it on dives as shallow as 170 ft? [edit: and it gives the target 1.2 at 90m]

You are right. But I also target 5.2g/L at the bottom; 12/65 is close enough with 5.8 g/L so would be a good second choice for me.

One of the arguments in favor of 12/65 is that you have a better pO2 in case of bailout. While this is definitely true, it's not a major factor for me because in case of bailout I go as quickly as possible up and switch to my second gas typically before reaching any deco ceiling. This discussion becomes much more complicated in a cave. Luckily al my deep diving is in the ocean.

 

[rddvet]

What PO2 and GF high do you run for that dive? That seems like a lot more deco than most people do (not critiquing just asking)?


I'm sure there is less to carry with a rebreather but is it really that much less? I'm guessing you're still carrying 2 bailout tanks, and an o2 bottle plus the rebreather and those dive times seem pretty close to what you can get out of a 2 bottom stages dive and an O2 + double 130s

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. The difference is they just don't get used with the CCR. But If I'm way back in Ginnie and I lose my rebreather completely (unlikely but possible), I don't want to swim back not knowing if I have enough stages to get me back. Add to that the fact that the ccr has had a slight negative effect on my OC sac rate

Yeah I can swim all the way to mainland and spend 2 hours back there and have basically 35mins of (50/70 GF) deco on 25/25 and Sp of 1.2. 90 minutes is crazy amounts of deco for anything exit side of the Hinkle

Agreed. I'm still running 50/70 (went to it from 30/85 when I started getting pfo-related hits) and using 30/30 and maintaining around a 1.0-1.2. I've never gotten 90 minutes of deco playing around back around the Hinkle-area and closer.
I've been back and forth on 25/25 and 30/30 for my dives around Fl. 25/25 has a nicer little cushion for when the rivers are high and your depth is a bit deeper than the expected 100ft. But haven't had enough of a good reason to change what I bank yet.

 

[rjack321]

I've been back and forth on 25/25 and 30/30 for my dives around Fl. 25/25 has a nicer little cushion for when the rivers are high and your depth is a bit deeper than the expected 100ft. But haven't had enough of a good reason to change what I bank yet.

The times I used 25/25 in FL I started with a richer mix in deeper systems and then topped with 32 to water it down for Ginnie dives later in the week.

Up here 25/25 knocks the edge off tiny, dark, 4C caves down to 110-120ft although only >90ft ish if its friendly (large ish, silt free) at depth. Most times its original exploration, max depth is uncertain, and we bump up to 18/45 cause I am not spending months and weeks and $1000s of dollars in fuel, food, and vacation time off prepping for a cave dive to have to turn around due to a gas MOD. If it ends up being >200ft which is my limit for brand new freezing cave, we know it's a bigger going project requiring more resources.

 

[AeroSpartacus]

The times I used 25/25 in FL I started with a richer mix in deeper systems and then topped with 32 to water it down for Ginnie dives later in the week.

Up here 25/25 knocks the edge of tiny, dark, 4C caves down to 110-120ft although only >90ft ish if its friendly (large ish, silt free) at depth. Most times its original exploration, max depth is uncertain, and we bump up to 18/45 cause I am not spending months and weeks and $1000s of dollars in fuel, food, and vacation time off prepping for a cave dive to have to turn around due to a gas MOD. If it ends up being >200ft which is my limit for brand new freezing cave, we know it's a bigger going project requiring more resources.

A bit off topic, but what's the cave situation over in/around the PNW? I've been meaning to check in with people in the area, I'm moving from Atlanta to Seattle soon (August) and the drive to Florida isn't going to be quite as nice after that. From my understanding, there wasn't much in the way of caves but sounds like there might be a little bit of something? It's gonna be a good bit colder, but I want to stay in practice as much as I can.

 

[rjack321]

A bit off topic, but what's the cave situation over in/around the PNW? I've been meaning to check in with people in the area, I'm moving from Atlanta to Seattle soon (August) and the drive to Florida isn't going to be quite as nice after that. From my understanding, there wasn't much in the way of caves but sounds like there might be a little bit of something? It's gonna be a good bit colder, but I want to stay in practice as much as I can.

Washington State itself is a cave desert. We have a tiny sliver of limestone at Snoqualmie Pass and another sliver outside the town of Concrete (the name is a hint lol). Otherwise it's 99% lava tubes in the Mount Saint Helens / Mount Adams area closer to Portland, OR. None of those are flooded. There is a privately owned flooded lava tube in Malheur / Eastern OR which has been dove decades ago. That site is potentially open to the public once a year.

There are divable systems up on Vancouver Island, also interior BC mainland but far far north closer to Alaska, and a few in Alberta. There are significant caves in the Alaska panhandle.

Closest cave diving from Seattle is a 10-12hr drive & ferry, requires a high clearance 4WD, and is highly seasonal. We have about a 3 month diving window annually, the remainder of the time those caves are inaccessible due to snow or raging with rainfall or both. You have to *want it* badly and it's a huge investment in camping, driving & diving for potentially tiny bottom times in extremely remote places. But I've been either the original explorer or extended a known cave significantly a dozen times. So that is driving my dil choices to some extent. "Coming back with a deeper gas" literally means planning a whole new trip the following summer.

All but one are sidemount systems and marginally doable on OC. Lots of vertical profiles potentially going from the surface to 250ft depths. So full cave SM CCR trimix territory is common.

Our caves are for sure not practice for FL kind of things. FL and MX are ridiculously easy picnic table access warm bare hands cave diving (IMHO) practice for BC. Although even at that they are not especially relatable honestly.

 

[justinthedeeps]

18/35 can happen when you're partial pressure blending for ~45-50m and want a few less dollars and few less deco minutes in the tank.

Air dil CCR shallower than 35 metres is simple, universally available, worthwhile, common, and here to stay (obviously)

Dil 0.32 can make sense for the same reasons SCR uses it

Have the impression that "standard gases" is a reaction by some (instructors, agencies, shops, teams expeditions?) to frustrations over variety in mixes, and not always having or wanting the time to discuss it, or have dives occur with inconsistent gases.

Very glad the experience and trainings many of us get more interestingly spend time discussing the actual characteristics and merits of various mix options, and why or when they work, objectively as well as operationally (as in this thread)

 

[rjack321]

Have the impression that "standard gases" is a reaction by some (instructors, agencies, shops, teams expeditions?) to frustrations over variety in mixes, and not always having or wanting the time to discuss it, or have dives occur with inconsistent gases.

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)