Standard gasses

[mdwalter]

I've heard of 18/35 as well as wondered if that is because of ease of blending.

That would typically be more difficult to blend than 21/35 or 18/45 - at least if you were using 32% (another "standard" gas) as the top off gas.

 

[tbone1004]

That would typically be more difficult to blend than 21/35 or 18/45 - at least if you were using 32% (another "standard" gas) as the top off gas.

@formernuke don't use 18/35, using EAN28 is a bandaid for people not wanting to get on helium early enough and can lead to gas density issues if you are pressing it for MOD. Use 21/35 or 18/45 and keep your gas densities low. EAN28 is not a standard gas, EAN32 is.

 

[mdwalter]

Standard OC bottom gasses don't necessarily work for diluents because they are designed around a ppO2 of 1.4-ish at max planned depth (and not 1.0 or 1.1) and they are designed to be blended using Helium and Nitrox32 and gas density isn't as important.

For example, 18/45 is generally used for depths up to 200'/60m. The "problem" with using that "standard gas" for a diluent gas is that 18% O2 has a ppO2 of about 1.3 at 60m.

I like to use the rule of thumb where I choose as my dil the standard gas I would use for OC to the next depth range below the one I plan to go to. For example, RB using 18/45 dil to depths where I would use 21/35 on OC. This puts your PO2 closer to 1.0 after a dil flush at your planned max depth. This also allows some margin for "accidentally" dropping below the planned max depth without actually exceeding the MOD of my dil. So I still use standard gasses, which are easier to find or blend, but lower the likelihood of hitting an excessive PPO2 when adding dil at max depth.

I feel like this method gets a bit hairy when going to 10/70, 12/65, or even 15/55, but the same would hold true for non-standard hypoxic gasses.

 

[mdwalter]

If a shop has EAN28 banked, then mixing 35% helium and 65% EAN28 will yield 18/35 (actually, FO2 = 18.2, FN2 = 46.8, and FHe = 0.35). Straightforward.

rx7diver

So first you mix a non-standard Nitrox (28%) in order to make it easier to mix a non-standard trimix (18/35)? Seems a bit complicated to me. Or are there really shops that bank unusual nitrox blends like EAN28?

 

[mdwalter]

21/35
18/45
15/55
12/65
10/70

With the exception of the obvious, these can all be made using preblended 10/70 and 32/00

Or straight He + 32%, or any of the other (lower O2 / higher HE) standard trimixes + 32%

 

[mdwalter]

This is maybe a GUE standard gas for suit gas and tires, but not for dil... ;-)

Well, 21/0 is a good dil for dives up to 100' or so (like when you get your first RB and are putting hours on it at a "safe" depth) or even up to 130' if you're not yet trimix certified, since the RB (or yourself) is basically blending your EAN28 to EAN36 continuously on the fly to maintain your set point.
That said, I ended up just filling all my dilout tanks that used to have air with 21/35, since after the initial hit of filling tanks it is not costing much more for the minimal amount of He used on dives where it's not really needed, but it's still usable to deeper depths as well. The down side is now I need that little suit bottle of air on EVERY dry dive, not just the deep ones. Sometimes I wish I had kept at least one real (full size) tank for use as air dil. Oh well.

 

[grantctobin]

Well, 21/0 is a good dil for dives…even up to 130' if you're not yet trimix certified,

Oh come on, you should know better than this

 

[lostsheep]

Well, 21/0 is a good dil for dives up to 100' or so (like when you get your first RB and are putting hours on it at a "safe" depth) or even up to 130' if you're not yet trimix certified, since the RB (or yourself) is basically blending your EAN28 to EAN36 continuously on the fly to maintain your set point.

I don't understand why anyone would do air dil, just go straight to helitrox. It seems like an instructor telling you no is just trying to sell more classes. Maybe I am way off base on this one and always reserve the right to be wrong, I'd love to hear justification for that intermediate class.

If air dil is good enough for your planned depth, just dive OC. For TDI at least, why even go CCR if you haven't exhausted all of the dive sites you are interested in that can be done on OC? Maybe I'm the odd man out here but I went CCR because it was time for helium. There were dive sites I could see and I knew I couldn't / shouldn't go to them without Tx.

 

[formernuke]

I don't understand why anyone would do air dil, just go straight to helitrox. It seems like an instructor telling you no is just trying to sell more classes. Maybe I am way off base on this one and always reserve the right to be wrong, I'd love to hear justification for that intermediate class.

If air dil is good enough for your planned depth, just dive OC. For TDI at least, why even go CCR if you haven't exhausted all of the dive sites you are interested in that can be done on OC? Maybe I'm the odd man out here but I went CCR because it was time for helium. There were dive sites I could see and I knew I couldn't / shouldn't go to them without Tx.

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

 

[mdwalter]

I don't understand why anyone would do air dil, just go straight to helitrox. It seems like an instructor telling you no is just trying to sell more classes. Maybe I am way off base on this one and always reserve the right to be wrong, I'd love to hear justification for that intermediate class.

Yeah, that's basically what I said.

If air dil is good enough for your planned depth, just dive OC. For TDI at least, why even go CCR if you haven't exhausted all of the dive sites you are interested in that can be done on OC? Maybe I'm the odd man out here but I went CCR because it was time for helium. There were dive sites I could see and I knew I couldn't / shouldn't go to them without Tx.

I, too, initially went with the RB to break past the I can't carry / afford enough trimix to do the deep dive I want to do (especially with a buddy already on a RB). However, I did find that once I had it, it also solved the problem of I can't carry enough gas to do the shallowish (air / nitrox range) cave, wreck, or long OW dive I want to do.
Once I had my RB, I was able to do that half mile dive up the gold line at Ginnie because I only needed to carry 2/3 the gas for bailout as I would have needed for the same dive on OC. For those who don't know, that's about 90 min in, 90 min out and 90 min deco at 20'. That's pretty cumbersome on OC, and I would feel rushed on the exit as I'm breathing the second third of my gas. With the RB, I'm actually building more safety gas reserve minutes as I exit, because I'm not actually using any gas. So when I'm half way to the exit, I now have 4x the gas I should need to exit. When I'm 3/4 to the exit I have 8x the gas needed to exit from there, etc.
Similarly, on "2 tank" OW or wreck dives where we stay at the same wreck for both dives, I just do one really long dive on the RB while the OC divers have to get out, take a surface interval, change tanks, and get back in for the second dive. I've also done this with other divers on OC AL80 doubles where they get about 40 min at 100' then go up to switch to a second set, while I continue my dive and wait for them to return for their second 40 min dive. All this can be done on air, but like I said, I now just use 21/35 dil because it lasts "forever" when you only need a couple CF per dive.