MOD Stickers

[tbone1004]

@stuartv there is some flexibility there since you're not diving within a GUE team, but even then, there is some flexibility. The point of standard gases is again to have EAN32+helium. Anywhere you're diving helium, there should be banked nitrox. I.e. if they have helium and a booster, but don't bank nitrox, I'd probably go to another shop...

Yes there are wide depth ranges for the gases, but the pressure range isn't that big. In your first example you'd probably use 21/35. Easy gas, still at ppO2 of 1.3 at 170ft and 1.1 at 140ft.
Your 90/130 is a bit weird because of GUE's push for helium below 100ft and there is no real good gas for 130ft. That said, for that dive using the blending method for standard gases you'd end up with like 26/20. You lose the advantage of ratio deco with it, but you get the blending advantages of standard gases.

What are you using for your ppO2 and ppN2 for your "best mix" calculations?

In either situation we are talking a couple of minutes of deco.
Your first example, the "200ft" gas is 18/45
At 170' for 30 mins, on 50/80 with EAN50 as your deco gas, you have 48 mins of deco. With 21/35 it drops down to 41. Not that big of a deal, but I'd probably dive 21/35 if the planned depths were 170 or 140
At 140ft for 30 mins you have 22mins on 26/20, 28mins on 21/35, and 33 mins on 18/45. For 6 minutes of deco on this dive, I'm not going to complain about diving 21/35. Even the 11 minutes of deco against 18/45 isn't really that big of a deal.

The deeper you go, the smaller the benefit gets because the total pressure differential is small. Your last example has 160/180 ft against a 200ft gas. That gas is 18/45, and at 200ft. Staying with a max ppO2 of 1.2 because you should at that depth, then the 180ft gas is 19% O2, and the 160ft gas is 20%. Not really a huge difference. Staying on 18/45, at 180ft your ppO2 is 1.17 so that's fine, and at 160ft your ppO2 is 1.06 which is a bit low, but not huge and you can see that the deco advantage isn't really going to do much.


So I'll ask. in your examples, what are the "best mixes" that you'd use?

all of this is a bit pedantic for me now since I have gone to the silent side, but for that especially keeping standard gases is MUCH easier than best mixes because you rarely if ever need fills so you want a really wide depth range so you aren't shuffling bottles around or dumping gas

 

[Jack Hammer]

It's like the Standard Gases people just don't want to hear it.

Best Mix does not mean I HAVE to use a gas that gives me a ppO2 of 1.4 at my planned site. It means that I CAN use whatever gas is "Best" (in my opinion) for my total overall dive plan.

If I have to sit out a dive because something changed and my mix is too rich for the new plan, that is a fault of my planning, not a fault of Best Mix.

It sounds like you're arguing semantics...
I believe "most" tech divers understand "best mix" to mean you plan the intended dive and select the gas that will minimize your deco time without crossing your O2 limit.

 

[boulderjohn]

Anywhere you're diving helium, there should be banked nitrox.,

Most of the diving I do is an exception to that rule.

In addition, when I am diving in South Florida, the shop where I get the trimix has a computerized system that can give you any mix you want with equal ease.

I pretty much never do any trimix dives where my mix is made by combining helium with banked nitrox.

 

[tomfcrist]

I pretty much never do any trimix dives where my mix is made by combining helium with banked nitrox.

Same here. I see the merits of doing so, but it’s just not a thing here locally.

The only time that’s an option for me personally is when I’m filling at Wayne’s.

 

[stuartv]

So I'll ask. in your examples, what are the "best mixes" that you'd use?

In both, I would use an FO2 that gives me a ppO2 of 1.4 at the deeper of the two sites.

Plan for 140, but contingency for 170? I'd use 23% O2.

Plan for 90, but contingency to 130? I'd use 28% O2.

Amount of helium (if any) is irrelevant, since the FO2 already dictate that I would not be using Standard Gases.

Would the difference in NDL or deco time be large, compared to using the SG for those dives? Probably not. But, that is not the point. The point was to address @PfcAJ 's assertion that using Best Mix would result in using gases that are less "optimal" than somebody else on the same boat who is using Standard Gases. In fact, it's the opposite.

Presuming my team has Best Mix and the other team on the boat has Standard Gases, and we all have gas that will work at the primary or contingency site, then, if we go to the shallower site, my team's gas will be closer to optimal than the SG team. And if we go to the deeper site, my gas will be exactly optimal, and the SG team will still have sub-optimal gas.

 

[tomfcrist]

I also question the idea of bailing out to standard gases adds “a ton of deco”.

Considering the logistics of a "TON", it relative.

Lets assume a couple scenarios based off the same depth, bottom time and dil. 130ft, 75 minute bottom time, 21/35 dil (yeah, standard gas..i know...I'm doing it deliberately) at 1.2. Buhlmann 50/80. This should be a 145 minute run. We will look at the difference between 21/35 bailout and a more optimal bailout. We can assume a worst case scenario of a completely ****** rebreather. To aid in deco...we will also carry O2. All gas Requirements will be based on getting 1.5 divers out...Team Bailout..

Scenario 1: Open ocean wreck dive with little to no penetration. Figure bailout from the end of bottom time...75 minutes.

Bailout with 21/35 and switch to O2 at 20'. What do we need?
-O2 : 91Cuft 21/35: 190Cuft Runtime: 186

Bailout with 32% and switch to O2 at 20'. What do we need?
-O2 : 98 Cuft 32: 102 Cuft Runtime: 146

Now that's 40 minutes of deco savings. Is it a "Ton"...Maybe not if you are used to doing 12 hurs of deco in a habitat...but its a significant amount. The amount of tanks you need to drag along are affected as well. A 2 man team in the standard gas crowd isn't going to do it with 2 80's each.

Move the same scenario to a cave or a wreck where non traverse penetration is needed. Consider bailout from max penetration....at around 37 minutes.

Bailout with 21/35 / 02
O2: 98 Cuft 21/35: 378 Cuft Run: 193

Bailout with EAN28 and O2

O2: 52 cuft EAN 28: 346 Cuft Run: 127

That's an hour difference...That's very significant. If you add a staged 80 of 50% you can make this a lot easier...145 minute run with 21/35, and a 119 run with EAN 28. In a wreck though, I want to sling as few bottles as possible....particularly if I'm going inside it. I refuse to drop stages on a wreck dive.

None of this says that your gas choices are invalid...they certainly are...for your application. however if I can save 40 minutes to an hour of deco on an open ocean dive..and carry fewer cylinders...I will every time.

 

[PfcAJ]

Considering the logistics of a "TON", it relative.

Lets assume a couple scenarios based off the same depth, bottom time and dil. 130ft, 75 minute bottom time, 21/35 dil (yeah, standard gas..i know...I'm doing it deliberately) at 1.2. Buhlmann 50/80. This should be a 145 minute run. We will look at the difference between 21/35 bailout and a more optimal bailout. We can assume a worst case scenario of a completely ****** rebreather. To aid in deco...we will also carry O2. All gas Requirements will be based on getting 1.5 divers out...Team Bailout..

Scenario 1: Open ocean wreck dive with little to no penetration. Figure bailout from the end of bottom time...75 minutes.

Bailout with 21/35 and switch to O2 at 20'. What do we need?
-O2 : 91Cuft 21/35: 190Cuft Runtime: 186

Bailout with 32% and switch to O2 at 20'. What do we need?
-O2 : 98 Cuft 32: 102 Cuft Runtime: 146

Now that's 40 minutes of deco savings. Is it a "Ton"...Maybe not if you are used to doing 12 hurs of deco in a habitat...but its a significant amount. The amount of tanks you need to drag along are affected as well. A 2 man team in the standard gas crowd isn't going to do it with 2 80's each.

Move the same scenario to a cave or a wreck where non traverse penetration is needed. Consider bailout from max penetration....at around 37 minutes.

Bailout with 21/35 / 02
O2: 98 Cuft 21/35: 378 Cuft Run: 193

Bailout with EAN28 and O2

O2: 52 cuft EAN 28: 346 Cuft Run: 127

That's an hour difference...That's very significant. If you add a staged 80 of 50% you can make this a lot easier...145 minute run with 21/35, and a 119 run with EAN 28. In a wreck though, I want to sling as few bottles as possible....particularly if I'm going inside it. I refuse to drop stages on a wreck dive.

None of this says that your gas choices are invalid...they certainly are...for your application. however if I can save 40 minutes to an hour of deco on an open ocean dive..and carry fewer cylinders...I will every time.

The whole plan you’ve described for why standard gases are no good is totally bunk.

First off, a 75min bt at 130’ in the ocean is ridiculous. But I’ll play along for sport.

Your gas requirements for oxygen alone preclude the plan. 90-98 cubic feet? No one is dragging 100cuft of oxygen with them. 50% is a much better gas choice. Reduces bottom gas bailout and gets you on a deco gas way sooner, totally negating your points about gas volume. Switching to 50% in your first example reduces the deco time by about 20mins, further closing the gap.

Bailing out to a dense gas and no helium at 130’ is also such a bad idea I can’t believe I have to draw it to your attention. Actually... yeah I can believe it. And then you want to do that on a penetration wreck or cave dive? Good grief.

Your last example is preposterous for a wreck dive. You’re going to drag 2x lp120s in the ocean plus an al80 of oxygen? Because that’s what you need to get those bottom gas volumes. Not happening. Not even close to a real life thing. A cave dive that might be reasonable, but even then that dive calls for the addition of a 50% bottle.

Come back with non made up fantasy land scenarios pls.

 

[RyanT]

I haven't always been in agreement with @PfcAJs views on things, but his post above is spot on.

 

[tbone1004]

@stuartv why ppO2 of 1.4?
remember that IMO the biggest benefit of standard gases is mixing convenience *provided you don't do ratio deco, where the ease of that is definitely the biggest advantage* so while you may end up with 28% for your ideal fO2, if you blended it the way we blend standard gases you'd end up 28/13. That would be perfectly fine to choose for that depth if you want 1.4, though I don't.

Two big arguments against standard gases are the fact that they target a bottom ppO2 of 1.2 instead of 1.4, and they put a lot of helium in compared to others by keeping the ppN2 around 2.7.

The question was to figure out what you thought was the "best" ppO2 and ppN2 to target to see what real benefit you thought you were getting.

@tomfcrist 130ft isn't something that I know of anyone that would use 21/35 on that, but that depth is part of why 25/25 and 30/30 exist. That said on those dives for that depth you should be using 50% instead of O2

 

[tomfcrist]

First off, a 75min bt at 130’ in the ocean is ridiculous. But I’ll play along for sport.

It’s not unheard of. But I agree it’s unusual. I wanted to streamline the pattern for you...fewer variables as you would say.

Your gas requirements for oxygen alone preclude the plan. 90-98 cubic feet? No one is dragging 100cuft of oxygen with them. 50% is a much better gas choice. Reduces bottom gas bailout and gets you on a deco gas way sooner, totally negating your points about gas volume. Switching to 50% in your first example reduces the deco time by about 20mins, further closing the gap

I agree that it’s unlikely. But if we are going to use an extreme example of unlikely dives and their outcomes...I’ll refer to Simons reasoning for the rediculousness of the NEDU profiles...”it’s all for science”. If we are going to highlight the disparity between gas efficiencies, we need to make it stupid enough to note the difference.

The only reason I added deco gasses at all was because it’s completely unrealistic to do a dive like that without having some kind of bailout deco gas.

Bailing out to a dense gas and no helium at 130’ is also such a bad idea I can’t believe I have to draw it to your attention. Actually... yeah I can believe it. And then you want to do that on a penetration wreck or cave dive? Good grief.

Yeah I get that...so add a helium percentage to the back side...it doesn’t change the deco obligation.

You’re going to drag 2x lp120s in the ocean plus an al80 of oxygen? Because that’s what you need to get those bottom gas volumes. Not happening. Not even close to a real life thing.

You aren’t good at math evidently...

A 2 person team slinging 2 85’s each can make that dive. 1 with O2, 3 pumped up with bottom gas.