Travel Gas labeling and Counter Diffusion

[doublesunder]

What is everyone's suggestion for labeling or marking permanently a travel gas, Just MOD? I currently label my deco gas 70 and 30 respectively. What is the general consensus on using say 20/25 travel gas with a 18/45 bottom gas. Necessary or not?Thanks ahead for any input

[Joshua Smith]

I usually just put the mix on some blue painters tape, but I think I'm going to start adding the MOD as well.

[kitsapdiver]

Well first and foremost let me preface my answer by saying that I'm not qualified to dive hypoxic 3mix gasses. However that being said 18/45 is not hypoxic. Generally I've hear of gasses less than 16% O2 being label as hypoxic. I don't believe you need a "travel gas" at all for an 18/45 mixture.

When diving with bottom stages (which is very different from travel gas, because I can safely breath a bottom stage at the deepest planned depth of the dive, not necessarily true with "travel gas") I have heard a couple schools of though.

1. It doesn't need to be marked at all. However, the basis for not marking the bottle is because it was safe to breath from at the bottom, so I don't think it's applicable in your case.

2. Mark the bottle with a temporary label (i.e. white duct tape, and bold sharpie marker). And when doing this method it's not as simple as finding the MOD of the gas based on PP02 and marking it. For instance with a mixture like 20/25 (which seems like a weird mixture to me btw), I wouldn't mark the bottle as having an MOD of 198, I would mark the bottle as having an MOD of 130 based on the Narcotic Potency of mixture.

My question would be why not use a deco bottle just to reduce the number of bottles you're carrying. If you're making a dive using a hypoxic mixture, or even 18/45 you should be carrying 50%, and you can honk off that down to 20 fsw and then switch over, that way you don't need have an extra bottle with some gas that is not useful except for the first 20 feet of your dive.

If gas planning is your concern and you wanted to switch over deeper to save the backgas I would suggest a bottom stage before travel gas because that way the extra bottle you're carrying has gas that's usable across all the depth ranges of your dive.

As I said earlier I don't have the training for diving hypoxic mixtures, and according to your training listed as Normoxic Trimix neither do you. So I would expect that you've dove 18/45 in the past without a travel bottle. If you are going to incorporate "travel gasses", "bottom stages", or any type of extra bottles into your diving I believe you need to get that training from a qualified instructor.

Adding a bottom stage, or travel gas is not as simple as jump in, breath gas down, switch to backgas. There are other oollies worth talking about with an instructor.

[doublesunder]

Certainly food for thought, which is appreciated. I guess what I was trying to ask was not necessarily about doing a dive with a hypoxic mix and a travel gas to transition to depth. I was pondering using as you more correctly stated a bottom stage, but with less helium to transtion from a high helium gas to an intermediate helium gas as pertains to Isobaric counter diffusion. Im asking if say my bottom gas is 18/45 and I switch to 50% at 70 feet thats a huge change. Wouldnt it be better to switch to even 20/30 in between or on way down to depth? Sorry my original question was pretty vague.

[Joshua Smith]

Certainly food for thought, which is appreciated. I guess what I was trying to ask was not necessarily about doing a dive with a hypoxic mix and a travel gas to transition to depth. I was pondering using as you more correctly stated a bottom stage, but with less helium to transtion from a high helium gas to an intermediate helium gas as pertains to Isobaric counter diffusion. Im asking if say my bottom gas is 18/45 and I switch to 50% at 70 feet thats a huge change. Wouldnt it be better to switch to even 20/30 in between or on way down to depth?

My personal belief is that Isobaric Counter Diffusion is a non issue until you start looking at profiles deeper than 350-400'. I dont factor it in to any of my bailout plans.

[kitsapdiver]

Alright, so now I'm a little confused, I want to make sure that I understand what you're asking, you're completely talking about the accent right? So you would dive your back gas first, and then switch to this intermediate gas to prevent an ICD hit?

There are some general rules about ratios for gas switches, and people do use Helium in Deco mixes for that purpose (among others), but as Josh said this doesn't really become an issue until the dives are much deeper. But at that point the intermediate gas is a DECO gas and would be marked as a deco bottle. For instance 35/25 is a common deco gas, but I would expect that to marked appropriately as a DECO bottle.

I think what you're getting after is that the Buhlman ceiling for a diver may not be the best first stop, and intermediate stops with the appropriate gasses can be far more rewarding from a deco standpoint, but once again that is something that would be covered in the next series of trimix courses for you, and I think with Normoxic mixtures (including 18/45) it's not a real concern for you yet.

[BDub]

We use 15/55 with no travel gas. Granted, it's not too fun when you're waiting on the surface to descend in strong current, etc.

Bringing a bottom stage for just a few moments on the descent seems to add a lot of unnecessary complexity.

IMO, ICD isn't a concern on profiles using 18/45. I'd be more concerned with getting winded or lightheaded at the surface waiting for the team to descend.

[kitsapdiver]

Brian,

When you do use a mixture more lean than 15/55 do you use your deco gas to drop to 20 feet or what's your method. I was under the impression that the idea of travel gas was going away.

That question is completely for my own edification.

[BDub]

Brian,

When you do use a mixture more lean than 15/55 do you use your deco gas to drop to 20 feet or what's your method. I was under the impression that the idea of travel gas was going away.

That question is completely for my own edification.

I have no idea if travel gas in its traditional form is still being taught or used or not.

Personally, I'll use my deepest deco bottle if the backgas isn't breathable at the surface. Obviously, you must get off your "travel" bottle and back on to backgas ASAP, and definitely before exceeding the MOD of the bottle/gas you're breathing.

I'm not going to bring an extra bottle for the sole purpose of just getting me down to 20 or 30 feet. That's going to turn into a major PITA later on when you're moving a full stage back.

[doublesunder]

I guess my logic behind the matter was that I could speed up decompression times as well as ease transiton per ICD. Did some Vplanner work with a 30/25 mix for both descent and ascending. The Gains seem to be there, but definitely adds to the complexity of the dive. The third bottle isnt much of a hindrance, but will probably just stick to filling with the same as bottom gas. Also everyone seems to be saying ICD is " a non issue with these profiles". Thanks everyone for your input its all greatly appreciated. These are the things I think about at home sick on the couch

[kdupreez]

+1 on not taking extra "travel" gas stages that doesnt really do much for you except add complexity.

If you are on a 15/55 (or even as low as a 10/90 heliox), just use your 50% deco gas to get you to 20-30ft..

PO2 of a 15/55 at 20ft is 0.24, and that's even more than 0.21 of the air we breath at the surface..

So, use your 50% and take few swags of it down to 20ft, then switch to back gas.

Keep it simple and dont take what you dont need

(BTW - ICD have a much higher likelihood if you deco on gas that as higher helium PP than your back gas.. ie. diving 21/35 and deco on 50/50 - if you want to do anything, add a little helium to your 50% deco gas.. but usually not more than half of the helium PP of your back gas)

K

[CaptnJack]

+1 "travel gas" is old skool unnecessary. Certainly unnecessary with 18/45.
Just put the 15/55 or 12/65 reg in your mouth and drop. Your respiratory rate will go up if you breathe these shallower than ~10ft but it will take quite awhile for worse adverse effects than that to appear (eg cyanosis). Obviously if you are working real hard on the surface those gases are not going to work well for that. But if you are working real hard on the surface you might want to think about whether you should be doing the dive at all at that point.

[Joshua Smith]

Since I skipped OC trimix in order to buy my Meg, I'm not really qualified to speak here....but that never stopped me or anyone else from answering a question like this.

FWIW, I've buddied with OC divers on some deep dives. They usually take a deep breath at the surface and drop down to 20' as fast as they can before getting on their backgas.

[lamont]

My personal belief is that Isobaric Counter Diffusion is a non issue until you start looking at profiles deeper than 350-400'. I dont factor it in to any of my bailout plans.

I know of one inner ear hit on a 330, and inner ear hits are thought to be IBCD related.

Also, addressing the rest of the thread, I believe that hyperbaric techs don't see inner-ear DCS on dives much less than 200 feet, so breathing 21/35 or 18/45 its unlikely to be necessary to worry about it.

I'm confused why anyone would worry about IBCD with travel mixture at all... You're just ongassing everything as you go down, and you won't have ongassed enough to cause an IBCD issue when you swap over. Everything is undersaturated at that point.

And, yeah, no point to travel mixture with 18/45, that isn't hypoxic.

[kdupreez]

agreed.. and if you are worried about inner ear DCS hits on the ascend and want to add an "intermediate" travel mix to alleviate some of that instant helium over super saturation gradients, rather just add some helium to your deco gas. problem solved and it makes for a nice "travel" gas without adding any complexity.

FYI - good writeup on inner ear DCS hits http://jap.physiology.org/content/94/6/2145.long

K

[doublesunder]

Thanks everyone for your replys on ICD discussion, I know now that I am not at these depths yet to be a factor. My intermediate gas question went in a different direction than meant, which was partly my fault by the word "Travel" However, yes I know 18/45 isn't hypoxic and that you dont need a travel gas to dive it. What I wanted to know is, would it be worth adding a third bottle of say 30/25 Triox to optimize your decompression schedule and increase your total gas availability. Or would a stage of bottom gas better serve the purpose with less complexity.

[CaptnJack]

Thanks everyone for your replys on ICD discussion, I know now that I am not at these depths yet to be a factor. My intermediate gas question went in a different direction than meant, which was partly my fault by the word "Travel" However, yes I know 18/45 isn't hypoxic and that you dont need a travel gas to dive it. What I wanted to know is, would it be worth adding a third bottle of say 30/25 Triox to optimize your decompression schedule and increase your total gas availability. Or would a stage of bottom gas better serve the purpose with less complexity.

What dive are you planning?
I have done both a 3rd deco gas and a bottom stage (on different dives). While I only have a few dives in the 3rd deco gas range, which strategy works best depends on the dive IMO.

[kdupreez]

Good question.. You said you played with v-planner.. What profile are you intending to dive? Also, what dive are you going to do i.e. dive site? (Environmental issues could also affect gas selection)

[doublesunder]

200,30, 18/45 with 50 and 80% 108 Faber's and 40's and maybe 20-30 foot descent rate. I know a scooter I was trying to plan a dive on the Steele Dodge.

[BDub]

200,30, 18/45 with 50 and 80% 108 Faber's and 40's and maybe 20-30 foot descent rate. I know a scooter I was trying to plan a dive on the Steele Dodge.

20ffw, for :30 on the Steele Car?

Why the slow descent rate?

[doublesunder]

200,30, 18/45 with 50 and 80% 108 Faber's and 40's and maybe 20-30 foot descent rate. I know a scooter I was trying to plan a dive on the Steele Dodge.

20ffw, for :30 on the Steele Car?

Why the slow descent rate?

Is 30 min to long and 20-30 to slow? I havent dove there before. I was also looking at using this profile for one of the minesweepers in Lake Wash too with a faster descent rate. Any info is greatly appreciated.

[BDub]

200,30, 18/45 with 50 and 80% 108 Faber's and 40's and maybe 20-30 foot descent rate. I know a scooter I was trying to plan a dive on the Steele Dodge.

20ffw, for :30 on the Steele Car?

Why the slow descent rate?

Is 30 min to long and 20-30 to slow? I havent dove there before. I was also looking at using this profile for one of the minesweepers in Lake Wash too with a faster descent rate. Any info is greatly appreciated.

Eh, personal choice. I'd be bored after 12-15 minutes on the car. It is a car, ya know? It's a cool dive, but it's very small.

As far as descent rate...for me personally, the clock's ticking and every breath of 18/45 = cha ching. I prefer to get down as quickly as possible.

[kdupreez]

I'd bump those LP108's waaay up, doing the math here in my head for a 20min descend and 30min bottom time..

That slow 20min long descend alone will cost you 40cft of backgas..
(Assuming 30min of actual bottom time.. not runtime..)
+ Your 30min bottom time will cost you about 160cft,
+ The ascend to 70ft about another about 45cft + 45cft for your buddy (mingas = 90cft)
= you're looking at 290cft of total back gas.. (thats a very expensive dive!!)


If its 30min total runtime (until you thumb the dive).. you are using 66% of runtime on descend and only getting 10min of bottom time? thats a very expensive descend if you pay $1.00 per cft of trimix, you are paying $40 for that nice slow descend

My opinion, do a 20-25min runtime, boogy on down to 200cft stay there about 15min.. and forget about travel gas

[CaptnJack]

No bottom stages and no travel or 3rd deco gas for 200ffw dives. I wouldn't consider a 3rd bottle until >220ft.
EAN50 and 100% are all I've ever used for any sort of reasonable profile. For 200ft 95s/119s/130s etc when decently filled are all plenty of backgas.

There are very few dives in either Lake WA or Crescent where'd you'd be at 200ffw for 30mins. The minesweepers aren't that deep on average for instance. The Steele car is 20ftx8ft, you can go around it 5 times in 15mins. The only dive I can think of where you possibly want to do 200ft for 30mins is the MT6. But its a barge afterall. At least to me, one half looks basically like the other, minus crumbling deterioration differences. For most of the other deep stuff, 30mins is not really do-able within slack. The Al-ind-eska-sea being a possible exception.

As others have noted, the 20min descent is killing you on gas and deco.

[doublesunder]

Was thinking more a 20 -30 foot per min descent , sorry for the confusion. Plus it hard to get 3500 psi in the 108's everywhere Was thinking more like a low 80 min runtime with 16 min or so at depth.