30 replies to this topic

[Diverbrian]

In this type of diving, we are using high PO2 mixes or even pure O2 to do our decompression stops.

What do you personally accept as acceptable risk on PO2 limits and how do you plan your gases in that regard?

For instance, I have moved over to preferring leaner mixes for bottom mixes (typically 1.3 PO2 or less) and trying to maintain 1.6 PO2 through most of my decompression. My tendency has become to use use O2 at 20 ft. for my shallow deco.

I have one primary reason for this. One is that if I get blown off a planned shallower dive, my tanks are still likely to have an acceptable mix if the next dive planned is deeper. This is not necessarily the case if I planned for a bottom mix already at the limit of 1.4. Let's just say that blown off dives in the Great Lakes are not uncommon. I can make up for the slightly extended decompression during my hangs.

[drbill]

1.6 PO2 if there is no significant work to be done at maximum depth. This is fairly easy for me since when I go down to such depths, it is to photograph a known feature or species I know is there (or should be).

[PerroneFord]

Both Agencies I most closely associate with are 1.4. When I dive with the rebreather guys, it's 1.3 just like they do. Fortunately, getting "blown off" a cave or spring is highly unlikely. The rebreather guys suck. They are custom mixing on the fly. So if they have air as diluent, they can make any nitrox mix they want up to near 80/20 I guess. I'm not a blender or rebreather guy so that's just a guess.

If they are using lean trimix (as per usual), then they can make any trimix they want from diluent (10/70, 15/55) to pure 02. They cannot get heliox though unless they use it as a diluent. Which can get pricy. Fortunately, they are only doing 40cuft per 4-6 hours. Slightly less time on a Kiss or other constant bleed system.

But it's 1.4 or 1.3 for me given my druthers.1.35 has been my upper limit and that was in the Adv. Nitrox portion of class when we took EAN34 to 100ft.

[PerroneFord]

DrBill,

One of the things I've noted from speaking with some of the deep divers around here is that they tend to keep the pp around 1.2 or 1.3 until they are at 70ft and the bulk of their decompression is done. The thinking being that if they ox-tox deep and have to surface, chances of survival are quite low. If they ox-tox at 70ft with 80+% of their decompression obligations complee, their chance of survival is MUCH higher

Again, I'm not talking about me here, but talking about guys who tend to spend HOURS at 300ft or below. These guys are pretty safety conscious.

[gcbryan]

.....
The rebreather guys suck. They are custom mixing on the fly. So if they have air as diluent, they can make any nitrox mix they want up to near 80/20 I guess. I'm not a blender or rebreather guy so that's just a guess.
......

I assume you mean EAN80, 80/20 would be 80% oxygen and 20% helium using standard mix notation and would be impossible with only air as the diluent.

Correct me if I'm incorrect.

DrBill,

One of the things I've noted from speaking with some of the deep divers around here is that they tend to keep the pp around 1.2 or 1.3 until they are at 70ft and the bulk of their decompression is done. The thinking being that if they ox-tox deep and have to surface, chances of survival are quite low. If they ox-tox at 70ft with 80+% of their decompression obligations complee, their chance of survival is MUCH higher

Again, I'm not talking about me here, but talking about guys who tend to spend HOURS at 300ft or below. These guys are pretty safety conscious.

I'm curious, what guys tend to spend HOURS at 300 ft?

[Walter]

It depends on the diving. Bill is correct, there's little danger in a single, non-working, non-deco dive with a PO2 of 1.6. OTOH, if you'll be pushing your O2 clock, it makes sense to back way off on the mix. I always plan my mix not for the planned depth, but for the hard bottom. When there is no hard bottom in reach, I leave my O2 no higher than 21%.

[PerroneFord]

I assume you mean EAN80, 80/20 would be 80% oxygen and 20% helium using standard mix notation and would be impossible with only air as the diluent.

Correct me if I'm incorrect.

No, you are correct. Hard to shift from nitrox mode to helium mode and back!

I'm curious, what guys tend to spend HOURS at 300 ft?

WKPP. Also some of the rebreather guys I know. Though my instructor tends not to do this any more. He does spend a few hours below 200.. and sometimes below 250 though.

[drbill]

It depends on the diving. Bill is correct, there's little danger in a single, non-working, non-deco dive with a PO2 of 1.6. OTOH, if you'll be pushing your O2 clock, it makes sense to back way off on the mix. I always plan my mix not for the planned depth, but for the hard bottom. When there is no hard bottom in reach, I leave my O2 no higher than 21%.

Thanks, Walter. I should have clarified something for the rest in my previous post. My deep dives are almost always a single descent, non-deco and non-exerting. I go down to my max depth of 180 ft, stay long enough to film what I need without exceeding my non-deco limit and then do a multi-level ascent.

Most divers who descend to these depths are diving a completely different profile with longer bottom times and different equipment. I would follow their advice were I doing the same type of dives.

[ScubaDadMiami]

I assume you mean EAN80, 80/20 would be 80% oxygen and 20% helium using standard mix notation and would be impossible with only air as the diluent.

Correct me if I'm incorrect.

No, you are correct. Hard to shift from nitrox mode to helium mode and back!

With CCRs, you have one diluent tank that has either air, heliox or some form of trimix. (There are also other experimental gas mixes out there with things like neon.) It will be a mix suitable for use on the bottom (not hypoxic at depth) so that, in an emergency, you could breath this mix in semi-closed mode. In no case would a diver put in something like pure helium since that can not be breathed by itself at any depth.

For example, if you somehow ran out of oxygen (i.e., due to failure of the first stage), you could inject some of this diluent into the loop, take a number of breaths (the number depends upon depth), and then you would flush the loop with fresh diluent, repeating the process. This is particularly helpful when having to exit an overhead environment by making a long swim near the deepest portion of the dive (such as in a cave or wreck). Even though you have carried enough open circuit bailout gas with you (You did, didn't you? ) for your return to the surface, staying on the loop, operating in SCR mode, can extend the time before you will need to switch over to open circuit, giving you that much more cushion in the event of such an emergency.

Back to the PPO2 matter, when you get to the shallows, you can flush the loop with pure oxygen. However, as soon as you exhale, this will start to drop the PPO2. So, repeating this process so often will use up your oxygen quickly. Because of this, many CCR divers will opt for a lower PPO2 during decompression, usually somewhere around 1.2 to 1.4. Some will even carry a small cylinder of open circuit oxygen, going to that since oxygen is relatively inexpensive compared to the price of helium.

As time goes by and more divers are using CCRs, we are learning some things and altering what was done in the past. Lately, many have been considering lowering the PPO2 exposure during the bottom portion of the dive. (Right now, I am running at between 1.0 to 1.1.) Some are starting to ponder if the higher PPO2 exposures are too much for the body to handle over the longer dive times being carried out with CCRs. The cost of lowering the PPO2 is just some more deco time. The cost of too high of a PPO2 could be an ox-tox hit.

I am willing to spend more time underwater especially if it helps contribute to getting me back from the dive. After all, the whole purpose of diving is to spend time in the water.

[Lubold8431]

1.4 for the bottom mix and 1.6 for deco mixes. The 1.4 gives you some breathing room. I like to plan a dive and stick to the plan.

[Diverbrian]

Funny, I was initially trained with 1.4 as the limit when I did my initial nitrox course during my open water training. Then, I noted the fact that I normally dive cold water. Conservatism forces you to back off of the PO2 a bit even for no deco in that circumstance. An Oxygen hit at depth is very likely to be the last thing that you ever feel so it is something that an intelligent diver takes some effort to avoid.

I started placing anchors on or near wrecks shortly after starting into dive leader training as the deeper wrecks out here are not always moored. That constitutes a working dive. Drop the PO2 back even more for a bottom mix. I was taking Advanced Nitrox at the time which allows for minimal deco, so I was still interested in running a little higher PO2 to stay within the limits of my training though.

Then I hit last year. I took Normoxic Trimix which required me to plan and execute dives with fairly long decos. Long hangs no longer bothered me. I got to the point of saying that I would rather run a leaner bottom mix to further avoid the possibility of a CNS hit and do the slightly longer deco.

Throw in the fact that my dives were done in Presque Isle/Alpena area with 150-200 ft. of water. If I planned best mix for a wreck in 150 ft. of water and got blown off, I couldn't use that set of doubles for a 180 ft. dive a week later. This constituted a problem. I've already ran into that this year. I got blown off of a wreck on January 1 in Lake Erie. Again, now what do I do with that mix?

So lately, if it involves a boat, most of the time my doubles don't see an O2 percentage above 21 percent.

[PerroneFord]

Given those parameters, 21/35 seems prudent. And you can pick up 50% at 70ft.

I do have a question for you. In your deco diving, are you shooting a delayed bag? I'm just wondering if agencies see it as a crutch when you shoot a bag and use the spool/reel as visual reference. Right now, it's pretty difficult for me to deco just looking at my watch. I'll get there, but I'm not there yet.

Winding in the spool also keeps my ascent rate where I want it.

[Dive_Girl]

I always plan my mix not for the planned depth, but for the hard bottom. When there is no hard bottom in reach, I leave my O2 no higher than 21%.

I do the same.

In regards to the question, I plan my more involved dives with 1.3-1.4 and then 1.6 for at-rest deco.

When at deco with a deployed bag, I fold my hands and monitor my bottom timer/depth gauge. I don't focus on the line. I like to look around (when not in mid-water) and work on finning techniques or making faces at my buddies...

[PerroneFord]

I always plan my mix not for the planned depth, but for the hard bottom. When there is no hard bottom in reach, I leave my O2 no higher than 21%.

Something about this is curious to me.

Lets say you are going to do a dive in an area with a 500ft bottom. I am assuming that you consider this a bottom out of reach. So you mix for your planned depth. I get that.

What I don't understand is what changes when there is a hard bottom closer. Maybe at 130. What is it about the hard bottom that tempts you to change your mix? Is it that you won't follow your dive plan and might drop down to the bottom? Is there another reason?

I guess I see this scenario for me now where I will be diving ships in 100-130ft of water. I'll go to 100 or 110, but I won't go to 130. Even though the hard bottom is just 20ft down, I don't want to go there. If that was a murky abyss, I wouldn't even be considering it.

[Diverbrian]

I always plan my mix not for the planned depth, but for the hard bottom. When there is no hard bottom in reach, I leave my O2 no higher than 21%.

Something about this is curious to me.

Lets say you are going to do a dive in an area with a 500ft bottom. I am assuming that you consider this a bottom out of reach. So you mix for your planned depth. I get that.

What I don't understand is what changes when there is a hard bottom closer. Maybe at 130. What is it about the hard bottom that tempts you to change your mix? Is it that you won't follow your dive plan and might drop down to the bottom? Is there another reason?

I guess I see this scenario for me now where I will be diving ships in 100-130ft of water. I'll go to 100 or 110, but I won't go to 130. Even though the hard bottom is just 20ft down, I don't want to go there. If that was a murky abyss, I wouldn't even be considering it.

Perrone,

If I have to go down and get something like another diver or a piece of equipment that got dropped, I like the comfort of knowing that my mix isn't too rich for the hard bottom. You might never plan to be down there, but Mr. Murphy will foul up the best laid plan in a heartbeat. You had better be prepared for that.

For instance, I am looking at diving the Daniel Morrell this August in 200 ft. of water. The nice stuff is at 160 ft. and there really isn't much to see over the side, so one would think that I would plan my mix for 170 or so, right? Wrong. My bottom mix will be slightly hypoxic to allow me the comfort of knowing that I can recover something off the bottom if need be. Maybe I will have to hook the wreck and have 20 ft. vis. to do it in, Maybe... In any case, my bottom mix will be suitable for the hard bottom with a dive plan made up for that continguency.

Wall diving illustrates a difference with myself and some of the others. If there is no hard bottom, I will dive nitrox for the planned depth. I won't be going after equipment as it will fall way out of reach and it is not my responsibility to go after divers who feel that going deeper than me sounds like fun. On those dives, I am not likely to be allowed my deco bottle or have tri-mix (which is my deep mix of choice) so I am diving for myself on my plan. Air limits my bottom time too much .

By the way, I am a diver who is known to use the tension of the spool and reel somewhat if I have to do an ascent that way. I use every tool available. But, I also do these ascents all of the time from 130 feet in the quarry (where I don't have to worry about boats finding me) and don't waste time shooting a bag until later when I don't have to swim with the thing once deco is done. On a boat dive, you hover your way up and don't swim with the bag. Once up, you just wait for the boat to pick you up . So that makes it a free ascent with wrist computer/timer. I don't have a problem in the world with that.