Reasons NOT to use Enriched Air?

[PCBCaptChris]

That is an interesting vantage point and has clearly been well thought through. However, given your strong presentation of the point I don't think I would gain from presenting an alternative perspective. (With the kindest of intentions)

 

[Bismark]

The whole _ppO₂ thing has changed over time. It used to be a _ppO₂ of 2.0 ATA, after a test to not quite 3.0 ATA was standard. Then it got cut back to a _ppO₂ of 1.8 ATA, then 1.6, then 1.4, then just to do one better GUE went to a _ppO₂ of 1.3.

Everyone needs to grasp a few realities:

1. Diving an elevated ppO2 has hazards associated with it.
2. The higher the elevation the greater the potential hazard.
3. The longer the exposure the greater the potential hazard.
4. The higher the work load the greater the potential hazard.

So the question is, why are you using an mix with an elevated _ppO₂ and does its use offset the increased level of potential hazard?

For me it does, and from past experience (which is not a perfect guide) I am willing to accept (for myself only) a _ppO₂ of almost 2.0 for decompression, 1.8 for resting and 1.6 for "normal" diving. But I'd not recommend those exposures to others ... its just that I'd allready done that for years before the agencies got into the topic with recommendations that they had to run through their docs and their lawyers and the accountants

Thanks Thal. Now would everyone just print this out and stick the four points to their foreheads. In all fairness, your personal P02s are a little on the high side for me, but that is a personal choice made by both you and I based on our own level of risk calculations after studying the implications. That simple.

 

[Bismark]

That is an interesting vantage point and has clearly been well thought through. However, given your strong presentation of the point I don't think I would gain from presenting an alternative perspective. (With the kindest of intentions)

While you may not gain from anything, perhaps other readers would? I would simply like you to substantiate your statement. I think it would be an enlightening discussion for everyone. You clearly felt strongly enough about something to post your opinion, so why not defend it? There are a lot of new divers reading this forum who I am sure would be interested in our experiment.

 

[John_B]

If you understand and appreciate the math, and you simply communicate between buddies before each dive there is no real need for having the standardized depth labels/mixes. Other than being able to justify owning a herd of tanks...

Since this is the "Basic Scuba Discussions" forum, I'll give you a basic scuba discussions answer. There is basically no recreational dive that you will plan where 32% isn't flexible enough to change the dive plan at the last minute and where you still won't benefit from additional bottom time (either for one or two deep-ish dives, or for several dives in the 60' range). The only limits are the MOD, the diver's SAC, and whatever the NDL/MDL is for the plan.

While you could technically call a 130' dive a "recreational dive", even PADI⁽¹⁾ would recommend taking more gas, redundancy, etc. in the planning for those depths. It definitely changes things from a Rock Bottom perspective.

⁽¹⁾ PADI "Adventures in Diving" Text, Deep chapter.

 

[Blackwood]

Lets explore this further.
[snip]
Game?

I think the stock answer will be that you two will agree beforehand on the gases/deco schedule. In other words: you'll be diving the same profile.

If I were planning that dive with someone and he wanted to deco out on 40% and 80%, I'd find another team mate.

 

[Thalassamania]

Thanks Thal. Now would everyone just print this out and stick the four points to their foreheads. In all fairness, your personal P02s are a little on the high side for me, but that is a personal choice made by both you and I based on our own level of risk calculations after studying the implications. That simple.

In all honesty they're a little high for me. I'f I hadn't been diving them for years I'd never have started out (or ended up) that high and those are my upper limits, not every day practice.

 

[Bismark]

I think the stock answer will be that you two will agree beforehand on the gases/deco schedule. In other words: you'll be diving the same profile.

If I were planning that dive with someone and he wanted to deco out on 40% and 80%, I'd find another team mate.

I think it would have been pretty obvious that anything else would be ....sub-optimal ... Unfortunately, rather blase statements like that post are often made without actually thinking through the repercussions. That was what I wanted to show. I am trying to be a kinder, more gentle diver......

 

[Bismark]

That is an interesting vantage point and has clearly been well thought through. However, given your strong presentation of the point I don't think I would gain from presenting an alternative perspective. (With the kindest of intentions)

Since you decided not to provide an example, I thought I would do it for you I have taken a few liberties to make this interesting and light hearted. I don't know you personally and I am sure you are a swell person so don't take this approach to seriously. Pretend we are doing this over a couple of beers in a pub, after watching the All Blacks smash France...........

To keep everything nice and simple, I am using a brand spankin' new V planner just downloaded to my Mac! Finally.....(thanks Ross).

I am going to dive 18/45 for 20 minutes at 180'. You pulled out your abacus and calculated that you were quite happy with a max bottom P02 of 1.35 and since narcosis is highly overrated, you were happy with an END of 116'. Presto, instant 21/30 mix and saved a bundle on helium.

I chose to stay with my 50% and 100% 02 mix for deco (I am a simple guy and don't have abacus) while your astrologer told you that 40% and 80% were perfectly acceptable deco gases. We chose to neglect those pesky deepstops and get on with the real business of deco with according to V planner starts for you and I at 90'. However, you immediately switch to your 40% bottle (I think it was your 40% bottle but I couldn't tell because it wasn't labeled as such) and I spend a minute then move to 80' where you are still on your deco gas. Then I spend another minute and move to 70' with you, where I finally switch to my 50% bottle (hope it was, were you sure?). Then we both spend one minute and move to 60' where I have to spend two minutes while you only spend one minute. You moved to 50' while I was still at 60' and I toxed because I was pretty tired, and dehydrated from last nights 6 nations game and my tolerance for 02 wasn't what it normally was. Bummer for me as I am 10' below you. You have to make a decision: do you come back down and help or send an FTD floral bouquet to the widow when you get to the boat? You are pretty sure you heard me say I was using 50% and 100% deco, but my bottles are not marked so what happened? Makes sense that I went on to the wrong gas...but maybe not? Crap, now what. Okay, so you come down after me because I owe you money. You stick a regulator in my mouth (which one?...please don't let it be the one that just fell out..) and now what? Okay, so this situation is getting kind of silly.......

How about this one then? We are diving together but our deco calls for different stops at different depths because even though we have the same bottom gas (we got a great deal on helium at Walmart) but the viz is for crap and about 8'. Tough luck that we are spending some time at least 10' apart........where did you go? Now we have some current......bummer....and fog on the surface.........double bummer. Does the boat follow your SMB (or was it mine) or not? Triple bummer. One of us may need a taxi ride home.

So the point of this example was to emphasize that while we both have calculators and can do gas calculations to 10 significant digits, when the rubber hits the road, merde happens. Moreover, not bothering to label our gases can add further drama to an already dramatic situation.

But heh, I am open to anything. If you can provide me a rational, reasoned argument to support your statement, then, as the rabbit said..... I am all ears.

 

[Thalassamania]

It's just a common sense rule that a buddy pair of divers either dive the same mix (preferable) or follow the more conservative decompression schedule (e.g., you dive air and I dive EAN, we follow your deco schedule). You'd have to check the more aggressive nitrogen exposure's deco schedule against the less trying dive.

 

[Gombessa]

It's just a common sense rule...

Sorry, I thought we were on Scubaboard here...