Reasons NOT to use Enriched Air?

[Catt99]

I tried to resist responding to this thread again.

Diving to 130 fsw on EAN 32 is no sweat. This would give you a ppO2 of 1.6 ATAs for which the safe oxygen exposure time is 45 minutes. At that depth, your NDL limit is going to be a whole lot less than 45 mins.

If 1.6 ATAs is safe oxygen exposure for 45 mins (and I am not disputing that it is otherwise because I simply don't know whether it is "safe"), then why wouldn't EANx36 be the ideal gas to 110 ft instead of 90 ft? You're not going to get 45 mins of NDL time at 110 ft (nor at 90 ft though getting close).

Diving with EAN 36 should be limited to 95 fsw, as I said originally, and which you apparently misunderstood.

Why, exactly? EANx36 at a ppO2 of 1.6 should give you a max depth of ~113 feet and you've already said that 1.6 ATAs is fine for up to 45 mins.

Also, you're being disingenuous by saying " . . . as I said originally, and which you apparently misunderstood." You originally said EANx36 should be limited to 90 ft (see my quote of your post from yesterday) and you edited your post today, almost 24 hours later, without noting the reason you had edited your post (I haven't read closely to see if you made other changes). No biggee if you meant 95 ft and originally typed 90 ft -- but editing your post and then saying "as I said originally" leaves a pretty bad taste for readers paying attention.

Novice divers are recommened to limit their ppO2 exposure to 1.4 ATAs, and if you are a novice, that is what you should also do.

You're responding to a thread titled "Reasons NOT to use Enriched Air" in a forum titled "Basic Scuba Discussions" and throwing around phrases like " . . . the general rule is that EANx is ideal and better than air for any depth between 50 ft and 150 ft" and "for dives in the range of XX to XX EANxXX is the best mix." And now you want to make the point that "oh, by the way, novice divers should do something ENTIRELY DIFFERENT than I originally suggested" and "it normally requires a tech-deco course to fully explain oxygen exposure time limits" so you won't provide more detail on your basis for the statements in this thread?

I'm really not trying to flame you personally (though my disapproval of your approach to posting is clearly evident) -- whatever motivates / causes your approach, I'd urge you to be more sensitive to the scope of readership on these boards and be cautious about making declarations regarding what is appropriate and safe for the general readership on something as critical as breathing gases.

 

[ianr33]

If 1.6 ATAs is safe oxygen exposure for 45 mins (and I am not disputing that it is otherwise because I simply don't know whether it is "safe"), then why wouldn't EANx36 be the ideal gas to 110 ft instead of 90 ft? You're not going to get 45 mins of NDL time at 110 ft (nor at 90 ft though getting close).

If everything goes perfectly (no current,no exertion,perfect buoyancy.high performance reg) then 36% at 110 feet would be fine.
But,start working hard kicking into a current,producing lots of CO2 and retaining more of it (due to gas density at 4ATM),maybe drop down to 120 without noticing and things could start to go bad. High PO2 and high CO2 is a REALLY bad combination.
Its all about risk management and where you choose to draw the line. In the past some divers would routinely use a PO2 of 2.0 Most of them did not die from it.

Another point is that when using a PO2 of 1.6 for deco most divers will not breathe this for longer than 12-15 minutes without taking a break to lower PO2's. High PO2's for any length of time will cause lung irritation/coughs.

 

[Hoyden]

For tech-deco diving, the oxygen exposure limit is an overall factor for the entire dive, and in that case, you would normally be limiting your ppO2 to 1.2 ATAs during the bottom phase, due to the decompression at 20 ft on 100% O2. And you would normally need V-Planner software to determine the overall limit, and it is common to create a margin of error with 80% of the CNS Ox Tox total. But that is a different topic.

There are several statements here that are open to debate. None of them are pertinent to a discussion of nitrox in a basic scuba forum.

 

[String]

Lets just keep this simple.

Provided you know your maximum depth and are sure of your ability not to violate that the best mix for that dive is whatever would give you a ppO2 of 1.4 at max depth. Simple as that.

 

[String]

W
For tech-deco diving, the oxygen exposure limit is an overall factor for the entire dive, and in that case, you would normally be limiting your ppO2 to 1.2 ATAs during the bottom phase,

Why? When every agency i can think of says 1.4. Where are your sources for this?

And you would normally need V-Planner software to determine the overall limit,

No you don't. Every single Adv Ean course ive seen by any agency covers manually calculating CNS% (and OTUs).
V-planner is nothing more than a VPM-B/E planner. There are many others around using many different models and not one of them is essential to calculating CNS.

and it is common to create a margin of error with 80% of the CNS Ox Tox total. But that is a different topic.

"Create a margin of error" ?! What type of crap is this. NOAA and others recommend 80% max exposure. Nothing to do with an error margin.

Some of the things you've posted here and other threads lead me to believe you've never received any formal technical training or if you have you didnt listen to it or the instructor wasnt qualified.

 

[Blackwood]

Why? When every agency i can think of says 1.4. Where are your sources for this?

GUE recommends (or demands, depending on how you look at it) 1.3 for bottom mixes. Though the texts list the same 'industry standard' 1.4, the standard gases reflect 1.3 for backgas (and 1.6 for deco gas excepting the 190 bottle which is a 1.4).

 

[String]

GUE recommends (or demands, depending on how you look at it) 1.3 for bottom mixes. Though the texts list the same 'industry standard' 1.4, the standard gases reflect 1.3 for backgas (and 1.6 for deco gas excepting the 190 bottle which is a 1.4).

GUE is about the only agency i can think of though that doesn't allow a diver to pick the optimum mix for every dive - it limits them to "standard gases" which aren't going to be perfect for every dive.

 

[Blackwood]

Yah, unless we count AG's UTD as an agency, GUE is the only one I know of as well.

And I'm going to resist the temptation to derail the thread into a pissing match about the merits of standardizing gases

 

[Thalassamania]

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

 

[John_B]

GUE is about the only agency i can think of though that doesn't allow a diver to pick the optimum mix for every dive - it limits them to "standard gases" which aren't going to be perfect for every dive.

Are you aware of the reasons behind that?

But I'd not recommend those exposures to others ...

If the new-ish diver is flying the NDL number on their computer, the chances are that the computer was hardwired to the 1.4 and/or 1.6 pO2 limits anyway.