Why not air?

[SparticleBrane]

"unbendable rules" and "thinking divers" don't necessarily fit in the same sentence if you ask me...


Situation:
You're doing a meg-tooth dive on a ledge 40ish miles out in 110ft of water. Vis is 100ft+, temps in the mid-high 70s. Does this dive require trimix since the ppO2 is >1.2 using 32%?

 

[Vie]

EAN32 reaches a PPO2 of 1.2 at 91'. Doesn't a MOD of 100' for EAN32 break their own acceptable PPO2 levels?

1.4 is the limit for recreational dives (which is what EAN32 would be used for)... This would actually make the MOD 33m (approx. 108') - GUE is more conservative and places the MOD for EAN32 at 30m/100'

 

[Rainer]

1.4 is acceptable for recreational dives (which is what EAN32 would be used for)...

Sorry, but when I took GUE-F, the slides said nothing about that. And why then scale 30/30 back (Rec Triox) to 100' from the old 120'? That was a recreational class (and might still be if it ever comes back).

 

[Rainer]

"unbendable rules" and "thinking divers" don't necessarily fit in the same sentence if you ask me...


Situation:
You're doing a meg-tooth dive on a ledge 40ish miles out in 110ft of water. Vis is 100ft+, temps in the mid-high 70s. Does this dive require trimix since the ppO2 is >1.2 using 32%?

For some, I guess it does. Personally, I'd just shave a few points on the O2 and do it as a nitrox dive. Mods, since this is clearly not "DIR" feel free to delete.

 

[mer]

If it was a wreck with relief in 110' of water, I would probably do the dive on 32% since I won't need to break 100'. If there was something to see right on the bottom and/or any potential for strong currents, I'd be on triox. I definitely notice the gas density around 70-80' and CO2 rentention and the resulting narcosis can be an issue if you have to work at that depth.

Bottom line is that we must respect PPO2 and narcosis. Any dive can require work to handle an emergency. While you can get away many times with pushing PPO2 or "managing" narcosis, it is not one of those things we play russian roulette with when we have the tools to drastically increase the safety margin. Your team may adopt more conservative practice, but going beyond PPO2 1.4 or END of 100 is just not something we do purposefully.

We are thinking divers because we know WHY we do what we not, not just blindly using a gas for a specific depth range because the standard gas table says so.

 

[Vie]

Sorry, but when I took GUE-F, the slides said nothing about that.

I took Fundies when it was still "DIR-F" so I don't have the latest version of the Fundamental slides/notes.

From my Tech 2 workbook by Richard Lundgren (page 44):

"GUE recommends that great care must be taken when exposing the divers to elevated PO2. GUE does not believe in the theoretical decompression benefits of constant PO2 diving (CCR) rather the inherent limitations of this practice. GUE propose the following PO2 limits;

Decompression PO2 = 1.6 ATA
Recreational MD diving PO2 = 1.4 ATA
Technical diving PO2 = 1.2 ATA or less depending of the exposure"

Note/disclaimer - I left the typical GUE written material's grammatical errors and typos uncorrected...

And why then scale 30/30 back (Rec Triox) to 100' from the old 120'? That was a recreational class (and might still be if it ever comes back).

I think GUE's reasoning is along the line of not wanting divers with no deco training (i.e. Rec TriOx trained but not Tech 1) to use 30/30 beyond 30m and then being tempted carry an O2 bottle to use as deco gas... I think the feeling was that deeper/deco dives should be done using 21/35 or 18/45 (and using EAN50 as deco gas) at the T1 level and only after one has gained enough experience (doing T1 level dives) and taken T2 would deeper profiles requiring stage(s) and multiple deco gasses be attempted.

 

[OkiMike]

In addition to Vie and Mer's excellent points on theory, I might add that simplification of logistics is another great consideration for 32% over air.

Here in Okinawa most of us log between 100 and 200 dives per year. This means that each day we go out, we are doing 2 or 3 dives per day within a period of 6 hours or less for all dives (often with double duty on weekends).

Not wanting to take a whole day just to do 3 dives, enriched air becomes a necessity for non-stop diving. If, however, you are simply doing one dive per day then, by all means, use air unless you can get Nitrox for free or close to it.

Another thing to consider is, if you buy gas from a fill station and don't already have them, get (or ask for) bulk purchase fill cards. For instance, usually our shop charges $6/tank for Nitrox but if you buy a 20-use card, you can get it for $100 which brings it down to $5/tank. We also have a bulk purchase option of buying four cards at once ($400) and getting one more card for free which brings the price down to $4/tank or gives us 20-free fills whichever way you prefer to look at it. And buying it this way lasts a half a year or more depending on one's dive schedule.

Lastly, if you own your own tank(s), then you should only use it with Nitrox since you will inevitably want to use it for a variety of depths and likely do not want to incur "custom mix" charges from your local fill station each time you want to switch between EANx 21 or whatever "best mix" you happen to want to use each time.

 

[Atlantis Diver]

Wher have you been hiding? The Nascar media is selling nitrogen to the public, people are buying it! I'm openning a helium tire fill station, add wiil state "Make your car lighter on the road and save gas money"

 

[Rainer]

In addition to Vie and Mer's excellent points on theory, I might add that simplification of logistics is another great consideration for 32% over air.

Here in Okinawa most of us log between 100 and 200 dives per year. This means that each day we go out, we are doing 2 or 3 dives per day within a period of 6 hours or less for all dives (often with double duty on weekends).

Not wanting to take a whole day just to do 3 dives, enriched air becomes a necessity for non-stop diving. If, however, you are simply doing one dive per day then, by all means, use air unless you can get Nitrox for free or close to it.

Another thing to consider is, if you buy gas from a fill station and don't already have them, get (or ask for) bulk purchase fill cards. For instance, usually our shop charges $6/tank for Nitrox but if you buy a 20-use card, you can get it for $100 which brings it down to $5/tank. We also have a bulk purchase option of buying four cards at once ($400) and getting one more card for free which brings the price down to $4/tank or gives us 20-free fills whichever way you prefer to look at it. And buying it this way lasts a half a year or more depending on one's dive schedule.

Lastly, if you own your own tank(s), then you should only use it with Nitrox since you will inevitably want to use it for a variety of depths and likely do not want to incur "custom mix" charges from your local fill station each time you want to switch between EANx 21 or whatever "best mix" you happen to want to use each time.

You make some very good points. As I've tried to note in this thread, however, they just don't apply to the OP's situation here in MA. Like none of them. I really do wish they were pertinent.

 

[b1gcountry]

Lastly, if you own your own tank(s), then you should only use it with Nitrox since you will inevitably want to use it for a variety of depths and likely do not want to incur "custom mix" charges from your local fill station each time you want to switch between EANx 21 or whatever "best mix" you happen to want to use each time.

I don't really understand the point you are trying to make here? Does your local shop charge you more for a nitrox fill if it wasn't filled with nitrox before?

Tom