Nitrox vs Air

[in_cavediver]

May I change a little bit the focus of this discussion to ask other thing about Nitrox...

Here in Brazil we have a paradise called "Fernando de Noronha". It is a island an our flight far from the shore.

There is a war ship sunk at 63m, but usually we go to maximum 53m. Sometimes has current, but you descent in a rope and water is 40-50m visibility and 27°C.

The point is that they asks you to have a nitrox certification and charges you about USD200, but when you dive, you do 15min at the bottom with air and ascend till 6m when the DM gives you a pure O2 from his 15l cilinder.

What do you think about this?

Given what you said in post #1:

I hate to use tables because till now I do just recreational dives as the name says, recreational means to enjoy a dive, not do guessing whether you will die or not if you do not use the tables. So I bought a computer which will do every calcs to me, even better than I could, because multilevel dives you will never know how will be.

I personally see this as nothing short of reckless. Its a deep (175ft) dive without proper planning, training or equipment. I know it done places and some say all you need is AOW but it doesn't make it smart or safe. Skip the dive until you have the proper training.

Mike

 

[DA Aquamaster]

A SAC of 10 lpm would be equal to .35 cfm. But that is pretty optimistic for an open water dive with a current where real honest to gosh swimming is involved. A SAC of .6 is more realistic (about 17 lpm if my math is correct).

Also, you could do the dive per US Navy tables where a 180 ft dive for 15 minutes on air would require the following stops:
20 ft for 3 minutes
10 ft for 6 minutes

But nobody I know does deco that liberally and the US Navy does not even do it without an on board recompression chamber to treat the divers who will fairly frequently get bent on that type of profile (and in my opinion substituting 100% O2 for the deco gas will not make that much difference on what is basically an antiquated "bend and treat" profile.

So...using DPlan with a much more conservative bubble gradient deco model and more reasonble and conservative SAC rates of .6 for the bottom portion of the dive and .4 for the deco portion of the dive to 173 ft for 15 minues using air and 100% O2, I get:

67 cu ft of air required
5 cu ft of 02 required

and the following deco stops:
80 1 min
70 1 min
60 1 min
50 2 min
40 2 min
30 4 min
20 4 min
10 5 min
(20 minutes total deco)

With air for back gas but 50% for a deco gas I get:

58 cu ft of air required
13 cu ft of 50% required

and the following deco stops:
80 1 min
70 1 min
60 1 min
50 1 min
40 1 min
30 3 min
20 4 min
10 7 min
(19 minutes total deco)

With both 50% and 100% for deco I get:

58 cu ft of air required
7 cu ft of 50% required
4 cu ft of 02 required

and the following deco stops:
80 1 min
70 1 min
60 1 min
50 1 min
40 1 min
30 3 min
20 3 min
10 4 min
(15 minutes total deco)

In the event you could not access your deco gas and had to do the deco on air, this is what you would have:

79 cu ft of air required

and the following deco stops:
80 1 min
70 1 min
60 1 min
50 2 min
40 2 min
30 4 min
20 8 min
10 14 min
(33 minutes total deco)

So... assuming your SAC rates for the swimming and deco portions of the dive are at or below .6 and .4 respectivley you could actually do the dive with a 100 cf tank and still have exactly a 1/3 reserve left if you use 100% O2 for deco and you could have even more in reserve if you used 50% for a deco gas (a normal advantage to using 50%). Your reserve would also be adequate to do the deco on backgas if the need arose.

In contrast with an 80 cf tank (which only holds 77 cu ft) you are going to be 2 SCFM short under ideal circumstances in the event you have to do the deco off backgas, so an AL 80 won't cut it.

And you still need to address the redundancy issue. Doubles (or at a minimum an H valve on the single tank) where you would have the ability to quickly shut off a malfunctioning regulator would be essential to do the dive safely. There are also some very obvious and serious training needs that need to be addressed first to ensure proper gas planning and proper responses to various emergencies that may arise. I won't even get into those but it is safe to say that the responses and equipment required exceed what you are going to get and use in an AOW training situation.

However, given all the things that can go wrong in open water, it is absolutely not something I would ever consider doing as a "trust me" dive, as a single tank dive, or without proper training, even if you do theoretically have enough gas to complete the dive on a single 100 cu ft tank.

 

[LavaSurfer]

A SAC of 10 lpm would be equal to .35 cfm. But that is pretty optimistic for an open water dive with a current where real honest to gosh swimming is involved. A SAC of .6 is more realistic (about 17 lpm if my math is correct).

I am sure there are DM's and lots of other people with SAC rates of 0.35 but I have never met them. Maybe a DM in Czumel might have come close. I was at .65 and surfaced with 700 PSI and he surfaced with 1600 PSI.

However, given all the things that can go wrong in open water, it is absolutely not something I would ever consider doing as a "trust me" dive, as a single tank dive, or without proper training, even if you do theoretically have enough gas to complete the dive on a single 100 cu ft tank.

Exactly what I was trying to get across. Not a good dive in my mind even on 100 CF. unless I had some DECO gas hanging on my side or on the line somewhere

 

[DandyDon]

unless I had some DECO gas hanging on my side or on the line somewhere

Wouldnt count on that. I want the gas I plan to use and my reserves both on my body. Finding the bottle left behind is too iffy...!! :11:

 

[LavaSurfer]

Wouldnt count on that. I want the gas I plan to use and my reserves both on my body. Finding the bottle left behind is too iffy...!! :11:

I was just thinking, deco dive whee the gas is placed on the mooring line.
Personally, I would want to do this dive with EAN50 hanging on my side. Although EAN36 would work fine.
Then maybe some 100% hanging under the boat.

No way in the world I would want to find the DM and get my DECO gas

 

[DandyDon]

I was just thinking, deco dive whee the gas is placed on the mooring line.
Personally, I would want to do this dive with EAN50 hanging on my side. Although EAN36 would work fine.
Then maybe some 100% hanging under the boat.

No way in the world I would want to find the DM and get my DECO gas

You may well be better at finding the boat than I am, but there will be times you won't find it. Never count on that.

 

[DA Aquamaster]

Exactly. Even if your navigation is perfect and visibility is great, there are too many possible situations that can happen to keep you separated from your deco gas. For example if you end up down current from the ascent line and have to swim up current to get back to the ascent line (a really bad plan, but it can happen) you may have a problem. If the current is too strong you may not be able to make headway or may use too much gas doing it (making even a .6 SAC hopelessly optomistic.) At least if you have your deco gas on you, you can shoot a bag and ascend with enough gas to make a safe decompression. This of course also assumes you have a bag and a reel and know how to deploy it at depth.

Leaving the deco gas on the ascent line is problematic from another standpoint. If the hook pulls free of the wreck and the boat drifts off, the boat is going to take your deco gas with it. If you are going to stash your gas on the wreck, tie it off on the wreck, not to the ascent line, so at least if the boat pulls free, you can still make a normal ascent - assuming you can get back to the gas. If you cannot, this is where knowing what you need for deco on the backgas and having enough with you to do it becomes very important.

Your best bet is to always keep your deco gas with you. If you are concerned about the extra baggage on a penetration, tie it off at your exit point on the wreck so you don't have to spend time looking for it or retrieving it if you get delayed inside the wreck.

 

[LavaSurfer]

You may well be better at finding the boat than I am, but there will be times you won't find it. Never count on that.

Good Point

 

[DandyDon]

Good Point

It's happend to me many times, even when I was being careful - uncommon that that is.

 

[miketsp]

Exactly. Even if your navigation is perfect and visibility is great, there are too many possible situations that can happen to keep you separated from your deco gas. For example if you end up down current from the ascent line and have to swim up current to get back to the ascent line (a really bad plan, but it can happen) you may have a problem. If the current is too strong you may not be able to make headway or may use too much gas doing it (making even a .6 SAC hopelessly optomistic.) At least if you have your deco gas on you, you can shoot a bag and ascend with enough gas to make a safe decompression. This of course also assumes you have a bag and a reel and know how to deploy it at depth.

Leaving the deco gas on the ascent line is problematic from another standpoint. If the hook pulls free of the wreck and the boat drifts off, the boat is going to take your deco gas with it. If you are going to stash your gas on the wreck, tie it off on the wreck, not to the ascent line, so at least if the boat pulls free, you can still make a normal ascent - assuming you can get back to the gas. If you cannot, this is where knowing what you need for deco on the backgas and having enough with you to do it becomes very important.

Your best bet is to always keep your deco gas with you. If you are concerned about the extra baggage on a penetration, tie it off at your exit point on the wreck so you don't have to spend time looking for it or retrieving it if you get delayed inside the wreck.

Before everybody overdramatises this situation.

The wreck has a permanent large buoy and cable.

..snip..
water is 40-50m visibility
..snip..

It is most unlikely that you would be able to lose your DM even if you wanted to. As you start down the line you see the whole wreck before you. Group size is maximum 5 or 6 with 2 DMs.

The operator normally requires you to do 3 dives with them in the 40m range before taking you on this dive so your SAC and other abilities are established beforehand.