Air / Nitrox / Trimix etc

[ratcliffe]

Can someone answer a question for me please.

When I used to dive regularly, air was the only way to go, Nitrox was around, but if one diver in 30 was using it, then I'm probably being generous. Nowadays, dives which everyone did on air as standard (it being the only way) are now considered "taboo".

Could someone out there give me a guideline as to what depths each apply.

For example:

What is considered the safe depth limit for standard air diving ?
Nitrox's best range is from 0 to what depth ?

etc.

I'm just trying to learn about all these "new" things (new to me). Thanks

[Mark Chase]

Do what you feel comfortable doing but be aware of what you are doing. Nick some mix off a M8 and switch to it at your personal max depth to see what its like.


But to answer your question from my personal perspective.....

IN THE UK

I would use air in the pool of on the 6m shelf at stony cove. Given the choice I would use Nitrox down to about 25-30m and then switch to Trimix at 30m+

However: I can aford it, I have little or no problem getting it, I am already qualified to dive it.

If I couldent aford it couldent get it and wasent qualified Id dive air down to 40max in the UK and would keep the dives simple. Bigger dives with penitration or in demanding conditions id probably stick to 30-35m on air.


In the old days it was OK not to ware a seat belt and to do 170mph on the M1... things change for good reasion on the whole.


ATB

Mark Chase

[zepp]

It depends on how much repetitive diving you are planning. Nitrox is certainly better if you are planning 2 or 3 dives/day over a weeks trip. So I personnly would dive Nitrox down to 40mts then use trimix/Nitrox for anything deeper.
So really air doesn't come into it as there is always a better gas to use.

[Freeflow]

What may appear to be a somple question has lots of answers depending on the percentage of oxygen in the mix.

If you are familiar with the term 'partial pressure' skip to the next paragraph. If you aren't read on. The term partial pressure [pp] applies to the pressure [usually in bar] in a mixture of gasses such as air of each individual gas. For air at sea level the partial pressure of oxygen is 0.21 bar as air consists of 20.9% oxygen, 78% nittogen and sundry other gassed, however for simplicity we round it to 21% O2 [oxygen] and 79% N2 [nitrogen]. Now as depth increases so does the partial pressure, at 10m depth the absolute pressure is two bar, so you have .42 bar oxygen [ie two lots of .21 bar] and 1.42 bar nitrogen [2 x 0.79 bar].

The limit for air diving depends on the oxygen content of the air. Most training restricts you to a pp of oxygen of 1.4 bar, although some agencies allow 1.6. I believe that the Navy are still prepared to go to 2 bar, but that is second hand information and I am prepared to be corrected. With this restriction, and knowing the percentage of oxygen in the air, you can use the following formula to work out your maximum operating depth [MOD]:

MOD= allowable ppO2 divided by the % of oxygen [as a decimal, ie 21%=.21, 42% = .42 etc], then this result multiplied by 10 before subtracting 10. You multiply by 10 to convert the pressure in bar to depth in metres and subtract ten to remove the 1 bar at the surface from the equasion.

So at a ppO2 of 1.4 and 21% oxygen we get:

MOD= 1.4 x 10 - 10 [sorry couldn't get all the correct maths
.21 brackets in]


= [6.66 x 10] - 10

= 56.6 metres

So 56m is the maximum for air diving. BUT nitrogen has the narcosis effect and I for one usually dive trimix below 40m because of this.

You can use the formula above to check the maximum depth of a nitrox mix, so the common mix of 32% has a depth of 33.8m, and 36% 28.9m.

There are many other things you need to be aware of to dive nitrox safely, so if you are planning on using it you will need to do the appropriate course, and you can't buy ntrox without the qualification anyway. As a basic nitrox course the PADI one is better than most.

Hope this helps.

[zepp]

Originally Posted by Freeflow

Now as depth increases so does the partial pressure, at 10m depth the absolute pressure is two bar, so you have .42 bar oxygen [ie two lots of .21 bar] and 1.42 bar nitrogen [2 x 0.79 bar].

.

1.42 BAR?? I think you have mis-typed, it should be 1.58 BAR

[ratcliffe]

Originally Posted by Freeflow

What may appear to be a somple question has lots of answers depending on the percentage of oxygen in the mix.

You're not wrong! Good grief Charlie Brown! It just goes to show just how much diving has changed whilst I was away. Its just so different these days... when I dived regularly before, a pony was rare, now I get strange looks for diving without one and relying on two regs off the one tank (And yes, I am saving for a Pony as fast as I can!)

= 56.6 metres

So 56m is the maximum for air diving. BUT nitrogen has the narcosis effect and I for one usually dive trimix below 40m because of this.

You can use the formula above to check the maximum depth of a nitrox mix, so the common mix of 32% has a depth of 33.8m, and 36% 28.9m.

So based on this, I must have completely the wrong end of the stick, but the limit for air is actually deeper than the limits for the two mixes of Nitrox you mention ?

The reason I asked the question was based on this. At the moment, my diving consists of a couple of dives at Stoney to the 21M section, and a surface interval of 1:30 plus. This leaves me close (in my terms) to the tables, and I'm guessing Nitrox would make it a little safer ? (Not that my dives are unsafe, but hey, if there is a safer option then I'm interested.

Sorry if I am being thick, but I really don't understand all the modern stuff, I'm fine and understanding of the kit I use, but want to learn about the newer stuff and perhaps eventually go down one of those routes.

Thanks again.

[zepp]

Originally Posted by ratcliffe

So based on this, I must have completely the wrong end of the stick, but the limit for air is actually deeper than the limits for the two mixes of Nitrox you mention ?

.

Correct. Air is Nitrox, but with 21% o2. The more oxygen you have in the mix the shallower your limit is. eg. pure o2 has a limit of 6mtrs. This is why we use helium to dive deeper; one you can lower the o2 and also the Nitrogen to prevent Narcosis.

[ratcliffe]

I would have thought that that would have negated any benefits that Nitrox would have had, or does it basically mean that for dives of less than 28 metres, that you can dive for longer on nitrox to that depth ?

[zepp]

Originally Posted by ratcliffe

I would have thought that that would have negated any benefits that Nitrox would have had, or does it basically mean that for dives of less than 28 metres, that you can dive for longer on nitrox to that depth ?

Exactly, you can dive for longer, have less Nitrogen uptake, therfore extend any 2nd dive time before decompression limits, or go into deco but have less time on stops and more on your dive. A large %age of people also say they feel better for it after diving, ie less tiredness and fatique.

[zepp]

Originally Posted by ratcliffe

I would have thought that that would have negated any benefits that Nitrox would have had, or does it basically mean that for dives of less than 28 metres, that you can dive for longer on nitrox to that depth ?

But it isnt just for dives less than 28 mtrs, it extends right down to a mix of 22% and 53 mts.