Switching gas mixes at 100'

I have a question regarding switching from air to nitrox at 100ft.
First some background:

I was invited to do a 100ft. dive with some others. They had ST 130s while I had a AL80. To make up the short fall in gas it was suggested I borrow one of their ponies and use it to extend my bottom time (scuse me?). My AL 80 held nitrox 32% and the pony just held air. For various reasons (this question being only one of them) I listened to that little voice in my head and politely declined the dive but I was curious about the effects of switching from air to nitrox (or visa versa) at that depth. Because I didn't know the effects I didn't dive it.
Anyone know what effects (if any) there could be from switching like that and would it be considered mixed gas diving?

Please don't bash my buddies BTW. They just wanted me to join them and I take full responsibility for my own dive plan and safety out there. If I don't understand it - I don't dive it. I'd like to know the answer though so I could share it with them.

There is nothing wrong with this dive plan, given that the dive was not overly deep and provided you have a computer that is capable of gas switching, such as the Dive Rite Nitek Duo. You don't mention how big the pony bottle offered to you was, and if we're talking a 40 cu ft bottle or bigger, sure, why not? As long as you're comfortable switching regs, you can do whatever you want. Of course, it is never ideal to use a pony to extend bottom time, but it really depends on your plan. Only if you use the pony as an emergency supply, which is what most people do, then it would not be acceptable to factor it into the dive plan. But if you treat it like a stage bottle, I don't see a problem. This is a good example of "no scuba police" - as long as YOU are comfortable with what you're doing and plan accordingly, you can do whatever you want.

Oh, and as to "effects" of switching from nitrox to air: I practiced an ascent once or twice from 100 feet, switching from a main supply of 30% nitrox to air, and I didn't notice any effects. Of course, once you start using a different mixture as a bottom gas, you have to factor in added nitrogen loading and added effects of narcosis, but as long as your computer allows you to switch gases, go for it. Now, one thing you have to keep in mind, of course, is that once you switch to air, you will approach NDLs more quickly than while breathing nitrox.

Ok, just looking at the gas side of the question first, I'd just plan the entire dive as if on air. Unless you plan on doing a bunch of dives, it's going to be tough to run up OTUs on 32%.

Now, as to the other issues, you did exactly the right thing declining the dive. If you weren't comfortable with the parameters stay on the boat or on the shore. No faulting you for that. What I would have LIKED my buddies to do in your situation, is dive to the least common denominator. When you hit your gas limit, everyone ascends together. Get new tanks, and get a second dive in. Or leave you up top if you don't have any more tanks, and they do their second dive together. To me, that's where things went sideways. The gas mixing should never have entered the equation.

I often dive doubles even when some of my buddies are on singles. No biggie to me. I just don't have to change tanks that day. My single tank buddies are never made to feel like they are holding me back just because they are carrying less gas. Nor should they be. It's just diving. And I'm not knocking your friends here, but examine the mindset and act accordingly.

pteranodon:

There is nothing wrong with this dive plan

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I disagree completely. It was sideways from the jump. Trying to get a diver to "extend their bottom time" with a pony bottle containing a different gas, so that they didn't have to shorten their dives tells the tale.

And just to point out, the BEST method if he was going to use the second bottle, would have been to do the bottom portion on air, and switch over to nitrox on ascent to help speed decompression. Not that I'd expect him to know that.

PerroneFord:

And just to point out, the BEST method if he was going to use the second bottle, would have been to do the bottom portion on air, and switch over to nitrox on ascent to help speed decompression. Not that I'd expect him to know that.

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That is not correct from an inert gas loading point of view. Where does the nitrox have the biggest benefit? When the PPO2 is higher (O2 window) so I disagree on that basis.

DaleC, no bashing here, you have brought up an interesting topic and it's useful to see what people have to say.

wedivebc:

That is not correct from an inert gas loading point of view. Where does the nitrox have the biggest benefit? When the PPO2 is higher (O2 window) so I disagree on that basis.

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But it IS correct from an inert gas UNLOADING point of view. Hence why we consistently switch to higher o2 level gases on deco as we ascend. Of course this doesn't apply as readily to you constant ppo2 guys...

Yes, you are right that if we use 32% as bottom gas we will load up less, and honestly, on a recreational length dive it might be more beneficial to do it that way. But switching on ascent from a higher ppo2 to a lower ppo2 gas is "backwards" and I wouldn't run my dive that way.

From a procedural standpoint, dives involving gas switches are normally made with the greater percentage of oxygen and lower percentage of nitrogen being used as the deco gas. For example, switching from gases like trimix, nitrox or air used in the deeper portion of the dive to gases such as 32% nitrox, 50% nitrox or 100% oxygen will aid in decompression by reducing the amount of inspired nitrogen allowing more nitrogen to be eliminated through respiration as a diver ascends. Switching to a higher percentage of oxygen and lower percentage of nitrogen will also change the gradient and the driving force from a decompression theory standpoint with the gradient being theoretically more favorable if switching to gases with lower nitrogen percentages as pressure on the tissues is reduced during ascent. This is usually a more advantageous and safer gas switch than switching to a higher nitrogen percentage. However, even for dives in which a gas switch is planned to accelerate the decompression, it is possible to lose the deco gas and find yourself decompressing on your back gas or on another gas source. The other gas source might be air. This would be a fairly common scenario for a recreational diver in an emergency. You can imagine a diver at 100 feet on nitrox running low on gas during a wreck dive in Florida and gaining the attention of another diver using air. Once the emergency is handled the nitrox diver ascends breathing air and should probably lengthen safety stop times as a precaution for avoiding DCS.

As for using air over nitrox to lengthen dive time, when running the possible strategies on V-Planner (nominal conservative settings), we can compare the following dives:

If we use 32% nitrox for 20 minutes at 100 feet then switch to air for 10 minutes to extend our bottom time and ascend on air, V-Planner asks us to do a 40 second mandatory decompression stop at 30 feet, another mandatory deco stop for 3 minutes at 20 feet and finally a mandatory deco stop for 7 minutes at 10 feet.

Reversing that to using air first for 10 minutes at 100 feet then 32% nitrox for 20 minutes at 100 feet and ascending on the nitrox 32%, V-Planner asks us for a mandatory 2 minute decompression stop at 10 feet.

Had you made that dive, you would have been better off using the pony bottle first. Not only that, but if you encountered a problem such as a bcd malfunction and you needed to lighten your load, you could shed the pony bottle and have the greatest amount of gas saved for the end of the dive on your back. Most divers tend to exhaust their primary gas supplies and then switch to a pony. But, one technique for deep diving is to carry a stage bottle and use that for the bottom portion of the dive leaving the gas on your back untouched and employing another bottle or other bottles for decompression.

You did the correct thing by not diving without adequate training and understanding. It's commendable to be able to say, "No thanks!" when faced with peer pressure. Sometimes our friends mean well, but if we mean anything to our friends we always hope they'll be supportive and understanding when we thumb a dive whether underwater or at the surface.

TraceMalin:

Had you made that dive, you would have been better off using the pony bottle first. Not only that, but if you encountered a problem such as a bcd malfunction and you needed to lighten your load, you could shed the pony bottle and have the greatest amount of gas saved for the end of the dive on your back. Most divers tend to exhaust their primary gas supplies and then switch to a pony. But, one technique for deep diving is to carry a stage bottle and use that for the bottom portion of the dive leaving the gas on your back untouched and employing another bottle or other bottles for decompression.
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I completely agree with this and if I was to do this dive I would definitely start with the pony first. In addition I would not breath the pony right down. I would leave enough in the pony to be able to use it for an emergency ascent if the need were to arise. If I am going to have two tanks with me I would like to know that I had enough air in either of those tanks to get me safely to the surface if one of my systems failed.

TraceMalin:

You did the correct thing by not diving without adequate training and understanding. It's commendable to be able to say, "No thanks!" when faced with peer pressure. Sometimes our friends mean well, but if we mean anything to our friends we always hope they'll be supportive and understanding when we thumb a dive whether underwater or at the surface.

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Good post Trace,

Thanks.

Kenny