Switching gas mixes at 100'

[Tortuga68]

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

In this scenario, you wouldn't "need" the pony at all, unless you were solo diving

I would dive back gas on the descent and bottom, then switch to the EAN pony at the appropriate time during the ascent (as PerroneFord said), for a number of reasons

The higher the oxygen content, the lower the MOD

So I don't see why you would use EAN at a deeper depth then switch to air on the ascent (as someone suggested earlier)

 

[Charlie99]

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.

If they called it a stage bottle, would it be OK then?

 

[miketsp]

In this scenario, you wouldn't "need" the pony at all, unless you were solo diving

I would dive back gas on the descent and bottom, then switch to the EAN pony at the appropriate time during the ascent (as PerroneFord said), for a number of reasons

The higher the oxygen content, the lower the MOD

So I don't see why you would use EAN at a deeper depth then switch to air on the ascent (as someone suggested earlier)

Did you read the OP before answering?
"My AL 80 held nitrox 32% and the pony just held air."

 

[h90]

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.

Or consider it as "only air dive" than a computer without switching will do the job.
Or if you breath the main tank down instead of leaving a safety so you shouldn't need the pony.

 

[PerroneFord]

If the OP had training in the use of stage bottles, then you can call it anything you want. But then he would not have needed to post his question here.

If they called it a stage bottle, would it be OK then?

 

[Diveral]

Fascinating discussion on uses of pony bottles and gas switching. Not that's there's anything wrong with it with proper planning and equipment. I do it too. However, could you have rented or been loaned higher capacity tanks so that all of you would be diving with close to the same size tanks?

AL

 

[InTheDrink]

Or consider it as "only air dive" than a computer without switching will do the job..

Wouldn't what you suggest leave the OP somewhat vulnerable to getting the PO2 level on the wrong side of safe? If the planned dive was around 100ft, then it wouldn't leave a lot of head room on EAN32 if they strayed down a little further - which would be no problem except your computer's not telling you you're between the 1.4 and 1.6 margins???

 

[PerroneFord]

Wouldn't what you suggest leave the OP somewhat vulnerable to getting the PO2 level on the wrong side of safe? If the planned dive was around 100ft, then it wouldn't leave a lot of head room on EAN32 if they strayed down a little further - which would be no problem except your computer's not telling you you're between the 1.4 and 1.6 margins???

So don't rely on the computer as a nanny.

If you have the skill to actually dive safely at 100+ feet, and you have the skill to perform gas switches at that level, then you should certainly have the skill and presence of mind to not violate MOD. And 1.4 on EAN32 is ~110, and 1.6 is ~130. Seems like a pretty fair margin to me. If you can't maintain between 100 and 130ft, take the EAN off your back.

 

[S. starfish]

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?

Switching from air to 32% is roughly the equivalent of a 20% change in depth except without the change in ambient pressure. So if you were diving 100ft on 32% and then switched to air it would be pretty much the same thing as if you were diving air at 80ft and suddenly going to 100ft, or going from 100ft on 32% to 125ft as far as nitrogen loading is concerned (keep in mind this is rough). If you were doing the reverse and switching from air to 32% it would be something like hopping from 100ft to 80ft, but without the change in ambient pressure so no real worry about bubble formation.

A lot of people on here are talking about switching to EAN for the ascent, which makes sense in basic theory - higher 02 content = faster off gassing. Fair enough. However as wediveBC pointed out nitrox is really only advantageous when its P02 is highest 1.4 or 1.6. Switching to 32% nitrox for an ascent from 100ft really doesn't make sense since you're starting with a P02 of 1.3 ( already losing the advantage) and then rapidly decreasing that. Since in NDL diving the only stop you're likely to do is a safety stop at around 15ft the major driving force for offgassing is the reduction in ambient pressure rather than the artificially increased gradient afforded by breathing a higher 02 content. The only time nitrox would be benefiting you would be right when you make the switch, assuming that you're switching at the MOD for the nitrox and since we're talking NDL diving, you'd only have this advantage for a very brief time. So bottom line: Advantage of switching from air to nitrox for an ascent in a recreational dive = diddly squat.

Personally if I had to make a switch between air and nitrox for a recreational dive, I would use air for the ascent. Diving nitrox at depth would reduce the overall nitrogen loading in my tissues, making for easier off gassing. Switching to air would provide me with a quick spike in PN2 but that would be quickly reduced as I ascended and wouldn't present much of an issue as I started off gassing. Depending on how close to my NDL I was I might stay an extra minute or so at 10ft. This is if I HAD to switch, I don't really see why anyone would bother.

 

[ianr33]

Surprisingly enough PADI has the perfect course that will answer all these questions!
DSAT Tec Diver Level One Course Details

<<You'll learn to make gas switch, extended no-decompression dives using air and enriched air to 40 metres/130 feet >>

Would also be the perfect opportunity to use a shiny new multiple gas recreational computer such as a Galileo Sol.