Do I want a Spare Air

[woodcarver]

That is a good point. Of course, it may not be universally applicable. Personally, I have both handled one (actually multiple), AND I have used one, actually several, for training purposes. And, I recommend against the use of a SA as a REDUNDANT AIR SOURCE. It simply is not adequate to serve in that capacity. Even 6 cf of air, and certainly 3 cf of air in the configuration primarily available until recently, at depths equal to or greater than 90 ft, is insufficient to make an 'normal' ascent to the surface in case of a catastrophic gas failure. That is reality.

You are certainly welcome to use one, exclusively. That is your choice. The very good news is, catastrophic - abrupt, complete - gas supply failure is exceedingly uncommon . Even though 'Insufficient breathing gas' was cited as the most common 'mechanism of injury' for scuba fatalities reported in US and Canadian divers - accounting for 15% of the incidents - in the 2018 DAN Annual Report (https://www.diversalertnetwork.org/medical/report/AnnualDivingReport-2018Edition.pdf)- which was based on 2016 data, there is no evidence that these incidents were solely the result of catastrophic gas supply failure. Actually, that is not correct. What IT boils down to is whether a diver is serious about carrying a REDUNDANT gas supply. If they are, then a SA is simply inappropriate. If they are NOT serious about that, the issue of what to carry is moot. Carry a Spare Air. Plan to breathe the air from your BCD. Carry a rabbit's foot. Whatever. It makes no difference.I couldn't have said it better myself. That is exactly the issue. That is exactly the VALID objection to use of the SA system. And, the fact that the volume has been doubled, to 6cf, is inconsequential. As you so appropriately stated, it is still a 'small tank'. Whether it travels well, or not, is of no consequence. How easily it deploys is irrelevant. Whether it functions as a REDUNDANT gas supply IS the issue. You are quite correct. Of course it doesn't. But, that was not the OP's question in this thread (70-90 feet was specifically mentioned). If I am carrying a gas supply that is intended to be REDUNDANT, it must function appropriately throughout the dive, not just for part of it. Moreover, if I am at 30 feet, I can reasonably do a CESA - and, yes, I have also 'handled' that procedure, and actually used it once. But, if I am at 90 feet, and my buddy is not accessible, a CESA is really not a viable option, and I am left with an emergency buoyant ascent - a very unappealing option. And, a gas supply that will not get me to the surface without an excessively rapid ascent is little better. Notably, after 'Insufficient breathing gas', the next two most common 'mechanisms' of injury cited in the DAN Report were 'Rapid Ascent', and 'Panic'. You go out of air at 90 feet. You don't have enough gas in you 'redundant' supply to get to the surface using a normal, controlled ascent. You panic, you ascend too fast.

If a piece of gear is not going to meet the need for which it is being carried, don't carry it. Having a regulator on my primary gas supply that will only function from the surface down to 60 feet is of no value to me if I plan to dive to 120 feet. Having a REDUNDANT gas supply that is only adequate to 30 feet, is of no value if I plan to dive to 90 feet. It doesn't mean that the particular redundant supply is 'BAD", rather it means that it simply cannot serve the function for which it is being used.

Several posters in this thread have questioned the appropriateness of the discussion in a 'Basic' forum. The point has been made that we should be emphasizing good buddy technique, good diving procedures. I am quite comfortable with that. But, the OP's question was directed toward what he might carry as a back-up for dives to 70-90 feet. So, let's address that issue.

I have no particular bias for or against a Spare Air, per se. But, I do not delude myself into thinking that is can serve as a REDUNDANT air supply at 90 feet. In the 1934 movie, The Scarlet Pimpernel, there was a very insightful comment voiced by the main character, Sir Percival Blakeney (played by Leslie Howard): 'and nothing in the world is so bad as something which is, "not so bad".' A Spare Air is 'not so bad'. It MAY even be 'better than nothing' - at 30 feet, maybe at 60 feet. But don't say that it is an appropriate REDUNDANT gas supply for deeper dives. It is not.

This is a better response than what most were putting out there, certainly more useful as we are in the basic arena.

It took me a several trips to figure out how to prune the extra weight out of the suitcases to accommodate a larger pony. Actually, it took convincing the GF she didn't need that many shoes and changes of clothing. So I traveled with a 3cu SA. I understood its limitations, i.e., the volume of air it contained. But I still hold that a CESA from 15'-30' is a better option than one from 90'+, which is what that 3cu would do for me. A 6cu will get me to the surface, but without a safety stop, or not much of one anyway, depending on depth.
If I was unable to fit my larger pony in for a future trip, but the SA fit, you can bet the SA would go.

Hopefully, the OP now sees his was not so simple a question. Whether or not to carry any secondary air source, and what that might look like, is a very individual thing. Keep asking questions and read. Learn about air consumption. Then he can decide if a SA is a viable option for him.

 

[UTscuba]

You know... that is a good point about the perils of using a Spare Air that I rarely see addressed in the pros and cons of these debates. Unless its secured to the diver somehow, there is a non-zero chance of dropping the thing before it gets in the diver's mouth.

that's probably why mine filled with water, to make it neutrally buoyant. Makes total sense now

 

[bletso]

I got one when I was a newbee diver. It didn't take long to up to a pony. I did find it useful when I was grinding some fiberglass under the boat. I could get about enough time before I had enough at a go...