Spare air - or not?

[doctormike]

Interesting side note: Those in favor of Spare Air all seem to be SoCal kelp divers. Op is a So Cal diver.
Let's see: Kelp, poor vis, easy to get away from your bud, high chance of entanglement, most dives less than 40 feet, "interesting" shore entries at times.

It almost sounds like 3 cf of detachable air could come in handy.

P.S. I have a friend (who had more than 400 dives at the time) who came up without his unit, using his Spare Air. (Kelp entanglement.)

That's an interesting point, maybe you have something there. I hadn't thought of that.

However, if entanglement was the issue, and the backup plan is to unbuckle and ditch the unit, couldn't you just CESA from that point (especially since you could still be breathing from it until it was time to go).

That is, assuming that you aren't going to get entangled by the knife strapped to your calf - but no one wears those any more, right?

 

[coldwatercanada]

i have a spare air becuase one the year i got my DM card they had a deal for Spare Air and i got one for about 1/3rd of the cost. If i remember to bring it i will or if i dont trust a strange buddy. I did take it too a dive tank and figured out i get 4 full breaths off it at 20feet. So thats how i treat it! If i had to pay full price i prolly never would have got it!

 

[tadawson]

@coldwatercanada -

For the 3cf model? Following that, you should only get 106 breaths off an 80cf tank. Sitting here noting that I breath about every 4 seconds or so, that is 15 breaths per minute. Following your figures, that would mean that the 80cf would only last 7 minutes.

Methinks your figures are woefully broken . . . .

Looking at some figures, it looks like a pretty typical SAC rate is about 1cf/minute, and considering that 20 feet is about .6 atmosphere, then you should see about 50 minutes on the AL 80, not 7 as per your figures . . . Working with the 4 seconds per breath approximation, that would be about 750 breaths, not the 106 supported by your math. Working back to the Spare Air, then with 3cf and 20 feet, we get a tad less than two minutes at a normal breathing rate, and at 15 breaths per minute, that is closer to 30 breaths than 4 . . .

Looking at it a different way, a typical humal tidal volume (volume of a breath) is 500ml, which equates to about .018 cubic feet per breath. That comes up to about 100 breaths on the Spare Air at 20 feet, although I suspect stress and workload would likely halve that to 50, but that is still closer to what I got above with SAC rates than what you got . . .

One of these days, I'll work this up in detail . . . have seen too many flawed attempts that did not consider varying depth, tidal volumes, etc. correctly that gave bad data to count . . .

One of the other things that I don't hear discussed, but which I feel is valid, is that even if the Spare Air won't surface you from 60 feet or whatever), it will get you a hell of a lot closer to the surface than nothing . . . and a Spare Air ascent to 30 and then a CESA beats the heck out of a CESA from 60! No, not as good as a pony, but a hell of a lot less to lug around too . . The big issue is not to consider the SA as a "get out of jail free card" that will easily get you out of anything you may get into . . . and that, it likely the limiting factor for most casual divers who don't bother to work out what it can really do.

Not saying that the Spare Air is necessarily good as it is advertised, just that your figures are, well, very broken . . .

- Tim

 

[bpotkin]

As it turns out, mostly all of my dives have been around 15-45 ft. in dense kelp forests.

 

[blackvans1234]

I think what people are forgetting is that you can never have enough air!

 

[Crush]

I think what people are forgetting is that you can never have enough air!

That is why I love double HP 130s.

IMHO if you plot cost vs. amount of air (don't forget to factor in your extra 1st stage reg) and weight vs. amount of air (don't forget to factor in your extra 1st stage reg) you will quickly see that doubles rock. However, sometimes doubles are overkill or just too expensive. In that case, a large pony is better bang for the buck than a Spare Air.

 

[Bob DBF]

@coldwatercanada -

For the 3cf model? Following that, you should only get 106 breaths off an 80cf tank. Sitting here noting that I breath about every 4 seconds or so, that is 15 breaths per minute. Following your figures, that would mean that the 80cf would only last 7 minutes.

Methinks your figures are woefully broken . . . .

Looking at some figures, it looks like a pretty typical SAC rate is about 1cf/minute, and considering that 20 feet is about .6 atmosphere, then you should see about 50 minutes on the AL 80, not 7 as per your figures . . . Working with the 4 seconds per breath approximation, that would be about 750 breaths, not the 106 supported by your math. Working back to the Spare Air, then with 3cf and 20 feet, we get a tad less than two minutes at a normal breathing rate, and at 15 breaths per minute, that is closer to 30 breaths than 4 . . .

Looking at it a different way, a typical humal tidal volume (volume of a breath) is 500ml, which equates to about .018 cubic feet per breath. That comes up to about 100 breaths on the Spare Air at 20 feet, although I suspect stress and workload would likely halve that to 50, but that is still closer to what I got above with SAC rates than what you got . . .

One of these days, I'll work this up in detail . . . have seen too many flawed attempts that did not consider varying depth, tidal volumes, etc. correctly that gave bad data to count . . .

One of the other things that I don't hear discussed, but which I feel is valid, is that even if the Spare Air won't surface you from 60 feet or whatever), it will get you a hell of a lot closer to the surface than nothing . . . and a Spare Air ascent to 30 and then a CESA beats the heck out of a CESA from 60! No, not as good as a pony, but a hell of a lot less to lug around too . . The big issue is not to consider the SA as a "get out of jail free card" that will easily get you out of anything you may get into . . . and that, it likely the limiting factor for most casual divers who don't bother to work out what it can really do.

Not saying that the Spare Air is necessarily good as it is advertised, just that your figures are, well, very broken . . .

- Tim

Considering .6atm, I highlighted in red, is about 4100' altitude, I would suggest checking your math as well as the person you are correcting.


Bob
----------------------------------------------
I may be old, but I’m not dead yet.

 

[GrumpyOldGuy]

That is why I love double HP 130s.

IMHO if you plot cost vs. amount of air (don't forget to factor in your extra 1st stage reg) and weight vs. amount of air (don't forget to factor in your extra 1st stage reg) you will quickly see that doubles rock. However, sometimes doubles are overkill or just too expensive. In that case, a large pony is better bang for the buck than a Spare Air.

Spare Air is not about getting any bang for the buck. Its about getting a gooey warm fuzzy feeling of security at a reasonable price in a small, light package. The only problem is that warm fuzzy feeling tends to evaporate at depth when you actually need it and you are left with over-confidence in your undersized toy when cold hard reality and physics control your destiny.

 

[DevonDiver]

I think what people are forgetting is that you can never have enough air!

Which is actually a perfect argument for not using a Spare Air, in favor of a larger capacity pony cylinder which will contain a calculated volume of air to permit a full normal ascent from your planned depth, with margins.



Spare Air is basically a CESA bottle - it gives a few (...few...) breaths during the course of an emergency ascent. There is no margin for delay, planning, problem resolution or orientation. You have to immediately CESA and deploy the bottle on the way up...

In contrast, a pony cylinder will allow a normal ascent, including safety stop. If the volume is properly calculated (search 'Rock Bottom Gas Management'), then you have a margin for orientation/resolution at the outset...can maintain a desired/slow ascent rate... complete a safety stop...and reach the surface with gas to spare.

Using Spare Air necessitates the continuation of an emergency situation - and that is laced with both physical and psychological hazards. A pony cylinder ENDS the emergency... thus eliminating the spiral of knock-on hazards to the diver.

For the record: If I need gas redundancy, I use doubles. For recreational/lite-deco/lite-wreck, that means strapping 2 AL80's onto my wing as indies; takes a few seconds to set-up from my normal single cylinder configuration. For open water rec diving, I'll do 2 dives on the 2 cylinders, with sufficient air remaining in both for an handsome contingency, if reqd.

 

[BDSC]

Question for anyone that can answer this.

What does it feel like to actually run out of air? Do you have a nice full breath and then the next there is nothing or can you began to feel like you are having to "suck harder" on the last few breaths thus letting you know you have a real problem?

Thanks.