Spare air

[BIG Tiggz]

I know this topic might have been beatin to death but I have not seen any threads on this topic.

So...


I hear spare you can asend from 99ft and do a three min deco stop at 15ft with it.

I also hear it only gives a few breaths before it is empty.

I have heard this from dive shop owners to dive masters on both sides.

What do the people of scubaboard have to say on this topic?

Thanks for any input

 

[Randy43068]

There is nothing more one can say on this topic of a Spare Air ... Not being a smart-Alec but it's been beaten to death here on SB and a few other boards. You'll have HOURS of reading if you search it out.

 

[El Orans]

Here's one: http://www.scubaboard.com/forums/tanks-valves-bands/201226-spare-air-advice.html

This forum contains a lot of them: http://www.scubaboard.com/forums/tanks-valves-bands/

 

[muddiver]

One of the problems is that the people on this bullitin board use depth specific static caculations to show that a any cylinder smaller than 40 cubic feet will not perform as a redundant breathing gas supply for SCUBA diving. In reality, the consumption of gas as one is ascending during an emergency ascent or aborted dive is a non-linear curve that is based on the change in depth as you ascend. It requires setting up a caculus equation that any Mechanical engineer or scientist could figure out. I'm just a dumb Civil Engineer and I sucked at Caculus or I would have already done a study to prove some of the tables use to argue the theory that a SpareAir is too small to be wrong for the argument.

I have personally come up from 130' on a 6 cube pony because I was not paying attention to my gas supply. I had plenty for a :03 at 20' and there was more in my primary cylinder that I could use when I reached that depth (sorry don't know why).

 

[Randy43068]

One of the problems is that the people on this bullitin board use depth specific static caculations to show that a any cylinder smaller than 40 cubic feet will not perform as a redundant breathing gas supply for SCUBA diving. In reality, the consumption of gas as one is ascending during an emergency ascent or aborted dive is a non-linear curve that is based on the change in depth as you ascend. It requires setting up a caculus equation that any Mechanical engineer or scientist could figure out. I'm just a dumb Civil Engineer and I sucked at Caculus or I would have already done a study to prove some of the tables use to argue the theory that a SpareAir is too small to be wrong for the argument.

More to the point. If one is so sloppy in dive planning as to depend on this contraption then they probably shouldn't be diving.

Really.

 

[PerroneFord]

Awww... lets do it for fun.

A basic rule of thumb is that the average stressed diver breathes about 1 cuft per minute on the surface. So lets use that. And yes, an experienced diver can do it with less, a newbie might breathe more. I know my first cave dives brought me to about .91-1.2 breathing rates. Now they are in the .65-.72 range.

At 99ft or 4ATA we are breathing at 4 times the normal rate. If we ascend at the recommended 30fpm we will need 1 minute to get from 99 to 66 feet. So 4 ATA times 1 cuft per minute means it will take about 4 cuft to get from 99ft to 66ft. (Yes I know this isn't exactly right).

Similarly at 66ft we are at 3ATA so it will take us 3 cuft to get from 66ft to 33ft.

And to go from 33 to about 15 ft we'll call that 1.5 cuft per minute.

So 4+3+1.5 = 8.5cuft just to get to the safety stop.

At the safety stop we are at 1.5 ATA or so, so we'll be breating 1.5 cuft per minute for 3 minutes. So another 4.5 cuft. Add that to our 8.5 cuft to get there, and we're at a total of 13 cuft.

This does not take into account getting from the safety stop to the surface, or dealing with any surface waves, etc.

So ~13cuft to get from 99ft to 0 under ideal conditions. How much does the Spare Air hold?



(and again, with depth averaging, we could probably do this with 8-10cuft but the example still holds.)

 

[Randy43068]

What are you thinking Perrone??

Don't confuse anyone with the facts, they have already made up their minds!!

Awww... lets do it for fun.

A basic rule of thumb is that the average stressed diver breathes about 1 cuft per minute on the surface. So lets use that. And yes, an experienced diver can do it with less, a newbie might breathe more. I know my first cave dives brought me to about .91-1.2 breathing rates. Now they are in the .65-.72 range.

At 99ft or 4ATA we are breathing at 4 times the normal rate. If we ascend at the recommended 30fpm we will need 1 minute to get from 99 to 66 feet. So 4 ATA times 1 cuft per minute means it will take about 4 cuft to get from 99ft to 66ft. (Yes I know this isn't exactly right).

Similarly at 66ft we are at 3ATA so it will take us 3 cuft to get from 66ft to 33ft.

And to go from 33 to about 15 ft we'll call that 1.5 cuft per minute.

So 4+3+1.5 = 8.5cuft just to get to the safety stop.

At the safety stop we are at 1.5 ATA or so, so we'll be breating 1.5 cuft per minute for 3 minutes. So another 4.5 cuft. Add that to our 8.5 cuft to get there, and we're at a total of 13 cuft.

This does not take into account getting from the safety stop to the surface, or dealing with any surface waves, etc.

So ~13cuft to get from 99ft to 0 under ideal conditions. How much does the Spare Air hold?



(and again, with depth averaging, we could probably do this with 8-10cuft but the example still holds.)

 

[PerroneFord]

I have personally come up from 130' on a 6 cube pony because I was not paying attention to my gas supply. I had plenty for a :03 at 20' and there was more in my primary cylinder that I could use when I reached that depth (sorry don't know why).

What was your ascent rate?

 

[Sideband]

I have personally come up from 130' on a 6 cube pony because I was not paying attention to my gas supply. I had plenty for a :03 at 20' and there was more in my primary cylinder that I could use when I reached that depth (sorry don't know why).

That last sentence scares me. Actually, the first one scares me too, but you are a Dive MASTER and don't understand how ambient pressure effects your first stage and available air? Please tell me that isn't what you meant.

Joe

 

[hlsooner]

So I prefer 30 cu ft of gas for backup. But I decided to whip up a spreadsheet to calculate the gas consumption on ascent for the old Spare Air. It is not a true integration, but I did calculate it down to 1 ft increments.

Assumptions:
Starting at 99 ft. Ascending at 60 ft/min to 60ft and then 30 ft/min to the surface. Assuming an immediate ascent when going onto bailout, and no safety stop.

At a SAC rate of 1.0 as described above, you need exactly 6 cu ft of gas to get there. So with the large Spare Air, 3.0 cu ft, you would need to make an immediate ascent, 60 ft/min to 60 and then 30 ft/min to surface, and keep your SAC down to 0.5 in order to not run out of gas. No Thanks.