Spare Air: some thoughts

[Hatul]

Pretty amusing thread. My only Spare Air incident happened in Cozumel Aug 2008 when i was insta-buddied with Ivan. About 30 minutes into the dive Ivan turned to me and gave me the out of air signal and pulled out a Spare Air and left me floating at 60 feet with a truly dumb/shocked look on my face. Ivan clearly did not understand the purpose of the Spare Air.

So what happened to Ivan?

 

[idocsteve]

So what happened to Ivan?

Ivan was at 60 feet and had a Spare Air.

He was back on the boat halfway through his second beer before JimmyBoy got back to the boarding ladder.

 

[iztok]

Oh.

I thought that the measured tank pressure increases as you ascend from depth.

Since you claim that it does not...and for all I know you may be right about this..then why does my AI computer give me "more dive time remaining" as I ascend- and yes it bases that information on the most limiting factor and that in my case is almost always gas remaining not Nitrogen absorption.

idocsteve, what kind of doctor are you?!?

Or is this "doc" just something you've attached?

Whoever explained you basic physics principles in 7th or 8th grade did a poor job. Tank is pretty much a non-flexible container (for suitable values of "pretty much") and with change in surrounding pressure the pressure in tank does not change.

 

[idocsteve]

idocsteve, what kind of doctor are you?!?

Or is this "doc" just something you've attached?

Whoever explained you basic physics principles in 7th or 8th grade did a poor job. Tank is pretty much a non-flexible container (for suitable values of "pretty much") and with change in surrounding pressure the pressure in tank does not change.

Yes, Thal clarified that for me and he wasn't even insulting about it.

I know where I screwed it up.

Of course I knew at one time that tank pressure doesn't change with depth, all other things being the same, that's Boyle's law and it's covered in the PADI manual. But that was over 25 years ago, and I've become a bit too complacent since using my AI computer and throwing my SPG in the closet.

What also threw me is the common statement that "as you ascend, more gas becomes available". While that's true, the gas is more available not because it increases in pressure but because it increases in volume.

If you want to know what kind of a "doc" I am, check the link in my signature.

 

[Tanked]

idocsteve, what kind of doctor are you?!?

Or is this "doc" just something you've attached?

\.

idocsteve. i+doc+steve= an eye doctor named steve?
Its just a guess of course

 

[idocsteve]

idocsteve. i+doc+steve= an eye doctor named steve?
Its just a guess of course

Now you're reaching

 

[DCBC]

I am not sure if it has been said yet, but I would think that due to decreasing pressure that someone that goes OOA around 100-140' and does a Emergancy Swimming Accent from depth will get an extra breath, or two while the accend due to decreasing pressure in the water, the tank could have for ex 30psi left in it, and not give air at 140' due to the water pressure being greater then that in the cylinder, as you go up, and the pressure decreases the gas in the tank becomes breathable again due to the cylinder's pressure once again being greater then the sourounding water pressure....

If your regulator is functioning properly you will, but that may not be when you require it. If it's not functioning, you're out-of-luck without a redundant system.

 

[Blackwood]

That's because the gas laws work in Kelvin, so a difference of 277 K (thermocline temperature) to 303 K (hot tropical day) is only a 9% change in temperature.

Oh boy, now I get this thread in my email.

They work in absolute temperature, not necessarily Kelvin.

FYI, you can turn off notifications. I did that a LOOONG time ago

 

[Atomic_Diver]

I will not lie, I tried to enter to win a Spare Air just for the purpose of having it to use in the pool, and for the hell of it, or to sell it to get money... But hey Spare Air may be good for Helo pilots, rafters, other (above) water sports where the risk of being pinned under something u/w, or whatever exsists, mabey in a car incase it accidently gets into the water from a bridge, or rolls in at a boat ramp, or mabey for the Freediver to have for an emergancy use only, for these situations a Spare Air could be a lifesaver, but in the scuba realm its just another marketing gimmick, and a waste of money, as i said earlier you can get an AL 40 cuft tank and a (for ex) a single OMS, or Dive Rite 1st Stage for less than $199.99, and hook up your octo to the 1st stage, and wa la... A regulator for the pony, if your buddy goes OOA give him the regulator to the pony, and make your accent...

 

[iztok]

Yes, Thal clarified that for me and he wasn't even insulting about it.

I know where I screwed it up.

Of course I knew at one time that tank pressure doesn't change with depth, all other things being the same, that's Boyle's law and it's covered in the PADI manual. But that was over 25 years ago, and I've become a bit too complacent since using my AI computer and throwing my SPG in the closet.

What also threw me is the common statement that "as you ascend, more gas becomes available". While that's true, the gas is more available not because it increases in pressure but because it increases in volume.

If you want to know what kind of a "doc" I am, check the link in my signature.

Complacency kills. Or seriously injures people (see SG accident).

More gas is available is general statement that does take into account pressure. You have finite amount of molecules available and lungs work due to small pressure difference between surrounding environment and lungs. So regulator has to provide gas at that pressure (slightly above) so one can breath. At a depth more pressure is needed (thus more air is consumed) for one to breath. During the ascent regulator can provide gas at lower pressure (thus consuming less molecules of available gas). So technically you don't have "more gas available" per se but you can "stretch it longer".

In case you didn't read the previous one. I wasn't bashing you I was bashing your physics professor. It has been a while since I had to learn these basic principles in school but I do still remember the principles (while I might not remember formulas).