running out of air - at what point is it not subtle

[dlwalke]

Let's say, for the sake of argument, that your SPG is stuck on 1200 or so, or maybe you are just spacing out and let your air run down to rock bottom. I assume that breathing effort does not go from normal to the feeling I get when an incompletely pureed strawberry gets jammed in the inlet of my smoothie straw. Is this right? If so, at around what point (i.e., how low do you go in terms of remaining air) before it starts to become quite obvious that you are nearing the end of your tank?

Thanks,
Dave

 

[mars2u]

dlwalke once bubbled...
Let's say, for the sake of argument, that your SPG is stuck on 1200 or so, or maybe you are just spacing out and let your air run down because you haven't been checking your SPG. I assume that breathing effort does not go from normal to sthe feeling I get when an incompletely pureed strawberry gets jammed in the inlet of my smoothie straw. Is this right? If so, at around what point (i.e., how low do you go air in terms of remaining air) before it starts to become quite obvious that you are nearing the end of your tank?

Thanks,
Dave

1) It isn't immediate...you actually notice that it's harder to breathe. To test this, I suggest doing this in a pool and have your buddy almost close your tank valve...you'll notice it gets harder to breathe, but it's not immediate.
2) If you are at depth and notice that it has become harder to breathe, as you ascend there is less pressure on your tank and therefore you will GAIN a couple of extra breaths. Remember the balloon and soda can example from your class? The physics are also happenning on your tank.

 

[Spectre]

mars2u once bubbled...
To test this, I suggest doing this in a pool and have your buddy almost close your tank valve...you'll notice it gets harder to breathe, but it's not immediate.

I wouldn't suggest that. You can 'test' the same sensation out of the water, and it really doesn't help with the question that Dave is asking.

Personally, I don't know the answer. I've never been there. The closest I've been is when I forgot to open my tank the rest of the way after cracking it slowly [got distracted while gearing up]. I started to notice something was up around 500 psi. However that doesn't answer the question either.

In lieu of a real answer, let me make sure to point out that 'spacing out and not checking' is not a good idea. Also, if you check your SPG and it hasn't changed from the last time... this probably doesn't mean your air consumption miraculously dropped.

Keep track of your SAC rates from your dives. Use those SAC rates plan when to ascend. Another thing I like to do is use my SAC rate to 'spot check' myself at different points in the dive. If my plan has me at 100 feet for 20 minutes, I like to calculate out what my estimated gas will be 5, 10 and 15 minutes into the dive. If it's way off your estimates, and you can't explain it, then you might want to get out of there. I've made adjustments in the past in-water where I noticed I was using more air than I estimated, due to stressful dives. I'll tack a little more onto my rock bottom and end the dive a little earlier than planned.

 

[FredT]

The "choke" point where you'll notice it is when the gas demanded cannot be supplied through the second stage. Typically this won't happen to a relaxed diver breathing on a good reg until the tank pressure is below the IP pressure, but just how far below that point it gets before you notice it will depend on how much air you're using.

If breathing slow while stalking a fish or picture you can sip a tank all the way down to ambient before you notice it. BTDT! OTOH if you are hoovering while swimming hard into a current you may notice an increase in breathing resistance slightly before tank pressure makes it to the IP point. The flow passage restrictions in the first stage start to be the choke at lower tank pressures and high flow rates. Little things like partially clogged regulator inlet filters can raise the tank pressure where you'll notice it earlier too.

FT

 

[scuberd]

only once (and it was for a lark), I had my tank down to about 300 after a dive and at about 15-20 feet. It felt like I was trying to create a negative pressure space (similar to sucking the air out of a glass bottle) I could still take a breath but I had to work for it.

 

[Big-t-2538]

Spectre once bubbled...

let me make sure to point out that 'spacing out and not checking' is not a good idea.

I'm going to have to second that...I wouldn't condone "spacing out" underwater

Also, if you check your SPG and it hasn't changed from the last time... this probably doesn't mean your air consumption miraculously dropped.

What...you mean gear can malfunction??? Proposterous...I don't believe you...


As for the original question, I have never been there either, but I have had my air turned off in a controlled environment doing situation handling skills. In about 7 feet of water, you can tell that it is gettting hard to breath about 1 breath before you have no air left. I would imagine that you get a few more breaths when the tank is on b/c there is more volume of air to breath, at that particular pressure, but like I said, I'm only guessing.

 

[Hoppy]

Hi guys,

Having OOA'd last week (don't ask !)

I got three breaths before NOTHING !

The first dragged a little , the second and third I had to suck out of the tank like a b*stard.

It's a twitchy moment I can tell you. !

Before anyone wades in to flame , the sitrep is as follows :-

Having had a problem with cramp on the dive (I) we turned it, it was safer to swim under the surface than on it, due to swell and proximity to rocks. Thats IMHO & the instructor qualified diver I was with. (Comments welcome)

I was HOOVERING big style at this point, but at least aware of it and knew i was very low when we hit the stop. My buddy was with me all the way (thanks Martin), the exit was up a ladder to the quayside in a 6 foot swell, so the reg was in while going up the ladder.

I lost the air while on the ladder. So exited with gulps of air and the Mediterranean :upset:

So thats my story, I would welcome comments or questions. Did I handle the dive properly ?

Cheers
Hoppy

 

[devilfish]

mars2u once bubbled...



2) If you are at depth and notice that it has become harder to breathe, as you ascend there is less pressure on your tank and therefore you will GAIN a couple of extra breaths. Remember the balloon and soda can example from your class? The physics are also happenning on your tank.

Hmm. "As you ascend there is less pressure in your tank".
Would you explain that, how pressure is less in a rigid container.

 

[redacted]

25+ years ago. As I recall, we were around 30 ft (depth shoud be majopr variable). I took a breath and suspected it might be time. The next breath left little doubt and I hit the lever. But that was probably with a double hose reg with much lesser performance than modern HP regs.

If I crunched the numbers right, an AL80 should have 3 cu ft of air under pressure when you hit an IP of 135 psi. With an unbalanced first stage, you might get some warning before this point as performance will suffer slightly if deep. A balanced first should give you no warning. The performance of an unbalance 2nd should fall pretty quickly when you don't have the set IP helping you crack that valve. With a balanced 2nd, the effect should be more subtle. So with a SAC of .5 (a pretty cool head); you've got 3 breaths left at 30 ft, 2 at 60, 1.5 at 90 - without considering change in depth. Factoring in change in depth, coming up from 30ft should make another .4 cu ft available or almost another breath, 60 ft gets you .8 cu ft and 90 get 1.2. Something in the ballpark of about one more breath - not much. So, by the time you realize that damn gauge was lieing to you, planning on 2 to 4 breaths if your lucky looks about right.

REDUNDANCY and/or a good BUDDY sounds good. Hope I didn't screw up the math to badly. My fingers are sore.

 

[Hoppy]

how does a reduction in depth increase the amount of air available ?



Hoppy