Another PONY Q!

[NAM001]

If I were you, I'd qualify that by saying that I have had a chamber oxygen toxicity test, and I'm not prone to O2 toxicity so I would use the oxygen at a depth of 130 feet. Oh, you may not have had the chamber test; in that case, you're best off simply doing an emergency swimming ascent. You have enough air in your lungs and Scuba tank to take you all the way to the surface. And remember, we're in a "Basic Scuba" area, and are not doing overhead hazard diving or decompression diving.

SeaRat

dies it make any difference if youare going on 100% at 130 ft or not. it takes several breaths to get your lungs changes form 21 to 100% by then you can be at 20-30 ft. like breathing off your bcd i would not turn down any gas supply in an EMERGENCY.

No one in their right mind would ever say at deep depth that they RESUSE to breath an unsafe gas at 130 ft maintain an ascent rate of < 30 fpm tillt he got to a safe depth to breath that gas.

 

[2airishuman]

So your slinging a pony doing recreational dives. The pony is strictly for OOA emergencies for anyone in our group of divers...The divers are mostly on O2, i'm wearing a computer set to Ean 32.

I'm diving Nitrox....what air should go in the PONY? Why?

The official answer would be that it should contain the same gas you're using for your primary supply, because unless you have tech training you aren't prepared to perform a gas switch, or more generally, manage a dive that involves more than one gas available with the attendant risks for mixing them up.

And that's the main risk. If you lose your reg partway through the dive, sweep your arm, and manage to grab the pony reg and think it's the primary, you could end up bent, or at risk of toxing, depending on the situation. If you're slinging there's less of a risk of that, but it's still out there.

That said, I've been on dives where the pony or hang tank didn't have a mix that matched my back gas.

 

[northernone]

Actually that got me thinking: how much air does one have on his lungs when he has exhaled ?

The idea of CESA from depth is that air expands as you go up, that’s why you should be able to exhale a bit.

Also I think you can last a bit more if you can prevent the gag reflex and not inhale water.

I read as well that you’ll be able to usually take another breath from the reg mid CESA as the air in your cylinder will expand as well. If you inhale mid CESA the should you stop to avoid air expansion in your lungs ?

A lot.

Here's a better quantified answer:
Lung Volumes and Capacities

Even some experienced divers confuse "I just exhaled" with "my lungs have no gas in them".

On a deep CESA (slow swimming ascent without using scuba) practice after exhaling "all" my air prior to beginning the ascent I was suprised to find I needed to vent upon reaching shallow depths as my residual lung volume air has expanded and presented an over expansion risk.

As @John C. Ratliff reminded us, this is basic math.

I do prefer a closer airsource myself... And would like it to be air not 100% oxygen if it's a gas I intend to use in an emergency situation at depth.

There's a crude saying which suggests the sentiment that a breath is a breath when we need a breath and death is the other option.

Any port in a storm.
Cameron

 

[Diving Dubai]

Actually that got me thinking: how much air does one have on his lungs when he has exhaled ?

I can answer this from actual experience, not from internet diving hypothesising

I had a laryngospasm at 25m down in the Red sea - nice and warm with fantastic vis

When I first had no air from my reg I switched to my pony and when that didn't work I went vertical.

According to my computer I covered 15m in 45 seconds - it seemed a lot, lot longer. You chest goes in and out as you try to breath, Your mind is quite clear thinking of many things - one of which is "Is this the end?)

In my case the laryngospasm ceased at 10m and my buddy caught up with me while I was clearing my mask (the force of the exhale fully flooded it)

I think I could have made the surface a few more kicks and boyle's law would had increased my ascent speed - and I wouldn't' have slowed it down to a controlled ascent - Surface or bust! I can only think, I have no proof and I'd rather not have to do it again as a test.

I cannot imaging trying to do that swim from almost double the depth in the cold and dark. There is a huge gulf between making a CESA attempt from any depth safe in the knowledge that if you run out of air you can breath and try again, to doing it for real, knowing if you don't make the surface you die.

If I could have breathed I'd have breathed what ever I'd have had - I don't believe there would have been any conscious thought as my brain's self preservation mechanism would have overridden and determined actions on my part

 

[uncfnp]

Bear in mind too that while the air in the lungs may expand, the amount of oxygen particles does not. So while you may be able to continue to exhale the carbon dioxide, at some point the lack of O2 is going to drive you to want a breath. And if you start with lungs depleted of O2, end exhale, the amount of useful oxygen will be significantly reduced already.

 

[uncfnp]

I know this is basic but since the subject has come up...

When I started, for health reasons, to use O2 at my safety stops diving
Florida, I had to decide if I also wanted to carry my pony bottle. That would mean carrying 3 tanks on most recreational dives. What a pita. So I gave it some serious thought and research and decided to just carry the O2 knowing that I could ascend to a "reasonable" depth, take a breath from the "pony" continue my ascent and finish at my safety stop with the O2.

And yes, I did this once. I think I was at about 80 feet when I started.

 

[Fastmarc]

For those of you that top off using a whip, how long does it usually take to equalise if say it's at 2800 and the tank 3200?

 

[uncfnp]

For those of you that top off using a whip, how long does it usually take to equalise if say it's at 2800 and the tank 3200?

Never timed it but should be quick. Probably less than a minute. To estimate final pressures.

Transfill calculations?

Wouldn’t be much for your efforts.

 

[John C. Ratliff]

Bear in mind too that while the air in the lungs may expand, the amount of oxygen particles does not. So while you may be able to continue to exhale the carbon dioxide, at some point the lack of O2 is going to drive you to want a breath. And if you start with lungs depleted of O2, end exhale, the amount of useful oxygen will be significantly reduced already.

Okay, a couple of things here. First, I think you need to revisit the physics of partial pressures. Second, it is not lack of oxygen which triggers the "must breath" signal in the brain. That is why hyperventilating prior to a breath-hold dive can lead to shallow water blackout. It is the buildup of carbon dioxide that triggers "must breath" in the brain, and by blowing off CO2 by hyperventilation, the breath-hold diver may black out when oxygen is depleted.

Now, about the laws of partial pressure.

The partial pressure of a gas is proportional to the number of molecules of the gas present in a specified volume at a given temperature. If a container were filled with pure (100 percent) oxygen at normal atmospheric pressure, then the partial pressure of oxygen in that container would be 14.7 psi, or 1 atmosphere. The partial pressure of oxygen in this case would equal the total pressure because no other gas molecules were present. If an equal number of nitrogen molecules were then added into the container without letting any oxygen escape and without changing the temperature, the total (absolute) pressure would become 2 atmospheres. We would now have a gas mixture consisting of 50 percent oxygen and 50 percent nitrogen. The number of oxygen molecules in the container would be the same, so the partial pressure of oxygen would remain the same: 1 atmosphere. This pressure now would be only half of the total, because the partial pressure of nitrogen also would be 1 atmosphere. If the size of the container were reduced one-half without letting any gas molecules escape, and if the temperature were kept constant, the total pressure would be doubled again: it would now be 4 atmospheres. The number of all gas molecules per unit volume, and the number of molecules of each gas per unit volume, would also be doubled. Therefore the partial pressures o for both nitrogen and oxygen would become 2 atmospheres. Changes of temperature cause changes in partial pressures in proportion to the change in absolute pressure. For example, if a container of gas filled with nitrogen and oxygen were heated enough to double the absolute pressure, the partial pressures of the nitrogen and oxygen would also be doubled
U.S. Navy Diving Manual, March 1970, page 34

Yes, the total number of oxygen molecules in the lungs will remain the same until the ascending diver begins exhaling as the volume of gas changes. At 132 feet, the oxygen partial pressure is 1 atmosphere, and although that decreases as the diver ascends in a controlled swimming emergency ascent, there is plenty of oxygen to sustain the diver throughout the ascent. Remember that as the oxygen is depleted by the body, the volume is also increasing with the lessening pressure.

Now for those who say this is an academic exercise and not real life, I have simulated a full exhale in a 18 foot deep pool, then by swimming up from that depth to the surface, but covering 25 yards underwater at the same time on an exhale. Even from 18 feet, there was enough expansion to feel it. I feel very confident that even today I could swim up from 130 feet fairly easily.

SeaRat

 

[uncfnp]

Okay, a couple of things here. First, I think you need to revisit the physics of partial pressures. Second, it is not lack of oxygen which triggers the "must breath" signal in the brain. That is why hyperventilating prior to a breath-hold dive can lead to shallow water blackout. It is the buildup of carbon dioxide that triggers "must breath" in the brain, and by blowing off CO2 by hyperventilation, the breath-hold diver may black out when oxygen is depleted.

Now, about the laws of partial pressure.

Yes, the total number of oxygen molecules in the lungs will remain the same until the ascending diver begins exhaling as the volume of gas changes. At 132 feet, the oxygen partial pressure is 1 atmosphere, and although that decreases as the diver ascends in a controlled swimming emergency ascent, there is plenty of oxygen to sustain the diver throughout the ascent. Remember that as the oxygen is depleted by the body, the volume is also increasing with the lessening pressure.

Now for those who say this is an academic exercise and not real life, I have simulated a full exhale in a 18 foot deep pool, then by swimming up from that depth to the surface, but covering 25 yards underwater at the same time on an exhale. Even from 18 feet, there was enough expansion to feel it. I feel very confident that even today I could swim up from 130 feet fairly easily.

SeaRat

Hi John,

Actually there are three receptors that drive respiration, thoracic neural, peipheral chemoreceptors, and central chemoreceptors. As you mentioned, the primary drive are the central chemoreceptors in the brain that moniter pH which is an indirect measure of carbon dioxide. But there are oxrgen chemoreceptors as well that has been estimate to control about 15% of respiratory drive. I believe this drive is triggered when PaO2 reaches about 60 mmHg.

I will be honest, I really don’t know at what point the O2 drive would take over in a CESA, if it would at all, especially since the peripheral chemoreceptors are slower to respond to changes.

I did consider freediver blackouts as well as hypoxic CCR divers. I seem to recall reading that .16 was hypoxic? But I still wonder if the remaining oxygen in the aveoli is sufficient to maintain the average diver (not someone with freediver skills) from 130 to surface. Would they maintain the partial pressure to the surface? It would seem logical that the point in the respiratory cycle, exhalation, and the gas itself would be factors and that hypoxia may eventually be a factor. As well as time to surface. Anyone ever done the math?

Interesting discussion and admittedly well beyond my grade level.