Nitrox in 300bar pony?

[tamas970]

I am planning to buy a pony and wondering about two things:

#1: What about filling it with Nitrox36? In a nasty OOA scenario it can happen, that
an unplanned deco stop has to be completed. For that, nitrox is supposed to be
better than air.

The risk of breathing Nitrox instead of air is oxygen toxicity, but I
can hardly imagine a scenario, where I run out of air deeper than 50m/150Ft. If so, I can
still take 1-2 breath of the nitrox mix and ascend to the MOD. If I happened to be that
deep for whatever reason (saving a mermaid, etc.), I will need a "strong mix" anyway
to avoid DCS...

#2: What about small carbon tanks? I found a 4.7l/52cf one in a german store, its only 4kg.
Quite a compact package for bringing a lot of air underwater.

I know, 50+cf sounds a lot, and nitrox sounds to be a hassle, but I feel I can greatly improve my safety margin with that. If i dive with a nitrox primary, I need a nitrox pony in order keep up with the deco planning of my computer.

 

[elan]

#1 The MOD for 36 is around 95ft 150ft is way over that limit. Oxtox results in seizures and drowning and potentially death, DCS will not kill.

Put the mix that accounts for your deepest dive.

#2 Carbon tanks are too positive in water. You will need to add lead to sink them down so that's the reason they are not used in scuba

I am planning to buy a pony and wondering about two things:

#1: What about filling it with Nitrox36? In a nasty OOA scenario it can happen, that
an unplanned deco stop has to be completed. For that, nitrox is supposed to be
better than air.

The risk of breathing Nitrox instead of air is oxygen toxicity, but I
can hardly imagine a scenario, where I run out of air deeper than 50m/150Ft. If so, I can
still take 1-2 breath of the nitrox mix and ascend to the MOD. If I happened to be that
deep for whatever reason (saving a mermaid, etc.), I will need a "strong mix" anyway
to avoid DCS...

#2: What about small carbon tanks? I found a 4.7l/52cf one in a german store, its only 4kg.
Quite a compact package for bringing air underwater.

 

[rhwestfall]

Are you trained in deco procedures?

deco is specific planning, management, and gas selection, not just "It should work, right?".

redundant gas is something completely different..... (and 21% has been somewhat established as the appropriate "mix" for pony use for the "OOPS!" in some past threads)

"just in case" is not a dive/gas management plan.... (or a deco dive plan for that matter)

 

[halocline]

I am planning to buy a pony and wondering about two things:

#1: What about filling it with Nitrox36? In a nasty OOA scenario it can happen, that
an unplanned deco stop has to be completed. For that, nitrox is supposed to be
better than air.

The risk of breathing Nitrox instead of air is oxygen toxicity, but I
can hardly imagine a scenario, where I run out of air deeper than 50m/150Ft. If so, I can
still take 1-2 breath of the nitrox mix and ascend to the MOD. If I happened to be that
deep for whatever reason (saving a mermaid, etc.), I will need a "strong mix" anyway
to avoid DCS...

#2: What about small carbon tanks? I found a 4.7l/52cf one in a german store, its only 4kg.
Quite a compact package for bringing a lot of air underwater.

I know, 50+cf sounds a lot, and nitrox sounds to be a hassle, but I feel I can greatly improve my safety margin with that. If i dive with a nitrox primary, I need a nitrox pony in order keep up with the deco planning of my computer.

This post perfectly illustrates my personal ambivalence towards pony use, actually mis-use in this case. If you want to "greatly improve your safety margin" simply do not dive in the manners you're describing (saving a mermaid, really?) and instead, take control of your dive behavior.

Talk about delusional diving....sorry, no offense intended, but you have to realize what responses you're going to get if you post that you're looking for advice on how to handle 'unplanned deco' or 'running out of air at greater than 150ft' by asking what blend to put in your HP pony.

Best of luck!

 

[tamas970]

It will come, but I need to collect some more experience before. "saving a mermaid" was obviously a
joke, but reading the accident reports, it DOES happen, that someone accidentaly goes too
deep or gets entangled at depth and when he manages to free himself, he exceeded NDL.

The Pony is not standard equipment and I know its meant to be used in emergency = "just in
case" application.

"just in case #1" you need enough air to reach the surface from 50-60'/15-20m - this can be easily done with a 6cf, 200bar bottle.
"just in case #2" you are in trouble at depth near the recreational limit (120'/40m), get entangled at the end of your bottom period. Freeing yourself takes 3-4 minutes, which means, you need an extended safety stop. - For this, I am afraid, 6cf won't be enough. 19cf (2.5L/200bar) should be OK, but that bottle is almost as heavy as the 4.7L carbon which holds more than twice as much gas.

As for bouyancy: true, carbon tanks are positive, but for this one (4.7L/3.8kg) shouldn't need more than 1-1.5kg lead. They are being used for scuba, the site I linked lists a handful of carbon tanks between 4.7 and double 15. (translated to imperial units: 333cf together)

Regarding the mix: If the main threat is oxygen toxicity (=staying below 100' while on pony), then I agree, air is the right gas. However, I personally would ascend immediately if my main tank depleted. As far as I remember from my nitrox course, O2 doesn't kill immediately, it has to accumulate in the CNS, which takes some time (~20 minutes are needed to reach 50%CNS at 1.6bar ppO2 according to the NOAA table). Anyway, you are right: in water the diver is safer on air. If he missed a deco stop, he can still be taken into a chamber.
Besides, its is simpler to arrange an air-refill than a nitrox.

Are you trained in deco procedures?

deco is specific planning, management, and gas selection, not just "It should work, right?".

redundant gas is something completely different..... (and 21% has been somewhat established as the appropriate "mix" for pony use for the "OOPS!" in some past threads)

"just in case" is not a dive/gas management plan.... (or a deco dive plan for that matter)

 

[rhwestfall]

"just in case #2" you are in trouble at depth near the recreational limit (120'/40m), get entangled at the end of your bottom period. Freeing yourself takes 3-4 minutes, which means, you need an extended safety stop. - For this, I am afraid, 6cf won't be enough. 19cf (2.5L/200bar) should be OK, but that bottle is almost as heavy as the 4.7L carbon which holds more than twice as much gas.

I'd suggest you read up on "Rock Bottom", and implement it on your dive plan. Your gas planning for this dive was not proper (and a pony is not the answer for this situation).....

side note: flying the edge of NDL's without proper equipment/training/and planning is just waiting for a big problem.

 

[KD8NPB]

I would advise switching to some small doubles.

 

[tamas970]

At the depth limit of recreational diving (120'/40m) EANx32 gives you 1.6 bar,
which is safe for emergency/short term use.
If you rise to 80', then you are in a safe 1bar ppO2 and nicely decreasing your nitrogen
load in the meantime. The only risk I see, is residual oxygen, which is an issue, if the subsequent dive is on nitrox.


Even the best equipment can default in the hands of a well-trained diver on a well-planned dive.
E.g. your first stage can freeze at 120' (=no matter, how much air you have in your tank, you won't get it). i.e: you need a pony/BB.
Your fault? no, if you bought an appropriate, cold-suitable 1st stage.

I might be too paranoic, but if things go really bad, they like to unleash a chain of events, leading to disaster.
My sole intention is to look up viable escape points.

I'd suggest you read up on "Rock Bottom", and implement it on your dive plan. Your gas planning for this dive was not proper (and a pony is not the answer for this situation).....

side note: flying the edge of NDL's without proper equipment/training/and planning is just waiting for a big problem.

Good idea. A bit heavy for flying though, if I don't find doubles to rent locally (I am an air-hog, usually dive with a 15L/110cf tank on an 80' dive).

I would advise switching to some small doubles.

 

[Doppler]

#2 Carbon tanks are too positive in water. You will need to add lead to sink them down so that's the reason they are not used in scuba

One small "correction."

Carbon tanks ARE buoyant. Gravity loses once they are taken into the water. However, they are not too positive. The cylinder in question has a displacement of a little more than 5.25 litres (4.7 litre internal volume). This translates as 5.25 kg buoyant effect when empty. While this IS a bit of a pain in the ass, because it would require carrying that much ballast, the overall mass walking around on the boat would hardly be "too much" compared with a steel tanks with similar capacity (a smidge over 1400 litres). Of course, the common item in both a steel and carbon cylinder would be the mass provided by the gas. Filled to 300 bar, the weight of the gas (let's assume air) will be about 4.7 X 300 X 1.2 gm or around 1.6 kg.

I have a buddy who uses carbon cylinders in her CCR. Does just fine with them.

P.S. to a NA centric diver, carbon cylinders may seem exotic and OTT but there's a whole world outside of North America folks...

 

[Lemna]

Even the best equipment can default in the hands of a well-trained diver on a well-planned dive.
E.g. your first stage can freeze at 120' (=no matter, how much air you have in your tank, you won't get it). i.e: you need a pony/BB.
Your fault? no, if you bought an appropriate, cold-suitable 1st stage.

Tamas,

Don't worry about not getting enough gas when your first stage freezes... But even if something really catastrophic happens, you still should have your buddy close to you. IMHO, good buddy skills and good dive planning are a much better solution than a pony.

Some of NWgratefuldivers articles would be interesting reading material, I think: NWGratefulDiver.com