Couple questions on a pony bottle for bail out

[John C. Ratliff]

mrdre,

I'm going to do it a bit differently, using the U.S. Navy Diving Manual and their tables. According to the U.S. Navy, use of nitrox can be conducted using air decompression tables and the Equivalent Air Depth Table 10-1. Using their procedure, you can calculate the Equivalent Air Depth too, using this formula:

EAD = ((1-0₂%)(D+33) / 0.79) - 33

Where
EAD = equivalent depth on air (fsw)
D = diving depth on mixture (fsw)
O₂% = oxygen concentration in breathing medium (percentage decimal)

So when diving EAN32 at 100 feet, the formula goes like this:

EAD = ((1-0.32)(100+33) / 0.79) - 33
EAD = ((.68)(133) / 0.79) - 33 = 81.5

Because this is over 80 feet, for the U.S. Navy Dive Tables we go to the next higher, or 90 feet for the decompression schedule. Looking at this decompression schedule, we find that at 90 feet for 30 minutes, there is no stage decompression, and a 3 minute ascent time. Many will use a 20 foot safety stop for 3 minutes, giving a total ascent time of 6 minutes (not by the tables though).

The U.S. Navy Diving Manual does have, in paragraph 10-4.3, a "Special Procedures" statement for nitrox diving:

Special Procedures. In the event there is a switch to air during the NITROX dive, using the diver’s maximum depth and bottom time follow the Air Decompression Table for the actual depth of the dive.

Therefore, when diving an air pony bottle, the actual decompression table should probably be carried. In this case, it would use the 100 feet for 30 minutes schedule, which requires a stop of 3 minutes at 20 feet. Add the safety stop of 3 minutes, and that is six minutes decompression.

My original reservations about the impact of using air on ascent had to do with uptake of nitrogen when the ascent had already begun. There is a time lag between when breathing air, with a slightly increased nitrogen content, and the tissues receiving it. My thought was that going on air at 100 feet when initiating an ascent would not additionally load the tissues, and the diver ascending would make the equivalent depth on air (10 feet higher in this case) in 20 seconds at an ascent rate of 30 feet per minute (37 seconds to 81.5 feet). Beyond that, he would be off-gassing (maybe not at the same rate as on nitrox, but ascending nonetheless). So I feel that the U.S. Navy method of using the actual depth for computing decompression when going on air to be rather conservative, but it is there and therefore is a standard.

Dive computers may or may not handle this, but the diver should know about it.

SeaRat

 

[mrdre]

OK.. so let's see if I understood you right. Assuming 90 feet/30 minutes the diver on EAN32 is on NDL and doesn't need 20' stop, but let's say he/she chooses to.. so we have 3 minutes ascend + 3 minutes optional stop. With air we have 3 minutes ascend + 3 minutes optional stop + 3 minutes mandatory stop at 20 feet. Did I understand it right? So basically with air 19 cft pony the diver just must remember "stay on 20 feet 3 more minutes". Right?

Second question I have.. (I just don't want to hijack the thread, maybe it's not the place for this discussion).. Why US Navy tables and manuals? As I understand US Navy was first, did extensive study, etc. but... I'm using computer software (not dive computers) to plan the dives, there are different models, I'm using ZHL16. My examples were from this. Why US Navy tables and not more modern computer software? If it's a long discussion, we can move to some other thread.

 

[John C. Ratliff]

mrdre,

I think you've got it correctly, but follow the links I provided and look at the U.S. Navy Diving Manual. I don't have access to any computer programs for calculating Nitrox dive profiles, and so used the only other way I know of, the Navy diving manual.

The whole idea was to look at the physiology behind switching from nitrox 32% to air for a bailout, and the U.S. Navy Diving Manual has things to say about that. Hence the discussion of Equivalent Air Depth and their procedure for switching. You are using computer software, something called ZHL16, but not providing any readout information other than the description you are using. We know nothing about the underlying assumptions of the program, the number of tissues it is using, or how it is making this "switch" from nitrox to air. At least, with the U.S. Navy Diving Manual, we had their methodology documented.

I see that NOAA has their own Nitrox 32 No-decompression Table too.

I'll look at my Suunto Dive Manager program, and see what it can do too. I also see that your ZH-L16 model is explained on-line at this link:
http://www.achim-und-kai.de/kai/tausim/saett_faq_e.html#k6
It says much the same as the U.S. Navy Diving Manual, but does not show switching to air from nitrox during a dive (but it is a 2000 publication).

The whole idea here is to get people to think, and not simply rely on their computer without knowledge of underlying assumptions. I am also attempting to "demystify" nitrox, as it is not a magic mix but can be handled with both computer/decompression meters or using tables in the more traditional manner with a bit of knowledge.

Concerning the op ed's question, it appears that there may be a bit of an extra decompression obligation switching from nitrox 32% to air in a pony bottle for immediate ascent, but not much. I think both of us have shown that.

SeaRat

 

[mrdre]

mrdre,

I think you've got it correctly, but follow the links I provided and look at the U.S. Navy Diving Manual. I don't have access to any computer programs for calculating Nitrox dive profiles, and so used the only other way I know of, the Navy diving manual....

OK, I'll read through the Navy manual about switching from Nitrox to air.. (it's interesting anyway).

I use this algorithm:

Bühlmann decompression algorithm - Wikipedia, the free encyclopedia

The other:

Varying Permeability Model - Wikipedia, the free encyclopedia

There is a list of computer software. Some are free. I use the iPhone version, so I can plan right before diving.

 

[kwinter]

Ok, let's try this another way. Forget about equipment failure. You are diving nitrox at 100 feet or so and getting near the end of NDL. Time to end the dive and start your ascent. You still have nitrox left in your main tank and you also have a slung tank of air as your pony. What do you do?

Is there anyone in his right mind that would choose to abandon his main tank of nitrox and switch to air because he thinks it is a better choice for the ascent? I can't imagine this. So if the right choice is to stay with nitrox for the ascent, why isn't it the right choice to have nitrox in the pony bottle in the first place? Fill it with 32% and be done with it. Top it off when needed. And forget about the extra $10 a year it might cost.


iPhone. iTypo. iApologize.

 

[John C. Ratliff]

kwinter,

That is fine, but wasn't the question that was being discussed. To remind others, here is the discussion point:

Finally, to this post. Note the sentence I highlighted; you won't immediately go into a decompression situation because you switched to air--that is a fallacy. You get into a decompression situation by breathing nitrogen (or another inert gas such as helium) under pressure. This gas dissolves into the blood plasma over time, and simply switching from nitrox to air does not place further nitrogen into your system immediately. That takes time, and pressure. However, the person has begun an ascent, so this time is not counted as bottom time. So no further decompression obligation would be incurred if ascent immediately begins.

This is not true and dangerous.
Grab any decompression software, plug in a long deep dive to the end of recreational limits using 32% nitrox and note the decompression time to the surface. Now do the same thing but set your deco gas to 21% nitrox and switch deco gases at just above the maximum depth (will simulate using it as the gas for ascent). You will now have a decompression obligation.

As you ascend you have a gas gradient and a pressure gradient you can use to to help you decompress. If you actually increase the nitrogen/gas gradient going into your tissues (instead of reversing it out like is done in accelerated decompression) you increase the decompression time required to off-gas the nitrogen in your tissues during ascent. Nitrox tables calculate the NDL assuming you are using the same gas the entire time. Use the same gas in your "reserve" as your bottom gas if you diving within NDL - if not get the proper training for decompression diving (and learn what gases to properly use).
(emphasis added, jcr)

The point is that it is not dangerous if the diver knows the procedure to follow in making the change to air from nitrox 32%. It is a matter of understanding the diving physiology, and not trying to scare divers.

SeaRat

 

[Errol Kalayci]

Why carry a pony bottle in the first place? Plan your gas management such that you always have enough gas for two divers to ascend to the surface following the prescribed ascent rates while sharing gas. I posted an article a long time about this on the forum somewhere where I explained in detail the entire situation. If you run the numbers, it works out to roughly 40cft needed from 100', 30 cft from 80', 20 cft from 60'. Not telling anyone to do anything but if you planned enough gas to cover one diver losing gas for whatever reason, then you both make it up to the surface by donating a working regulator (mine is the one I am breathing attached to a 7' hose for logistical ease of traveling or ascending as a team in horizontal position to give us better stability and buoyancy control) to the diver needing gas and breathing from your other working regulator (mine is found just under my chin held in place by thin bungie). So if you are diving 100' and want more than 12 minute bottom time, if you run the numbers you will need double's anyway or big steel tank (check buoyancy and if it works for a balanced rig and how it handles in the water...double 80's often better choice, not always).

 

[dumpsterDiver]

Why carry a pony bottle in the first place? Plan your gas management such that you always have enough gas for two divers to ascend to the surface following the prescribed ascent rates while sharing gas. I posted an article a long time about this on the forum somewhere where I explained in detail the entire situation. If you run the numbers, it works out to roughly 40cft needed from 100', 30 cft from 80', 20 cft from 60'. Not telling anyone to do anything but if you planned enough gas to cover one diver losing gas for whatever reason, then you both make it up to the surface by donating a working regulator (mine is the one I am breathing attached to a 7' hose for logistical ease of traveling or ascending as a team in horizontal position to give us better stability and buoyancy control) to the diver needing gas and breathing from your other working regulator (mine is found just under my chin held in place by thin bungie). So if you are diving 100' and want more than 12 minute bottom time, if you run the numbers you will need double's anyway or big steel tank (check buoyancy and if it works for a balanced rig and how it handles in the water...double 80's often better choice, not always).

Maybe we don't want to be dependent on some ill trained diver to preserve enough gas for our use? Not too long ago a very experienced instructor and moderator on this board ran her tank to zero and drowned in 12 feet of water. Not only did she not save enough air for a buddy, she didn't have enough to survive in water no deeper than a pool. If we can not depend on super experienced instructors who dive daily, how can we be expected to bet our lives on an unknown insta-buddy on a charter boat? You comments seem so out of touch with the type of diving most people do.


Maybe we want to be self-sufficient? Maybe we want to do an advanced and challenging dive alone. Many, many reasons to carry a pony bottle .. and as training gets more and more "abbreviated" the need for redundancy becomes more important!!!!

---------- Post added July 25th, 2014 at 11:30 AM ----------

Ok, let's try this another way. Forget about equipment failure. You are diving nitrox at 100 feet or so and getting near the end of NDL. Time to end the dive and start your ascent. You still have nitrox left in your main tank and you also have a slung tank of air as your pony. What do you do?

Is there anyone in his right mind that would choose to abandon his main tank of nitrox and switch to air because he thinks it is a better choice for the ascent? I can't imagine this. So if the right choice is to stay with nitrox for the ascent, why isn't it the right choice to have nitrox in the pony bottle in the first place? Fill it with 32% and be done with it. Top it off when needed. And forget about the extra $10 a year it might cost.


iPhone. iTypo. iApologize.

Seriously this is a big question? The answer is I do whatever I want. It makes no significant difference as has been quantified by several people using different methods. If the diver wants to come up on his air pony he does it. My pony gets topped off with air for free anytime I want. Getting chrged 8 or $12 for a nitrox fill after a few cubic feet are consumed from a pony could cost hundreds of dollars a year if you dive often.

The whole point with nitrox is that you can NOT get it topped off if the shop is doing partial pressure blending. it must be completely dumped and re-filled EVERY time. We went over this before, didn't we?

 

[mrdre]

....Is there anyone in his right mind that would choose to abandon his main tank of nitrox and switch to air because he thinks it is a better choice for the ascent? I can't imagine this....

I don't think anybody argues that air in the pony is better. The *right* way is when the gas in the pony matches the main tank. It's just if somebody dives with air in pony it's *not that bad*, meaning in an emergency it will get the diver (with enough experience) safely to the surface.

 

[Errol Kalayci]

So let me get this straight "Dumpster Diver", I see you are an Instructor. Your rational is that planning a dive to include enough gas for 2 divers to make it safely to the surface is out of touch from how most people dive? As an instructor, are you incapable of planning that out and explaining it to students or your dive buddy? Don't you know how long your gas will last at any depth before you dive? Your justification is that poor training exists, training is becoming shorter and shorter and solo diving is a good option?
My strong suggestion is for divers to learn to dive more knowledgably and with more skill so that they enjoy are safer and have a lot more fun. It is really not that hard to learn to dive with greater skill, safety and efficiency. Your dive buddy is a choice, why not dive as a team? Anyone that is looking for information on how to improve their diving may want to check out a non profit organization called Global Underwater Explorers, diving is not rocket science. Invest in a Fundamental's course and I guarantee you will learn a lot and it will be worth every penny. I am not soliciting for students, just suggesting that those interested seek out better information. Some divers do amazing recreational and technical dives without need for a pony bottle, most do not. Decide which you wish to be and act accordingly.

Why does nitrox need to get dumped with partial pressure blending, never heard of a haskel or storing banked gas? Partial pressure blending should be almost gone by this point as much better alternatives exist and are in practice throughout the world.

---------- Post added July 25th, 2014 at 02:06 PM ----------

Found the Article
Proper dive planning includes consideration of many variables, in this article we will focus on gas management and tank selection for a drift dive using 32% Nitrox on a dive not to exceed 100 feet deep. Given that we are diving to 100’ using 32% Nitrox we know that our maximum bottom time can not exceed 30 minutes before entering staged decompression requiring a stop exceeding 1 minute at a certain depth during the ascent. Let’s make it conservative and say we are planning to dive for only 20 minutes of bottom time rather than our allowable 30 minutes from a decompression standpoint. Can we safely make this dive using a single aluminum 80?

In order to plan our gas management for this 100’ drift dive we must calculate the following:
1) how much gas we will need for a controlled proper ascent with consideration for our decompression strategy (see “Minimum Decompression” explanation below); and 2) how much gas we need for our planned bottom time; and 3) how long an emergency might take to sort out on the bottom.

Minimum Decompression Concept
If you agree that on any dive you do, you ongas to some extent, then every dive requires decompression. Obviously depth and time determine the extent of the decompression required. For our purposes, any dive requiring longer than 1 minute at a certain ascent stop is considered mandatory staged decompression and outside the Minimum Decompression definition and the scope of this article. With that said, for the sake of simplicity and brevity let’s say that there are two basic categories of decompression theory, the first is the old Bulhman model which basically says that you should drive the pressure gradient as far as possible to start decompression. For example, ascend directly from 100 feet to the 10 or 15 foot stop. The second theory is the Bubble theory which basically tries to prevent bubbles from forming while minimizing the size of any bubbles that do form during the ascent and eliminate them before they grow and cause damage. Perhaps the prudent thing to do is to combine both theories which is what we do and refer to as Minimum Decompression. For a Minimum Decompression dive, a diver ascends to 50% of the maximum depth at an ascent rate of 30’ per minute. This first stop is commonly referred to as a deep stop. Upon reaching the first stop, a diver pauses for 30” and then ascends for 30” up to 40’, pauses again for 30” then ascends for 30” up to the 30’ stop, this is repeated to the surface. In doing so, a diver would leave the bottom at 100’ and ascend at 30’ per minute up to 50’ which would take 1:40, then do a 30” stop and ascend for 30” to the 40’ stop, etc. That means the total ascent time would take say 7 minutes. For those thinking this is way too long of an ascent, under the traditional 3’ safety stop a diver would ascend at 30’ per minute from 100’ to 10’ which would take 3 minutes and then do a 3’ stop and then another 10” to reach the surface which is a total of 6 minutes and 10” so it is roughly the same amount of time, just spread out differently.

Gas Consumption Rate
An average diver has a surface consumption rate (SAC) of .75 cubic feet per minute. This means that on the surface a diver would use ¾ a cubic foot of gas per minute. To determine how much they would use at depth, you have to multiply by the ATA. At 100’ the ATA is (100/3) + 1 = 4 so .75cft/min x 4ata = 3 cubic feet per minute consumed on the bottom. We presumed a 20 minute bottom time so we will use 3cft/min x 20 minutes = 60 cft.

Ascent Gas Requirements (aka Minimum Gas&#8221
How many times has a boat captain said, be back on the boat with 500 psi or 750 psi? How do you do that and why that number? Those are good questions. Perhaps it is better to ask the questions of what happens if you or your buddy has an emergency and one of you runs out of gas and what is the minimum gas required to bring them up while following your decompression profile? For those that say, you just free ascend to the surface or to a shallow safety stop, I ask why? Once a diver is breathing appropriate gas with enough volume to properly complete the ascent, the emergency is over so why undertake the additional risk not following your planned minimum decompression ascent strategy to minimize the chances of decompression illness? Great, you agree so what how do we calculate the amount we need? I will now explain how to calculate the amount of gas you need to reserve and how to put it into practice.

Should an emergency happen on the bottom, it is likely that it will take you and your buddy a minute to sort it out and prepare to ascend. As we saw above, it will take approximately 7 minutes to ascend so that is a total of 8 minutes. As we learned an average diver will consume 3 cubic feet per minute at 100 feet without being stressed. For practical purposes, a dive team with an out of gas emergency will probably have an increased SAC rate so to increase the rate from .75 to 1cft/min is reasonable. For the ascent portion you would take the SAC times the average depth (in this case the average depth between maximum depth and the surface which is 50’ or 2.5ATA) Therefore, to plan gas for this out of gas diver ascent you take 8 minutes ascent time x 2 divers x 2.5 SAC = 40 cubic feet which is referred to as ‘Minimum Gas”. Therefore, a diver must depart the bottom when they have 40 cubic feet remaining. Using an Aluminum 80 that means when the pressure gauge reads 1600 psi it is time to ascend!

Gas Required for Bottom Time
Let’s start by determining how much bottom time we get with the gas we have available. If we must reserve 40 cubic feet for our ascent, that leaves us 40 cubic feet available for our bottom time. Given that an average diver uses 3 cubic feet per minute at 100’ as we discovered above, our bottom time would be 40 cubic feet available divided by 3 cubic feet per minute which is approximately 13 minutes. That means that even though we have 30 minutes of non staged decompression time available thanks to the 32% Nitrox, and we wanted to do a 20 minute bottom time, we only have enough gas for a 13 minute dive. If we must get back to the ascent line or really should, then we have to take that into account as well but that is outside the scope of this article. So what can we do? The answer is we need more gas volume. Choosing the right equipment selection is an article into its self and the balanced rig concept must be considered. However, a set of double 80’s is a good choice as it provides more redundancy and can be a very nicely balanced and stabile package. Using double 80’s with an isolation manifold would give a diver 160 cubic feet total – 40 cubic feet (Minimum Gas) = 120 cubic feet of available gas volume. A diver consuming 3 cubic feet per minute therefore (120/3 = 40) would have 40 minutes of gas available before hitting Minimum Gas and having to start and ascent. Given that they only have 30 minutes of bottom time allowable under Minimum Decompression, they would depart at the 30 minute mark of bottom time. Should the dive go without issue, the diver would climb aboard the boat having maximized their allowable bottom time and with enough gas remaining to do a regular second dive in the 60 feet or less range which is typical of this type of trip. As you can see, Doubles are not just for Technical Divers anymore but can substantially increase a recreational diver’s bottom time and safety.

Double Aluminum 80’s vs. Steel 100’s
For those thinking that a steel 100 is a better choice, you must consider that while the steel 100 would allow a diver to do the 20 minute bottom time, it does not have enough volume for a diver to take advantage of the 30 minutes (per decompression limit) bottom time available. Double 80’s would provide enough volume. Further, the cost to purchase 2 steel 100’s is approximately the same as the amount to purchase a complete set of double aluminum 80’s with a Halcyon Manifold and Bands. Manifold doubles provide greater stability and redundancy than a single tank and steel has potentially more maintenance concerns. Finally, when traveling you can easily pack your manifold and bands and set them up on aluminum tanks which are way more widely available then steel 100’s in most locations.