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Oriskany Gas Party March 30, 2009

Posted by Chris Sullivan in Technical Diving.
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On Sunday some of us got together at the shop to fill our tanks for diving the “Mighty O” 3 weeks from now. We had given some thought to using a light Nitrox mixture for our bottom gas but decided against it so we could have the option of going to the hangar deck, lying at 165-170 feet,  on our first dive. It didn’t seem worth it. Our PPO2 (partial pressure of Oxygen) at 170 would be 1.29 with air, which is a good conservative figure to work with.

We decided on EAN50 for our deco gas for a couple of reasons. The charter operator charges an extra $30 for a second deco cylinder, so it didn’t seem worth it to carry one for a richer mix. Our dive shop doesn’t have a booster pump (although these are starting to become more accessible with less expensive models coming on the market), and the large O2 cylinders that we get from the supplier are only at about 2200 PSI or so, making it very difficult to fill a rich mix, especially in an Aluminum cylinder with a working pressure of 3000 PSI. Finally a leaner mix has an advantage in an emergency, as it can be safely breathed much deeper than pure Oxygen or EAN80.

On this last point, as everyone learns in their Nitrox course, breathing high partial pressures of Oxygen can cause convulsions, which for divers without full face masks usually leads to drowning. We learn in basic Nitrox training never use more than 1.4 atmospheres partial pressure of O2 during the working phase of the dive, and 1.6 atmospheres during decompression. The toxic effects of Oxygen don’t turn on like a light switch, however. Experiments and accident data reveal that CNS Toxicity (Central Nervous System – the kind that leads to convulsions) is a function of both time and O2 concentration, plus several other factors like temperature, exertion, and so on.

During  WWII, some diving operations on pure Oxygen rebreathers had maximum depth limits of up to 90 feet, or a PPO2 of almost 4! The rate of O2 toxicity was shocking, of course, but it was also not a certainty that it would occur. Over time, depth limits were reduced, and time was introduced… so many minutes allowed at 60 feet, a greater number of minutes at 30 feet, etc. The bottom line (for me, don’t take this as advice) is that I think breathing EAN50 at even 170 feet, a PPO2 of 3, on a controlled emergency ascent probably won’t be fatal and would be preferable to drowning. A 60 fpm ascent would bring the mix to 2.2 within 60 seconds, and to 1.4 within another minute. That’s very likely to be survivable.

Pure Oxygen, on the other hand, would have a PP02 of more than 6 at 170 feet. That would be extremely dangerous, and an extra minute would be required to get to 1.6. You would also have to skip most of the deeper decompression stops to get yourself out of dangers. On EAN50, there would be few or no stops missed.

I will be using my two AL80 cylinders for decompression – one at a time for each day’s diving. These are filled to their standard pressure of 3000 PSI by first filling them with 1101 PSI of O2, then topping off with air. Of the 3 large cylinders of O2 at the shop, the first had been drained to about 600 PSI by time we got there. We had 3 cylinders to fill, my two plus another, so first we used the partially drained cylinder to fill each of the 3 to 500PSI or so, then used a full one to top them up to 1101. This is exactly the same procedure we use for the 6 air cylinders at the shop – using them in sequence so that high pressure gas is available for as long as possible.

In case you’re not familiar with gas blending, we arrive at 1101 PSI by looking at a chart on the wall. That’s the easy way. There are computer programs which will also do the calculations. The math for Oxgyen plus Air blends is as follows: If F is the desired total pressure of the cylinder, P is the desired partial pressure of Oxygen, and X is the pressure that you need to fill the cylinder with Oxygen before topping off with air (to the pressure F), then X = F(P-.21)/.79.

For our back gas, we’re using a pair of Faber 95 cubic foot cylinders. These cylinders have a working pressure of 2640, and we’re planning to fill to 3200 PSI, giving us 95 x 2 x 3200 / 2640 or 230 cubic feet of air. At my SAC rate this would give me about 35 minutes at 170 feet with thirds. Our dive plan will more likely be multilevel, but we’re still discussing exactly what we want to do. Likely we will drop through to the hangar deck and exit at the stern, then drift back along the deck with the current to the tower, exploring the tower at shallower depths until we need to ascend for deco.



1. rickheil - March 31, 2009

Have a great dive on the MIght ‘O’.
Good write up – I eagerly look forward to your trip report 🙂

I’m eager to give it a try probably later this year.

We’re off to do some dives locally in Lake Mead in a few weeks (a little colder than the Gulf).
Enjoy and be Safe!


deepstop - March 31, 2009

Thanks. There’ll definitely be several trip reports. The motel we’re staying at has free internet service so I’ll be able to write about it right after the dive. I’m debating taking my camera down there. It’s only rated to 130′ so I’d have to leave it somewhere on the tower before going lower. My other more likely option is to take a Nikonos but I’ll have to wait to get the film developed.


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