How deep are you willing to go?

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One of the benefits of a Deep Diving class (recreational - up to 130ft) is that is teaches gas planning and concepts like Rock Bottom. I recommend. It's one of the continuing ed classes that can be well worth it if you have a good instructor. (Unlike "Boat Diving" -- how is that a class? But I digress.)
 
The most important criteria for any dive is that whatever size cylinder you use and whatever your turn around pressure, both the size and the remaining volume of gas will have to be enough to get you and your buddy safely home from whatever depth or distance from safety you are at any given moment.

There are two schools of thought on emergency planning:

1. The first school of thought is that an emergency is an EMERGENCY and that it will be a high stress situation. Divers under stress will be able to recall the proper procedures from their training and put them into play when something goes wrong. As long as the divers make it back to the boat alive, somehow, it was handled successfully. This is the most common recreational diving philosophy.

Low gas reserves, depth, and lack of training or practice often aggravate the situation and stress cam quickly move to panic. The divers are psychologically unprepared, surprised when it happens, and may be confused and scared regarding how to sort it out. Open water ascents can become rapid, uncontroled ascents in such a situation and may lead to DCS or AGE and death.

2. The second school of thought is that while an emergency is serious and possibly life-threatening, there is no reason why we can't manage the situation calmly, slowly, and confidently. There is no reason why we can't take time on the bottom or at max penetration to sort out the problem, take a few deep breaths, think things through, then act accordingly. There is no reason why we can't swim home or ascend slowly at the proper rate, making all safety or decompression stops along the way. There is no reason why we cannot be in total control of the situation, the dive, and our personal skills due to practice. This is the most common technical diving philosophy.

The recreational diving market is one of inclusion. Most agencies believe everyone should be a diver and have access to the underwater world. I agree. In an old Sea Hunt, Mike Nelson teaches a woman who is in a wheelchair following a climbing accident to dive as water therapy. Even in the early days of the sport, with tougher training, thoughts went to allowing everyone a chance to experience scuba diving. However, the recreational marketplace is one in which the idea of reducing standards and making diving easier will be the way to make more money. Such an approach cheats a student out of true education and causes confusion over simple things such as how much gas do I need?

The technical diving market was once highly exclusive, The places divers can go and the depths they can reach have a long record of killing even recreational instructors who attempt them. Now, thanks to the experience of those who paved the way into the extreme environments, technical diving has become safer and is much more inclusive. But, standards are extremely high in some courses and generally higher than average in many. Training is more thorough and better prepares a diver more an emergency. Instructors can be tough on students because technical diving isn't the "cash cow" that recreational diving is at the moment. That will change. The quest for profit will further reduce quality and instructors will be swayed away from being like drill sergeants and more like Disney employees. Let's hope that change is somehow retarded by a diving public hungry for education, training and talent.

At the highest level, gas management can be figured out in one's head "on the fly" at any given depth. If I'm at 130 feet with 2000 psi in my tanks, I know that I have 100 cubic feet and 20 minutes of gas that my buddy and I have to share. That wopuld give me 10 more minutes before I'd have to begin ascent, move toward a gas switch, or get shallower and recalculate gas needs. To learn this skill, I paid $1500.00 for the course, my half of the instructor trainer's expenses, expensive helium fills, and worked my butt off for 7 intense and extremel;y stressful days. But, the pay off was that my diving is so much easier at the technical level.

Normally, to answer the question posed by the OP, I'd be most comfortable doing a 200 foot dive with twin AL80's on my back, using an AL80 bottom stage, and using an AL80 and AL40 deco bottle or two AL40's. I'd use the bottom stage for the dive, then switch to my deco gases. This would leave my backgas untouched for emergencies such as an OOG diver or loss of deco gas.

For the new diver who may want some gas management guidelines, a "no brainer" rule of thumb for a single 80 is to take whatever depth you are at and add a zero, and that is your reserve for you alone then X 2 for another diver. At 130 feet, a team needs to leave the bottom with 2600 psi. That's not much time, so they slow at 100 feet, check their gas supply, and stay until they reach 2000 psi. They move up to 80 feet where they remain until one reaches 1600 psi. At 50 feet, they can stay until 1000 psi. They move to the safety stop at 15 feet and absolute minimum gas is 500 psi - unless the computer is telling them that they need to remain. If you accidentally have deco, but have gas, please deco, by all means. This is slightly conservative and assumes an average RMV of .75 cubic feet per minute with an AL80.

There are more effective ways to manage gas consumption and cylinder needs, but if you really are the type of diver who wants the computer to manage the time, this might manage your gas suyply a little more safely than guessing.

If a team has plenty of gas in an emergency, they'll have plenty of time. If a team has plenty of time, then they'll be able to fix the problem slowly and not rush and make mistakes. If you can fix an emergency slowly, it can be done smoothly and much more efficientl;y than if you are fumbling and making mistakes.

Slow is smooth. Smooth is fast.

Time is gas. Gas is life. The more time and gas you have, the better your chances of keeping your life.
 
Depends. This past Saturday I went to 125ft. I was wearing double 80s with over half a fill. For added safety I carried a 40cuft pony. So I did the dive with approx 125cuft. 50% more than a AL80.
 
How deep are you willing to go on a specific size tank?

For example, on an Al 80 would you dive to 115 ft?

On an AL80, I won't go deeper than 100 feet, and I'll only go that deep with a reliable buddy beside me.

On my double LP95's, with my current training and experience, I'd probably go to around 160-170 feet or so.
 

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