Double Ponies?

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GET TRAINING!!!
Trying to learn deco diving on the internet is dangerous and stupid.
 
Before every single gas switch, I visually verify the MOD...although I only carry one deco gas right now, it instills good habits for when I start carrying two deco gases.

If your deco gas is back mounted, how are you going to visually verify that you have the appropriate gas? If you have a free flow of the deco regulator, how are you going to "feather" the valve?

And finally, do you have any deco training? I don't mean to flame you, because odds are, you're a lot more experienced of a diver than I am, but there's a good reason to take the training that's readily available. I thought I knew what I was doing when it came to decompression diving...until I took the course! There were a lot of things that I didn't know that had the potential to really hurt me.
 
Most deco divers carry 40CF stage bottles, or even AL80 stage bottles. I would assume you know this based on the training you SHOULD have to do deco dives.

If you are deco diving without training, I'd suggest you find some trained divers to help you. Unfortunately most believe in training, so maybe this is not easy to do. But you NEED to understand the training behind deco diving. This is not 1970, and training, while not cheap, is something IMO you should have assuming you do not. OTOH, if you had the training, you would know the answers to these questions?
 
My friend, just reminding once more the wise recommendations above... NEVER go on decompression dives without proper training, just some issues...

1 - If you use multiple gases with nitrox for deco and mix up the gases, you can get O2 intoxicated and drawn

2 - Get doubles

3 - EAN80? In the training they will teach you best gases for deco, usually EAN50 and/or O2 (I am not sure if any agency recommend EAN80), if using only one besides back gas, better EAN50, you can start your deco with a better gas for that purpose deeper, maximizing your oxygen window and saving some deco time

4 - Also in the training you will learn to carry the stage bottles, like someone said before, how to do it depends on the training, my preference is to carry both on the left side (I use full DIR configuration)

5 - Once again in the training you will learn to calculate not only decompression times with diferent gases, but the mix to use for your back gas, for a 150 ft dive, you should seriously consider using helium (trimix), for reference, I start with trimix below 30 ~ 35 mt (around 100 ft)
 
Yes, in 2005 I took the TDI Advanced Nitrox and the TDI Advanced Decompression classes. I have several hundred dives under my belt, in all sort of conditions from no viz, high current, cold, hot, rivers, lakes, oceans, pools, cenotes...etc etc am very comfortable in the water; ex Navy, love to swim, sail and scuba

The question here was MORE a hardware configuration question, then learning how to dive or training...and what is the norm with configuration and options.
Trust me I have a good handle on risk and acceptable risk. Thus my discomfort with relying on a bottle hanging from a boat that may or may not be there.

Thank you all for your comments.
 
The question you need to answer for yourself is:

How much gas do you need for your deco obligation?

Also... Don't make the dive without carrying enough gas for the dive. Relying on the boat as a gas location could be a error that could get you a chamber ride.

if you for some reason - you can't find the boat, and you've planned on using that 80% to complete your deco portion... would you have enough back gas to complete the deco if you didn't have that 80%?

Carry the gas you need to complete the dive. If that means slinging multiple stage bottles, then so be it.
 
Yes, in 2005 I took the TDI Advanced Nitrox and the TDI Advanced Decompression classes.

If you are still asking questions like these, then I think you need to re-take your Adv/Deco with a better instructor. Diving with a spare air at this depth is a pretty good giveaway that you shouldn't be doing these dives.

I have several hundred dives under my belt, in all sort of conditions from no viz, high current, cold, hot, rivers, lakes, oceans, pools, cenotes...etc etc am very comfortable in the water; ex Navy, love to swim, sail and scuba

Being ex-Navy doesn't make any difference for what you are doing.

Hanging a bottle off the boat, is not uncommon in NE wreck diving and has been suggested to me by others. The advantage is of course 30 lbs less physically on me during the dive...maybe I'm just being paranoid.

Your stages don't 'weigh' much during a dive, and hanging it off a boat or off yourself, you still have the same amount of weight to deal with getting in/out of the boat.

Tom
 
Many deco divers consider anything smaller than an AL40 worthless! :D I don't necessarily agree, but it is not difficult to carry a couple of 40CF tanks in addition to your primary tank.

Howard had it right when he suggested that one needs to carry the air necessary to complete the dive. It makes no sense to get bent over leaving your tanks behind. As you are diving a single, you could even side mount tanks to the primary if you don't want to bother slinging them. Most divers doing your profiles are using doubles, and it is as much about redundancy as it is about gas.
 

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