Why do I take a Buddy Bottle?

A friend of mine, who happens to be a newly minted Cave Diver and an experienced north east wreck diver, asked me the following:

I was wondering about your reference to a "buddy bottle". Are you diving doubles backmount and bringing and extra bottle for tertiaryredundancy?

The answer is, "Yes". I have a couple of reasons for this decision. But before we get into the reasons, let me tell you about my diving. I dive 3-5 days a week. Normally, all but one of those days I am alone. I dive CCR, side mount and backmount, so I am very familiar/practiced with carrying a sling/sidemount bottle already. Plus, I did a bunch of diving in NJ/NY that required carrying a stage bottle with a set of doubles.

I carry a buddy bottle when I am in backmount doubles because:

• It makes me feel more comfortable.
• I like to have the extra gas
• I consider the isolator valve a potential failure point.
• And it makes good practice for when I am staging.
• It isn't an inconvenience for me.

In the past, I used to cave dive without a buddy bottle. I considered doubles as redundant and safe enough for me. Then I started reading and listening to other OLD divers. And I decided I wanted to be an old diver. When I got to Mexico, I decided that with the frequency of my diving, the chance there could be a complete gas loss at the worst possible time, would increase with the increase in the total time underwater. So I committed to myself to carry a buddy bottle when in doubles. I have to carry the same bottle(s) when I am on the CCR, so what is the difference?

My risk assessment indicated to me, that I should carry the buddy bottle as a hedge against total gas lost. I don't want to drown because I broke the stem of my isolator valve.

It should also be considered, that you will suffer your uncontrolled gas loss while in a nasty restriction. The gas will rush out of the cylinders while you try to squirm out. You will not be able to get your hands up to close the isolator, because you are in a nasty restriction. Unfortunately, you will not suffer your gas loss some place convenient like a wide open cave, where you can reach the valve. It will happen at the worst possible moment, when you will be seriously delayed in doing anything about it. In comes the buddy bottle with the gas you need to exit in a sane and controlled manner.