Why a backup reg?
Normally we breath from our primary regulator, our longhose. During decompression we use one of our deco bottles and when diving stage bottles we breath from them. When the longhose is not used it is always clipped of with an attached boltsnap to our right chest D-ring.
If we have a problem we donate the only regulator we KNOW is working - the one we are breathing. That way we also know that we give away something that is safe to breath at that depth - important when you use multiple gases.
After donating our regulator we need something else to breath - our backup regulator. Since our backup is what we are going to use in an emergency we need to find it fast and we need to be sure that it is going to be there. Let's see how we can do that.
The backup second stage hangs around our neck like a necklace. The necklace is made of elastic bungee, aka shock cord or surgical tubing. Surgical tubing is smoother but will deteriorate and dry rot after a couple of years. Bungee however will hold for many years and can be found in most hardware stores or marine stores. If you want it in black you may have to lock around though.
Make the loop the length that you can just reach the regulator with your mouth while wearing it. That way it's going hang quite close to you but not so close that you can't look down while diving.
The cord or surgical tubing is put under the same tie wrap as the mouthpiece on the second stage. If you use bungee then make a knot when you have adjusted it so it can't be pulled through. With surgical tubing that is not necessary.
Let's say you get something wrapped around your backup regulator and you keep swimming without noticing. If your backup reg was not securly attached it would be pulled out and if you needed it it would not hang around your neck anymore.
Some people use a loop that is put around the mouthpiece. As you can understand from above that is not a good idea. I actually used to have it rigged like that when I started tech diving because that was how I was taught. After having had the backup coming undone twice so that I could not find it when I needed it I stopped using the loop.With the bungee secured under the tie wrap the bungee will stretch it is impossible to swim hard enough to pull it off. If you have any doubt I suggest you try and pull it off on land so you realize how much force is needed.
Picking a suitable second stage
Since the backup will hang upside down it will be prone to freeflow. Actually most second stages are suppose to be adjusted when serviced so that they freeflow in this position. To solve that problem you have three options:
Option number three is what I prefer. Now I can detune the second stage underwater and if I need to breath it for an extended amount of time I can tune it up again.
In the pictures above the cracking pressure adjustment is the metal knob to the left (Apeks TX50). The following second stages currently in production (2006) have this adjustment; Apeks TX50/100/200, Apeks ATX50/100/200, Apeks XTX50/100/200, Scubapro G250, Scubapro S600.
Look here for details about proper hose length and how to route the hose. Apeks and some other manufacturers put hose protectors near the second stage. Cut them off as they will only stop the hose from doing a nice soft bend.
When you decide on the proper length for the bungee don't forget that wearing a hood will make it shorter.
If you like something fun to do, you can practice getting the backup reg in your mouth without using your hands :)
With this setup we know where our backup is and we are going to be 100% sure that it is going to be there when we need it. What more can we possible ask for?