I was reading another post that got me thinking about the use of a pony. I dive mostly wrecks off the NC coast with a max depth of 120-130. I dive a single ST120. I have heard alot of people say below 100 they will use a pony. When do you use it? I know doubling up might be preferable, but I am not looking to take that avenue just yet. Do you feel comfortable/safe diving to 120/130 in a recreational (no deco) setting without a pony assuming you are respecting rock bottom?
One of them is the size of your single cylinder.
If you are diving a steel E8-130 pumped to 4550 psi, that is one situation. If you are diving an aluminum 80, thats another.
A second is calculating how much gas you might need in the event that something goes wrong on one of those 130 fsw dives about 3 hours out of Morehead City.
A third is the characteristics of your "dives off the NC coast" - e.g. whether you're diving off a commercial charter boat or not. If you are, the boat is likely anchored to the wreck (e.g. not a live boat). Moreover, you need to be on that upline when the time comes to ascend (currents, shipping lanes 40-70 miles offshore, etc.).
That is a different situation from one where you can basically ascend from any point, at any time you wish. You need to return to a specific location before beginning your ascent.
Many divers who absolutely must return to a known location before ascending will dive thirds, or some other conservative gas plan that builds in a contingency to respond to unplanned events. The problem becomes that, at depths of 120'-130' 70 miles offshore, there is insufficient volume in an aluminum 80 to safely plan even NDL dives. Once you back out 'rock bottom', there isn't enough gas left to do the dive.
(If you're using aluminum 80s) you basically have two options to boost available volume; go with doubles or sling another tank. Most guys would go with doubles. But you don't want to go that route yet. So at the moment you're looking at sling tanks.
Given the depths you're diving to, slinging an 80 would not be unheard of, and it gives you a 2nd 80 when the time comes to double them up. Example - do the first (deeper) dive using your sling tank as a stage. Breath from the slung 80 first, then switch to backgas as needed. If you don't use backgas, then that tank is available for the 2nd (shallower) dive of the day. If you don't want to sling an 80, fine, sling a 40 or even a 63. You can use it when you get into deco diving with a deco mix.
I suggest you read Bob's sticky on gas management planning. Its a good place to start. Bottom line is that diving to 130' on a single aluminum 80 is unwise. Your margins for error are nonexistent, and 70 miles offshore is not the place to be living life on the edge. If those are the dives you're doing, then first work on doing the math that tells you how much gas you're going to need - THEN worry about how you're going to carry it. Starting by considering different tanks before you know what amount of gas you need to carry is sort of putting the cart before the horse.
Best,
Doc
