A balanced rig is one that, when empty is a near neutral as possible, and when full you can swim to the surface.
We need to be able to swim a FULL rig to the surface without any air in the BC.....at 15ft there should be little to no air in the BC. (ie being properly weighted)
BTW, there has been plenty of talk about ditching weights and why it isn't stressed more......well lets go to this. Why isn't proper weighting taught.....and why aren't people taught that they need to change their weighting in different environments, gear, etc.
I've seen sooooooo many new divers over-weighted its ridiculous.....and no need for it.......that seems like a good starting point for me.
So lets take this one step further.
A 7mm wetsuit will lose about 10lbs of buoyancy in around 70-80ft of water. That is now 10lbs of just dead weight you need to be able to swim up which can be very difficult if you had a complete wing failure at the beginning of a dive......now if you ditch that weight, as you come to the surface, your wetsuit will expand and gain back those lbs of buoyancy essentially turning you into a meat missile as you get close to the surface. Not a desirable situation. A drysuit, IMO is better in this scenario.
If you look at dives where a thinner wetsuit can be used, doesn't compress as much at depth so you don't lose that buoyancy like in the 7mm because you didnt have it in the first place. So if you wear an alu tank that is positively buoyant at the end of the dive, you need a few lbs to offset that. If you dive steel that ends negatively buoyant, you may not need any weight. So now here comes the important part....with the alu, at the very beginning of the dive, if you have an issue, you can drop a weight and away you go....if you have steel then you have no ditchable weight then you MUST be able to swim the entire rig up. In this case, Alu tank may be a better choice unless you have the ability to swim the steel to the surface with no air in the BC.
At the end of the day, if you can swim your rig to the surface with full tanks and all the weight still on you, then your rig is balanced. If its too heavy, you better have enough ditchable weight to make it swimmable to the surface.
For what its worth, I have no ditchable weight on my sidemount rig. I wear a drysuit with undies, and 2 hp100 tanks. I can swim this rig from 100ft to the surface with an empty wing and squeezed drysuit.
If I am diving it in a thin wetsuit, I use only alu tanks....so again, I can swim it to the surface with an empty wing.
I have had wing failures where I needed to be able to do this.
In a drysuit, you should be wearing enough lead that you can descend with an empty suit. Ergo, your net buoyancy should be near 0 (ie drysuit with undies is +7, lead is -7)
Then comes the rig, enough lead, that with a near empty tank, and empty wing, it is neutral.
At this point, you are weighted properly and all the BC needs to do is be able to compensate for the change in buoyancy of gas in the tank from the first of the dive to the end of the dive, and the change in buoyancy of your exposure protection.
If you did all this, you should never have a bc so full that you also need the DS to have air in to to compensate for all weight you have on you. if you do, you are grossly overweighted.
