If this question is too off-topic I can post it as a separate thread, but how do folks define a balanced rig in the context of diving doubles?
A balanced rig is a rig that has been setup as a balance between 2 weighing extremes, these are:
- being so heavy that you cannot ascend anymore from depth.
- being so light that you cannot stay down, keep a specific stop depth.
This weighing is based on 2 factors, one of which can be calculated, the other not and needs to be checked in water.
- The weight of the gas you are breathing, taking with you underwater.
- The weight/buoancy shift of material you take down and can compress (become less buoyant the higher the pressure is and the more it compresses).
GAS weight: This is easy to calculate. First of all my appologies for using metrics and metric tank volumes, Imperial is a bit hard for me and I would probably make mistakes. So air and air related gasses (nitrox) weighs in at about 1.2kg per 1000 barliter. So a full double 12L or 80cuft set will be filled with about 5 to 6kg of air (quite a bit less when it's filled with trimix but I digress). You will breath this set down so you become lighter during the dive. This you'll need to compensate with weight.
MATERIAL that compresses: Typically we are talking about neoprene (a wetsuit or non compressed neoprene drysuit). Since the material loses volume the deeper you go, you will get heavier the deeper you go. How much heavier depends of course on the amount of neoprene. With a 3mm shortie the difference will be negligible, with a double 7mm neoprene wetsuit it could be quite a lot of weight.
If you would combine a big set of tanks (so heavy because of the amount of gas in it) and a double 7mm wetsuit (which loses buoancy when it compresses), you'll end up with an "unbalanced" rig meaning, it will be so heavy at depth that in case of a failure to your bcd/wing chances are you won't be able to swim back up. This is where a compressed neoprene/trilam drysuit comes in the picture (there are other solutions like a double wing, or using an smb but I won't go there now). Since the material doesn't compress it stays neutral (doesn't change weight) during the course of the dive at whatever depth. Next it can act as a 2nd floatation device (if you keep horizontal) in case of problem with your bcd/wing.
Now what is a balanced rig to summarize:
- A rig (not just your leadweight, everything on your rig has weight or is buoyant and will affect your weighing) that will be heavy enough so you can keep a shallow stop at 3m/10ft with almost no gas in it, but at the same time is light enough so in case of an emergency at max depth you can still swim up. If the latter is not the case you'll need to think of some weight that can be dropped (like a weight belt or a big battery of your light).
So typically you want to be as light as possible while able to maintain a stop depth, How do you check this. get in the water with all the equipment you'll use. Empty your set and try to stay at a 3m stop depth with the min amount of weight necessary. Let a buddy check your bcd/wing to make sure there is no gas in it any more, if there is still gas in it , dump it and check if you can lose additional weight.
Next you'll want to check what kind of dives you do in what circumstances and if you'll be able to get up in case of failure to your bcd/wing. In tropical water diving (no or very thin neoprene wetsuit) you'll probably have no issues (unless you are taking an incredible amount of gas with you). In cold water diving, specifically deeper cold water diving (lots of gas) you might want to re-evaluate the use of a wetsuit, or come up with other solutions.
Cheers