There’s been some discussion recently on how to trim out in a sidemount rig. The argument has long been whether to trim out using air in your wing or to trim out using weights placed somewhere on your rig, usually around the shoulders. Many of the commercially manufactured sidemount rigs are designed with this in mind, in fact. The original Dive Rite Nomad was designed so the bungees would trap air in the bottom area of the wing to create more lift closer to the feet. The Oxycheq Recon and Hollis SMS100 were designed with 360 wings and the largest area of the wing at the bottom so air would migrate to that area and provide more lift closer to the feet. While this is a good idea in concept, it could lead to a huge mess should the wing fail.
If a wing that is designed to keep the diver in trim should fail, then the trim of the diver will also fail to happen. With no air in the wing, the diver will revert to the position created by the center of gravity of the rig, which is usually lower along the main mass of the body. What this means is without air in the wing the diver will be in a feet down orientation. This may not matter much in open water, but it can make a big difference in a cave, especially a silty one! I’ve experience a couple of wing failures, both in very silty cave passages. Had I depended on my wing for my trim I would have left a silty mess behind me, and in some areas of the cave, left quite a few marks on the floor since the floor to ceiling height would not have accommodated me out of trim without touching something.
I’ve trained and mentored a lot of divers in sidemount diving. I start all of them out without any trim weight at all. We get in the water and I have them get neutral and relaxed. A majority of them trim out at about a 30-45 degree angle with their feet down. This occurs after I have already positioned their cylinders so they are as far forward under the armpits as possible. Any farther forward and the valves and first stages would have to go in front of the shoulders and throw the trim of the cylinders off. Sure, we can try to direct the air in the wing toward the feet to bring them up. This works if the wing isn’t already over half inflated to compensate for the weight of the cylinders (typical in wet suit divers). This also works if the wing is working properly (read hasn’t failed).
My recommendation is to place 1-3 pounds somewhere around the shoulders to bring the diver into a horizontal position. Trim weights don’t fail. No matter how much air is in or isn’t in the wing, the diver will remain in horizontal trim. If the diver is diving dry the dry suit becomes the buoyancy compensator and gets him/her neutral. If the diver is diving wet, then he/she better have a dual bladder rig or an SMB that can be used to get neutrally buoyant (properly place trim will be unaffected). But if the diver is depending on air in that wing to remain neutrally buoyant, a wing failure will likely lead to an out of trim diver.
Most divers need some weight to compensate for the gas being breathed down from the cylinders anyway. The gas being breathed weighs one pound for every 400-600 psi (depending on the cylinder size). A typical cave dive will result in the use of about 2000 psi, which equals 3.5-5 pounds per cylinder for a total of 7-10 pounds of lost “ballast”. But there’s still another 3-4 pounds per cylinder (6-8 pounds total) worth of gas that could be lost or used in emergency situations. Just like you learned in your open water class, you are supposed to be neutrally buoyant with an empty cylinder. We have learned to check for proper weighting at the end of the dive with 500 psi in the cylinder and then add 1 pound to compensate for that 500 psi of gas. This doesn’t change in technical or cave diving. We still need to be neutrally buoyant with empty cylinders. Many divers don’t think about this and are probably positive with empty cylinders, even if only by a few pounds. So adding trim weights will not only trim these divers out but also allow for them to be neutral should they end up still underwater with very little gas left in their cylinders.
So whatever method you decide to use to trim out, at least be aware of the possible consequences. If you’re strictly an open water diver, then it’s not going to matter much either way. But if you do any cave diving at all, then you need to consider what can happen to your trim if your wing fails. While you may be able to get out of the cave, it may not be without doing damage to the cave. So, if nothing else, keep cave conservation in mind.