I just got back from a trip to Mexico where I was taking a sidemount class with Roger Williams at XOC-Ha. It was a great trip – I learned a lot and saw some very cool cave.
I used aluminum 80 cylinders for all my dives in Mexico, but now that I’m back home I need to make a few changes to dive with my steel HP100 cylinders. This includes getting my weight dialed in.
The most common way of figuring out weighting is to start with a guess of how much weight is needed, then tweak the amount and location through trial and error. Experienced divers and dive professionals get pretty good at guessing how much weight a diver needs by looking at their body, exposure suit, tanks, etc. You can also get an estimate of how much weight you need with the eye level test. However, going from a first estimate to perfect weighting can be a frustrating experience, especially since weighting can be counterintuitive. And since I’m new to sidemount, I don’t even have a good sense of where to start.
Instead of trying to set up my new sidemount rig through trial and error, I figured this would be a good time to go back to First Principles and do what I call a Gold Standard weight check.
The Gold Standard weight check has three principles:
- The diver should have just enough weight to be neutrally buoyant with empty tanks and an empty wing. This is important because it keeps the amount of gas in the wing to a minimum. The more gas there is in the wing, the more the wing expands or contracts with depth changes.
When a diver has extra gas in their wing to compensate for being overweighted, they need to add or dump more gas when they change depth, and be more careful with their breathing just to maintain depth. - The diver’s weights should be positioned so that the diver can easily hold horizontal trim with with an empty tanks and an empty wing.
- The diver’s wing should be positioned as close as possible to the tanks’ “center of gas”. When the wing is placed correctly, it cancels out the weight of the air in the tanks, so that you have the same perfect horizontal trim with empty tanks, full tanks, and anywhere in between.
Here’s how I do it:
- Start with almost empty tanks and an empty wing. Add weight one pound at a time until you’re just able to submerge by exhaling. The weight doesn’t need to be attached yet – your buddy can add weight to a bag you’re holding. This is the correct amount of weight for you with this combination of tanks, exposure suit, etc. (If your tanks aren’t almost empty for this step, add weight to compensate for the weight that will be lost if you breath them empty. Air weights 0.075 pounds per cubic foot. For example, if you have 500psi in an AL80, you need (500psi / 3000psi) * 80cf * 0.075lb/cf = 1lb of additional weight.)
If you’re able to submerge with no weights, it means you’re already overweighted. Switch to aluminum tanks. - Figure out where the weight needs to be along your body to put you in horizontal trim. You can make this step a little easier by holding some or all of the weights to your chest and moving them up or down until you find the position that puts you in trim.
- Now that weight amount and position is figured out, the only thing left to adjust is wing position. Switch to full tanks, inflate your wing enough to be neutrally buoyant, and experiment with moving the wing up or down to find the position that puts you in trim. DON’T TOUCH THE WEIGHTS! They’re already where they need to be.
Because this method is based on basic principles, it works with any gear configuration: single tank, backmount doubles, or sidemount all work. It also works for all tanks, including tanks notorious for having weird buoyancy like AL80s. This method takes a lot more time than the traditional “float at eye level” weight check, but correct weighting is vital for safe and enjoyable diving, so it’s worth getting it right.
As a shortcut, you can eyeball the tanks’ center of gas for positioning the wing instead of setting up the weights and wing independently. The problem with this shortcut is that if the wing isn’t in exactly the right place, you’ll have proper trim with full tanks, but come slightly out of trim as the tank empties (or vice versa).
UPDATE: Some readers correctly pointed out that having your wing as close as possible to the center of gas in the tanks might not be right when diving with a thick wetsuit. Wetsuits compress as you descend, and the wing needs to compensate for the change in buoyancy of the wetsuit as well as the tanks in this case. Since the wing can only be in one place while compensating for two things, it may not be possible to have perfect trim at every combination of depth and gas remaining.