At the start of the dive you should be overweight by the same amount as the weight of gas in your cylinders.
Becoming a better diver can come down to fixing one problem, the single biggest issue in scuba diving: overweighting. When you dive, you’re attempting to maintain neutral buoyancy, and yet everything you’re carrying wants to sink, from your regulators and cylinders to the weights in your pockets. That’s all got to balance with what wants to go to the surface, which is basically any gas pocket, from tiny pockets such as your mask, to larger pockets such as your lungs and huge pockets such as the gas trapped inside a drysuit and your BCD.
If you add 5kg (11 pounds) of lead onto a perfectly neutral diver, in order to offset this weight and regain neutral buoyancy they are going to have to add the same amount of lift somewhere, which equals approximately 5 liters of gas. Thus, the more over-weighted a diver, the more gas they have to manage to keep themselves neutrally buoyant.
On the bottom this is more of an inconvenience than a problem. Five liters of extra gas in a BCD or drysuit is an annoyance that will make the diver feel unstable. If the extra gas is in a drysuit, a diver will feel particularly unstable when trying to trim flat in the water. One of the most common signs instructors see of over-weighting is that divers seem to be fussing in the water, as if they are fighting to maintain buoyancy control. Almost without exception, overweighting is the cause. Remove the excess weight, and the diver looks at you like you’ve just performed some kind of mystic sorcery. In reality, it’s a quick win for an observant instructor. A correctly weighted diver will rapidly calm down and find everything easier.
This hypothetical 5 liters of gas becomes an issue, however, when the diver is changing depth. I’m sure everyone remembers his physics lecture from his very first scuba diving training course? Were talking about Boyle’s Law. If we remember that the volume of a gas will increase as the pressure decreases then all of a sudden that 5 liters becomes a potentially dangerous side effect of overweighting. By adding extra weight and then having to offset it with more gas, the diver has actually created a higher risk of a rapid ascent.
If a student takes a long time to settle down after a small change in depth, such as swimming over part of a wreck, it’s more likely than not a problem related to weighting rather than a skills issue, especially if he or she gets it under control after a while.
WHERE DOES INCORRECT WEIGHTING COME FROM?
Sometimes it comes from poor initial instruction. I’ve heard instructors tell their students to relax because they are going to nail them to the bottom with 20kg (44 pounds) of lead. Simultaneously, a fear of ascending out of control is instilled in the students. The end result is a student who believes the way to avoid a rapid ascent is to add more weight. Now let’s introduce that diver to his first sea dives. How much weight does he add? Well, he knows he has to add “about 3kg,” (7 pounds), but he’s a nervous new diver, so he adds a little more lead just to make sure. Now he could be a few pounds overweight, and this is presuming his instructor taught him to weight himself carefully in the first place.
Now, don’t get me wrong; I’ve also seen a course taught with a huge focus on nailing buoyancy skills, and on the importance of not over-weighting. Overweighting is not an agency issue; it’s a dive industry issue. Sometimes the instructor has nothing to do with it. Sometimes it’s the diver himself, who doesn’t know how changing conditions affect his weighting needs, or perhaps he dives infrequently and cannot remember. When you boil all of this down to its core, you find it’s caused by fear of an out-of-control-ascent, which is ironically being made more likely by not understanding the underlying physics.
SO WHAT IS CORRECT WEIGHTING?
Correct weighting is all about balance. In a perfect world, a diver with 1 bar of gas (14.5 psi) remaining in his cylinder and absolutely no gas in his BCD will float neutrally buoyant at 10 feet (3 meters). This means if you had to breathe all of your gas down you could still hold a stop. So, just for clarification, you are aiming to be perfectly neutrally buoyant with almost no gas anywhere. You are in equilibrium. Think about that. This means at the start of the dive you should be overweight by the same amount as the weight of gas in your cylinders.
How do we work this out? Try this exercise. Get into the water and bring a friend — this is critical. You’re going to breathe your reg down to the minimum and there’s no guarantee that it won’t stop working, leaving you with no source of buoyancy and nothing to breathe. So again, bring a friend.
Drain your cylinders down to 30 bar (435 psi).
Descend to 10 feet (3 meters). Vertically. If wearing a drysuit, ensure that your auto-dump is completely open. Let the pressure squeeze all of the gas out of your suit. Now ascend, continuing to work the gas out of your suit. You want it to really squeeze you so there is minimum gas in the suit.
Fully inflate your BCD to keep you on the surface.
Now breathe the reg down to 20 bar (290 psi) or so. This is why you need a friend with you.
Now you’re ready to do the weight check.
When you are doing this, avoid waving your arms about, and avoid using your fins. I have seen countless people keep themselves on the surface with a nervous wiggle of the feet.
Cross your legs to stop yourself from finning.
Lift the BCD high and start dumping.
As it dumps take a deep breathe in and hold it in.
Keep the dump up and when it finishes dumping make a forceful breathe aggressively out and hold it out.
See what happens. Stop wiggling your feet and don’t breathe in.
If you are correctly weighted, you should just, and I really do mean just, be able to descend. By the time you get to 10 feet, holding a stop and breathing normally will be easy. Most people sink too quickly. A lot of people sink like a stone because they’re over-weighted. If this happens, simply remove weight and repeat. You will be astonished at how much easier everything becomes following this exercise.
Is that the perfect way to check your weighting?
It’s pretty good, as it has the benefit of checking what happens at the end of a dive when you would potentially be very low on gas. It’s not perfect, though. For one thing, we don’t actually want to be perfectly weighted — who wants to squeeze out every last bit of gas in order to hold a stop and freeze? I would rather have the flexibility to be able to add a bit of gas in the end to keep myself warm. So I do the above check and then add 2 pounds (1kg). It’s important that you keep doing this exercise because weighting requirements change over time. Peoples’ breathing patterns change over time as they become more relaxed; a drysuit’s undergarments compress over time and their buoyancy characteristics change. People also continually change shape and their weighting requirements change accordingly, so do this exercise as frequently as possible, both inland and in the sea, and you will find things like buoyancy control, trim, gas usage and stress levels suddenly seem to be less of an issue.