Think back to your Open Water class. Did your instructor tell you anything about how to determine how much weight you should wear on your weight belt? I hope so, because proper weighting is something each diver needs to know how to do for himself. Most certified divers remember their instructors saying something about how to properly weight themselves. The irony is, what they were told - or at least what they remember - is often wrong.
The most common problem we see among divers certified at other shops is that they are overweighted. When we ask them why they're wearing so much weight, almost invariably, the response comes back, "My instructor told me I need a weight belt equal to 10% of my body weight?" While 10% of body weight might be a good rule of thumb under certain circumstances, it must not be thought of as an inflexible rule. The reality is, the only way to know how much weight you need it to perform a buoyancy check. I'll discuss how to do this later. Right now, the point I want to drive home is that the amount a ballast a diver should wear depends on whether he's diving in fresh or salt water, his body composition, the thickness of exposure suit he's wearing and his level of experience. Let's have a look at these factors one by one.
Salt water vs fresh water
Salt water is more dense than fresh water, owing to all the salt that's dissolved in it. That means that an object which is neutrally buoyant in fresh water will float in salt water. The difference, as far as a weight belt is concerned, can be significant. A 160 lb person neutrally buoyant in fresh water is going to need 4 lb or more to be neutrally buoyant in salt water. That, alone, is a difference of 2.5% of the divers bodyweight.
Body composition
Muscle is more dense than fat, meaning muscle sinks and fat floats. If you're a 200 lb body builder, you're almost certainly going to need less ballast than a 200 lb couch potato. In fact, the body builder might not need much weight at all if he's wearing a light wetsuit. Telling him to wear 10% of his bodyweight while wearing a 3mm shorty will make him as heavy as a rock, reducing his efficiency in the water and possibly putting him in danger.
Exposure Suit
The wetsuits we use to keep us warm in the water have buoyancy. The thicker the wetsuit, the more buoyancy it will add to the diver. This is why divers in cold water generally wear more weight than in warm water. This is one of the biggest factors in proper weighting. Diving here in Okinawa, you'll use less weight in the summer when you're wearing a 3mm shorty, compared to the winter, in which you're more likely wearing a 5mm full-length with a vest and hood. We're talking about a difference of 6-7 lbs in required ballast between those two exposure systems.
Experience
Newer divers, often without realizing it, are reluctant to let out their breath all the way. This is because their survival instincts are still telling them "You can't breathe under water iditot!" Divers with more experience, on the other hand, tend to need less weight to get down and stay there. This is because they're a lot more comfortable in the water and don't feel the need to hold in their breath. When its time to sink, they push the air out of their lungs with confidence and down they go. So, the more you dive, the less weight you'll need.
So, to recap, there's no single rule by which you can calculate the amount of weight any give diver needs. But there is a relatively foolproof method. Wear your full scuba kit, fill up your BC and swim out to water that's a bit too deep to stand up in. Hold your breath and let all the air out of your BC. With your breath still in, note where you're floating in the water. If you are floating higher than eye level, you'll want to add more weight. If you're floating below eye level or sinking, you need to take some weight off. Ideally, you should float with the surface exactly at eye level on a full breath and empty BC, then slowly sink when you let out the breath. If that's not what happens, it's time to adjust your belt.
The most common problem we see among divers certified at other shops is that they are overweighted. When we ask them why they're wearing so much weight, almost invariably, the response comes back, "My instructor told me I need a weight belt equal to 10% of my body weight?" While 10% of body weight might be a good rule of thumb under certain circumstances, it must not be thought of as an inflexible rule. The reality is, the only way to know how much weight you need it to perform a buoyancy check. I'll discuss how to do this later. Right now, the point I want to drive home is that the amount a ballast a diver should wear depends on whether he's diving in fresh or salt water, his body composition, the thickness of exposure suit he's wearing and his level of experience. Let's have a look at these factors one by one.
Salt water vs fresh water
Salt water is more dense than fresh water, owing to all the salt that's dissolved in it. That means that an object which is neutrally buoyant in fresh water will float in salt water. The difference, as far as a weight belt is concerned, can be significant. A 160 lb person neutrally buoyant in fresh water is going to need 4 lb or more to be neutrally buoyant in salt water. That, alone, is a difference of 2.5% of the divers bodyweight.
Body composition
Muscle is more dense than fat, meaning muscle sinks and fat floats. If you're a 200 lb body builder, you're almost certainly going to need less ballast than a 200 lb couch potato. In fact, the body builder might not need much weight at all if he's wearing a light wetsuit. Telling him to wear 10% of his bodyweight while wearing a 3mm shorty will make him as heavy as a rock, reducing his efficiency in the water and possibly putting him in danger.
Exposure Suit
The wetsuits we use to keep us warm in the water have buoyancy. The thicker the wetsuit, the more buoyancy it will add to the diver. This is why divers in cold water generally wear more weight than in warm water. This is one of the biggest factors in proper weighting. Diving here in Okinawa, you'll use less weight in the summer when you're wearing a 3mm shorty, compared to the winter, in which you're more likely wearing a 5mm full-length with a vest and hood. We're talking about a difference of 6-7 lbs in required ballast between those two exposure systems.
Experience
Newer divers, often without realizing it, are reluctant to let out their breath all the way. This is because their survival instincts are still telling them "You can't breathe under water iditot!" Divers with more experience, on the other hand, tend to need less weight to get down and stay there. This is because they're a lot more comfortable in the water and don't feel the need to hold in their breath. When its time to sink, they push the air out of their lungs with confidence and down they go. So, the more you dive, the less weight you'll need.
So, to recap, there's no single rule by which you can calculate the amount of weight any give diver needs. But there is a relatively foolproof method. Wear your full scuba kit, fill up your BC and swim out to water that's a bit too deep to stand up in. Hold your breath and let all the air out of your BC. With your breath still in, note where you're floating in the water. If you are floating higher than eye level, you'll want to add more weight. If you're floating below eye level or sinking, you need to take some weight off. Ideally, you should float with the surface exactly at eye level on a full breath and empty BC, then slowly sink when you let out the breath. If that's not what happens, it's time to adjust your belt.