For once and for all, lets get this muscle contraction thing right. PLEASE READ BEFORE SPEAKING. (sorry I shouted).
It's NOT about the volume change, although the human body is a lot more compressible than divers are led to believe.
It's about body control. We're all taught that the main factors in buoyancy control are Archimedes' principle and Charles' Law. Not Boyle's Law, because - barring tropical diving - temperature also plays its part if you really want to start nitpicking. And water density of course, which can be a side effect of a temperature change.
Only thing is: these basic principles apply to motionless objects. A diver moves! A diver's equipment changes according to depth because of compression and temperature.
So let's dissect the process of submerging oneself in water:
You're properly weighted, your bc's inflated and you're ready to dive. When you hang vertically in the water, you go down. Change your body position, ie. go into a horizontal position, and you slow down. A buoyancy issue? Hell yes!
The air in your bc changes position as well. Depending on where your weights are positioned you'll need to add or vent air in order to maintain your position in the water column when you go from vertical to horizontal or vice versa. Don't believe me? Try it yourself.
If you drop half a metre and go under a thermocline, the air in your bc will change volume - it'll contract. Water density will change: the water will be denser, giving more lift.
It even gets worse in a dry suit. Since most dry suit divers dive with their dump valves (partly) open, hanging upright causes air to be dumped, not to mention the air that escapes from the neck seal.
Now the muscle thing. A novice diver usually dives overweighted. Having gained experience he/she will drop a considerable amount of weight from their weight belt. Partly because the neoprene of the exposure suit permanently compresses (let's say the suit loses 1 kg of buoyancy), but mostly because the diver in question has mastered two things: airway control and body control.
Airway control is very, very important. Everyone knows this and everyone understands the rationale - I hope.
Body control however is virtually unknown. As I've said before, a tense diver unconsciously moves hands, arms, legs and feet. All these movements combined cause propulsion. They don't cancel out.
By teaching novice divers to clench their abdominal muscles, I teach them to relax their arms and legs. Fortunate side effect is that they don't arch their backs, thus avoiding lower back pains and shifting tanks.
If you don't believe me when I say that this results in better buoyancy control, try this exercise: put on your thickest exposure suit and boots. Jump in the water (no weight belt). See what happens.
If you don't clench your abdominal muscles, you'll have a tendency to float horizontally. If however you relax your arms and legs, and clench your abs, you'll be able to float comfortably at the surface in a vertical position.
I have my open water students do this from day one. It really helps me avoid a lot of problems involving buoyancy. For instance, very very few of my students keel over backwards on descent (a common problem with open water students).
This form of body control teaches you that every movement you make in the water affects your position in the water column. This, combined with proper weight calculation and visualization techniques builds knowledge and by that builds confidence. A knowledgeable diver is worth ten divers who just try things out and see what happens.
Before anyone goes off on the visualization thing, I know it sounds new agey, but it works. And if it works, I'm not going to knock it just because it's a bit fluffy round the edges.
Phew, all this just to say that hydrodynamics is important. Moral of the story, a diver's body is the one piece of diving equipment that is generally overlooked. Controlling your body is controlling your position in the water. And buoyancy control is about controlling your position in the water, isn't it?
As for gear configs and changes thereof, I'm still advocating the adage "know your gear". Having to do the weighting procedure on every change is a pain in the backside.
And FIY: diving conditions in Belgium and in the UK are comparable, so over here, we also face incredible variations in configuration throughout the year.
It's a complex issue, and most changing factors don't have all that much influence on buoyancy, but all these little factors together do count. Which is probably why so many divers take so long to fully master buoyancy. If they ever master it completely, which I doubt. As a dive center owner and instructor I see so many people diving who haven't got a clue. The worst of it is, nobody ever seems to want to listen.
I sincerely hope this clears matters up a little.
It's NOT about the volume change, although the human body is a lot more compressible than divers are led to believe.
It's about body control. We're all taught that the main factors in buoyancy control are Archimedes' principle and Charles' Law. Not Boyle's Law, because - barring tropical diving - temperature also plays its part if you really want to start nitpicking. And water density of course, which can be a side effect of a temperature change.
Only thing is: these basic principles apply to motionless objects. A diver moves! A diver's equipment changes according to depth because of compression and temperature.
So let's dissect the process of submerging oneself in water:
You're properly weighted, your bc's inflated and you're ready to dive. When you hang vertically in the water, you go down. Change your body position, ie. go into a horizontal position, and you slow down. A buoyancy issue? Hell yes!
The air in your bc changes position as well. Depending on where your weights are positioned you'll need to add or vent air in order to maintain your position in the water column when you go from vertical to horizontal or vice versa. Don't believe me? Try it yourself.
If you drop half a metre and go under a thermocline, the air in your bc will change volume - it'll contract. Water density will change: the water will be denser, giving more lift.
It even gets worse in a dry suit. Since most dry suit divers dive with their dump valves (partly) open, hanging upright causes air to be dumped, not to mention the air that escapes from the neck seal.
Now the muscle thing. A novice diver usually dives overweighted. Having gained experience he/she will drop a considerable amount of weight from their weight belt. Partly because the neoprene of the exposure suit permanently compresses (let's say the suit loses 1 kg of buoyancy), but mostly because the diver in question has mastered two things: airway control and body control.
Airway control is very, very important. Everyone knows this and everyone understands the rationale - I hope.
Body control however is virtually unknown. As I've said before, a tense diver unconsciously moves hands, arms, legs and feet. All these movements combined cause propulsion. They don't cancel out.
By teaching novice divers to clench their abdominal muscles, I teach them to relax their arms and legs. Fortunate side effect is that they don't arch their backs, thus avoiding lower back pains and shifting tanks.
If you don't believe me when I say that this results in better buoyancy control, try this exercise: put on your thickest exposure suit and boots. Jump in the water (no weight belt). See what happens.
If you don't clench your abdominal muscles, you'll have a tendency to float horizontally. If however you relax your arms and legs, and clench your abs, you'll be able to float comfortably at the surface in a vertical position.
I have my open water students do this from day one. It really helps me avoid a lot of problems involving buoyancy. For instance, very very few of my students keel over backwards on descent (a common problem with open water students).
This form of body control teaches you that every movement you make in the water affects your position in the water column. This, combined with proper weight calculation and visualization techniques builds knowledge and by that builds confidence. A knowledgeable diver is worth ten divers who just try things out and see what happens.
Before anyone goes off on the visualization thing, I know it sounds new agey, but it works. And if it works, I'm not going to knock it just because it's a bit fluffy round the edges.
Phew, all this just to say that hydrodynamics is important. Moral of the story, a diver's body is the one piece of diving equipment that is generally overlooked. Controlling your body is controlling your position in the water. And buoyancy control is about controlling your position in the water, isn't it?
As for gear configs and changes thereof, I'm still advocating the adage "know your gear". Having to do the weighting procedure on every change is a pain in the backside.
And FIY: diving conditions in Belgium and in the UK are comparable, so over here, we also face incredible variations in configuration throughout the year.
It's a complex issue, and most changing factors don't have all that much influence on buoyancy, but all these little factors together do count. Which is probably why so many divers take so long to fully master buoyancy. If they ever master it completely, which I doubt. As a dive center owner and instructor I see so many people diving who haven't got a clue. The worst of it is, nobody ever seems to want to listen.
I sincerely hope this clears matters up a little.
