Welcome back to my "Becoming a PADI Instructor" Blog Series! Let's talk about buoyancy!
Let's start with an interesting tidbit of information. I've said it myself and I've heard other say it when people ask "What is Scuba diving like?", we often respond "It's like being an astronaut floating around weightless in space!" and everyone gets really excited. The excitement is real but the weightless statement is not. Even underwater earth's gravity continues to pull down on you as if you were on the surface. This is why you can still feel whether you are upside down or right-side up underwater. An astronaut does not have these feelings.
It all starts with Archimedes' Principle stating that An object wholly or partly immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object.
Imagine you have a large dunk tank filled with water, and that tank is inside a larger empty tank. You then submerge yourself in the water filled tank forcing the water to overflow in to the empty tank. After you get out and dried off you empty the outer tank in to a scale, the weight of that water is the amount of upward force (buoyancy) against your body.
Since we are diving we are mostly dealing with buoyancy characteristics of water. The more dense a fluid is, the more buoyant it is. When you first learned to dive you learned that more weight is needed when you are diving in salt water as oppose to fresh water. Salt water is more dense and weighs more because of it's dissolved salts. One liter of fresh water weights one kilogram (62.4 lbs) while one liter of salt water weights 1.03 kilograms (64 lbs per cubic foot ). These are the weights you would get if you were to perform the same experiment in the previous paragraph with a 1 cubic foot block. These numbers will come in to play when determining how much air or weight to add to an object to change it's buoyancy. (I'll be including a very good youtube video that will explain it much simpler than I)
Have you ever wondered why you may sink or float when you jump into a pool but your friend is effected the opposite? Specific Gravity is the tendency of an object to sink or float in water. Pure water has a specific gravity of 1.0 which means anything less dense than pure water will have a specific gravity less than 1.0 and will float in water and anything more dense than pure water will have a specific gravity greater than 1.0. When dealing with salt water the same applies but the magic number is 1.03. Our bodies are composed of lots of different fat and muscle tissues as well as bone, each with their own specific gravity numbers. Fat tissues are between .7 and .9, bone is ~1.9, and muscle is 1.08.Based on the ratio of muscle and fat and bone your number will typically hover around the 1.0 area. If you've got a higher fat ratio you'll be lower than a 1.0 and will be positively buoyant. If you've got a higher muscle ratio your specific gravity will be higher than a 1.0 and you'll be negatively buoyant. Of course this example leaves out the specific gravities of your equipment and wetsuits that tend to be less than 1.0 (positively buoyant). In order to become neutrally buoyant you aim to adjust your BCD and weights to adjust your specific gravity equal to the type of water you're diving in.
For the following portion of this article I'm going to direct you to watch the included youtube video to cover some of the things I've said above as well as explain how to use what we've learned in the practical application of lift bags and making objects neutrally buoyant.
Now go out there and use this knowledge to lift some treasures from the sea bottoms. Of course you're more than welcome to share it with me!
Please feel free to comment or ask questions or make corrections as I'm doing this to help teach as well as learn myself.
On to Becoming a PADI Instructor: Gases (Part 1)
Disclaimer: I am not a PADI Instructor and am writing this up as a learning aid for myself to better understand and explain the theories as I learn them. You should NOT be using these as your sole resource for learning how to dive or dive education. I suggest you sign up for a PADI or a respective company course to continue your diving education. If you find any errors in these posts, please feel free to contact me. Thank you.