Joe’s PADI IDC and Divemaster classroom - Physics
PADI Divemasters and Dive Instructors need to have an indepth dive knowledge. To help you get prepared for the dive theory for your PADI Divemaster course or PADI IDC on Bali, Joe's Gone Diving discusses various topics in this blog. This time, we dive into the very first physics topic in the PADI Open Water course: buoyancy.
What is buoyancy?
What is buoyancy? That’s a question you will hear quite often as a PADI Instructor or Divemaster. We could talk about it in general like Archimedes did, who by the way discovered the principle, but as divers we are only interested in buoyancy related to water.
The straightforward answer to this question would be: 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.
This means the denser the fluid, the more buoyant the object will get. In water that means:
Objects in salt water will be more buoyant than the same object in fresh water.
Seawater is more dense and weighs more than fresh water. Sea water is actually nothing more than fresh water with the addition of some salt. In general 1 liter of fresh water weighs 1 kilo, and 1 liter of salt water weighs 1.03 kilos.
When we estimate how much weight we will need, we need to take the water displacement of an object (the BCD), the weight of an object (tanks and weights) and the water type into consideration.
Let's look at an example. By accident you lost your outbound engine of your boat. After checking the engine's manual, you determine it weighs 100kg. The first thing you want to find out, is how much existing buoyancy your engine has. In this case we assume your engine displaces 20 liters of seawater. Which brings us to our first calculation, to figure out what the buoyant force of our engine is.
Step one Multiply:
20l x 1.03= 20.6kg
which is the buoyant force. So it's pulling down by 100 kg and it’s pulling up by 20.6kg.
Now we need to balance the opposing forces:
Step two Subtract:
100 kg - 20.6 kg = 79.4 kg
This means we need an additional force of 79.4 kg to make our object neutrally buoyant to bring it safely to the surface.
To get this additional force we will need a device like a lift bag to displace a certain amount of water in litres
Step three Divide:
79.4 kg / 1.03 kg per litre = 77.1 litres of water to displace.
So basically you now know you would need to bring a lift bag which displaces a minimum of 77.1 litres of water to be able to bring your outbound engine back to the surface.
TIP: An easy way to remember these three formulas is Master Scuba Diver = MSD = Multiply Subtract Divide.
To master buoyancy questions you will always need those three steps.
Stayed tuned for our next Blog in Joe’s PADI IDC and Divemaster Classroom when we’ll talk about Physiology.