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Wednesday, 5 March 2014

Buoyancy Basics Part 2- Salt vs Fresh Water

Scuba Divers in Seawater
Continuing on from our first buoyancy basics blog last week, we will move on to the difference between buoyancy in salt water compared to fresh water. If you haven't read last week's basics, please do, as it underpins what will be read here.

During our scuba diving beginner courses, we learn that if we dive in Fresh water, we need to use less lead weight on a set diving equipment rig than we would in salt water. Generally speaking, if we move from the salty sea to a swimming pool, we drop around 2kgs off our weight belts.

But why is this?

The answer is actually quite complex but we will look at it in the most simple terms possible.  Again, it relates back to Archimedes Principle.

Salt water is water with salt in it, therefore, it is heavier (for a given volume) than fresh water, which is water without salt!! 1 litre of fresh water weighs approximately 1 kg while salt water is more like 1.04kg per litre. This increased weight means the salt water has a greater upward or buoyant force.


Archimedes principle states that an object will float or sink depending on the weight of water it displaces. If the weight of water the object displaces is more than its own weight, the object floats, while if the weight of the water displaced is less than that of the object, it sinks.

So, imagine our 80kg diver is neutrally buoyant in fresh water. This is because s(he) displaces 80 litres of water giving an 80kg buoyant force counteracting the 80kg down force.

If we move our diver to the sea, the additional weight of the salt in the water causes the buoyant force of the water to increase to approximately 83.2kgs, (80 x 1.04) which is greater than the 80kgs down force of our diver, making them more buoyant and in need of a few extra weights.

Neutral Buoyancy in Salt Water Caves
However, the amount of salt in the sea varies depending on where in the world you are. Even diving in the Mediterranean, the seawater is much more salty around Cyprus than around the Western Med and of course, diving in the Dead Sea would require an awful lot more weight than pretty much anywhere else. So this isn't exactly a science that can be mathematically computed pre-dive.

Tried and tested is to get in the water, do a weight check and scuba dive. You will eventually get the knack for guess-timating the weight that you will need as your environment and diving equipment changes.