How to calculate the right amount of weights
Are you one of the divers coming to dive centres and asking for 10 Kg to 14 Kg because you are a floaty person? Drop that number of weights down drastically, it adds unnecessary workload to your body. In this article we will explain how to determine the right amount of weights for you.
Imagine taking a six-pack of water and carrying that for 45 - 60 minutes from the supermarket to your flat. That is the equivalent of diving with 12 Kg when in reality you needed 6 Kg for the dive.
Ideally you have just enough air in your tank that at the end of your dive, when you hopefully have a bit more than 50 bar in your tank, that you are still neutrally buoyant.
First factor: The Tank
You will start your dive with a full tank of air. We will have to calculate how much weight it is going to lose when it is at around 50 bar. Let's assume we dive with a standard S80 aluminum Scuba Tank (80 cubic feet), the most commonly used diving tank. Let's convert these cubic feet to litres which will give us 2265 litres. We could bore you with the ideal gas law equations here or you could believe us that 1 liter of air weighs around 1.2 gram at sea level. If you were to breathe your tank from 200 bar down to 50 bar you will lose nearly exactly 2 Kg (2265 * 0.75 * 1.2). A standard Luxfer S80 tank is 1.6 Kg negatively buoyant when full, leaving you with only 400 grams of positive buoyancy towards the end of the dive.
Different aluminum tanks have different characteristics, but the most positive buoyant one that a normal diver will probably come across is a Luxfer 80 which gives 1.4 Kg positive buoyancy at 50 bar. The myth that aluminum cylinders are very positively buoyant is widespread, but completely unfounded. Catalinas C-Series tanks for example are all negatively buoyant, even when completely empty. Luxfers S-Series is ever so slightly positively buoyant when completely empty that it hardly makes a difference in weight calculations.
Other factors: equipment and body mass
A Large 5mm long wetsuit will give around 3 Kg of positive buoyancy. Taking fins, mask and your BCD most combinations will give you not even a total of 1 Kg of positive buoyancy, if at all.
Let's add up those numbers above. We realise that equipment towards the end of our dive adds a little over 5 Kg of positive buoyancy. Of the people that take 12 Kg on their dives who here really thinks that they can casually swim along in nothing but their swimwear with 7 Kg tied around the waist? The numbers above swiftly shift into the direction of taking less weights when we assume a smaller, shorter or thinner wetsuit or a steel tank. 5 Kg of positive buoyancy for equipment is fairly generous for diving in the tropics and depending on the equipment the real numbers will probably be closer to 2 or 4 Kg.
Body composition plays another role in how much weights you need. Everyone heard the phrase "fat floats" which is very much true.Although everybody is built different the standard human body density to water is 0,98 which means we very slightly float. In salt water very few people will need more than 1 Kg to sink.
Buoyancy check
We cannot stress enough how crucially important a proper buoyancy check before your first dive is. Take your time and find out how much weight you need. Jump into the water fully equipped and take just enough weight that you float at eye level while holding a normal breath. We repeat, a normal as in you being at rest, breath. The difference between a normal breath of 0.5 liter and a full breath can easily add up to more than 3 or 4 Kg. Once you added enough weight to float at eye level add an extra 2 Kg for the end of your dive when your tank gets towards your reserve. There are very few people that would genuinely need anything close to 10 Kg using the above calculations. Most end up with between 3 or 8 Kg.
Of course the above numbers are a generalization and we assumed diving in a tropic environment but the point we are trying to bring across is that most people tend to take more weights than they really need. We made this point to further reduce our effort underwater, resulting again in an improved air consumption.