Question: Why do we need about 2 kg more lead with the aluminum device than with the steel device?
The most given answer is: Because aluminum is lighter than steel. WRONG!!!
The reason why we need about 2 kg more lead with the Alu PTG explains us the law of good old Archimedes, which reads:
A body immersed in a liquid apparently loses as much weight force as the amount of water it displaces weighs.
What does this mean using the example of a 12 liter PTG (compressed air diver)?
Before we address this question the following must be known:
Steel has a density almost 3 x higher than aluminum. As a result, the wall thickness of an Alu PTG must be 3 x thicker to withstand the same pressure. This in turn results in an aluminum PTG being larger and heavier than a steel PTG.
A 12 liter steel TG weighs (empty, i.e. not filled) about 14 kg and displaces a total volume of about 13 liters.
A 12 liter aluminum TG weighs (empty, i.e. not filled) about 16.5 kg and displaces a total volume of about 17 liters.
If both PTG are filled with 200 bar air mixture, they weigh about 3 kg more.
(1 liter air weighs about 1.25g results in 12 x 200 = 2400 x 1.25 = 3000 g resp. 3 kg)
In the following table the effect of Archimedes is shown when diving in salt water with a 12 l steel and 12l Alu PTG, filled with 200 bar and almost empty with 20 bar.
What does this tell us as “Otto Normal” divers?
1. It is correct, when diving with an Alu PTG you have to carry around 2 kg more lead.
2. The Alu PTG is around 2.5 kg heavier on land than the steel PTG. With the consequence that the diver must drag more kg.
3. The aluminum PTG has under water, at the beginning of the dive, still a small downforce.
4. Towards the end of the dive, the aluminum PTG has a buoyancy. This is then felt by divers as an uncomfortable “pull up” behind.
Diving with a steel PTG is four times more comfortable and enjoyable.