Why it Works | 5 min

Steve Martin2 Comments

2 Comments on “Why it Works | 5 min”

  1. Neville Douglas

    Thanks for this gents, training for my Divemaster at the moment and like the simplified way that you explain the science, makes it simpler to explain to my students. In regard to the reduction of air at depth ie not what is in the cylinders as that does not compress until its breathed, could you clarify that as there is a lot of confusion in this area, could be another great whiteboard session. thanks

  2. Vas Proud

    Hi Neville,

    Great to hear you are taking Divemaster training – I really loved mine. I can imagine you are enjoying the dive theory from it too – I really enjoyed it – especially the Physiology, actually.

    I don’t fully understand your question, I’m afraid. Are you asking about what actually causes the compression of air spaces? If so, we cover that in a later video where we talk about “What is Pressure” – ultimately it’s the force of (several billions) of particles above an object all collectively weighed down by gravity.

    You are right about the non-compressibility of the tanks. In general, the “whole diver” does not compress much – we are broadly the same size – but we get small volume changes due to our suit and BCD compressing – this is why we have a “compensation” device – the BCD, to add back the volume we lose on descent (or to vent the volume we have gained on ascent).

    We do have a dynamic volume change as we breathe in and out – but that, on average, is a variation around a “middle” value (normal lung) and doesn’t need compensating for by the BCD.

    We also do not undergo a big weight (mass) change as divers. Sure, we may lose some weight from sweat and urine, but this is the same density as the surrounding water and doesn’t impact our buoyancy.

    The most significant change is the weight we lose from the gas we are carrying in the cylinders. We use a typical value of between 1.2 and 1.3 grams per litre of Air (at Room Temperature and Pressure – RTP) and so we can say that 150bar of Air in a 12L tank weighs close to 2.3kg – and this weight you start a dive with, but you are lighter at the end by this amount. With more tanks (sidemount, dry suit inflation etc.) you start with even more weight and lose that too by the end. This is what drives the principle of an end of dive weight check – we must check we can stay down and not have any/much lift from our BCD when the diver is at their lightest (i.e. 50 bar reserve).

    Please let me know if that answers your question, otherwise if you can say more specifically I will help for sure.


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