# Minimum operating depth for lean SCUBA gas

when doing deep decompression SCUBA diving where a travel gas is used I am curious how to calculate the minimum operating gas for a lean O2 mixture (like 10%). Obviously the maximum is calculated by some maximum PO2 figure (either 1.4 or 1.6 usually). I assume the same is true for the minimum where there is some minimal acceptable PO2, but I dont know what that figure might be.

My answer is based on my experience and training, and your choices should be based on your experience and training. I am working off the assumption that we are talking about SEA LEVEL (1bar at surface) and we are talking seawater (10MSW = 1bar) Furthermore we do not aim to dive to minimum or maximum levels, we always err towards safety in a large margin, especially with hypoxic mixes. As an simple example you would start your dive on normal air and use that down to 20msw or 30msw before switch over to your hypoxic mix, giving a large margin of safety. I am not aiming to give a perfect answer and do not use this to plan your dives on. (If I remember correctly 12% tend to be considered the minimum O2 Fraction in any diving mix, except for very extreme reasons/depths)

So let me just quickly start off with stating that the maximum partial pressure of oxygen in a diving gas for the "working" part of the dive is 1.40 bar (in rebreathers this is more conservatively brought down to 1.2) For the decompression part of the dive we tend to work closer to an absolute max of 1.60 bar

The minimum partial pressure of oxygen on any part of the dive is 0.18 bar (and this is at an absolute push, do not dive with 0.18 bar as your aim EVER!), but we tend to stay well above this, why switch shallower when you can switch a little deeper and be safer?

So as far as calculations go, you should already have the correct formulas.

PO2 = FO2 * P Partial Pressure of Oxygen is equal to the product of the Fraction of Oxygen in the mixture and the absolute pressure ((Depth/10)+1)

So this will give you: P = PO2 /FO2 and P = (Depth/10) +1 Depth = 10(P-1)

Pressure = Partial Pressure of Oxygen / Fraction of Oxygen Depth = (Absolute Pessure minus 1) times 10

since we do not want a minim PO2 less than 0.18 we have that value fixed and then you add the fraction of o2 in your mixture as the FO2

So for a mixture of 12% Oxygen we would have: P = 0.18/0.12 P = 1.5

Depth = 10(1.5-1) = 10 * 0.5 = 5m Absolutele Minimum Operating Depth

The partial pressure of the oxygen in the fresh air is 0.21 and it's what people have adapted to. The maximal amount of oxygen our body is able to use is 0.05, so the partial pressure in the fully used air by the body in the good shape is 0.16. The mixture having partial pressure of oxygen of 0.16 or below is guaranteed to kill any individual even in the state of minimal oxygen consumption.

The partial pressure of 0.17 is normally the minimum for the mixtures that you start breathing from the surface (normoxic - but notice, it means that you start breathing on the beginning of the descent, so you breath partial pressure 0.18 only for seconds.

Everything below 0.21 means that your body gets less oxygen that it needs, and is potentially lethal (low level of oxygen means risks of fainting without pre-warning).

• Just a correction on your answer the partial pressure of 0.208 for oxygen is mainly at sea level and it is lower as the altitude increase. So no humans have not adapted to partial pressure of 0.21 Oct 9, 2021 at 16:11
• No I am not referring to 0.21 or 0.208. I am referring to the fact that humans at altitude where the pressure is less than 1 bar has Oxygen partial pressure below 0.208 :) And never mind the fact that air also contains polutants, organisms, pollen, water, etc which also makes the partial pressure bellow 0.208. It is one of reaasons athletes do high altitude training. so tehy acclaimatise to the lower oxygen partialpressure. Oct 10, 2021 at 12:12
• Partial pressure below 0.16 isn't "guaranteed to kill" -- people work at altitudes above 2500 meters/8000 feet all the time -- the reason to avoid it is your judgement and coordination suffer, and it's easy to get into a situation you can't get back out of.
– Mark
Oct 15, 2021 at 2:18
• @AquaAlex, end of the first paragraph of the answer: "The mixture having partial pressure of oxygen of 0.16 or below is guaranteed to kill any individual..."
– Mark
Oct 21, 2021 at 20:43
• Sorry I forgot about the answer containing that part :) Yeah that is very wrong Oct 23, 2021 at 10:50