minimum operating depth

[mikemill]

Ok let me preface this post by saying I'm asking just out of curiosity and not looking to bypass any type of training

I know that for gases there is a maximum operating depth before it becomes toxic/narcotic. The MOD would be the maximum depth that you could safely breath that gas.

I was wondering if there was any gases with a minimum operating depth above which it was not safe to use. The scenario I have in my mind is one where you were using a gas with a low % of O₂ and some type of other filler gas. At a certain depth the increased pressure would (I think) allow you to inhale enough O₂. However, in too shallow of a depth you wouldn't be taking in enough O₂ to maintain bodily functions and would have to switch to a richer mix.

 

[Gombessa]

I was wondering if there was any gases with a minimum operating depth above which it was not safe to use.

There are indeed hypoxic mixes which cannot support life at the surface or shallow depths. 10/70 comes to mind, but I'm guessing any mix with an fO2 below ~16-18% counts?

 

[mikemill]

Thank you. Now that my curiosity has been satisfied I can get back to work

 

[kjvander]

While not being a tech diver myself there would stand to reason that there are some trimix mixes that would be hypoxic. A 15% O2 mix would not be good to breathe at shallow deapths and may cause harm and unconscienceness (sp?). So the easy answer is yes there are some mixes that are not recommended for shallow depths and this is why you get gasses described as travel gas and bottom mix.

Again...I'm not a tech diver but have started dabbling in reading about it and talk to many so I might not have all the exact information so I tried to leave it general.

 

[Ytsejam]

According to Dalton's law of partial pressures, as long as you breathe the same gas mixture in different depths, you will still get the same percentage of any gas making up the mixture, irrespective of depth.

 

[Thalassamania]

It is not the percentage that is important, but rather the partial pressure (percentage times ambient pressure).

 

[Gombessa]

According to Dalton's law of partial pressures, as long as you breathe the same gas mixture in different depths, you will still get the same percentage of any gas making up the mixture, irrespective of depth.

Virtually similar percentage at depth, but increasing partial pressure. It's the partial pressure that matters.

 

[mikemill]

Is the percentage what is important or the amount? I would think it'd be the amount.

So assuming constant volume and tempature:
pv = nrt

An increase in p would give an increase in n.

 

[Thalassamania]

Is the percentage what is important or the amount? I would think it'd be the amount.

So assuming constant volume and tempature:
pv = nrt

An increase in p would give an increase in n.

Correct. The critical item is the partial pressure, which varies directly with the number of moles of a gas in the mix (all else constant).

 

[mikemill]

Hmmm that makes sense. By looking at the partial pressure one could adjust the total pressure and the gas mix to obtain the desire ranges.