minimum operating depth

[Gombessa]

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.

Well, not so much the "total pressure" as I understand it, but definitely the mix (fO2). ppO2 at 1.2-1.4 generally defines MOD. 32% nitrox has 0.32 ppO2 at the surface (1ATA), and 1.4 ppO2 at 110ft (just over 4ATA).

Similarly, a mix with 10% O2 is not breathable at the surface, but should become so before you hit 33ft or 2ATA (at which point the ppO2 becomes 0.20, or roughly the same as the ppO2 would be at the surface).

 

[mikemill]

Ok by total pressure I meant the ambient pressure.

 

[LastManOut]

This is a very in depth thread for a very simple answer

 

[mikemill]

Well I liked the responses. Sure the question was a yes/no but the answering of why is also useful.

 

[Gombessa]

Well I liked the responses. Sure the question was a yes/no but the answering of why is also useful.

Doing otherwise is also kind of obnoxious.

"Are there mixes that are breathable at depth but not at the surface?"

"Yes."

"Um, OK. how would they affect you at the surface?"

"Physiologically."

"Uh, thanks a ton."

 

[ae3753]

Doing otherwise is also kind of obnoxious.

"Are there mixes that are breathable at depth but not at the surface?"

"Yes."

"Um, OK. how would they affect you at the surface?"

"Physiologically."

"Uh, thanks a ton."

Isn't that how you're supposed to answer to lawyers?

All kidding aside, I liked your answers, Gombessa.

 

[diverdowndude]

Most training agencies state 16% minimum O2 on the surface, I prefer to use a travel gas or some of my deco gas for anything below 18% O2 on the surface for an added safety margin when using hypoxic mixes. As previously stated "get proper training prior to using any mixed gases". Hope this is helpful.

 

[mikemill]

Isn't that how you're supposed to answer to lawyers?

All kidding aside, I liked your answers, Gombessa.

I thought the proper answer to lawyers was "I do not recall any such information at this time"

 

[DA Aquamaster]

16% versus 18% is pretty much a moot issue. If you arwe divign deep enough to use them, you will also be using a travel/deco gas with an O2 percentage in the 30-32% range and you will have at least 1.5 times the amount you plan to need for the dive. So it makes more sense to save the back gas for depths below 100' and just use a travel mix from the surface to 100'.

----

It is also the partial pressure that matters, not the mix. For example 100% O2 can be hypoxic at low enough partial pressures.

In an aircraft at 12,000 ft the pressure is only .64 ATA. so on 21% the PPO2 is only .13. At any altitude higher than that and you definitely want to be on supplemental O2 even at rest as your cognitive abilities begin to seriously degrade at lower partial pressures.

Now, at an altitude of 47,000 feet on 100% O2 even with a perfectly fitting oxygen mask, you are going to have that same partial pressure of .13 and if you fly any higher than that you need either a pressurized cabin or, preferably, a pressure suit in the event the cabin pressurization fails. At partial pressures that low, your lungs just can't transfer O2.

 

[rjack321]

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.

Its less than 15%. You might pass out after a LONG time breathing 15%, otherwise you just get winded and suck down your expensive trimix on the surface for no reason. Climbers can acclimatize to <15% without too much trouble at all.

Travel gas is really old skool thinking, many people just descend fast. Or breath a deco gas for the upper 20ft. Bringing a whole seperate cylinder just to descend a little isn't as popular nowadays as it was in days of yesteryear.