minimum operating depth

[DA Aquamaster]

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.

To some degree it is just semantics as travel gas = first deco gas for most deep OW trimix diving situations where you start your deco fairly deep. The major difference would be at what depth a diver decides to make the switch and on a fast descent to 100', it makes very little difference in terms of how much of what gas is used.

One argument for using back gas from the surface is that it eliminates a gas switch and eliminates the potential to switch to the wrong gas - and also eliminates the work of stowing a stage reg.

On the other hand, a hard swim in strong current from the stern to the anchor line on the bow has caused more than one diver to go unconsious on mixes lower than 16%. Consequently, you need to weigh the costs and benefits of one approach against the other depending on the circumstances.

The "really old school thinking" comment however is not accurate.

Travel gas can still be "travel gas" in a cave. Consider a dive in P3 to Hendly's Castle. You have 30-35 minutes of swimming at depths of 50-60' to get to the jump where you drop down to 135 feet then after a few more minutes passing through the sand slide, you drop further to 185'.

Doing 60-70 minutes in and out on bottom mix with a 190' MOD just to get to/from the jump where you can go deep makes no sense. Similarly, doing the eventual deco on a 32-36% mix you'd normally use at 50'-60' also makes little sense.

At the other PO2 extreme, using 50% for the travel gas pushes the CNS % a bit too high, uses a PPO2 of 1.4 for long periods while swimming with stages, and is then also less than ideal for the majority of the deco.

So you end up doing the dive with an O2 bottle for deco, an 80 with a travel mix in the 30-36% range and bottom mix in the back gas.

 

[Cave Diver]

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.

Yes. As others have already mentioned, this can easily happen with with trimix mixes and the 10/70 (10% O2) mentioned is a good example. Some divers advocate "fast drops" where they hit the water negative and quickly descend to a depth where the partial pressure makes this a breathable mix. I've seen this most often with those that use scooters to descend.

Doing dives requiring trimix will generally require several deco gasses as well. Many divers will use their deco gas as a "travel gas" to breathe from until they hit a depth that is appropriate for the switch to their bottom gas.

Use of a hypoxic mix on the surface can lead to unconsciousness then possible death by drowning.

 

[JeffG]

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.

What a waste.

I would just exhale and drop.

 

[LiteHedded]

What a waste.

I would just exhale and drop.

i know of at least one story where someone almost died doing that.
luckily they had a team and support divers in the water. I forget the specifics. ear trouble or something kept him shallow just a little too long
...of course this was with an fo2 of less than the 16% or 18% people are talking about here

 

[LastManOut]

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."

Easy tiger

 

[JeffG]

i know of at least one story where someone almost died doing that.
luckily they had a team and support divers in the water. I forget the specifics. ear trouble or something kept him shallow just a little too long
...of course this was with an fo2 of less than the 16% or 18% people are talking about here

Then I would guess he was breathing the gas before he even descended.

If he was having issues descending he should of just switched to a deco bottle.

Either way...he screwed the pooch.