Which do you think is less dangerous at 160ft? Open-circuit air or CCR trimix?

[inquis]

People should have a look at some videos of Buford Spring a straight in and out free diving cavern that some need a CCR+trimix. Plenty on YouTube

You're assuming it's Buford. The question was also, "which is safer?" -- NOT whether either was "safe enough".

 

[flymolo]

People should have a look at some videos of Buford Spring a straight in and out free diving cavern that some need a CCR+trimix. Plenty on YouTube

strangely a number of people have died there, mostly ow divers afaik

 

[Celt]

strangely a number of people have died there, mostly ow divers afaik

People have drowned in swimming pools on holiday,

 

[flymolo]

People have drowned in swimming pools on holiday,

but did they forget to check their gauges or get disoriented in a 160' cavern?

 

[flymolo]

People have drowned in swimming pools on holiday,

what is your point anyways? your location says ireland so i assume you never plan to dive at buford but i would imagine those who frequent north florida would like it to remain open. unnecessary deaths put that at risk

 

[dylanfromwinnipeg]

A curious stunt and probably still even tossed into some courses, but it seems like a bad idea in practice. It probably shouldn't be done or taught, not even as a demonstration or curious experiment. Minutes are like seconds, if there are sufficient distractions or task loading.

Idk maybe you are just saying watch it go from 1.3 to 1.0, but why

Just believe your instructor when they tell you there is typically enough oxygen left in the loop to take your time carefully bailing out or doing some other life-saving drill that restores oxygen flow or ppO₂.

Better training and safety point to promptly switch to another breathing source (e.g. BO) if you know that oxygen flow to loop has ceased. There are too many things that could happen on a real dive in between the time you discover this and the time you pass out.

[Manual additions via MAV to maintain setpoint is different, you do not sit there watcing ppO₂ drop]

The whole 'turning off the O2 and swimming around' needs to be scrubbed from courses, and I haven't seen this personally but have heard people have drowned doing this, on courses no less. This is just terrible protocol when divers should be busy MAVing, plugging an offboard O2, or bail out

If you lose your oxygen deep in a cave, being able to exit in SCR mode - and having practiced it in training - is useful.

 

[nadwidny]

The idea of the exercise is to watch how long your loop lasts, not to observe how PO2 changes with a change in depth, so you’re supposed to maintain a constant depth.

I'm pointing out the flaw in the exercise.

 

[flymolo]

I'm pointing out the flaw in the exercise.

There is at least one death attributed to shutting down oxygen during a training exercise, no?

 

[broncobowsher]

There is at least one death attributed to shutting down oxygen during a training exercise, no?

I recall one. Instructor shut down a student that they had not properly trained to handle a shut down. Forgot why the instructor couldn't get back to the student to correct his mistake before it was too late.

 

[justinthedeeps]

If you lose your oxygen deep in a cave, being able to exit in SCR mode - and having practiced it in training - is useful.

You make a good point, but for the sake of lively SB.com discussion, I am going to counter-argue as usual

Most divers in emergencies, or 'pre-emergencies,' are going to be safer the sooner they just bail out.

We all like to credit our ability to go into 'expert mode' (as trained) and keep diving on a compromised rebreather as if it were our only or best option, but this is usually not the safest, best or fastest option to get home that we have (super deep/advanced cave dives possibly excepted)

For someone calm enough and untasked enough to do SCR properly, their OC bailout exit will still be simpler and easier, at a relaxed SAC breathing rate, completely free to do whatever they need to do like follow lines, signal buddies, tidy up, etc etc without worrying about SCRing the loop. There is also extreme risk of rapidly dropping loop ppPO₂ in diluent-only ascents, with other tasks and distractions (this could easy kill)

SCR is a suboptimal and unnecessary complication most of the time, except for ok yeah [cave] alpinists who are not carrying enough bailout, loss of bailout, or where bubbles might cause ceiling litter/failure and loss of visibility etc. For some of these niche reasons yeah we rightly practice it in courses.

TL;DR
SCR shouldn't be considered a first or wise choice in loss of oxygen scenario, unless bailout isn't an option