Is it OK to turn off O2 in Rebreather Training?

[sea_ledford]

I am guessing we will find out the o2 was shut off, but all the talk of checking the controllers -- wouldn't the HUD usually notify the operator with at least red indicators? The controller knows the depth.

In shallow, clear water on a bright day both the HUD and controller can be hard to read, the ambient light is just too bright, or the glare can be high.

My primary issue with shutting of O2 at depth, is what is it actually testing? If the diver is paying the minimum attention to their pO2 to notice it isn't maintaining? That is drilled over and over and over if they are driving the loop manually. It certainly isn't testing the ability to turn on the O2 at the surface, which is when the fatality is going to happen, the conditions are just not the same. If a diver jumps in the water with their O2 off they have already had several failures before even getting wet, I don't see how shutting down the valve at depth does anything for that.

I don't think that it is overly dangerous in general, it is just overly dangerous for the efficacy of the learning outcomes. Much like doing couch testing of a CO2 hit, it is dangerous and doesn't provide any useful feedback.

-Chris

 

[boulderjohn]

My primary issue with shutting of O2 at depth, is what is it actually testing? If the diver is paying the minimum attention to their pO2 to notice it isn't maintaining?

Not being a rebreather diver, I did not want to ask that question, even though I have been wondering about it from the start. Is it just to teach paying attention to PO2? If so, isn't there another way of doing that?

In my tech training, on nearly ever dive a student does from the very beginning of tech through trimix, the student is required to write down current dive information (depth, pressure, etc.) at different time intervals in the dive.

When I did my cave training, my instructor simply said that if I ever missed my turn pressure, the class would be immediately over, with no refund. That was all it took for me to develop a healthy interest in my relation to turn pressure.​

 

[doctormike]

My primary issue with shutting of O2 at depth, is what is it actually testing? If the diver is paying the minimum attention to their pO2 to notice it isn't maintaining? That is drilled over and over and over if they are driving the loop manually. It certainly isn't testing the ability to turn on the O2 at the surface, which is when the fatality is going to happen, the conditions are just not the same. If a diver jumps in the water with their O2 off they have already had several failures before even getting wet, I don't see how shutting down the valve at depth does anything for that.

No O2 is a common failure mode, and the drill does replicate what might really happen in that situation better than reading cue cards. O2 valve shut off isn't the only way that can happen - hose rupture, first stage failure, stuck closed solenoid, etc... Dropping PO2 causes the student to work through what is wrong and come up with a solution. Does the MAV work? Is the valve on? What does the SPG say?

If you are diving an eCCR at low setpoint and maintaining PO2 manually, you still have the parachute. In this drill, the instructor is the parachute.

 

[doctormike]

Not being a rebreather diver, I did not want to ask that question, even though I have been wondering about it from the start. Is it just to teach paying attention to PO2? If so, isn't there another way of doing that?

In my tech training, on nearly ever dive a student does from the very beginning of tech through trimix, the student is required to write down current dive information (depth, pressure, etc.) at different time intervals in the dive.

When I did my cave training, my instructor simply said that if I ever missed my turn pressure, the class would be immediately over, with no refund. That was all it took for me to develop a healthy interest in my relation to turn pressure.​

Yes, "always know your PO2" is drilled into you again and again. The classic way it's done in the beginning for eCCR is that you use a low setpoint, but you are required to maintain a higher PO2 with manual addition. The Instructor watches your PO2 and yells at you if you don't maintain it.

This is a different lesson, and it more accurately simulates real world conditions. Many (most?) eCCR divers let the rebreather do what it was designed to do and maintain the high setpoint for them while they go about diving, taking pictures, etc... So unlike the above manual flight drill, this technique lets the instructor be sure that the student will pick up on the dropping PO2 when they AREN'T constantly being required to maintain it with the MAV.

 

[rjack321]

You either don't dive a rebreather, or don't understand the rebreather you dive. At 80' do you think the oxygen is consumed faster than on the surface?

I am certified on 2 units and I can absolutely tell you that you are wrong here. At 80ft a given loop fO2 has more oxygen in it. Even more the deeper you go.

I will illustrate how:
4liter counterlungs, air diluent, no oxygen added (or oxygen off)
Fill your counterlung with air diluent at the surface. 1ATA * 4L * 0.209 (fO2 in the air) = 0.84L of oxygen available to sustain life. At an average of 0.7L per minute base metabolism that will last a student on the surface just over a minute.

Same 4L counterlungs filled with air diluent at 80ft. 3.42ATA * 4L * 0.209 = 2.86L of oxygen in the loop. Divided by 0.7L/min = about 4 minutes oxygen (if the student were able to use it all)

Technically this exercise should use moles of O2 not surface liters but works the same. Unless you have a student running 100% o2 on the surface and some alternative low setpoint at depth you always have more O2 molecules at depth and the loop will sustain life longer than if you shut down the O2 on the surface.

 

[rjack321]

You know, I really respect all of the people objecting to this teaching method, but some these objections have a straw man quality to them. Even on the surface, it takes six minutes to become hypoxic. And Superlyte is not advocating just "turning off the O2" and leaving.

Let me ask you something. If the objection to the O2 shutoff is that (1) the student might be incapable of responding appropriately, and (2) the instructor might not intervene before the drill becomes lethal, why don't you have the same objection to a loop recovery drill? Wouldn't that situation become lethal much faster?

6 minutes is depth dependent. as illustrated above. On the couch shutting off O2 is fine, I don't care. On the surface its a super risky move that has almost no teaching value. At a modest depth and anticipated (ie student shuts their own O2 down and watches), I don't see it being a big deal. Surprising students by actually "breaking" their machine so it is no longer capable of supporting life is really the antithesis of RESA. Doing lights out drills while students are distracted or other types of combination (and unannounced) failures is just stupid, that's how students die (and at least one has).

Loop recovery is no different than regulator remove and replace. Can't get it back and you bail to OC.

 

[sea_ledford]

No O2 is a common failure mode, and the drill does replicate what might really happen in that situation better than reading cue cards. O2 valve shut off isn't the only way that can happen - hose rupture, first stage failure, stuck closed solenoid, etc... Dropping PO2 causes the student to work through what is wrong and come up with a solution. Does the MAV work? Is the valve on? What does the SPG say?

But shutting down the valve doesn't test any of those other things. Once the valve is back on, drill is done. With the additional risk of flooding your O2 first stage when you use the MAV and drain the line. Is using cards exactly like the real world? Of course not, no drill is. You can work though the process in greater detail with feedback from the instructor:

Diver told PO2 is dropping -> manual add
Instructor shakes head - that wasn't the solution
Diver checks valve
Instructor shakes head - that wasn't the solution, but valve is on
Diver plugs in offboard O2 if available, bails out, or goes SCR which ever is appropriate

If you are diving an eCCR at low setpoint and maintaining PO2 manually, you still have the parachute. In this drill, the instructor is the parachute.

Right, so what is the difference except the instructor is able to let you run down your PO2 to less than 0.7? At what point is the instructor to intervene? 0.5? 0.3? 0.2? What is defensible in court?

 

[doctormike]

6 minutes is depth dependent. as illustrated above.

Right, 6 minutes is on the surface.

On the couch shutting off O2 is fine, I don't care. On the surface its a super risky move that has almost no teaching value. At a modest depth and anticipated (ie student shuts their own O2 down and watches), I don't see it being a big deal. Surprising students by actually "breaking" their machine so it is no longer capable of supporting life is really the antithesis of RESA. Doing lights out drills while students are distracted or other types of combination (and unannounced) failures is just stupid, that's how students die (and at least one has).

Loop recovery is no different than regulator remove and replace. Can't get it back and you bail to OC.

We are sort of going over and over the same points here, but it's an interesting discussion. Thanks for that...

Loop recovery is quite different from OC regulator remove and replace, since the loop is flooding even if you just give up and bail out.

By your standard, loop recovery drill is "breaking" the machine so it is no longer capable of supporting life. Yes, in both cases, there is something that the student can do to recover, and they either will or will not be able to do that. And that's why the instructor is there, in case they cannot. Why do you feel that a student should be able to bail out to OC, but should not be able to turn on their O2? I'm not an instructor, but if I was I would far prefer to just quietly open the O2 valve if my student's PO2 dropped to 0.7 after a few minutes without them noticing, than try to deal with a student in panic with a flooding loop, even if they did manage to bail out to OC.

 

[doctormike]

But shutting down the valve doesn't test any of those other things. Once the valve is back on, drill is done. With the additional risk of flooding your O2 first stage when you use the MAV and drain the line. Is using cards exactly like the real world? Of course not, no drill is. You can work though the process in greater detail with feedback from the instructor:

Diver told PO2 is dropping -> manual add
Instructor shakes head - that wasn't the solution
Diver checks valve
Instructor shakes head - that wasn't the solution, but valve is on
Diver plugs in offboard O2 if available, bails out, or goes SCR which ever is appropriate

We keep making the same points, but I do appreciate the discussion. Yes, you can simulate failures with cards, or just over coffee on dry land. The point is that the O2 shutoff simulates it more accurately, in terms of preparing a diver for what actually happens in the real world. Sure, cutting an O2 LP hose with a shears would be even MORE accurate. C'mon, I think that you know what I'm saying here.

Right, so what is the difference except the instructor is able to let you run down your PO2 to less than 0.7? At what point is the instructor to intervene? 0.5? 0.3? 0.2? What is defensible in court?

Why all of this drama about court? It's such a straw man argument. Yes, I agree with you that the instructor should not turn off the O2 and then continue the simulation until the student is dead.

All the instructor has to do is make sure that the student picks up on what happened. If the setpoint is 1.2, and the PO2 drops to 0.7 without the student being aware of the problem, the instructor just turns the O2 back on and fails the student.

Is this slightly more risky than the cue card version? I guess, if the instructor dies during the drill and the student doesn't notice his PO2 dropping and doesn't notice his alerts flashing as the PO2 drops to 0.4, and doesn't turn on the O2 or bail out, then that would be a problem.

 

[rjack321]

Right, 6 minutes is on the surface.
.

They started at 0.7
As illustrated with air dil and no O2 its a lot less time. Squirt 10/50 dil in there and you don't have 6 minutes at all, its already hypoxic. Unless I missed it, we don't know the dil this young man was even diving so saying he had XYZ time to respond to his O2 being off (assuming that might be what happened) is misleading.

Loop recovery is quite different from OC regulator remove and replace, since the loop is flooding even if you just give up and bail out.

By your standard, loop recovery drill is "breaking" the machine so it is no longer capable of supporting life. Yes, in both cases, there is something that the student can do to recover, and they either will or will not be able to do that. And that's why the instructor is there, in case they cannot. Why do you feel that a student should be able to bail out to OC, but should not be able to turn on their O2? I'm not an instructor, but if I was I would far prefer to just quietly open the O2 valve if my student's PO2 dropped to 0.7 after a few minutes without them noticing, than try to deal with a student in panic with a flooding loop, even if they did manage to bail out to OC.

And if that student corks while their O2 is off? MOD1 students aren't known for stellar buoyancy. Swimming along and having the ppO2 drop from 1.2 to 0.7 is nothing like getting in with O2 off.

Loop recovery skills are not done midwater for this exact reason. So that a failed recover leading to a flooding unit, lungs compressing, ear drums popping and not getting to an OC regulator isn't overwhelming students to the point of panic. They settle to the bottom, eventually get OC gas from BO or the instructor, then head up.