CCR Cave - O2 shutoffs and HUDs

[Cio]

The mass of O2 is the same regardless of depth, assuming the same loop volume and PO2. While the total molecule count does increase at greater depth, the fraction of O2 decreases by the same proportion (since the same PO2 is present). As such, it wouldn't take longer. If anything, it could take a little less time, due simply to the choice of a diluent with lower O2 fraction (and less O2 added when restoring loop volume) on a deeper dive.

You are correct, I was incorrect in the available mass increasing.

33ft , 2 atm, PO2 1.2 = 60%EAN
66ft, 3 atm , PO2 1.2 = 40% EAN
99ft 4 atm , PPO2 1.2 = 30%EAN

Assume 4L volume (O2 6.5 g/l )
@33ft 0.6*6.5*2= 7.8 g of O2
@66ft 0.4*6.5*3= 7.8 g of O2 0
@99 ft 0.3*6.5*4 = 7.8g og O2

But;
Assume you pass out @ a PPO2 of 0.10
33ft 2 atm PPO2 0.10 = 5%EAN
66ft 3 ATM PPO@ 0.10 = 3.3%EAN
99ft 4 atm PPO@ 0.10 = 2.5%EAN

So @ 33f, there is a 55% decrease in gas volume
@66ft ~37% decrease in gas volume
@99 ft 27.5% decrease in gas volume.

Keep in mind that the total volume goes down but you can only use the lungs, so the effect is exaggerated as far as what the diver feels.

example;
Total Lung volume = 4L
Total dead space volume (scrubber hoses ect.) = 2 L
Lets say you are using 3L of lung space
total actual volume = 5L
@33ft you loose 5L*0.55= 2.75L whic is 91% of your lung vol You lost almost yout total lung volume
@66ft you lose 5L*0.37 = 1.85L (61%) Lost almost 2/3 of lung volume
@99ft you loose 5L* 1.375 (45%) lost, almost half.

The ratio of dead space to lung space will vary from unit to unit but the example above assumes a pretty worst-case scenario. (dead space is unlikely to be as low as 2L )

So, 12 minutes and those decreases in lung volume still feel beyond noticeable enough to rule this out as something that would go unnoticed IMO.

I would agree that on much deeper dives the risk increases due to the lower volume decrease, but if you are on a deep dive and not looking at your PPO@ for more than 5 min, you are probably asking for trouble.

 

[Wibble]

Total Lung volume = 4L
Total dead space volume (scrubber hoses ect.) = 2 L

Dead space will be more than that. Most CCRs take 2.5 to 4.5 kg of scrubber which would equate to around 2.5 to 4.5 litres of water. Loops will be at least a litre, if not more.

My Revo Mini takes about 10 litres of water when I'm filling it with antiseptic (needs to be mixed at 100:1 so use a jug). The lungs are supposedly 6.5 litres, i.e. about 4.5 litres dead space.

 

[Whitrzac]

I just did this exercise in training.
50 ft depth, 1.2 PPO2 O2 shut off.
12 minutes to get to a PPO2 of 0.7.
ADV fired 6 times to make up the loop volume.

I agree there is no need for an inline shut off.
I also agree that it seems far fetched someone would die from hypoxia if their O2 accidently shut off. PPO2 monitor and and ADV firing would have to be ignored for a prolonged period of time. 50 feet is not deep, there is way more mass of O2 in the loop at greater depths, making this take even longer.

There's a lot of gas in the hoses if you shut off the tank. A shut at the mav/orifice stops flow almost immediately.

It's not far fetched. It's happened multiple times.

 

[Cio]

Dead space will be more than that. Most CCRs take 2.5 to 4.5 kg of scrubber which would equate to around 2.5 to 4.5 litres of water. Loops will be at least a litre, if not more.

My Revo Mini takes about 10 litres of water when I'm filling it with antiseptic (needs to be mixed at 100:1 so use a jug). The lungs are supposedly 6.5 litres, i.e. about 4.5 litres dead space.

I didn't want to exaggerate the numbers so went conservative, the more dead space vs lung volume, the greater the loss of available lung volume.

 

[inquis]

99ft 4 atm , PPO2 1.2 = 30%EAN [...]
99ft 4 atm PPO@ 0.10 = 2.5%EAN [...]
99 ft 27.5% decrease in gas volume.

You're confusing terminology here. It's actually the change in O2 fraction that is 27.5%. The new volume is proportional to the new total molecule count, which has not decreased by 27.5% (since the inert molecule count has not changed).

99ft you loose 5L* 1.375 (45% [of the 3 L lung space]) lost, almost half.

That said, your bottom-line numbers work out to be slightly understated for the assumed conditions since you're just looking at differences. FWIW, the loop volume loss relative to lung volume (assuming no dil addition) is:

(V_start - V_end)/3L = V_loop / V_lung * (PO2_start - PO2_end) / (A - PO2_end), where A is the ambient pressure.​

At 4 atm and PO2 drop of 1.2 to 0.1, Vr = 47%.

I agree this should have been easily noticeable with the crucial stipulation that the diver was near optimal lung volume. As I recall, there was a restriction, and I can easily see the MAV getting bumped, increasing loop volume, and masking the issue. Or he may not have been able to take a full breath, making the detection more difficult. Or maybe he did realize there was an issue but couldn't reach the O2 MAV, leaving dil addition via ADV the only thing that was possible. As your test confirmed, one can still go hypoxic even when loop volume is being topped up. His metabolism/exertion may have been significantly higher than yours resulting in much less than the 12 minutes it took you (which also included multiple loop topoffs).

Your point is quite valid under some conditions, but we just don't know in this particular case. This sums it up in my view:

There are vastly more circumstances where you will pass out and die than those where loop volume, if the depth & dil stars align just right, will save you.

 

[Cio]

@inquis Your math is more accurate, I was using ballparks as it makes the point.

Interestingly enough, both sides of the argument still agree that inline shutoffs are not a good idea. so, I guess right or wrong, if someone thinks it's possible and ditches the shut off, that's a good thing.

 

[crofrog]

It’d be nice if a training agency or some other higher power could force these guys to issue a retraction for that video. The content is downright dangerous and illustrates a real lack of understanding of how a rebreather works. In 11 minutes he breathed the PO2 down by .6. He says the PO2 of the victim’s dil was around .45. The SW has a super small lung, all you need to do is clear your mask hard or vent the loop for any other reason and it will flush the loop down significantly. With a .45 PO2 dil, I’d say it’d drop from a 1.2 down to about .7. Do it again and it’ll be almost a complete flush, so down to that .45 range.

So about 7-8 minutes until lights out. Yes, the ADV will fire or you will need to manually add dil, but with a very hypoxic dil, it won’t drive the PO2 back up very much. So you’re maybe looking at about 10-15 minutes till hypoxic depending on the divers workload and metabolic rate. If a diver is seriously task loaded, and fairly inexperienced, they may not notice and out like a light.

I say should issue a retraction and say “sorry, we are the ones that should have known better than that, I guess this guy and many others have actually gone hypoxic at depth and it’s not some big conspiracy against inline shutoffs and rebreathers without HUDs.

What happens if you’re ascending with the O2 shut off?

My understanding is that the ppO2 would drop because the fO2 is decreasing as you breath it down and the ppO2 would drop as your ascending because of the pressure change.While the loop volume would expand because of the decreasing ambient pressure.

That seems like it could end up with a hypoxic mix in the loop fairly quickly without the loop volume dropping

 

[Wibble]

What happens if you’re ascending with the O2 shut off?

My understanding is that the ppO2 would drop because the fO2 is decreasing as you breath it down and the ppO2 would drop as your ascending because of the pressure change.While the loop volume would expand because of the decreasing ambient pressure.

That seems like it could end up with a hypoxic mix in the loop fairly quickly without the loop volume dropping

When ascending, the gas in the lungs and loop will expand and need venting off, thus the oxygen partial pressure will reduce. All CCR divers have to manage this by dumping loop gas and injecting oxygen.

The point raised in this thread concerns someone staying at a stable depth and controlling the loop oxygen content using diluent and loop volume.

 

[evandroairton]

What happens if you’re ascending with the O2 shut off?

My understanding is that the ppO2 would drop because the fO2 is decreasing as you breath it down and the ppO2 would drop as your ascending because of the pressure change.While the loop volume would expand because of the decreasing ambient pressure.

That seems like it could end up with a hypoxic mix in the loop fairly quickly without the loop volume dropping

That was exactly the fatality scenario i've mentioned

 

[rjack321]

What happens if you’re ascending with the O2 shut off?

My understanding is that the ppO2 would drop because the fO2 is decreasing as you breath it down and the ppO2 would drop as your ascending because of the pressure change.While the loop volume would expand because of the decreasing ambient pressure.

That seems like it could end up with a hypoxic mix in the loop fairly quickly without the loop volume dropping

That's a good way to die - fairly quickly too. All CCRs need O2 added as you ascend.