Prebreathing Survey

What is your prebreathing procedure?

  • None

    Votes: 16 18.8%
  • 5 minutes, wearing unit

    Votes: 19 22.4%
  • 5 minutes, before donning unit

    Votes: 16 18.8%
  • Less than 5 minutes, wearing unit

    Votes: 30 35.3%
  • Less than 5 minutes, before donning unit

    Votes: 4 4.7%

  • Total voters
    85

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

At what point does "many" change from survivorship bias to "data"?

If I do something dumb once or twice and don't have a problem, saying that it's safe is what I would call SB.

If I do something 10,000 times and don't have a problem and then say it's safe (for me), is that still SB?

If you do something 10,000 times and don't have a problem and then say "it's safe for me", that's not survivorship bias.

If you do something 100,000 times and don't have a problem and then say "it's safe", that IS survivorship bias. Which is the point that I'm making.

If you want "data" to produce a generalizable rule that you can publish for the diving community to prove the safety of anything, then you need n>1.
 
That's all I'm asking for. The body of evidence. You don't need to do CE testing to get numbers. But if you are going to categorically state that a 2.4 kg scrubber will last for a minimum of 6 hours in 25C water, I personally would like something more reproducible than "many excellent divers do that, and they are fine". Because until you actually do a real world prospective diver study, you just don't have that data.

I'm not saying that such studies don't exist. I'm just asking for them. And if they don't exist, then my point stands.

Hello Mike and others,

Many factors are important in determining the duration of a scrubber, but arguably none moreso than exercise levels and therefore the amount of CO2 presented to it. To bring some objectively to this debate about duration, there are some great examples in our tempstick efficacy study that will be published in Diving and Hyperbaric Medicine in March

Silvanius M, Mitchell SJ, Pollock NW, Franberg O, Gensser M, Linden J, Mesley P, Gant N. The performance of temperature stick carbon dioxide absorbent monitoring devices in diving rebreathers. Diving Hyperbaric Med. 49(1), 2019: In press.

We ran an Inspo Evo plus (2.6 Kg scubber - sofnolime 797) immersed in 20oC water at surface pressure on a test circuit that ventilated the rebreather and introduced CO2 to the exhaled gas to simulate two scenarios:

1. A diver exercising at 6 MET for the duration of the dive - this is pretty close to the CE test standard - maybe a little harder exercise-wise, until CO2 broke through the scrubber to 1 kPa (1% surface evquivalent CO2).

2. A diver exercising at 6 MET for 90 minutes, followed by a diver resting on deco (about 2 MET) until CO2 broke through the scrubber to 1 kPa. The latter was designed to be more in keeping with what happens on a real technical dive (hard work at first followed by a longer period of relative rest).

Everything was identical on all repetitions (6 on each protocol) except the simulated exercise.

The average duration over multiple repetitions on protocol 1 was 3 hours, and on protocol 2 it was just under 8 hours.

Please understand that these tests were NOT designed to establish expected durations for this scrubber in real diving. For example, the tests did not take place at pressure. But the data do illustrate the remarkable context sensitivity of scrubber duration where context in this case is exercise load. So, yes scrubbers probably can last 6 hours, but only if you are not working too hard!

The temp stick results (how good the temp sticks were at predicting CO2 breakthrough in inspo and revo rebreathers) will be published in the paper.

Simon

PS. Mike, you asked me what I do regarding prebreathing. I do a prebreathe. I do it for long enough to see that the controller establishes and begins to maintain surface PO2 setpoint. It usually takes a couple of minutes.
 
Thanks, Simon... that’s very helpful!
 
If you do something 10,000 times and don't have a problem and then say "it's safe for me", that's not survivorship bias.

If you do something 100,000 times and don't have a problem and then say "it's safe", that IS survivorship bias. Which is the point that I'm making.

If you want "data" to produce a generalizable rule that you can publish for the diving community to prove the safety of anything, then you need n>1.
Could not agree more, well said.
 
If you do something 10,000 times and don't have a problem and then say "it's safe for me", that's not survivorship bias.

If you do something 100,000 times and don't have a problem and then say "it's safe", that IS survivorship bias. Which is the point that I'm making.

If you want "data" to produce a generalizable rule that you can publish for the diving community to prove the safety of anything, then you need n>1.

Fair point. So, let me ask it a different way.

If 3 divers say "I dived my JJ scrubber for 5 hours* and it was fine, therefore I think it's safe", that's SB, right?

But, if 1,000 divers say the same thing, is it still SB?

If 1,000 divers say they did it 100 times each, is it still SB?

I imagine your response will be "yes, it's still SB because we are not hearing from ANY of the people who did the same thing but died because of it."

Valid.

But, it seems like, A, we do have lot of divers with lots of dives that are saying they did it and think it's safe. And, B, though the dead don't speak, it seems like there would be plenty of reports in the general community's awareness if there were statistically significant #s of people who have had real problems using the same approach.

I understand that you want something akin to a rigorous, scientific study. That doesn't exist (that we know of). The question I'm trying to ask is, at what point does the data that does exist (in the somewhat anecdotal form I described above) become sufficiently compelling to be considered "useful data" versus "insignificant anecdotes"? If your personal answer is "never", I certainly won't fault you for that.

"Everybody says" is not often a reasonable basis for a specific course of action. But, sometimes it is...

*5 hours is a semi-random spec chosen just for the sake of discussion. I'm not trying to make a case specifically for a JJ and 5 hours.
 
But Stuart, that's the thing. Just because logistically or financially we can't do a formal study, it doesn't change the validity of the scientific method.

None of us have anything like that sort of data set. When you say that 1000 divers did it 100 times, what do you mean? I don't know 1000 people, let alone 1000 divers, let alone 1000 CCR diver pushing their scrubbers in warm water.

When someone says that lots of Caribbean CCR instructors push their scrubbers, does that mean that they have asked 5 people what they do and have an accurate accounting of work rates, times, water temps, whatever? Or is it just a general impression of what people do, based on casual conversation. Seriously, the plural of anecdote is not data.

It's fine to come here and say "this is what I do and I haven't had a CO2 hit". But I object to saying "this is what I do, and it has an acceptably infinitesimal chance of causing YOU to have a CO2 hit".

Here's another thing. You said "yes, it's still SB because we are not hearing from ANY of the people who did the same thing but died because of it." But dying is not the only reason why you might get a survivorship bias. If people are more likely to report one result than the other in this thread, then your data will be enriched by one type of answer. It's the reverse with Yelp review of restaurants and hotels - complainers are much more likely to post than satisfied customers.

Or consider the thing that ALWAYS comes up about WAI computers, that drives me crazy. You have these threads with people swearing that their WAI is always great. But if you had 1000 WAI computers with a 20% failure rate, that would be totally unacceptable, but you could still point to 800 people who SWEAR that their WAI never has a problem.

So all I'm saying is that unless you are prepared to do what Dr. Mitchell does, and test things using the scientific method, I would be careful about coming here and implying that AS A GENERAL RULE, CE scrubber durations are overly conservative, and you should just work out whatever duration you want to use on your own.
 
So all I'm saying is that unless you are prepared to do what Dr. Mitchell does, and test things using the scientific method, I would be careful about coming here and implying that AS A GENERAL RULE, CE scrubber durations are overly conservative, and you should just work out whatever duration you want to use on your own.

I get all that. All I'm wondering is, is there ever a point where you read the CE scrubber duration (let's call it 3 hours at 4C) and then you hear instructors and the unit's designer tell you that it's fine for 5 hours at 20C, and you hear many people in the community affirm that as well, and you say "okay, I expect it to be okay for me for 5 hours at 20C and I am willing to try it"?
 
I get all that. All I'm wondering is, is there ever a point where you read the CE scrubber duration (let's call it 3 hours at 4C) and then you hear instructors and the unit's designer tell you that it's fine for 5 hours at 20C, and you hear many people in the community affirm that as well, and you say "okay, I expect it to be okay for me for 5 hours at 20C and I am willing to try it"?

Yeah, that is the better point. We do stuff all the time that isn't scientifically validated. Like the off-label use of medication, it always comes down to balancing risks vs. benefits.

So assuming that a CO2 hit is not going to be so insidious that you wouldn't be able to detect it early, handle it and survive (a big assumption!), I can see why people do the "listen to your body" approach and just push further to find their own limits.

I rarely dive in warm water. But if I were living in the Caribbean and doing a LOT of CCR diving, I might be more inclined to try to make that analysis. Right now, for me, the benefit would be using a little less sorb. It just doesn't seem like that much of a benefit.

One minor point. If the JJ's designer told me that it was OK, that would be a completely different story. But Jan wont even let me update the controller with the new Shearwater firmware until he has tested it out, so I doubt that he is going to tell me to just wing it on scrubber duration! :)
 
You could assemble a fatality database, since it's pretty rare that those don't get reported somehow. But you said it yourself - "all are possibilities that we can't conclusively know". So the burden of proof is on the positive.

Yea we have one of those at deeplife :p

To have survivorship bias you have to have two sided data, that is absent however. Hence we don't have a problem that can be parsed the way you want to parse it.
 
Something that occurs to me, especially with the results Simon posted, is that there is a really significant difference between doing 4 one hour dives on a scrubber and one 4 hour dive with a chunk of that being deco. Once you break those dives over multiple days, with all the variables in packing and storage, factoring in the low cost of sorb, I for one won’t be stretching my scrubber for the sake of more dives but I also am not too phased over going a few minutes over the CE numbers on a nice warm dive with no strenuous activity.
 

Back
Top Bottom