DCI and the perils of diving in a mixed EAN/Air Group

[dmaziuk]

...
The incident happened during the third dive of the day after two earlier dives to 30m. With still 10mins showing my dive computer I spotted that my back up computer (that I take along more in case of a failure that to use during the dive) was flashing a warning and had switched into deco mode. I signalled to the guide/leader that time was up and did the usual safety stop...

1. How many days and dives/day until that one?
2. What were the computers?

 

[guyharrisonphoto]

It seems to me there were several issues going on.

As to the computers, it seems the OP used computers with the same algorithm, but had (accidentally or deliberately) set a higher "conservatism" factor on one of them. But, according to the base algorithm, the OP was NOT in deco status at the end of the third dive, it was only the "extra" conservatism entered into one computer that triggered a deco warning, and, presumably, rendered his backup computer useless for at least 24 hours. But, it sounds like the OP did not understand what the differing information the computers were giving him meant.

The conclusion here is that, whatever the algorithm was, it was "too liberal" for the OP on that particular day, who learned the hard way, as even being within the base algorithm he received a hit.

But, in any case, if the OP believed that an ascent was critical, he should not have dallied, and alerted his buddy or the DM as he was approaching NDL on the more conservative computer.

As to diving with nitrox divers, enough said. That makes the OP the odd man out and impinging on the ability of divers who have paid for training and a fortune for a once in a lifetime trip to enjoy it to the max.

The procedures for ending a dive should have been coordinated with the dive master. The OP should either have been paired with a buddy who agreed to the shorter dive times, or the DM should have escorted the OP to the surface. Either way, there should have been a coordinated plan.

Finally, very little is known about skin bends and its link to more severe bends. It is possible that the OP could keep diving the original algorithm and not experience another hit. But (as someone who has had one hit of skin bends myself and saw a diving doctor about it), I would increase the conservatism by at least one level, and go to a 5 minute safety stop, and really, really carefully monitor ascent rates.

And, make sure both computers run the same algorithm and are set the same way!

 

[John the Pom]

Back when we dived using tables (and dinosaurs roamed the ocean) calculations were based on maximum depth, and we always rounded up, so for a typical dive there was a built-in safety factor. Nowadays I see divers pushing their computers close to the limit routinely. If they ask me, "Is it safe?" I reply, "Yes, the computer is perfectly safe. It is the person wearing it who is putting themselves at risk!"

 

[scubadada]

So, I assume these were Suunto computers with their P values. The fact that you were hit makes me think you are in a predisposed group. If you do not have a PFO, you must have some other predisposition. Good luck in your diving.

 

[kafkaland]

A simple solution might have been, once you were at the safety stop and knowing that you were pushing the limits based on the backup computer, to simply signal “five minutes”. I.e., signal “level off” for the safety stop, followed by a “five”. I’m sure the divemaster would have understood, and there is absolutely nothing embarrassing about asking for a bit of conservativism, especially since you were on air. And then very slowly ascend to the surface, being the last to the boat. This might have made all the difference, without even mentioning the D-word on the boat, or ever.

 

[chillyinCanada]

I’ve read of the very slow ascent from the safety stop to the surface. How slow is ‘slowly’.

I've never timed it but it's excessively slow and everyone else is already on the surface well before my head pops up.

 

[DandyDon]

A simple solution might have been, once you were at the safety stop and knowing that you were pushing the limits based on the backup computer, to simply signal “five minutes”. I.e., signal “level off” for the safety stop, followed by a “five”. I’m sure the divemaster would have understood, and there is absolutely nothing embarrassing about asking for a bit of conservativism, especially since you were on air. And then very slowly ascend to the surface, being the last to the boat. This might have made all the difference, without even mentioning the D-word on the boat, or ever.

That approach would help, but I wouldn't call it the solution. Generally diving more conservatively and/or getting nitrox certified would be better.

 

[Mike Walker]

In the context of this thread, I disagree.

(I have drifted a bit outside the context of this thread... this is something that's been on my mind for a while and this discussion provided a good platform to float it out there)

On the other hand, to address the "sheep" issue, the way that you communicate to recreational divers about what lies beyond that line is to tell them that they shouldn't plan on doing it without training, and appropriate redundancy and gas reserves.

With this I completely agree.

But you should also tell them that if for whatever reason they find themselves with a deco obligation, just follow the computer and do the deco. You don't need to do "a little bit of deco" to make that point. By suggesting de minimis cases ("what about just ONE MINUTE of deco? Can I do that?"), we normalize variance and make the next person push it a little further.

With this I do not - but in the sense of a 'which is the lesser evil' argument.

It relies on every single person - possibly with as little as one dive - using any one of the hundreds of computers available being able to:
a) identify they are in decompression and not experiencing some other error
b) know generally what this means (i.e. extra stops and longer ascent, not 'sky is falling')
c) understand and comply with the instructions their computer is giving them

(Yes, one dive is HIGHLY unlikely, but I'm sure you know what I mean....)

Some computers are verbose enough that this can be 'figured out' on the spot, even under some degree of stress. I doubt this could be said about all of them.
My diving experience is not yet that vast - but I've personally seen on two occasions someone coming on board and wondering what it meant that their computer was beeping at them. ("But I just followed the guide...")
My area of expertise does, however, include trying to make humans and technology communicate effectively with each other. Most people need some degree of explanation of even the simplest of interfaces.

There needs to be an effective means of making sure the first time someone sees deco behaviour on their computer is not also a situation where there are other sources of stress at play.

This is not about normalizing deco as 'ok'. It's precisely the opposite - experiencing it once in controlled conditions and then never doing it again without the proper training. Stepping through the process so that it is not unfamiliar or scary if it does happen at an inopportune moment (or due to some condition beyond the divers control at which point they are probably already in some sort of stressful situation). A 'line' is only appropriate when the likelihood of an experience is essentially a binary choice ('DO NOT ENTER' and there's a lock on the door to make sure that doesn't happen) - entering 'computer deco' is a big gray area (an extra couple minutes looking at a critter, an extra couple meters due to a down current, a conflict between algorithms with an instabuddy) and as such it needs to be treated as likely experience. One could go to effort of lobbying every computer manufacturer to implement a 'deco demo mode' on every computer to allow for adequate practical training (I doubt reading the manual would not be adequate for most learners), or, one could contemplate that moving a hair further into the gray in a controlled environment, on a single occasion to allow for first hand observation of the process on any computer model (past, present and future) might provide for more benefit than harm.

There will always be people who ignore the rules. Leave them to their devices and focus on what is best for those wise enough to willingly comply with justified limits but who might end up in an unexpected situation because there is not a big locked door keeping even the most careful of individuals out of the 'dreaded deco zone'.

(Going back to this thread - it does concern me greatly that there are suggestions being made about how to hide accidentally going into 'computer deco'. This is what happens when a 'line' is created and is not healthy. Mistakes and minor deviations from accepted practice must be allowed, accepted, discussed and learned from - as the OP has initiated with his post. Expecting arbitrary humans to bow down to the wisdom of the diving gods on the principal of 'cause I said so' is not really a recipe for success.)

 

[Graeme Fraser]

(I have drifted a bit outside the context of this thread... this is something that's been on my mind for a while and this discussion provided a good platform to float it out there)



With this I completely agree.



With this I do not - but in the sense of a 'which is the lesser evil' argument.

It relies on every single person - possibly with as little as one dive - using any one of the hundreds of computers available being able to:
a) identify they are in decompression and not experiencing some other error
b) know generally what this means (i.e. extra stops and longer ascent, not 'sky is falling')
c) understand and comply with the instructions their computer is giving them

(Yes, one dive is HIGHLY unlikely, but I'm sure you know what I mean....)

Some computers are verbose enough that this can be 'figured out' on the spot, even under some degree of stress. I doubt this could be said about all of them.
My diving experience is not yet that vast - but I've personally seen on two occasions someone coming on board and wondering what it meant that their computer was beeping at them. ("But I just followed the guide...")
My area of expertise does, however, include trying to make humans and technology communicate effectively with each other. Most people need some degree of explanation of even the simplest of interfaces.

There needs to be an effective means of making sure the first time someone sees deco behaviour on their computer is not also a situation where there are other sources of stress at play.

This is not about normalizing deco as 'ok'. It's precisely the opposite - experiencing it once in controlled conditions and then never doing it again without the proper training. Stepping through the process so that it is not unfamiliar or scary if it does happen at an inopportune moment (or due to some condition beyond the divers control at which point they are probably already in some sort of stressful situation). A 'line' is only appropriate when the likelihood of an experience is essentially a binary choice ('DO NOT ENTER' and there's a lock on the door to make sure that doesn't happen) - entering 'computer deco' is a big gray area (an extra couple minutes looking at a critter, an extra couple meters due to a down current, a conflict between algorithms with an instabuddy) and as such it needs to be treated as likely experience. One could go to effort of lobbying every computer manufacturer to implement a 'deco demo mode' on every computer to allow for adequate practical training (I doubt reading the manual would not be adequate for most learners), or, one could contemplate that moving a hair further into the gray in a controlled environment, on a single occasion to allow for first hand observation of the process on any computer model (past, present and future) might provide for more benefit than harm.

There will always be people who ignore the rules. Leave them to their devices and focus on what is best for those wise enough to willingly comply with justified limits but who might end up in an unexpected situation because there is not a big locked door keeping even the most careful of individuals out of the 'dreaded deco zone'.

(Going back to this thread - it does concern me greatly that there are suggestions being made about how to hide accidentally going into 'computer deco'. This is what happens when a 'line' is created and is not healthy. Mistakes and minor deviations from accepted practice must be allowed, accepted, discussed and learned from - as the OP has initiated with his post. Expecting arbitrary humans to bow down to the wisdom of the diving gods on the principal of 'cause I said so' is not really a recipe for success.)

If the case is being made for knowing what information the computer will display under deco obligation and how to interpret and act on this information, I agree. However I do not agree that the first time this should experienced for real is by an untrained diver and without appropriate guidance from an instructor.

I have read the manual for a range of dive computers and a good proportion provide adequate deco instructions and display images so that the user is not faced with unknown data in this situation. There are also online simulators and YouTube footage for various models.

 

[dmaziuk]

So, I assume these were Suunto computers with their P values. The fact that you were hit makes me think you are in a predisposed group. If you do not have a PFO, you must have some other predisposition. Good luck in your diving.

I met a DM once who mentioned her chamber ride and when asked about the dive said: "the chamber people, when they heard how many dives I have and what kind, said to not worry about it: it was 'just my time'". You have to remember that it's a statistical line drawn through the fuzzy area, it doesn't work out the same way when you are the statistic.