Decompression Stop Guidelines - What we have to do if got deco alert?

[Lorenzoid]

IDCBali was last seen: Wednesday at 8:47 PM

In other words, he logged out two minutes after NetDoc made the first post and hasn't been back to the site since.

In at least one previous thread I can recall, when someone pointed out "the OP has not returned to his thread," the OP suddenly chimed in and explained that he usually reads SB as a guest, not logged in, and that he had in fact been following the replies.

 

[Diver-Drex]

A point on the "there is never an excuse to go into unplanned deco" comments. I've been involved in two dives where it has happened.
The first was a case where the grapple pulled out of the wreck and the boat was gone. We made the decision to shoot a bag, clip a reel into the wreck and get ourselves in order before ascent. This extra time incurred a stop but was done to avoid a full on drifting blue water ascent.
Second time I came up on an entangled diver. I knew I had enough gas to handle deco if I stayed to help, so I did.
There are countless other stories that are similar I'm sure. This is different then not paying attention and then suddenly realizing your computer isn't happy. Accidents happen, people make mistakes. Car accidents shouldn't happen but you wear a seatbelt because they do happen.
Intentional or not, either way it is important to know how to follow your computer if you are using one or read your tables. This needs to be taught to all divers. If you're making the decision to exceed the NDL you need to be able to calculate your gas needs on the fly. I think it's more prudent to stress in training that new divers stay within NDLs.

 

[TrimixToo]

A point on the "there is never an excuse to go into unplanned deco" comments. I've been involved in two dives where it has happened.
The first was a case where the grapple pulled out of the wreck and the boat was gone. We made the decision to shoot a bag, clip a reel into the wreck and get ourselves in order before ascent. This extra time incurred a stop but was done to avoid a full on drifting blue water ascent.
Second time I came up on an entangled diver. I knew I had enough gas to handle deco if I stayed to help, so I did.
There are countless other stories that are similar I'm sure. This is different then not paying attention and then suddenly realizing your computer isn't happy. Accidents happen, people make mistakes. Car accidents shouldn't happen but you wear a seatbelt because they do happen.
Intentional or not, either way it is important to know how to follow your computer if you are using one or read your tables. This needs to be taught to all divers. If you're making the decision to exceed the NDL you need to be able to calculate your gas needs on the fly. I think it's more prudent to stress in training that new divers stay within NDLs.

A perfectly fair point. So let's differentiate between deliberately changing the plan on the fly (if you know what you're doing), and the real villains: accidental deco obligations by untrained divers, and deliberately changing the plan on the fly without a realistic assessment of the risks involved (again likely by untrained divers).

John

 

[gfaith]

@Diver-Drex good post two excellent examples.

 

[The Chairman]

They are great examples, and as @TrimixToo pointed out: they weren't 'accidental'. Very rarely does a single issue result in an injury. However, the inability to recognize when you need to make a decision and verifying your new limits against the resources on hand. It can be as simple as glancing at your gas supply and your guestimate of how much you'll need. Just don't cut it too fine. That's when the cascade can start.

 

[KevinNM]

And if you do cut it too fine, when you run out of gas you had best be on the surface. People can sometimes get away with screwing up decompression and come out fine, sometimes without even a symptom of DCS having skipped quite a lot of deco. You can't breath water.

 

[guyharrisonphoto]

As everyone has noted, the original post was so wrong because it did not address the elephant in the room, which is whether there is enough gas to perform the unplanned deco. The two examples above (intentional but unplanned deco), or even accidental unplanned deco, are where an AI computer with a "gas time remaining" algorithm can be a real safety benefit. This hasn't been discussed here yet.

On my computer (galileo sol) the GTR is based on beginning your ascent and performing all stops, to arrive at the surface with your set reserve. The GTR also adjusts in real time during the ascent. If you go into deco, the GTR will adjust downward at your current depth to account for the additional ascent time. There is no need for an "on the fly" estimate of whether you have enough gas, as the computer will tell you with a very high degree of reliability. If you go into deco and have GTR remaining, you have a reliable calculation that you can perform all the stops and still reach the surface with your reserve, and this will hold true even if you stay at depth, until the GTR reaches zero. If GTR goes down to "zero" immediately upon going into deco (or you are in deco and stay at depth until GTR hits zero) then you know you must start the ascent immediately and monitor your tank pressure (once GTR is zero, the display only shows pressure because you are eating into your pre set surface reserve, and gives you an alert at 200 psi because AI transmitters (and SPGs) have questionable accuracy below that).

The best thing is that these calculations are done in real time, based on actual dive conditions and actual gas consumption at the time. If the computer, during the ascent, gives you "back" some GTR (ie it again shows more than zero) then you are fine, especially if it gives you "back" some deco time as well by reducing your obligation during the ascent. If not, you know you will have to surface, maybe before you finish your deco stops, based on monitoring your tank pressure. But, at least you will know what to do and not have to guess or risk going OOG, and take the appropriate steps once you surface (getting on 02, monitoring for symptoms etc).

I have never put this to the test, as I don't do accidental unplanned deco and have not faced a situation where something compelled me to go into deco (a rescue, assist, etc). But, I know what my computer will be telling me if that happens, and how to apply it.

But, of course, this requires detailed understanding of what your computer is telling you, which is a whole topic by itself.

This also is no substitute for training in deco, but that is a different story as this thread seems to contemplate what happens in the rare instance of unplanned deco.

 

[The Chairman]

I have never put this to the test, as I don't do accidental unplanned deco and have not faced a situation where something compelled me to go into deco (a rescue, assist, etc).

Guy, one of the underlying tenets of overhead or deco diving is being conservative and redundant to the point of almost being ludicrous. If I am contemplating a deco dive, I'll be adhering to the rule of thirds, and possibly quarters. Why? At those depths, gas gets consumed quite briskly and your deco obligation increases exponentially. Ergo, I am incredibly uncomfortable relying on the base "reserve" used by any AI PDC. In fact, if I am going to willingly enter into any mandatory deco obligation, I want redundant gas as well as a deco gas. If I am begrudgingly entering in that situation due to an unforeseen circumstance, then my entire focus will be to get out from under that obligation as soon as possible with the least amount of risk possible.

When you're in the thick of it, it's one thing. Entanglements, finding the anchor and other issues might make deco a necessity and you need to assess as well as use all of the resources at your disposal to be sure it doesn't turn into an emergency. Just don't let that "gas time left" figure lull you into complacency. You should be alarmed that you've eroded your safety buffer and be taking steps to get it back safely. I'm not taking exception to what you wrote as much as adding the caveat, that divers shouldn't enter into deco casually and they really shouldn't use the "gas time left" as an excuse to enter into a mandatory deco obligation casually.

One last thing: If you're on an 'unplanned' deco, use as much gas as you can to fulfill your obligation. That talk about getting on the boat with at least 500psi left in your tank ceases to apply anymore. Just don't drown trying to get that last breath! Running out of gas at depth can be distressing. If I still had deco left, I would ascend at 50PSI and get on oxygen when I'm on the boat. I would be on that O2 at least 20 minutes or 5 times the amount of deco I missed, which ever is longer, monitor myself for DCS (taking appropriate steps) and staying out of the water for 24 hours.

 

[Lorenzoid]

@Diver-Drex good post two excellent examples.

Completely! The "emergency" or "accident" is not the deco itself but rather what prompted the diver to deliberately change his plan on the fly. It's not "emergency deco," it's "emergency-related deco." And the diver really needs to understand what he's doing.

A related thought is that keeping a generous buffer of no-deco time is also useful because it allows time for handling emergencies. If you let your no-deco time reach just about zero and THEN discover the boat has unhooked and you need to shoot an SMB, that's not so great.

 

[guyharrisonphoto]

Guy, one of the underlying tenets of overhead or deco diving is being conservative and redundant to the point of almost being ludicrous. If I am contemplating a deco dive, I'll be adhering to the rule of thirds, and possibly quarters. Why? At those depths, gas gets consumed quite briskly and your deco obligation increases exponentially. Ergo, I am incredibly uncomfortable relying on the base "reserve" used by any AI PDC. In fact, if I am going to willingly enter into any mandatory deco obligation, I want redundant gas as well as a deco gas. If I am begrudgingly entering in that situation due to an unforeseen circumstance, then my entire focus will be to get out from under that obligation as soon as possible with the least amount of risk possible.

When you're in the thick of it, it's one thing. Entanglements, finding the anchor and other issues might make deco a necessity and you need to assess as well as use all of the resources at your disposal to be sure it doesn't turn into an emergency. Just don't let that "gas time left" figure lull you into complacency. You should be alarmed that you've eroded your safety buffer and be taking steps to get it back safely. I'm not taking exception to what you wrote as much as adding the caveat, that divers shouldn't enter into deco casually and they really shouldn't use the "gas time left" as an excuse to enter into a mandatory deco obligation casually.

One last thing: If you're on an 'unplanned' deco, use as much gas as you can to fulfill your obligation. That talk about getting on the boat with at least 500psi left in your tank ceases to apply anymore. Just don't drown trying to get that last breath! Running out of gas at depth can be distressing. If I still had deco left, I would ascend at 50PSI and get on oxygen when I'm on the boat. I would be on that O2 at least 20 minutes or 5 times the amount of deco I missed, which ever is longer, monitor myself for DCS (taking appropriate steps) and staying out of the water for 24 hours.

We agree 1000%. I would never consider deliberately going into deco without proper gear or training, and certainly not based on GTR as the basis for gas planning. But, I was talking about if the unforeseen (accidental overstay of NDL or having to deal with an emergency) happens on a single tank rec dive, then the GTR would be a very valuable measurement of the "resources at your disposal" (gas and time) for completing the dive. You still have to make your own intelligent decisions as to how to use those resources, including using your reserve to satisfy the deco obligation (which is what the the reserve is for--a cushion for the unexpected). I also assume without question that, in every case of unplanned deco, the main goal is to get headed to the surface just as soon as possible.

But, I wouldn't dismiss the value of having the GTR info. Good information is the enemy of panic, and is valuable just for that.The alternative is a guess about "can I make it" (made at depth, under stress, wondering did I remember my SAC? What is the formula at depth??? OMG!! PANIC!!!) versus solid information about your status at a glance ( "GTR says I have an extra couple of minutes to help my buddy and still make my stops with a reserve at the surface, so don't have to abandon them" or "GTR zero, If I stay here, I am eating into my reserve and I have to watch my PSI, or maybe skip part of my obligation").

How people use that info is up to them, but at least they can make an informed choice based on real information.

Also, if I wasn't clear about my description, GTR does not send you up at your reserve, but well in advance of your reserve, so that you have the reserve intact at the surface. On a 100' dive with an AL 80, my computer usually will send me up at about 700-750 psi