Just wanted to thank RTodd for one of the better posts on this subject that I've read, through multiple threads.
120/20 rule or a 130 rule?
- Thread starter JessH
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dbulmer
Contributor
Floater,
Personally I would skip the dive(s). I want to relax on the boat and would want to take a drink and chat about the dive I'd just done - I'd feel rushed and would fear the possible consequences of peer pressure but that's me.
*Floater*:
What if it is the second Tuesday after the full moon? How do you factor that into your deco? Again, if you understand the rules, this can be done. It is fairly far from a conservative profile, but very doable. Particularly, if in your example the other divers are diving computers. Most computers on the market are going to freak out on dive 3 and would be doing far more deco than someone that knew how to calc this on the fly.
In saying the computers are going to freak, keep in mind that bottom time is just that, actual time on the bottom. Most divers like to pad the numbers and count far more of the dive as bottom time than really is. There aren't that many people that can actually follow rock bottom and spend 30 minutes at an average depth of 100' on an al 80. So, in figuring out your deco situation you also need to really understand at what depth you are actually spending the bulk of your time. This has a huge effect on your "NDL" status and is what most computers do for you - albeit poorly. This is an important part of the "understanding all of this stuff" caveat.
Also, think about real world. How often do a dive ops logistics allow you to do three quick dives. (And, why would you, this is supposed to be fun not torture.) In open water situations, my wife and I routinely do back to back dives to 100' with a 30 minute surface interval for the first two dives due to operator timing requirements. We do tend to back off depth and time or increase deco slightly for the second dive, but that is in the details that are important. In general, it is the same dive. Having said that, if we can get 60 to 90 minutes of SI we take it. Why wouldn't you?
Almost everywhere you dive you will then get a longer surface interval for lunch and can do a 3rd dive to 100' again if you wish. Again, backing off in certain areas increases the safety. Three hours of bottom time is generally the limit of the fun factor for open water diving for me. But, for a fourth dive, I would try to get a longer surface interval, dive a little shallower, and or back off in other areas.
Charlie99
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Actually, the profile (specified by *Floater* and approved by his instructor) was 4 dives back to back with 20 minute SI. The whole subject came up because in the Computer vs. No Computer thread, he gave wanting to ignore residual nitrogen as a reason for not wanting to use a computer.limeyx:
In addition to just generally being aggressive, the problem I see with the specified profile was that the ascents were just 1 minute each on the shallow stops, and the only extension of stops for the 3rd and 4th dives was to start the 1 minute stops at 40' rather than 30'. The 30/20/10' stops were still just 1 single minute, with a 30 second move between them. Diving agressively and doing the needed stops is one thing. Diving agressively while doing inadequate stops is yet another.
If one applies average depth calculations to the entire sequence, including SI, it exceeds reasonable limits. (I guess for DIR guys, that is more PC than saying that the repetitive profile has too high of loading in the slow compartments.
)It goes back to the basic question of whether or not residual nitrogen can be ignored. I choose to take it into account. A few divers apparently believe that a good ascent somehow magically unloads the slow compartments. The link to the e-mail exchange in one of my earlier posts.
It appears that *Floater*'s instructor has a different view of deco than other DIR-F instructors.
*Floater*
Contributor
RTodd:
I do not. I've never heard about factoring moon phases into your deco? Please elaborate on what you have in mind.
Doing 3 of those dives with 25 min of BT (including 3 minutes for descent) on EAN32 is okay with GAP-RGBM run on nominal (middle setting) with 5thD-X style min deco ascent (i.e. doubling shallow stops).
However, I was just wondering how people using the minimum of 60 minutes SI system would deal with this example because their system is not well defined for repetitive dives with less than 60 minutes SI, and I picked the example to be a borderline case.
The system I was taught in DIRF (no 60 minute SI requirement) is well defined for all situations, though it can produce very aggressive dives if applied literally and taken to extremes (many repetitive dives with short SI's and each to NDL limits), but as you point out below those extremes almost never arise in the course of normal diving, so the system works pretty well even assuming that residual nitrogen is not a non-issue.
I agree with what you say, which is why I think the system works in practice even though in theory you can find examples where it may not.
In practice I think I would double the shallow stops for repetitive dives with SI's between 60-90 minutes, triple them for 30-60 minute SI's when diving near the limits, and play it by the ear for less than that. That satisfies both what I was taught in DIRF and also what my deco software says for some examples I worked out. Also, it's only aggressive for the borderline cases which hardly ever arise in my normal diving. For the vast majority of my dives it'll be conservative (by GAP-RGBM standards anyway).
limeyx
Guest
Charlie99:Actually, the profile (specified by *Floater* and approved by his instructor) was 4 dives back to back with 20 minute SI. The whole subject came up because in the Computer vs. No Computer thread, he gave wanting to ignore residual nitrogen as a reason for not wanting to use a computer.
In addition to just generally being aggressive, the problem I see with the specified profile was that the ascents were just 1 minute each on the shallow stops, and the only extension of stops for the 3rd and 4th dives was to start the 1 minute stops at 40' rather than 30'. The 30/20/10' stops were still just 1 single minute, with a 30 second move between them. Diving agressively and doing the needed stops is one thing. Diving agressively while doing inadequate stops is yet another.
If one applies average depth calculations to the entire sequence, including SI, it exceeds reasonable limits. (I guess for DIR guys, that is more PC than saying that the repetitive profile has too high of loading in the slow compartments.
It goes back to the basic question of whether or not residual nitrogen can be ignored. I choose to take it into account. A few divers apparently believe that a good ascent somehow magically unloads the slow compartments. The link to the e-mail exchange in one of my earlier posts.
It appears that *Floater*'s instructor has a different view of deco than other DIR-F instructors.
OK, I must have mis-remembered (and I didnt want to trawl through the threads -- lazy old me).
I think some of the confusion might come from people believing that residual nitrogen still "exists" but that the reason for the tables being the way they are is due to "poor" standard ascent behavior, so a lot of the conservatism in them is present due to bubbling cause by getting shallow too quickly (just guessing).
This would mean that the body can actually tolerate a lot more dissolved N2 *if* you ascend properly to avoid the bubbles.
However, I'm still not sure I'm willing to do 4 30 min dives to 100 with a 30 min SI
Like RTodd said, 2 followed by a long SI for dive 3, sure.
And following 5thd-x rules, 6 or 7 dives per day is fine.
limeyx
Guest
*Floater*:
Um, how else are you going to apply it other than literally?
For sure a 30 min SI is *metaphorically* OK because you aren't the one actually doing it ?
limeyx
Guest
*Floater*:
RTodd basically answered for me, except
1) I dont dive with anyone that uses a computer (not saying I wouldn't, just that it works out that I don't)
2) If the site was awesome, I'd go back in and do the dive (noting that you have already backed it from 30 mins to 25
I would definitely double the 30, 20, 10 stops and my standard ascent already includes 1 min stops from 50 feet (so already more conservative than the other profiles in question)In this case, as RTodd said, I'd make sure I had a longer interval before dive 3
3) If the dive was "so so" I'd either skip it, or ask the others if they'd wait for me.
*Floater*
Contributor
limeyx:Um, how else are you going to apply it other than literally?
For sure a 30 min SI is *metaphorically* OK because you aren't the one actually doing it ?
No, I just meant that even though the (minimum) rule is to do a linear ascent from 80% ATA at 10'/min there's no harm in spending a little extra time at the shallow stops if you feel it's warented.
OP
This is what I personally belive, but it is based on my own assumptions and logic; i don't really have any data to back it up. It seems to me that some people are being taught that under the right circumstances they can ignore resigual nitrogen and they are taking this to mean that they don't have any residual nitrogen. It seems to me that minimum deco style ascents eliminate bubbling. That doesn't mean that the residual nitrogen doesn't exist, it simply means that residual nitrogen doesn't matter because you are keeping it from forming bubbles and they are what cause all of the problems.limeyx:
Under normal circumstances if you are doing proper minimum deco ascents I don't forsee any problems with this. Where I see it causing problems is when things don't go exactly as planned. Say you are doing recreational single tank diving. You have already done multiple dives today. You do a dive to 100ft with your buddy and on the way up you blow an oring and lose all of your gas. Everything else goes right and you get your buddys reg in your mouth, but that needle on your buddies spg seems to be going down a little fast for comfort. You realize that that aluminum 80 doesn't really hold all that much reserve gas for two people, you are also a little freaked out by the situation. So you decide to simply call the dive instead of doing all of that minimum deco you would normaly do. Besides it is the last dive of the day and most divers don't do minimum deco so you should be fine, right? Well all of that nitrogen loading you were previously ignoring now becomes very important. A minimum deco ascent would keep you from bubbling, but a straight ascent to the surface, even a slow one might not.
This is just one example of what could happen, I have seen a lot of divers blow off their safety stop because they were having some sort of problem, most of them were a lot less serious than losing all of your back gas.
This is why I personally feel that with most of the recreational, single tank diving that I do I need to keep my residual nitrogen level at a reasonable level. Minimum deco ascents are one way that I do this. I feel that they do result in faster offgassing and do get you out of the water cleaner, but not clean enough to totally disregard residual nitrogen unless you are in a situation where you would feel comfortable with doing a minimum deco ascent after the **** has really hit the fan. I think that most people doing single tank diving are not prepared to do this.
Now if you do a 90min SI between every dive combined with a minimum deco ascent that MAY keep your residual nitrogen level low enough that you don't have to worry about it. That is why I prefer to do a 90+ minute SI. This does not mean that I won't do a shorter SI at times, but I will keep in mind what it is doing to my residual nitrogen level and compensate accordingly. I do this in order to keep myself out of situations that I am not prepared to handle.
I think that the whole deep dive first vs deep dive last thing is related to this. It is not that one group is right and the other is wrong, it is simply that they have different goals. Doing deep dives first and then moving shallower keeps your residual nitrogen at a reasonable level. Doing shallow dives first and then moving deeper disregards residual nitrogen in favor of minimizing bubbles. By moving deeper you crush any bubbles that may have formed on previous dives. If you combine this with a proper ascent any extra residual nitrogen won't matter. The problem is that this assumes a proper ascent. As stated above I feel that this might not be a valid assumption when doing single tank diving, even for very good and highly trained divers. Of course I don't know for certain that a deep last sequence of dives does result in more residual nitrogen, and if it doesn't then it might actually be better for all dives.
~Jess
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