120/20 rule or a 130 rule?

[limeyx]

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.
...

~Jess

Extremely well put (IMO)

I propose a moratorium on this thread until Floater posts 4 back to back 30 min dives to 100 feet on 32% with just 1 min stops from 40 and 20 min SI's

(also noting that Floater seems be backpedaling on some of the original claims "oh yeah, you can add more stops if you feel like it" Come on now, you got to post all the "rules" and not mention some kind of vague exceptions that only you know about...

 

[lamont]

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.

Even when single tank diving, experience divers are going to be very unlikely to hit a situation where you don't have enough gas for two divers for 5 minutes at 10 fsw. With a single 130 you need about 250 psi for 2 divers for 5 mins, 500 psi will get you 10 minutes.

You can find edge conditions where you may not be able to stay at depth with two experienced divers on single tanks, but it involves things like having gas two gas loss issues on the same dive at the same time (ie. something drains one divers tank at depth and the backup reg on the buddy free-flows -- which is a situation that experienced divers will minimize because they're testing their backup regs and doing s-drills frequently).

I'm comfortable with 10 minutes of deco on backgas on single tanks -- because its a very small amount of gas you need to complete the deco, and you're actually very unlikely to get signficiantly hurt if you blow it off. One thing to realize is that the NDL line is not a magic barrier where if you cross it and screw up that you'll immediately suffer paralysis and need a chamber ride. At first when you make a mistake, its just your risk of a hit that increases, it may go from 1 in 10,000 to 1 in 100. Then it gradually becomes more certain that you'll get a pain hit, then it gradually becomes more certain that you'll get a more severe hit.

So, what we're talking about here is that if everything possible goes optimally wrong on the dive I might have a 1 in 100 chance of getting a shoulder hit after I get out... Okay, that's actually really unlikely and a minor risk overall...

Richard Lundgren posted his experiments with doing 1s from 50 fsw after doing dives to 100 fsw on 30/30 and he found that he could go to (IIRC) 50 minutes without getting any issues, and up to 60 minutes he started getting fatigue-hits, and then at 70 minutes he took a definite pain hit. Granted this is one lab rat and a small N, but it does illustrate that you don't immediately take a chamber ride for neurological symptoms if you blow off 5-10 minutes of mandatory deco...

 

[*Floater*]

I propose a moratorium on this thread until Floater posts 4 back to back 30 min dives to 100 feet on 32% with just 1 min stops from 40 and 20 min SI's

I think you misread the 4 dive example with 20 min SI's. The 1 min stops effectively start from 80% ATA (with travel time included). So ascent rate is 30'/min until 80% ATA and 10'/min from there on including the pause or stop. On top of that, you can do actual 1 min stops at 30'/20'/10' or 40'/30'/20'/10' for repetitive dives with short SI's. That's what I was taught to use in conjunction with average depth and the usual table.

It's almost same as the 5thD-X min deco rules as posted on their DIRF forum. The only significant difference is the 60 minute minimum SI rule (though that's is not part of their min deco rules as posted by Joe). However, it's the minimum deco; there's no rule against extending the shallow stops if you want. The problem is that I don't completely trust the linear ascent (though I'm open to experiment with it) so I prefer to extend some of the shallow stops in my actual dive profiles so that the dives are also within GAP-RGBM's limits.

I believe Charlie posted an interesting study on SB previously with various ascent profiles. One of the profiles was a direct ascent at 33'/min to surface after a NDL dive close to the limits. About 10% of the subjects came up with only Grade 0 Doppler bubbling (same with 10'/min ascent by the way). Assuming bubbles cause DCS, then those 10% could probably get away with some pretty aggressive profiles in general and most computers and tables would be way too conservative for them.

For those interested, the same study showed 65% Grade 0 bubbling for people who did a 33'/min ascent with both a 5 min deep stop and a 5 min shallow stop.

 

[JessH]

lamont- You make some good points. While there may actually be very few situations that would force you to skip your stops, I feel that in order to ignore residual nitrogen one needs to be comfortable with considering some sort of stop as mandatory and not simply a "safety" stop. If you don't consider a stop mandatory than a diver may skip the stop in situations where they may have not been forced too and as such incur a higher chance of DCS than necessary.

I have seen divers skip safety stops because of nose bleeds, drysuit leaks, lost buddy, etc.... With typical table diving this is no problem, but if you are ignoring residual nitrogen you may end up in a situation where skipping your stops would result in a higher likelyhood of DCS than is necessary. As long as you make sure to consider stops mandatory in most situations this will not be a problem. There are some situations in which I would have no problem needing to consider a stop mandatory and others in which I would prefer to not need to consider stops mandatory. I would still do stops if at all possible but I would like to keep the option of a direct ascent if the **** hits the fan. Most of my diving lately falls into this second category as I have been diving in a lake that currently has very poor visibility and I have been doing it with buddies that I don't know very well. I have not had any reason to doubt their diving abilities but I don't have enough experience diving with them to be certain that they will be able to handle an emergency well. As such I would prefer to keep the option of direct ascent open without having to incur too high of a risk. In order to do this I feel that it is necessary to keep residual nitrogen at a reasonable level.

~Jess

 

[JessH]

I just wanted to point out that my situation above is not exactly DIR since it involves diving with divers that I don't trust enough to dive with in a required stop situation. I am beginning to see how if you are truly diving GUE's DIR style, then within the NDL limits you will most likely have no problem ignoring residual nitrogen within reasonable limits. If I was doing such I would still try to maintain a 90minute SI whenever possible, but I do see how it could be possible to dive safely with a shorter one without specifically addressing residual nitrogen, but at some point the stops will become unquestionably mandatory.

~Jess

 

[limeyx]

I think you misread the 4 dive example with 20 min SI's. The 1 min stops effectively start from 80% ATA (with travel time included). So ascent rate is 30'/min until 80% ATA and 10'/min from there on including the pause or stop. On top of that, you can do actual 1 min stops at 30'/20'/10' or 40'/30'/20'/10' for repetitive dives with short SI's. That's what I was taught to use in conjunction with average depth and the usual table.
...

Not so fast Mister

From Charlie99's link:

For the example, I'd prefer 100' for 30 min with 20 min SI and repeat on EAN32. Also, I'd come up slower than what you have there; starting at about 80% ATA, say 70' for this example, I'd slow the ascent from 30' a minute to 10' a minute, or 30 second pause followed by 30 seconds to go up another 10'. Then increase the pauses to 1 minute at 30', 20' and 10'. And for 3rd/4th dive I'd start the 1 minute stops already at 40'. Nothing strenuous afterwards.

Admittedly Charlie kind of mischaracterized the ascent a little - although your ascent profile doesn't really look (IMO) like the one I learned from Joe on the deeper end. 30 second pause + 30 second move is ... 10' per min. 1 min "pause" is exactly the *same* as 30 sec pause 30 sec move (10' per min). The pauses we were taught were just long enough to OK the team and get the ascent under control. Then 1 min stops from 70% depth with travel time included (so not 1 min stop + 30 seconds move + 1 more min -- the travel time is included in the stop)


So when are we going to see profiles for 4 dives following the above "rules" ?
(please dont take this as me actually trying to get you to do them and then come sue me when you get hit. I would defnitely not do those profiles, probably not even on 30/30).

 

[limeyx]

It's almost same as the 5thD-X min deco rules as posted on their DIRF forum. The only significant difference is the 60 minute minimum SI rule (though that's is not part of their min deco rules as posted by Joe). However, it's the minimum deco; there's no rule against extending the shallow stops if you want. The problem is that I don't completely trust the linear ascent (though I'm open to experiment with it) so I prefer to extend some of the shallow stops in my actual dive profiles so that the dives are also within GAP-RGBM's limits.
...

One more thing (now I feel like I'm beating on you, sorry...)

I am not going to post the 5th rules here because they are in a protected area for use by people who have taken (but not necessarily passed ) DIR-F precisely because they get misquotes, mis-understood and people take only part of the picture away.
There definitely *is* a rule about SI in there (I am going to PM it to Floater and see if he agrees with me ...)

 

[lamont]

Most of my diving lately falls into this second category as I have been diving in a lake that currently has very poor visibility and I have been doing it with buddies that I don't know very well. I have not had any reason to doubt their diving abilities but I don't have enough experience diving with them to be certain that they will be able to handle an emergency well. As such I would prefer to keep the option of direct ascent open without having to incur too high of a risk. In order to do this I feel that it is necessary to keep residual nitrogen at a reasonable level.

Right there you've hit on some environmental conditions which no deco table, computer, algorithm or rules can possibly take into account.

Any GUE/5thd-X based rules/tables/whatever will inherently be assuming that you've got divers that can at a minimum deal with 5 mins of deco on backgas (aka 1 min ascent from 1/2 max depth), hold their stops to within +/- 5 feet, deal with air shares underwater, and manage an acceptable level of risk on those dives.

 

[lamont]

BTW, around here if someone needs to take a dump, you've got more than a 20 minute SI right there -- even assuming they hop out of the water and get on the can as fast as humanly possible, they still need to finish their business and suit up and gear up completely...

I don't think I've ever seen an SI shorter than around 45 mins... You can theorize about 20 minute SIs, but typically divers aren't in that much of a rush, and its really kind of a bad idea to be hurrying gear switches and gearing up that much... I don't know any divers that do that...

And the problem with trusting what any one DIRF instructor says in class is that they may be speaking off the cuff, non pedantically, and may be mentally assuming that nobody would come up with anything as stupid as two back-to-back 100 fsw / 30 min / 5 min ascent dives with only 20 mins in between...

 

[*Floater*]

One more thing (now I feel like I'm beating on you, sorry...)

I am not going to post the 5th rules here because they are in a protected area for use by people who have taken (but not necessarily passed ) DIR-F precisely because they get misquotes, mis-understood and people take only part of the picture away.
There definitely *is* a rule about SI in there (I am going to PM it to Floater and see if he agrees with me ...)

Reply sent and I still think those sets of rules are basically the same except for the 60 minute restriction.