Safety stop/Deco stop .Whats the difference?

[Dr. Lecter]

you're missing the point.

Take two buddy teams A and B. Team A has conservative computers, Team B liberal.

Both teams do the same dive profile, Team B holds their safety stop and surfaces, however Team A has their computer telling them to hold some extra deco stops, and they do.

Now, was Team B's dive safer, since their computers didn't go into deco?

I'll agree with you that it wasn't any safer in terms of DCS risk, but it was arguably safer in that neither one would have hesistated to head straight for the surface in the event of an emergency. Team A, however, could not do so. You picks your algo/tables and you follows the rules.

 

[nimoh]

I'll agree with you that it wasn't any safer in terms of DCS risk, but it was arguably safer in that neither one would have hesistated to head straight for the surface in the event of an emergency. Team A, however, could not do so. You picks your algo/tables and you follows the rules.

I agree with the terms of your agreement with me

I think Hatful made a valid point, but the distinction of which is safer, is somewhat trivial.

 

[scubadada]

You picks your algo/tables and you follows the rules.

Simple, and correct

 

[AlexL]

this is deep into the grey area of the statistical curve fitting that is compartment models, does not take into account unpublished 'revisions' to the base profile by noted agencies, and is proposed by a complete newbie diver with only 13 dives in the log, a basic armchair understanding of deco theory, and software written from said understanding thereforth. consider the following more a validation of understanding, rather then a statement of fact.

To complicate matters, at least one agency (the big one) says certain safety stops are required.

i've only personally ran SSI, NAUI and PADI tables through as published ZH-L16[bc], but strictly speaking, all of their NDLs invoke a deco obligation ( deco ceiling / tolerated ambient pressure (TAP) of over 1.0 ATA) at the edge of the table around 80' to 100' fsw. with an 80' u-profile, NDL is 30 ~ 35 minutes depending on the tables in use; if you constrain your deco ceiling to 1.0 ATA, it bails into required ascent around 26 minutes in. thing is, even with the full 35 minutes BT, any deco stop incurred, is cleared by time you hit the safety stop depth during a nominal ascent of 33f/10m per minute.. but only just.

if you take base ZH-L16c, leave off the deco ceiling limit, apply a GF of say .3 / .8 to a 35 minute BT dive to 80', and you've ended up with a constrained ascent rate of ~10 fpm from 35' fsw ~ 10' fsw, and 3 more minutes of obligated deco at 20 (1 min stop + travel time ) and 10 fsw ( 2min stop + travel ), in addition to a 5 minute stop at 15'. upshoot is your deco ceiling is almost double (-4.9 fsw) that of the straight 30fpm down, 35 at bottom, 30fpm up (-2.44 fsw), but you're spending a lot more time hanging out on a line. the winner for most conservative tissue loading, unsurprisingly, is to trim BT to keep the deco ceiling above 1.0 ATA (or whatever factor you wish to gamble on), at which point you're out of the water with a 7' fsw ceiling to spare; thats with no additional GF factors applied!

also of note, unless you've got gills, you're not gonna hit these situations in a single dive with a single al80. with my best calm rmv on a single al80, rock bottom pushes you into ascent a minute before you pull the deco ceiling below the surface.

# base NDL checks
$ ./plan --ceil 0 --gf-low 1.0 --gf-high 1.0 --m-value 1.0 --plan plans/ref/naui/test-naui-ndl-080.yaml
diving plan: 80' NAUI NDL test al80, al80
start   end     fsw in  fsw out ps in   ps mid  ps out  time    state   notes
0       3       0       80      3000    2962    2923    3       +27
3       38      80      80      2923    2232    1541    35      ~
38      40      80      15      1541    1501    1462    2       -32
40      45      15      15      1462    1363    1264    5       sstop
45      46      15      0       1264    1244    1224    1       -15
leading tissues: 18:0.926 27:0.917 38:0.893 54:0.823 12:0.818 77:0.742
0.926 ata, -2.44 fsw leading tap 18.5 ht, 0.853 ata, -4.85 fsw avg leading tap

# applied gradient factors
$ ./plan --ceil 0 --gf-low .3 --gf-high .8 --m-value 1.0 --plan plans/ref/naui/test-naui-ndl-080.yaml
diving plan: 80' NAUI NDL test al80, al80
start   end     fsw in  fsw out ps in   ps mid  ps out  time    state   notes
0       3       0       80      3000    2962    2923    3       +27
3       38      80      80      2923    2232    1541    35      ~
38      40      80      35      1541    1501    1462    2       -23
40      41      35      25      1462    1442    1422    1       -10
41      42      25      20      1422    1402    1383    1       -5
42      43      20      20      1383    1363    1343    1       stop
43      44      20      15      1343    1323    1303    1       -5
44      49      15      15      1303    1205    1106    5       sstop
49      50      15      10      1106    1086    1066    1       -5
50      52      10      10      1066    1027    987     2       stop
52      53      10      0       987     968     948     1       -10
leading tissues: 38:0.850 27:0.835 54:0.806 18:0.787 77:0.738 109:0.668
0.850 ata, -4.93 fsw leading tap 38.3 ht, 0.781 ata, -7.23 fsw avg leading tap

# with a hard deco limit of 1.0 ATA
$ ./plan --ceil 1.0 --gf-low 1.0 --gf-high 1.0 --m-value 1.0 --plan plans/ref/naui/test-naui-ndl-080.yaml
diving plan: 80' NAUI NDL test al80, al80
start   end     fsw in  fsw out ps in   ps mid  ps out  time    state   notes
0       3       0       80      3000    2962    2923    3       +27
3       28      80      80      2923    2430    1936    25      ~       deco limit (26 of 35 min planned)
28      30      80      15      1936    1896    1857    2       -32
30      35      15      15      1857    1758    1659    5       sstop
35      36      15      0       1659    1639    1620    1       -15
leading tissues: 18:0.797 27:0.771 38:0.750 12:0.731 54:0.695 77:0.634
0.797 ata, -6.69 fsw leading tap 18.5 ht, 0.730 ata, -8.92 fsw avg leading tap

# single tank rock bottom eviction
$ ./plan --ceil 0 --gf-low .3 --gf-high .8 --m-value 1.0 --plan plans/ref/naui/test-naui-ndl-080.yaml  --tanks al80 --rmv .5
diving plan: 80' NAUI NDL test al80
start   end     fsw in  fsw out ps in   ps mid  ps out  time    state   notes
0       3       0       80      3000    2936    2872    3       +27
3       27      80      80      2872    2082    1292    24      ~       rock bottom (25 of 35 min planned)
27      29      80      30      1292    1226    1160    2       -25
29      30      30      15      1160    1127    1094    1       -15
30      35      15      15      1094    930     765     5       sstop
35      36      15      0       765     732     699     1       -15
leading tissues: 18:0.775 27:0.754 38:0.736 12:0.704 54:0.685 77:0.627
0.775 ata, -7.43 fsw leading tap 18.5 ht, 0.713 ata, -9.46 fsw avg leading tap

so, for the OP's question: a safety stop is a stop taken to increase your safety margin vs a direct ascent, but is not required to provide a deco ceiling above conditions at the surface; a deco stop is a stop required to raise your deco ceiling above conditions at the surface. whats more fun, is that the line between the two can be extremely fuzzy at times, and is somewhat based on the diver's risk aversion level.

 

[victorzamora]

My wife is a new(ish) diver with zero interest in Deco/Tech diving. She just knocked out her 30th dive. I asked her what she thought. She said that safety stops were for recreational divers, and deco stops were for technical divers. "Deco" stands for decompression. Safety stops should be done while deco stops have to be done. I couldn't have agreed with her more. That may be overly simplified, but it's a good generalization.

As for the two divers with different algorithms: I have to agree with nimoh on this one. I'll agree that more stops is better IF they go into deco on one computer but not the other, but I disagree that that SHOULD be a scenario. Neither diver should have gone into deco at all if planning a NDL dive. When the first diver's computer gets close, everybody ascends. When the last diver's computer clears for the surface, everybody surfaces. This should hold true for all recreational OR technical dives ("computer" could mean cut tables, I was trying to be general).

 

[Skittl1321]

SHHHHH you guys I wanted to hear what new divers thoughts on this were.

I'm a new diver, here was my thought before I read.
A safety stop is an optional stop that adds a safety margin to your dive to help with your decompression.
A deco stop is a mandatory stop necessary because surfacing without it would result in a much higher chance of DCS.

Both safety stops and deco stops are planned (part of my plan is "do safety stop at 15 feet for 3 minutes) but deco stops require knowledgeable planning, as they vary based on the dive time and depth.

 

[tomfcrist]

A safety stop is optional, whereas a decompression stop is mandatory, with regard to complying with the chosen decompression algorithm's goal of limiting, but not eliminating, the risk of decompression sickness

The point is that all dive involve compression and decompression. The "safety stop" is just adding a deco stop on a recreational dive to minimize the chance of DCS.........

For what its worth, deco stops arent mandatory either......and in some cases it is safer to finish deco on the boat. Before someone chastizes me for saying that, let me explain. I was diving St. Lawrence river a few years back and the teams deco schedule called for a 12min stop at 20' on 02 for our final stop. The water was so cold that by this point in the dive we were all extremely cold and I was shivering at the 20' stop. I decided to slowly ascend, and get back on the boat.....Which i did very slowly and calmly. i then sat down and breathed my O2 bottle till it was dry......Practically meditating the whole time. The other 3 members of the team finished the 20' stop.......they got bent.....i didnt. Kinda hard to offgas when you are nearly hypothermic. Just saying.

 

[victorzamora]

I would consider that a 0fsw deco stop you did on the boat, so you still did it. I would also like to mention that that is well out of the realm of "Basic Scuba Discussions." Plus, it's a very rare case (but a good point overall) in the grand scope of things. Also, as you know, it's possible that they would've gotten bent doing your profile as well. It's not unheard of for two people with the same profile to react differently, or for the same person doing the same profile on different days to react differently. That could've just been a case of luck on your part.

 

[Dr. Lecter]

For what its worth, deco stops arent mandatory either......and in some cases it is safer to finish deco on the boat. Before someone chastizes me for saying that, let me explain. I was diving St. Lawrence river a few years back and the teams deco schedule called for a 12min stop at 20' on 02 for our final stop. The water was so cold that by this point in the dive we were all extremely cold and I was shivering at the 20' stop. I decided to slowly ascend, and get back on the boat.....Which i did very slowly and calmly. i then sat down and breathed my O2 bottle till it was dry......Practically meditating the whole time. The other 3 members of the team finished the 20' stop.......they got bent.....i didnt. Kinda hard to offgas when you are nearly hypothermic. Just saying.

What do you want, a cookie? All of you made the wrong call, did a shoddy job of dive planning, and one of you happened to get lucky. The right choice would have been to plan your exposure pro for the cold water dive you'd be doing well enough to avoid being in the position of flipping a coin to see if you get bent for blowing a stop or get bent for being so cold you couldn't offgas. Remind me to never suggest you to anyone as a tech instructor.

 

[tomfcrist]

I would consider that a 0fsw deco stop you did on the boat, so you still did it. I would also like to mention that that is well out of the realm of "Basic Scuba Discussions." Plus, it's a very rare case (but a good point overall) in the grand scope of things. Also, as you know, it's possible that they would've gotten bent doing your profile as well. It's not unheard of for two people with the same profile to react differently, or for the same person doing the same profile on different days to react differently. That could've just been a case of luck on your part.

I dont disagree with you. I was merely pointing out that scubadada's choice to try to correct me with a very textbook response is not the only way something can be done......I had a choice to make, and was prepared to deal with the consequences if they had occured....thus the stop wasnt exactly "mandatory".