Safety Stop - 3 min or 5 min?

[Lorenzoid]

As far as the relationship between depth and effectiveness of the safety stop, here's an interesting quote from the PADI Encyclopedia, which I find is a good resource in addition to Deco For Divers, since it helps me relate the more general theory to how I was taught to practice rec diving:

"[F]or dives greater than about 30 metres/100 feet, the 5-minute compartment controls. With its short halftime, it eliminates nitrogen rapidly during a safety stop. However, as the depth becomes shallower, control shifts to slower and slower compartments, thereby rendering the 3-minute stop increasingly less effective."

 

[halocline]

The slower compartments start to control more and more on repetitive dives, so a nice feel-good recommendation might be to do a 3 minute stop on your first dive, then longer stops on subsequent dives on a given day. But in the case of slow controlling compartments, I don't think five minutes will matter much, not when the compartment has a half life of 120 minutes. There is that statement from the PADI encyclopedia that someone quoted, indicating that a longer stop at 10 ft (I thought it was 10 minutes, but I could be wrong) eliminates nitrogen in a very effective way for no-decompression diving. What I do on repetitive NDL dives is a 3 minute stop at 20 feet and another at 10. Sometimes I'll hang at 10 ft for an extra few minutes. This has really changed my level of fatigue after multiple air dives on a given day.

 

[NAM001]

Good post halocline. I do lunch after a few dives to give the bod a chance to catch up. I would think that with a 120 minute compartment's. if you fill one of them its time to quit for the day.

 

[Basking Ridge Diver]

Good post halocline. I do lunch after a few dives to give the bod a chance to catch up. I would think that with a 120 minute compartment's. if you fill one of them its time to quit for the day.

If I am understanding what I am reading - I can't fill up the 120 minute compartment easily or at least in Recreational diving - it would take a single dive at 720 minutes / 12 hours (6 times 120 @50%) or 6 dives (pretty much back to back) at 120 minutes or more... Any compartment fills at 50% at that compartment time-frame (120 min). So 50% saturation at 120 min, 75% at 240 min, 87.5% at 360 min, 93.75% at 480 min, 96.88% at 600 min and 98.44% or full saturation at 720 mins.
Or yes time to quit for the day - .

 

[EFX]

Based on this wouldn't it be better served to recommend a 5 min safety stop for all no stop dives (1 min @ 6 meters and 4 min @ 3 meters) to get the fast compartments to reduce by 50%? Why is the recommendation 3 mins, does it matter?

The fastest way to offgas any compartment and limit ongassing of slower compartments is to maximize the tissue to lung pressure gradient (drop). You do this by ascending as fast as you can to the surface. However, fast ascent rates tend to increase bubble size and quantity and some fast tissues could be overwhelmed. So, there needs to be an ascent rate limit. The main reason for adding a safety stop is just that -- for safety. It adds a little conservatism to the dive at the expense of additional ongassing of very slow tissues. The problem is it adds to the RN for repetitive dives which will limit BT on those dives. Your computer may not lock you out for missing a stop but it may penalize you on additional dives so you need to make the stop.

My recommendation is to follow your computer. If it wants you to make a safety stop, make the stop. But, if you have nothing to do or see at 15 ft. and if you are making repetitive dives limit it to 3 minutes. If it is the last dive of the day make it as long as you want. If you really want to be conservative, rather than adding arbitrary safety stops or lengthening them, set your computer to the next conservative setting.

If you're interested in calculating the N2 tissue pressure for different lengths or depths or safety stops see my example in the thread "Buddies kept grabbing /pulling me to ascend ...." in this forum.

 

[Basking Ridge Diver]

The fastest way to offgas any compartment and limit ongassing of slower compartments is to maximize the tissue to lung pressure gradient (drop). You do this by ascending as fast as you can to the surface. However, fast ascent rates tend to increase bubble size and quantity and some fast tissues could be overwhelmed. So, there needs to be an ascent rate limit. The main reason for adding a safety stop is just that -- for safety. It adds a little conservatism to the dive at the expense of additional ongassing of very slow tissues. The problem is it adds to the RN for repetitive dives which will limit BT on those dives. Your computer may not lock you out for missing a stop but it may penalize you on additional dives so you need to make the stop.

My recommendation is to follow your computer. If it wants you to make a safety stop, make the stop. But, if you have nothing to do or see at 15 ft. and if you are making repetitive dives limit it to 3 minutes. If it is the last dive of the day make it as long as you want. If you really want to be conservative, rather than adding arbitrary safety stops or lengthening them, set your computer to the next conservative setting.

Understood but since my PDC (and I assume most) will allow a user to change to 3 min or 5 min @ 10 feet, 15 feet or 20 feet... I also understand the benefit of a 10 foot stop if you can hold it versus a 15 or 20 - but more curious about the recommendation at 3 mins. I can only assume at this point that most recreational dives have shown 3 mins to be safe enough... I was more curious about keeping the same bottom time for dives (I use 0 conservationism) and perhaps using the 5 min safety based on what I have read.... I also saw where some folks use conservatism to plan for gas reserves... Seems like this is an individual choice based on individual knowledge and experience...

Thanks

 

[EFX]

I wasn't aware that any rec DC's safety stop settings could be changed. I know the Suunto's deep stops can be set for either 1 or 2 minutes. My Mares Puck, a basic rec DC, has no safety stop settings and will only ask for the safety stop if any portion of the dive was deeper than 10 m (33 ft). The limited deco stop-time dives I've done with the same computer called for a deep stop and a deco stop at 10 ft rather than the safety stop. My guess as to why a 15 ft safety stop is preferable over a 10 ft. stop for non-deco dives is that it is a little harder to maintain that depth given the poor bouyancy of new/inexperienced divers. A 10 ft stop increases risk of hitting a boat if you are not on top of your buoyancy.

Regarding the 3 minutes vs. longer time. I agree with your assumption that it is safe enough. You mention gas reserves. Since new divers aren't doing any gas planning other than getting back on the boat with 500 psi, hanging out for longer times at 15 ft. increases the risk of running OOA.

 

[NAM001]

Your general premis is sound. The problem is that so many think of the safety stop as a mandatory stop. It is not. It is taught some what to be treated as such. Its a "MANDATORY RECOMMENDATION". Because its a catch all for any mistakes you may have made. No only errors in the tables but use of them and when it comes to computers use of conservative settings and ones physical condition environment ect places so many safety margins int the system it is a shame to treat the SAETY stop like a madatory stop. It is a catfch all for the many variables that can not be accounted for. I have dove with people who sill not dive more than 5-10 minutes from the NDL. As long as you have not exceeded NDL a safety stop is not required. Now when it comes to computers one can compare different models and get different results. Not only in the NDL issue but the DECO required which i presume is a byproduct of the NDL difference of that computer. I have actually had people ell me you cant do scuba in a pool cause you dont have enough water to do a safety stop. This level of non understanding is exactly a reason why we have safety stops. New divers are not required to understand deco to a working degree so it is replaced with "one rule fits all's) like safety stops, that you have to do. If a NDL is 15 the table will say 12 for a margin of safety. Set to super conservative on the computer and it will say 10.

The fastest way to offgas any compartment and limit ongassing of slower compartments is to maximize the tissue to lung pressure gradient (drop). You do this by ascending as fast as you can to the surface. However, fast ascent rates tend to increase bubble size and quantity and some fast tissues could be overwhelmed. So, there needs to be an ascent rate limit. The main reason for adding a safety stop is just that -- for safety. It adds a little conservatism to the dive at the expense of additional ongassing of very slow tissues. The problem is it adds to the RN for repetitive dives which will limit BT on those dives. Your computer may not lock you out for missing a stop but it may penalize you on additional dives so you need to make the stop.

My recommendation is to follow your computer. If it wants you to make a safety stop, make the stop. But, if you have nothing to do or see at 15 ft. and if you are making repetitive dives limit it to 3 minutes. If it is the last dive of the day make it as long as you want. If you really want to be conservative, rather than adding arbitrary safety stops or lengthening them, set your computer to the next conservative setting.

If you're interested in calculating the N2 tissue pressure for different lengths or depths or safety stops see my example in the thread "Buddies kept grabbing /pulling me to ascend ...." in this forum.

 

[nimoh]

Your general premis is sound. The problem is that so many think of the safety stop as a mandatory stop. It is not. It is taught some what to be treated as such. Its a "MANDATORY RECOMMENDATION". Because its a catch all for any mistakes you may have made. No only errors in the tables but use of them and when it comes to computers use of conservative settings and ones physical condition environment ect places so many safety margins int the system it is a shame to treat the SAETY stop like a madatory stop. I have dove with people who sill not dive more than 5-10 minutes from the NDL. As long as you have not exceeded NDL a safety stop is not required. Now when it comes to computers one can compare different models and get different results. Not only in the NDL issue but the DECO required which i presume is a byproduct of the NDL difference of that computer. I have actually had people ell me you cant do scuba in a pool cause you dont have enough water to do a safety stop. This level of non understanding is exactly a reason why we have safety stops. New divers are not required to understand deco to a working degree so it is replaced with "one rule fits all's) like safety stops, that you have to do.

I think (in the PADI system anyway) that if you come within 2 pressure groups of an NDL, then the safety stop is a mandatory otherwise it's optional. It's been about 4 years since I have taken a PADI course, and don't dive with the RDP or a computer anymore so I could be wrong about that.

 

[NAM001]

You hit on some inportant obervations. Look at a shearwater computer there is no safety stops that appear on the screen. Only mandatory. You do a safety stop on top of what the computer says if you are INCLINED to do so. The computer also does not lock up to PREVENT you from further diving if you miss a stop. other computers have no safety stop untill you exceed 60' and then it offers 3 min and if you exceed 100 ft or 90% of NDL it adds another 2 minutes to the recommendation. I seriously doubt that any new diver knows whether the NDL's privided are valid or how much they are padded for divers physical safety / manufacturers liability safety. As you have mentioned there is so much more involved in the systems process of recommendation establishment. When you are diving on the REC side of the sport, most everything has been figured out for you, with the greatest margin of safety built in for the worst situation scenereo's in mind. The tech side of things are different in so many areas.

I wasn't aware that any rec DC's safety stop settings could be changed. I know the Suunto's deep stops can be set for either 1 or 2 minutes. My Mares Puck, a basic rec DC, has no safety stop settings and will only ask for the safety stop if any portion of the dive was deeper than 10 m (33 ft). The limited deco stop-time dives I've done with the same computer called for a deep stop and a deco stop at 10 ft rather than the safety stop. My guess as to why a 15 ft safety stop is preferable over a 10 ft. stop for non-deco dives is that it is a little harder to maintain that depth given the poor bouyancy of new/inexperienced divers. A 10 ft stop increases risk of hitting a boat if you are not on top of your buoyancy.

Regarding the 3 minutes vs. longer time. I agree with your assumption that it is safe enough. You mention gas reserves. Since new divers aren't doing any gas planning other than getting back on the boat with 500 psi, hanging out for longer times at 15 ft. increases the risk of running OOA.