Why 15 ft for a safety stop?

[murdrcycle]

I was just wondering - it seems a little arbitrary. Does that mean that a safety stop at 20 feet is less safe? Or one that begins at 20 ft and ends at 10 ft 3 minutes later won't allow proper off-gasing? Or, what if there is a very convenient place to stop at 25 or 10 feet, but not at 15? Should I try to hover at 15 anyway?

 

[Henryville]

The search feature suggests here's a good place to start...

http://www.scubaboard.com/forums/basic-scuba-discussions/200849-best-safety-stop-depth.html

 

[shurite7]

When I was going through my DM training I came across some info about the safety stop, but I can't remember where I found it, so I'll do a short recap.

The source stated 10' (3m) was the ideal depth for a safety stop, however, if the seas are rough 10' may not work. It also suggested 20' (6m) was acceptable, but not "ideal", therefore 15' (4.5m) was recommended. That depth was also considered more managable by the average recreational diver and it could be used as an all around 'standard' depth for a safety stop.

When I worked in the islands we alway's instructed the divers to keep their safety stop between 15-20' (4-6m). During the winter months the sea state could be a bit rough therefore 15-20' was easier to maintain.

chris

 

[Greavsie]

Here in the UK (with the BSAC) we teach 6m as the safety stop depth, to account for the rough seas situation as mentioned above.

Computers can complicate things if you have compulsory deco stops to do - if your computer is expecting them to be done at 3m (as many do) you can still do them at 5m/6m (whichever your training agency recommends), they just take an awful lot longer! But that's a story for another day.

 

[Pseudocyber]

I thought a 20' stop was a deep stop, just a few minutes, and a 10' stop was a safety stop for 10 minutes?

 

[underwasser bolt]

the greatest fractional change in pressure occurrs in the 1st 33 feet of depth. Starting from the surface at 15 feet the fractional pressure change is huge, where as going from 99ft to 114ft the fractional difference is small. Same principal on the way up. stopping at 15 feet offgasses at a fractionally high rate, and is a good depth for a safety stop, which is actually a decompression stop. keep in mind that all dives are decompression dives, even if you stay within the "NDL" guidelines.

 

[Blackwood]

It makes very little difference to your deco. Do what's convenient, what's pretty, whatever.

I personally prefer doing a minute at 30, 20, and 10 for NDL dives. If nothing else, it keeps me occupied.

 

[underwasser bolt]

It makes very little difference to your deco. Do what's convenient, what's pretty, whatever.

I personally prefer doing a minute at 30, 20, and 10 for NDL dives. If nothing else, it keeps me occupied.

agreed; 20, 15, 10 are all acceptable; they are all less than 33 ft and allow good offgassing rates. due to drysuit compression and rolling waves, 10ft can be difficult to maintain. sometimes if the waves are rough 15 ft can be hard. i would not do a safety stop at 30ft unless i planned to do another one at more shallow depths too.

 

[DandyDon]

I thought a 20' stop was a deep stop, just a few minutes, and a 10' stop was a safety stop for 10 minutes?

The "deep stop" is generally accepted to be a minute at half depth. It may not be covered in your OW, might be in your AOW - depending on your Inst, as many of the agencies don't really endorse it I don't think, unless I missed a change?

 

[underwasser bolt]

The "deep stop" is generally accepted to be a minute at half depth. It may not be covered in your OW, might be in your AOW - depending on your Inst, as many of the agencies don't really endorse it I don't think, unless I missed a change?

I think the idea of deep stops come from an improved understanding of bubble formation. shorter stops at deep depths and incrementing the time of the stops up with frequent stops as you go more shallow is the newer idea. v-planner is a good way to get a good grasp of this idea and is free to try. by the time you get to 20 feet, you may be hanging on to the line for a very long time, where as at 90 ft the stop may be 30 seconds. all due to the fractional pressure differences as you ascend. v-planner allows the diver to set the depth of the final stop: 20,15,10 ft for example. I am not sure that dive agencies are generally teaching this directly, but ascent rate and safety stops are certainly related to bubble formation, even though the beginning diver may not be ready to grasp all of this., especially if they are getting certified in 3 days.