Stop at 15'-Why that depth?

[Tortuga James]

My first 100 dives (save my AOW dives) were at 60' or a less. I was diving with people who just had the attitude that at that depth, it was almost impossible to get bent, given the gas supply of an 80. The plan was always, dive until you have 700 psi, take a couple of minutes to surface and then figure out how long we needed in the boat before repeating that plan. The more I study and learn about nitrogen absorption and expulsion, and decompression, the more I want to take my planning to a more refined level.

Ok, none of that is the question, I can't seem to find a suitable explanation (or probably don't need it in practice) for why is 15' is the magic depth for a safety stop?

 

[TheTrickster]

Its due to (I'm lead to believe) that the Pressure Gradient between 15' and the surface is the most severve.

People please correct me..

 

[Aquamaniac]

A safety stop assists the body in rapidly eliminating nitrogen!

Why?
The reason is very simple. Bubbling does not occur in a diver under pressure, it only occurs when the pressure is reduced "too much". Once bubbling occurs, gas elimination is reduced. This is because the driving force for nitrogen elimination has diminished. The bubble now has to be reabsorbed - which can be a time consuming process. However, if bubbling can be minimized or prevented, then the nitrogen stays in a gaseous solution (i.e. dissolved) in the body. If this can be maintained while surfacing, then the nitrogen pressure is maintained while the pressure surrounding the diver is reduced. When the nitrogen pressure is allowed to stay greater than the surrounding pressure, a driving force for nitrogen elimination is created and nitrogen will "zing" out of the body. To put it even simpler, a diver that performs a 3 minute safety stop after a dive will have less nitrogen in their body immediately upon surfacing as compared to a diver that did not perform a safety stop, but has been on the surface "off-gassing" for 3 minutes. Therefore, one of the best things a diver can do for themselves is to perform a safety stop, no matter how short, after every dive.

How Deep?
Since the goal is to promote nitrogen elimination prior to surfacing so the risk of DCI is lessened, the next question is how to calculate the depth of a safety stop. Years ago, a safety stop was recommended at 10 feet after every dive. This recommendation wasn't so much a depth recommendation other than a recommendation to just perform a safety stop. The depth of 10 was chosen because the final stop on decompression tables was at 10 feet. Later, the recommendation turned to 15 feet and is now currently 15 to 20 feet for 3 to 5 minutes. A deeper depth of 15 feet was chosen for several reasons, but a driving force behind this deeper recommendation was overwhelming information that deeper stops promote greater nitrogen elimination. My favorite example was a fairly well controlled study where two control groups were taken to the same depth for the same time and then performed safety stops for the exact same time, but at different depths. One group performed a safety stop for 5 minutes at 10 feet while the other group performed a 1 minute stop at 20 feet and then a 4 minute stop at 10 feet. Even though both groups did 5 minutes of safety stops, the group that started at 20 feet had significantly less bubbling not only upon surfacing, but also over the hours that followed the dive. While this violates standard half-time theory, the evidence is in . . . deeper is better. But how deep? Again, one of the common recommendations is 15 feet. A rule I like to live by is "keep it simple" and picking one single depth that a diver can remember definitely keeps it simple and makes it easy to perform. However, when it comes to a simple task such as picking a safety stop depth, I like to give divers a little more credit and believe that the main difference between sport divers and technical divers is the amount of information each has been exposed to. Therefore, I believe that since technical divers perform decompression stops over multiple depths, so can sport divers especially considering that these stops are not mandatory, but merely safety stops. Therefore I don't recommend a single depth. Instead I recommend, performing safety stops over a range of depths - which is extremely easy to do. The only calculation that needs to be done is to pick the depth of the first safety stop. All that is needed is a rule of thumb. The rule I use is to take your deepest depth of the dive and divide it by one-half and round to a deeper depth if necessary. For example, after performing a dive to 90 feet, the depth of the first stop would be 45 feet - which could also be rounded to 50 feet if desired. This calculation can be performed easily in the diver's head while underwater. The question is . . .

How Long?
You don't need to stay long at the first stop and 1 minute is fine. The next step is to calculate where to go from here and for how long. The answer is come up another 10 feet and stay there for 1 minute until 15 feet is reached. A 20 foot stop is also needed no matter what the profile is. Once you reach 15 feet, stay there for 3 to 5 minutes or longer . . . whatever you desire (within reason of course). A sample profile looks like this. After a NoStop dive to 80 feet for 20 minutes, surface to 40' for 1 min., then go to 30' for 1 min., then to 20' for 1 min., then finish at 15 feet for 3 to 5 minutes. Another example is a dive to 90 feet. Make the first stop at 45' for 1 min., then stop at 35' for 1 min., then to 25' for 1 min., but now to 20' for 1 min. before going to 15' for 3 to 5 minutes. Once you reach 15 feet, slowly come to the surface. Creep your way up and maybe stop at 5 to 10 feet on the way up for 30 seconds to a minute.

 

[TheTrickster]

Great answer! Aquamanic

 

[PhilEllis]

My first 100 dives (save my AOW dives) were at 60' or a less. I was diving with people who just had the attitude that at that depth, it was almost impossible to get bent, given the gas supply of an 80. The plan was always, dive until you have 700 psi, take a couple of minutes to surface and then figure out how long we needed in the boat before repeating that plan. The more I study and learn about nitrogen absorption and expulsion, and decompression, the more I want to take my planning to a more refined level.

Ok, none of that is the question, I can't seem to find a suitable explanation (or probably don't need it in practice) for why is 15' is the magic depth for a safety stop?

As to the 15 foot depth figure, I don't think there is any "magic" logic to that depth. In light decompression diving, the only decompression stop is often at 10 feet. When you are inexperienced and diving in the ocean, it is sometimes difficult to manage remaining level at 10 feet, so the practice gradually grew to using the 15 foot depth as the stop. The safety stop accomplishes several things. First, it actually stops you, which is a really good way to slow you down. The three minutes you spend there gives you tissues a little time to equalize and that may forestall a light case of DCS.

As to the practice of moving directly to the surface from you 60 foot dives. I personally think this is a bad practice. Safety stops cannot hurt you and can become an enjoyable part of your dive. In the ocean, there is a lot to see at 15 feet. I think you should develop the habit of doing a safety stop REGARELESS of your diving depth and bottom time. It is simply a little insurance. When you move from your safety stop to the surface, do it VERY slowly. No matter what anyone tells you, you CAN get DCS even from short, shallow dives. Better safe than sorry. Thanks.

Phil Ellis

 

[Meng_Tze]

My first 100 dives (save my AOW dives) were at 60' or a less. I was diving with people who just had the attitude that at that depth, it was almost impossible to get bent, given the gas supply of an 80. The plan was always, dive until you have 700 psi, take a couple of minutes to surface and then figure out how long we needed in the boat before repeating that plan. The more I study and learn about nitrogen absorption and expulsion, and decompression, the more I want to take my planning to a more refined level.

Ok, none of that is the question, I can't seem to find a suitable explanation (or probably don't need it in practice) for why is 15' is the magic depth for a safety stop?

IIRC,
Safety stops were put in at that depth to dramatically slow down the ascent. As you know the biggest change in pressure happens between 1 and 2 ATA (0-30ft, give or take with rounding factors). I guess 15ft is nicely in the middle. Historically the tables were pretty square profiles and reasoning was that a diver would go up in a linear line up to 15ft, hang at 15ft to extend the time spent in the last 30ft, making the ascent line lay flatter.

But I could be wrong, I am not a scuba instructor, so take my reasoning with some caution.

 

[matt_unique]

Because of the seas I sometimes dive, my shallowest deco stop is always 20'. 15' is not a magic number, nor is 10 or 20'. The point is a shallow stop to off gas absorbed nitrogen.

--Matt

 

[Meng_Tze]

Man you guys are quick, or I type slowly. when typing my response, 4 others were posted.

 

[Tortuga James]

Man you guys are quick, or I type slowly. when typing my response, 4 others were posted.

I will 2nd that motion!

Thanks....I am beginning to get it.

 

[lmorin]

The responses thus far neglect the effect of ascent rate. I was instructed to use 60 ft/min, but the computer I bought tells me to obey 30 ft/min above 60 fsw. This slower rate of ascent may accomplish the same thing as the 1 min stop at 20 ft. The slower ascent may be more important than an additional safety stop in normal recreational diving.