“You’ll always miss one hundred percent of the shots you don’t take.”
Wayne Gretzky, the only NHL player to have his number (99) retired across the league – Born January 26, 1961
With all the postings recently on the scuba forums asking about deep stops and gradient factors, I figured it might be time to get back to some basics. Let’s talk briefly about the five waypoints and ascent behavior, which is a fancy name for a couple of simple ideas designed to help divers manage decompression stress and get out of the water after a dive with all their fingers and toes in place, and all their faculties intact. For the record, decompression stress is something that affects every diver at the end of every dive regardless of what type of dive went before. That said, I believe that for this discussion, it makes most sense to concentrate on the simplest possible ascent: surfacing from sport dives.
In the hope of getting everyone to sing from the same song sheet, let’s define sport dives as dives that require only one decompression stop (sport diving textbooks call this a Safety Stop), and which are conducted no deeper than the common depth limits set by several of the major sport diving certification agencies: 40 metres or 130 feet.
Now, I have to make a small confession, I teach technical diving and I have been teaching students in my classes about Waypoints and Ascent Behavior for more than 14 years. Learning the basic guidelines behind this stuff is a precursor to getting Deco on the Fly squared away. And I include a little module on that as part of my decompression courses. It is easy to learn, helpful and a good mental exercise for folks who want to conduct dives that are deep or long or both.
But it is MORE important to emphasis that the guidelines controlling ascent behavior are NOT only for technical diving; and the five waypoints, which form the underlying structure on which ascent behavior is built, are shared by ALL dives, even sport ones.
Obviously, most sport divers do not need to be bothered by anything outside the stuff covered in their student workbooks. And you will not find anything about Ascent Behavior, The Five Waypoints or anything similar in any of those. Many sport divers are happy – and well-advised – to strap on a Personal Dive Computer (PDC) and follow its directions. However, some of us like to know more than the suggestion “to follow what your computer tells you to do.” For example, if you were one of those kids who took things apart – like the wall clock that Auntie Jane gave your mom and dad for a wedding present – please read on.
The first step is to understand a basic concept, as true in diving as in anything else: when something looks complicated, we can make it look much less threatening by slicing it up into bite-sized pieces.
OK, once we buy into that, let’s apply its logic to the journey from a dive’s maximum depth (or average depth if you want to be more precise) to the surface. This trip can be punctuated by Five Waypoints. And just to restate an important point, these five waypoints work for ALL recreational dives whether they take place in 100 feet or 100 metres. The only differences are that on an ascent from a deep dive, there will be one or more gas switches (from a mix breathed at depth to a gas designed to optimize off-gassing); and the “safety stop” or single decompression stop that is common and recommended for a sport dive, is replaced by two, three or a whole bunch of staged decompression stops ranging from three minutes to many, many times more. All that said, the five fundamental waypoints remain the same!
1. Planned Maximum Depth or Actual Average Depth
2. Off-Gassing Ceiling
3. First Running Stop
4. Staged Decompression Stop(s)
5. Surface and Surface Interval Time (an often neglected but important part of all dives)
Most technical divers will probably already have an idea what each waypoint is but let’s have a brief explanation for the sport divers reading this.
Number one is simple: how deep did you go or, if you swum a saw-tooth profile, what was your average depth. The second waypoint is trickier to get your head around, but try this for a start. The off-gassing ceiling is a theoretical point in the water column during a diver’s ascent where the net result of the on-gassing and off-gassing meringue that goes in inside a diver’s body, is that more inert gas is coming out of her body than is going in. In other words, the off-gassing ceiling is the point where decompression and true ascent begins. For sport divers doing sport dives to 40 metres or less, we can ballpark this point at a little less than one and a half bar / ata shallower than the average depth. So for a dive to 40 metres or 130 feet on an EAN30 for example, the off-gassing ceiling (or gas transition point) sits at around 25-27 metres or slightly shallower than 85 feet.
The next waypoint – number three – is the first running stop, and at this level of diving, we can fix this at about one bar / ata above our off-gassing ceiling. Once again this is something that the average sport diver may have an issue understanding. I use the term running stop rather than Deep Stop, because I believe Running Stop better describes what goes on in the water column when a diver switches her behavior (ascent speed) to comply with the guideline. For a sport diver, Running stops are not actual stops but rather a change in ascent speed. At the sport level, this translates to the diver rising through the water column at around three metres or 10 feet per minute. Another way to write this is to put a one-minute stop every three metres from the beginning of the running stops until the safety stop is reached. End result is the same… the diver’s ascent has been checked.
The second to last waypoint is the safety stop. I think everyone knows that this is a staged stop of three to five minutes. All I contend is that this is a staged decompression stop by another name and rather than being optional, should be de-rigueur on any dive involving more than a minute on the bottom. Shallower than 30 metres or 100 feet, and well within the NDL, a three-minute stop is fine, deeper or closer to the NDL, stopping for five-minutes makes more sense in my opinion and experience.
The final waypoint is the surface and the surface interval. The SIT is the final stage in a diver’s little gamble with DCS. It is during this time that our diver has to be aware of any strange messages from her body. And it is during this time that she has to prepare herself for her next adventure. In the final analysis, this stage is as important as the rest of her ascent.
OK, those are the Five Waypoints, now let’s take a quick look at how a diver should behave moving from one waypoint to the next. This article is about Ascent Behavior after all!
Here’s the simple pattern for a sport dive. The diver ascends at nine metres or 30 feet per minute but no slower, between waypoints one and two (depth and off-gassing), and nine metres or 30 feet per minute but no faster between waypoints two and three (from off-gassing to first running stop). The diver then moves at three metres or 10 feet per minute between three and four (running stop and safety stop), and once the safety stop is completed (usually at either six metres or three metres (that’s 20 or 10 feet), she will go slowly to the surface no faster than three metres or ten feet per minute.
This is a variable ascent speed: tricky to master but certainly doable for just about ANY competent diver.
What bears thinking about and certainly bringing to everyone’s attention is that following these guidelines, a diver who has finished conducting a 10-minute dive at 39 metres (a smidgen less than 130 feet) will take around 14 minutes to surface! (See illustration. Apologies for imperial users but the conversion to feet is: 39 m = 129 ft; 26 m = 85 feet; 17 m = 55 feet; 3 m = 10 ft.)
A note for the propeller-heads among us who thrive on the nuts and bolts of deco theory: Following these guidelines makes for a slightly slower ascent than all but the most conservative PDC running a dual-phase algorithm. What is key to making this work is understanding that any time spend below the off-gassing ceiling is adding to bottom-time, and that running stops at this level is just a fancy name for slowing one’s ascent to three metres or 10 feet per minute.
Also important is to realize that decompression theory and the algorithms generated by those theories are not grounded in a perfect science. The maths are exquisite. How closely your body and mine follow the tracks laid down by those mathematics is something else more akin to a crap shoot to quote a decompression expert.
Sure, your brand-new fourth-generation PDC is a thing of beauty, has a gaming console and a CPU more powerful and programming more complex than the computers that helped land Neil Armstrong and Buzz Aldrin on the Sea of Tranquility. But decompression theory is just a stab in the statistical dark, and I believe that having an understanding outside of your PDCs user manual — the Five Waypoints and Ascent Behavior for example — is a useful nugget of knowledge. Dive safe folks.
This essay is based on an article that first appeared in Diving Adventure Magazine in 2006. A version was also used as the basis of a chapter in the Six Skills and other discussions called The Deco Curve: Controlled Ascent Behavior and contingency decompression on the fly.