Safety stop/Deco stop .Whats the difference?

[scubadada]

Neural tissues. Neural fluid and blood have almost the same half time of about 5 minutes. While getting bent is little more than noise, statistically speaking, if a recreational diver does get bent, it's most likely to be a type II or neural hit. Most likely means a 95% chance. I adjust my stops to ameliorate the the type of DCS hit I am most likely to get. After five cycles (25 minutes for blood/neural fluid) my tissues are 96% saturated. While I might be ongassing slower tissues during a deep stop, I'll be getting rid of a lot of blood and neural nitrogen. I like that.

Sorry I'm a neophyte, won't you still be ongassing fast half live tissues at 50-65 feet during your deep stop?

 

[The Chairman]

Sorry I'm a neophyte, won't you still be ongassing fast half live tissues at 50-65 feet during your deep stop?

No, you won't. If I dive to 99 feet on air for 25 minutes, I will have accrued an additional 2.88 PP N2 in my blood and neural tissue for a total of 3.88 atm PP of n2. At half my depth, 50 fsw, I will be breathing a PP N2 of 2.5 atm. The tension would then be a negative 1.38 atm. If I spend 2 minutes, I'll probably only lose 20% (a guestimate) or .28 atm bringing me down to 3.6 atm pp of N2. At 10 ft I will do a full five minutes and the tension would be a negative 2.3 atm. That would bring me down to about 2.4 atm. A similar 3 minute only stop would have a tension of 2.58 and the three minutes would only reduce my arterial and neural N2 by about 0.8 atm to 3.2. That's almost a full atm difference.

Note, I didn't use calculus to compute the actual offgassing. I merely used some mental gymnastics and interpolated the figures. i think it was Uncle Ricky who said deco theory is measuring with a micrometer, marking with chalk and cutting with an axe.

 

[Tortuga68]

I guess Basic Scuba has been redefined now...

 

[Rhone Man]

Over a few beers at the end of a few days diving a bunch of folks were chewing the fat.
We got to talking about new divers understanding of a safety vs a deco stop and the real world concequences of blowing either.
My question is for divers just starting out or with under 25 dives.
What is YOUR understanding of the difference between a safety stop and a deco stop?
Do you treat a safety stop as a deco stop and why?

I don't have to lie to my wife about never doing dives that involve safety stops.

 

[The Chairman]

I guess Basic Scuba has been redefined now...

It's a shame that basic deco theory is not taught in most OW classes. I don't follow the "keep 'em in the dark" philosophy and want my students to understand gas planning as well as how they on gas and off gas nitrogen. It doesn't matter if you teach your students how to dive with tables or a PDC, they still need to understand concepts like half times and tissue compartments. You don't have to be able to do the calculus or even understand the logarithms used, but at least a tenuous grasp of the fundamental concepts are important for any diver.

 

[MXGratefulDiver]

Sorry I'm a neophyte, won't you still be ongassing fast half live tissues at 50-65 feet during your deep stop?

To answer your question simply ... no. But depending on the dive profile you may still be ongassing slow half-life tissues during your deep stop..

At the OW level I like to use an analogy. Imagine two sinks connected by a common wall, and with a hole near the bottom of that wall that allows water to flow between them. One sink is your lungs, the other your body. As you descend, it's like pouring water into the first sink ... the hole in the bottom of the common wall lets some water into the other sink which will continue to flow in until the two sinks equalize. That's ongassing. As you ascend, you take water out of the "lung" sink. When it reaches a point where the water in that sink is less than in the "body" sink, water will start to flow the other way. That's offgassing.

Now let's take that analogy a bit further. Since your body comprises lots of different types of tissues that ongas and offgas at different rates, let's imagine that there are a bunch of sinks connected to that "lung" sink and that they all have different size holes ... the slower the tissue ongasses/offgasses, the smaller the hole. As we descend, adding water to the lung sink, the rate at which water flows into those other sinks varies depending on the size of the hole. So as the dive proceeds, the level of water in those sinks will vary, with those connected to the smallest holes (the slow tissues) holding the least water while those with the largest holes (the fast tissues) holding the most. As we begin our ascent, and the water level in the "lung" sink gets reduced, those with the highest levels (the fast tissues) will begin to offgas first. And because they have the biggest hole, they will also offgas the quickest. The slow tissue sinks will still be at a fairly low level, and many will continue having water flowing into them because even though the "lung" sink is being lowered, it's still at a higher level than they are.

This is a simplified analogy, but easy to picture ... and a relatively good analogy of what happens inside your body as the dynamics of the dive change.

As we ascend, we want to manage the rate of change between the "tissue" sinks and the "lung" sink, to help our lungs better remove the excess gas. And one thing we can understand about those sinks is that the closer to equal levels they get, the slower the rate of water flow between them will become. Therefore, the deep stop is really to help our fastest tissues "catch up" by managing the level difference between their sink and the "lung" sink. In other words, the deep stop is for the benefit of our fastest tissues. The trade-off (there are always trade-offs) is that our slowest tissues may therefore continue ongassing while we're stopped. It's for this reason that a deep stop is relatively short ... so that we give the maximum benefit to the fast tissues while at the same time making a minimum payment to the slower ones that are still ongassing. The safety stop is more for the benefit of the slower tissues, because by then the "lung" sink has a relatively low level in it, and all of the other sinks are at a higher level and therefore are flowing back into it.

Using this analogy isn't physiologically accurate, in that there's lots going on that it doesn't talk about ... but it helps to conceptualize ongassing and offgassing for people who have never really thought about it before ... and does so in a way that skips all the math. It also puts into context why a safety stop is recommended ...

... Bob (Grateful Diver)

 

[AlexL]

Using this analogy isn't physiologically accurate, in that there's lots going on that it doesn't talk about ... but it helps to conceptualize ongassing and offgassing for people who have never really thought about it before ... and does so in a way that skips all the math. It also puts into context why a safety stop is recommended ...

i like your analogy. one thing i might add to it, to address why you need the faster compartments to 'catch up' is there is a layer of fine sand on the bottom the tissue basins, along with a similarly fine screen over the connecting holes. with small differences between the basins, the outflow is calm and controlled, but if the outflow from the tissue compartments to the lung compartment is too fast (too much tension) the sand will get sucked onto the screen and start to clog it up (in tissue bubble formation). this both slows the outgassing rate for that tissue, and may clog the screen completely (DCS). you then have to clear the clog by blowing the sand off the screen, this is done by reversing the process and dumping a bunch of water into the 'lung' basin (chamber ride). for minor clogs (silent bubbles), the sand will eventually fall back to the bottom once the flow has ceased.

 

[MXGratefulDiver]

That's a nice visualization ... I may figure out a way to work it into the presentation.

There are many potential questions that can come from using analogy as a teaching method ... most of them start with the question "why". One reason I teach this way is to encourage those questions.

... Bob (Grateful Diver)

 

[The Chairman]

Dang Bob... consider that analogy stolen!

 

[live&dive]

Semantics, potato, potatoe. I kid of course but it really is a subjective matter since they are both deco stops.