100ft deep dive, safety stop at a little below 20ft

[markfm]

Excellent, thanks.

 

[Don Burke]

This is one of those grey situations.. say your'e doing a safety stop from a 100ft deep, and you have a mandatory safety stop to do. 3 min at 10-20ft.. well, as you all know, 10-20 ft zone is ALWAYS crowded.. So, chances are, you're a little deeper as you wait for the conjestion to clear.

Say you were at 22ft for 2 minutes, and then at 19ft for 2 minute. Obviously, my computer doesn't register the 2 minutes at 22 ft, so it'll tell me I still have 1 minute left. But what about the actual nitrogen content in my body? Has the 2min at 22ft and the 2min at 19ft combined effectively done what the 3 minute saftety stop is supposed to do?

I mean, it's one of those "well, just do the full 3 minutes as a precaution and be conservative" kinda situation, but it's good to know what's REALLY going on in case of an emergency. Like, is it better to be deeper, or shallower if you had to dodge a bulge of people?

It is normally better to hang a little deeper than shallower.

Coming up from that 100 ft dive, some tissues will start offgassing at 99 feet and some will still be ongassing at 20 ft. It is all a matter of degree.

In the specific situation of 15 minutes (a guess) at 100 ft, a slow ascent to 22 ft at 2 minutes, and 2 minutes at 19 ft, I would expect zero problems.

As a side note, the gas loading does not go away when you get to the surface. I hang out on the surface for a few minutes before doing anything strenuous.

 

[MXGratefulDiver]

This is one of those grey situations.. say your'e doing a safety stop from a 100ft deep, and you have a mandatory safety stop to do. 3 min at 10-20ft.. well, as you all know, 10-20 ft zone is ALWAYS crowded.. So, chances are, you're a little deeper as you wait for the conjestion to clear.

Say you were at 22ft for 2 minutes, and then at 19ft for 2 minute. Obviously, my computer doesn't register the 2 minutes at 22 ft, so it'll tell me I still have 1 minute left. But what about the actual nitrogen content in my body? Has the 2min at 22ft and the 2min at 19ft combined effectively done what the 3 minute saftety stop is supposed to do?

I mean, it's one of those "well, just do the full 3 minutes as a precaution and be conservative" kinda situation, but it's good to know what's REALLY going on in case of an emergency. Like, is it better to be deeper, or shallower if you had to dodge a bulge of people?

Well, the simple answer is that two feet, plus or minus, won't make a bit of difference to your body ... even if it does to your computer. So you need to decide which matters to you.

Let's start with a simplified explanation of why the safety stop is recommended ... and keep in mind that as long as you stay within NDL it is a recommendation, and not a mandatory stop at all.

Think of your body like a set of sponges that absorb nitrogen instead of water. Different tissue types will absorb the nitrogen at different rates ... and like a sponge they will only absorb so much, then they become "saturated" and can absorb no more. When you dive, the pressure of the surrounding water has an effect on those tissues ... it enables them to absorb more nitrogen than they would if you were on the surface. The deeper you go, the greater this effect becomes. When you start to ascent, this effect reverses and the ability of your tissues to absorb nitrogen decreases with depth (and pressure). Therefore the nitrogen that's already in your tissues must come out. That's what is known as "offgassing" ... the nitrogen that your tissues cannot maintain due to decreased pressure come out as tiny bubbles.

If you ascend slowly enough, those excess bubbles are expelled from your body through the normal process of blood circulation and breathing ... and cause you no harm. And just as different types of body tissues absorb nitrogen at different rates, so the reverse is true and they also offgas nitrogen at different rates.

Now ... you probably recall from OW class the example of what happens to a balloon as you descend and ascend ... the balloon gets smaller as you descend and larger as you ascend. Same goes for the nitrogen bubbles ... if you don't allow sufficient time on the ascent to expel them from your body, they will increase in size as you get shallower. The real danger is when they grow large enough to start colliding with each other ... if you've ever watched what happens to soap bubbles when they collide, you will recall that every once in a while these collisions result in a really large bubble. Same principle applies. That's why they recommend you come up at no more than 30 feet per minute ... in fact, slower is better, and coming up at a slower rate allows those bubbles to get expelled from your body before they can grow and collide.

That's also the reasoning behind the "deep stop" at 1/2 your deepest depth ... to give your body a chance to eliminate the microscopic nitrogen bubbles in your blood before they can grow and collide to combine into large ones.

So how does this relate to your question?

Well, the purpose behind the safety stop is to enable your body to eliminate the majority of the excess nitrogen bubbles that have built up during your ascent before you expose yourself to atmospheric pressure (1 ATA at the surface). Remember that the tissues comprising your body will continue to offgas for a long time after you've left the water, so you want to give your body as much chance as you can to eliminate the nitrogen that has been offgassed into your blood stream by your ascent. You do this by stopping for a while when your body is still subjected to greater than surface pressure ... 15-20 feet is a recommendation, not a hard-n-fast requirement.

At 20 feet your body is at 1.6060 ATA, at 22 feet it's at 1.6666 ATA. Somehow I don't think that extra .06 atmospheres of pressure is going to make a bit of difference to how efficiently your body offgasses. What's far more important is ... (a) ascend as slowly as you can reasonably do in order to give your body as much time as possible to offgas ... and (b) if you need to stop a bit deeper than you'd like, stay a bit longer. If you need to stop at 22 feet (or 25 feet) to avoid crowds, then once you get a chance to move up the line, stay an extra minute at 10 feet ... your body will thank you for it.

As to the computer, remember that it's only telling you what to do based on mathematical models. In reality the computer hasn't a clue how much nitrogen is in your system ... because that depends on a lot of physiological factors that the computer was never designed to detect. Understand at a basic level what diving does to your body in terms of nitrogen loading, that coming up slower is better, that when doing a deep dive, making a deep stop increases your body's ability to offgas, and that increasing the length of your safety stop allows you to hit the surface "cleaner" of nitrogen bubbles than you would otherwise.

Then you can base your ascent curve and stops on knowledge, rather than on an LCD display ...

... Bob (Grateful Diver)

 

[DA Aquamaster]

On deco dives I will have my computer along, but I use a much more conservative deco schedule based on DPlan (using a bubble gradient model with deep stops). I usually try to be much more conservative than my computer on dives within the NDL's as well.

For example on a 100 ft dive within the NDL's, I'll normally do 1 minute at 50 ft and then another minute or two at 30 ft before doing the computer mandated 3 minutes at 20 ft. And then I'll do a very gradual ascent to the surface with another minute or so spent at 10 ft. - more if the ladder is in use as i'd rather hang out at 10 ft offgassing than to float on the surface.

It's overkill maybe, but if I do 3-5 dives per day over a three day weekend I find that I feel great with this approach as opposed to feeling overly fatigued and having flu like body ache symptoms if I push the computer to it's limit and just do the 3 minute stops.

With my computer, I have to break 20 ft for it to start the 3 minute safety stop count, but it allows me to go as deep as 29 feet before it stops the count and requires a new one. So if the 20 ft mark is crowded, I just need to get the computer to 20 ft and then can descend 5 or 6 feet below the crowd. I think it's critical to do a safety stop above 30 ft, but I don't think it is quite as critical whether you do it at 15 ft, 20 ft or even 25 ft. For a "safety" stop a little deeper than 15 ft you can add an extra minute or so to offset any reduction in offgassing that may have occurred.

I am usually the last one out of the water, but that works because I own the boat so they have to wait for me.

 

[mars2u]

That is not true at all... depending on the profile, offgassing may not start for a significant depth above your max depth. The shorter your time at max depth the less your tissues are saturated - offgassing will start at a shallower depth. You would have to be very close to saturated for offgassing to start 1' above your max depth - not likely on a single 80

Not my understanding...but I'm not a medical professional. My understanding is that offgassing is always happening albeit at a smaller rate....but I can see it being less significant if it's only 1 foot from being maximum saturated. I've heard this from some Medical Professionals, but again don't have the professional experience to discuss either way. Perhaps someone with a medical background such as Dr. Deco would be more knowledgeable in commenting on these topics.

 

[kensuguro]

not that I wanted to start a discussion at a medical level, but thank you all for the awesome info. Much more detailed than I'd expected. So I guess the rule of thumb would be to stay depper than shallower, but not deeper than 30ft. Also, it's much safer if I'd do a minute stop at half pressure. Of course, the ascending at a safe rate thing is understood.

It's not that I don't understand how the safety stop works, it's that I'm starting to wonder about these grey situations as I've just started DM training. (I know, I know, I'm not very experienced yet..) I find myself having to deal with situations that I normally would avoid in the first place, and also have to think of what to tell the problematic student afterwards. It's easy to say "don't do that, it's not safe", but I also want to be able to give him possible alternative solutions, and reasons to consider them. (instead of the unsafe act) With the instructor's understanding, of course.

 

[loosebits]

Not my understanding...but I'm not a medical professional. My understanding is that offgassing is always happening albeit at a smaller rate....but I can see it being less significant if it's only 1 foot from being maximum saturated. I've heard this from some Medical Professionals, but again don't have the professional experience to discuss either way. Perhaps someone with a medical background such as Dr. Deco would be more knowledgeable in commenting on these topics.

This is basic diving 101. You understand that tissues have a half-time? For example, if the delta-P between a tissue and ambient is 1 ATM and the half-time is 60 mins, after 60 mins, the delta-P will be 0.5 ATMs, after 120 mins, it will be .25 ATMs and after 3 hours, it will be .125 ATMs. That's the whole reason we have to decompress on the way up - if tissues equalized with ambient instantly (a half time of 0), we would never need to decompress.. shoot straight for the surface, wouldn't matter.

Now, it works the same way on the way down. If you go to 2 ATA (33 FSW), your slowest tissues are still very close to being at 0.79 ATM PN2. I believe most models use half times of 600 mins for the very slowest tissue - not sure. But if you go down there for 1 min and come back up, I promise you, you won't start offgassing until you are within several feet of the surface at best. Offgassing meaning that you are exhaling more N2 than inhaling.

I think most computer use a fast tissue half-time of around 5 mins which even for that tissue, our decent to 33 FSW, and staying there for 1 min, that compartment isn't even 50% saturated - and that's the fastest one, the slowest tissues will have negligible N2 over .79 ATMs.

Btw, I am using tissues and compartments interchangeably... probably shouldn't. Suunto or Uwatec don't really know the half-times for all the tissues in your body but they will make up some half times between several minutes (representative of fast tissues like blood) and many hours (representative of extremely slow tissues like bone marrow). The computers model these tissues as "compartments". Each compartment having a different half-time and tolerance to a PP gradient (the higher the faster the half-time, the higher the tolerance).

 

[PerroneFord]

Boy, this is a little deeper discussion than the normal "Which fins should I get"??

 

[mars2u]

Boy, this is a little deeper discussion than the normal "Which fins should I get"??

LOL...yeah no doubt. Loosebit and I are talking apples and oranges again!...lol. No biggie.

 

[loosebits]

not that I wanted to start a discussion at a medical level, but thank you all for the awesome info. Much more detailed than I'd expected. So I guess the rule of thumb would be to stay depper than shallower, but not deeper than 30ft. Also, it's much safer if I'd do a minute stop at half pressure. Of course, the ascending at a safe rate thing is understood.

It's not that I don't understand how the safety stop works, it's that I'm starting to wonder about these grey situations as I've just started DM training. (I know, I know, I'm not very experienced yet..) I find myself having to deal with situations that I normally would avoid in the first place, and also have to think of what to tell the problematic student afterwards. It's easy to say "don't do that, it's not safe", but I also want to be able to give him possible alternative solutions, and reasons to consider them. (instead of the unsafe act) With the instructor's understanding, of course.

There is a lot of good information out there and there is a lot of bad information out there especially concerning newer ideas like deep stops. Search google for "pyle deep stops" for a lot of good info. Also it would be helpful to understand M-values before getting into deep stops. Here's a great document on M-values: http://www.dive-tech.co.uk/resources/mvalues.pdf