Ascending without a dive computer

[BlueTrin]

First off, let me clarify, I am not challenging anybody's knowledge or expertise. Simply trying to be a bit geeky and dive into the details so that over time I absorb more and more of this information.



I guess I am. I am surprised that we have tissues that are so sensitive to pressure that even a few inches of water pressure will keep them on-gassing. It seems like those tissues would by default be the fast tissues. What compartment, in my scenario, or any scenario at a microscopic level cannot off gas during the apropriate partial pressure of an ascent? For my question, a _very_ slow, possibly impractical ascent? Even if the off gassing is incredibly slow, there has to be some depth along the ascent path where each tissue's partial pressure is met, no? At some point on the ascent, if slow enough, won't every tissue reach a partial pressure equilibrium for the current depth and begin to off gas if that depth is reduced? This is why I asked if we were talking about a "practical for scuba" sweet spot of ascent rate vs a law of physics sweet spot. I guess I figured that a really slow ascent rate would be the perfect form of decompression. Slow tissues are allowed to off gas slowly and near zero chance of a bubble forming. I am not suggesting that all tissues would reach equilibrium at the same time, only that if an ascent was slow enough, all tissues would reach ambient saturation seconds and inches below the surface.


Agreed. Wholeheartedly. This is also the assumption you said I made above.

But you and I just agreed that they would? At least at any practical level. If an ascent were slow enough, wouldn't any practically measurable tissue that had nitrogen in it at a depth of a foot or two be at ambient pressures?

I understand the different compartment idea, admittedly at a very amateurish level. At the extreme, even the fast tissues would have an elevated nitrogen equilibrium at a foot of depth. Maybe immeasurable, but there. This is why I asked about a _very_ slow ascent. I am still working my head around a "too slow ascent rate." I remember the SB thread that contained a really nice on-gassing computer simulation. (Which I didn't find with a cursory look. I think that you were a contributor.) It explained this really well but I don't think it addressed anything like a too slow ascent rate. I understand that equilibrium will be reached at different depths for different tissues. Again, in my proposed scenario, we are talking about a possibly impractically slow ascent. I was just surprised that a diver could ascend too slowly. Just digging for an understanding.

Let me ask it in a different way. This doesn't tie directly to my original question but it is possibly a more practical question. Is scenario two actually at a higher risk of DCS than scenario one?
Scenario 1) I am doing a 60 minute dive to 45' depth, I stay there the maximum amount of time to allow for a safe "industry standard" ascent to the surface, 53 minutes. Then ascend normally at 30ft per minute. (2 minute descent. 3 min safety stop, 90 second combined ascent, so a 53+ minute bottom time)
Scenario 2) I am doing a 60 minute dive to 45' depth. It take two minutes to descend to 45'. At that time, I take 58 minutes to at ascend 45 feet smoothly to the surface. Roughly an ascent rate of .8 feet per minute.

Are you asking if you can reach the surface with the same level of nitrogen than you would have an atmosphere ?

Possibly but have you looked at the surface interval tables ? It takes a long time to change letters during the interval, when you are at 1 atmosphere, so it would be not really practical.

 

[txgoose]

I'm not quite sure I understand the point of confusion in your post, but I think that you are wondering about increasing DCS risk from a slow ascent. When you are at your NDLs on a recreational dive, not all of your tissue compartments are fully saturated - specifically, the slower ones. So if you ascend slowly, there will still be a pressure gradient for these compartments during part of the ascent, and they will continue to ongas. So with a slow enough ascent, your N2 load will possibly be great enough for one of these to become the leading compartment and put you into deco.

Or am I misunderstanding the question...?

In the post I originally questioned, I saw no mention of being up against an NDL. I have had to "buy back time" on a single occasion so I understand the need to ascend in that scenario. (I am proud that I have, at least once, had my air last long enough push NDL, woot!) But, I will agree that you have made me see deeper into the guts of the question. I can see a place where starting at sufficient depth a person could ascend slow enough to reach NDL on a normal rec dive. In retrospect I was focusing on the physics, rather than practicality of an AL80 or similar air supply.

I will still say that 3m/min (which spurred my questioning) seems pretty fast for something like a hard rule. My deepest non-training dive was 94 feet. At 2ft a minute, buying back time all along the way, I'd surface with a fair amount of air and highly doubt I'd ever hit NDL. Someone could do the math and graph that I suppose. A good calculus derivative. Based on rec depths, for any give depth, what is the slowest ascent you could make and stay out of NDL. And that ascent rate would obviously balloon as you reduced depth.

 

[txgoose]

Are you asking if you can reach the surface with the same level of nitrogen than you would have an atmosphere ?

Possibly but have you looked at the surface interval tables ? It takes a long time to change letters during the interval, when you are at 1 atmosphere, so it would be not really practical.

I was. And I agree with you. I think that you and @doctormike have definitely answered the practicality portion of my question. Your mentioning of the tables and "dropping" a letter is a good example.

 

[doctormike]

In the post I originally questioned, I saw no mention of being up against an NDL. I have had to "buy back time" on a single occasion so I understand the need to ascend in that scenario. (I am proud that I have, at least once, had my air last long enough push NDL, woot!) But, I will agree that you have made me see deeper into the guts of the question. I can see a place where starting at sufficient depth a person could ascend slow enough to reach NDL on a normal rec dive. In retrospect I was focusing on the physics, rather than practicality of an AL80 or similar air supply.

I will still say that 3m/min (which spurred my questioning) seems pretty fast for something like a hard rule. My deepest non-training dive was 94 feet. At 2ft a minute, buying back time all along the way, I'd surface with a fair amount of air and highly doubt I'd ever hit NDL. Someone could do the math and graph that I suppose. A good calculus derivative. Based on rec depths, for any give depth, what is the slowest ascent you could make and stay out of NDL. And that ascent rate would obviously balloon as you reduced depth.

Yeah, I think that the bottom line is that the assumption that "slow is good, slower is better" eventually comes up against the N2 loading associated spending too much time on the ascent. It basically makes your dive into a multi-level dive. Even though you are ascending, you are still ongassing some of your tissue.

Not to open up a whole can of worms here, but this was one of the concerns about "deep stops". The question was if any decompression benefit of a deep stop in terms of controlling bubble growth would be offset by extra N2 loading.

 

[BlueTrin]

Yeah, I think that the bottom line is that the assumption that "slow is good, slower is better" eventually comes up against the N2 loading associated spending too much time on the ascent. It basically makes your dive into a multi-level dive. Even though you are ascending, you are still ongassing some of your tissue.

Not to open up a whole can of worms here, but this was one of the concerns about "deep stops". The question was if any decompression benefit of a deep stop in terms of controlling bubble growth would be offset by extra N2 loading.

I am not an expert but if you go slowly enough you’ll have done all your deco stops even if you are past the NDL isn’t it, doc ?

 

[BRT]

I am not an expert but if you go slowly enough you’ll have done all your deco stops even if you are past the NDL isn’t it, doc ?

May take several days.

 

[doctormike]

I am not an expert but if you go slowly enough you’ll have done all your deco stops even if you are past the NDL isn’t it, doc ?

Depends on the profile. Your deco stops are generated by an algorithm to allow efficient offgassing.

Safe staged decompression doesn't just mean that you stop at every stop, but that you spend the correct amount of time at every stop. If you spend long enough at any deco stop, you will start saturating tissues that are not already saturated. And unless you are a hard hat commercial sat diver, there will always be some slow tissues on a recreational dive that can ongas.

So yes, if you go VERY slowly, all of your tissues will eventually saturate and then offgas as you ascend, but that implies limitless gas supply (i.e. a sat diver in a decompression chamber, who may take two weeks to "surface").

 

[boulderjohn]

I am not an expert but if you go slowly enough you’ll have done all your deco stops even if you are past the NDL isn’t it, doc ?

If you ascend very slowly at the deepest part of your dive, your slower tissues are on-gassing, and you will need to decompress longer when you are at the shallower levels.

 

[BlueTrin]

If you ascend very slowly at the deepest part of your dive, your slower tissues are on-gassing, and you will need to decompress longer when you are at the shallower levels.

Thanks I never realised it would be THAT MUCH longer.

 

[boulderjohn]

Thanks I never realised it would be THAT MUCH longer.

Here is a way to think about it by carrying it to an extreme. Let's say you are using the PADI tables to guide your dive. (I am using that example because I can give you specific numbers.) You are at 100 feet for 20 minutes, the maximum NDL time for that depth. You begin to ascend at 5 FPM. After 2 minutes, you arrive at 90 feet and have now spent 22 minutes between 90-100 feet. You have essentially gone into deco, although a table as no way to calculate it. You reach the 90 foot mark at just about the NDL for 90 minutes, but you have spent that entire time below that depth.