What makes us think we can trust any of them

[CIBDiving]

.... For the sake of this discussion, Why would someone take so much time to ascend when accepted opinion says 30 ft/min is no problem, even from depth? Arguing that even the 15 ft safety stop is optional on a ndl rec dive.

The 30 ft/ min ascent is a maximum value Not a "you must ascend at this rate" number. Because the computer model Must have an ascent rate to calculate NDLs on the fly it assumes the fastest safe one. I really don't want to get to deep into the mathematical theory ( for one I couldn't properly explain it) but the 'Best' ascent rate is not a constant velocity, it varies with depth being faster deeper. Humans don't do things like that well, so a 'best fit' rate is used. There is an 'effective' minimum ascent rate also, go to slow and you increase loading in other tissue. You also have trouble controling a rate that is very slow, a good minimum ascent rate is about 10ft/ min, maybe a little slower.

Most tables are made so that the 15ft "safety" stop is recomended but is Not a required thing - if it was required that you won't be doing "No Stop" diving would you.
It does have a number of advantages. First, it forces you to SLOW DOWN, the most critical point in the ascent and where the most rapid pressure chagne happens is from 30ft up. Remember that 'Best Fit' 30 ft/min thing? Here is where it starts to get farther and farther from the optimal rate, but on the side of to fast ( down deep 30 ft/min is slower than it needs to be).

The other part is many seem to think that all this a black and white - as long as you do this and don't go over that line, you're safe. It's more like jumping from a height, you can jump from 8 ft and not break something Most of the time, you can jump from high points and not break something but it Hurts to land. Deco theory is simalar to this, Physics says a bone is this strong, and a jump from a given height puts this much force on that bone, Therefore it's 'Safe' to jump from a height that is just under the limit for that bone.

 

[Charlie99]

I am a fairly new diver with only around 70 dives logged. I have never been overly tired after diving. even after doing a three dive day including the first to nearly 100 ft.

Ok, let me put it this way, other than perhaps the last 10 feet, what is wrong with the first profile in Snowbear's post?

Please take this the right way. I am trying to learn but also need to be convinced.

Nothing is wrong with the first profile in Snowbear's post, but OTOH, you can improve it and gain additional margin by simply spending a bit longer on the ascent.

Although you have never been overly tired after diving, you might be surprised at the difference in how you feel if you do change your profiles a bit.

A more technical explanation, using just simple dissolved gas model, can be found in Erik Baker's article explaining deep stops.

 

[Diver0001]

Ok, let me put it this way, other than perhaps the last 10 feet, what is wrong with the first profile in Snowbear's post?

I think she intended for you to look at the general shape of the profile without looking too deeply at the details. Those two graphs are more relevant to the discussion when viewed in thumbnail form.

R..

 

[WarmWaterDiver]

My thanks to Snowbear, Diver0001, & UP for clarifying the general statements. For those who understand the basic mathematics behind the concept of a half-time, and apply similar thinking to the ascent profile, this does not violate the algorithm of even the most conservative dive computer (per Snowbear) and is absolutely fine to use in concert with a dive computer - not absolutely as an either this or a computer but not both issue.

This does not put my wife / dive buddy / fellow engineer at greater risk of DCS due to being female and using a dive computer. She also understands the mathematical concepts involved.

"Undeserved Hits" can and do still occur.

This is why I love wall diving - scenery all the way up & down enjoyed at a leisurely pace, especially when getting back to shallower depths taking time to see and hopefully photograph the "small stuff", as well as chances for things "in the blue" during descent & ascent.

And, Diving Preacher, my choice to rely more on Buhlmann algorithm in our current dive computer choices versus Haldanean was a progression from Recreational to Advanced Recreational / Beginning Technical diving (depending on any particular agency's definition). Our tech dive instructor commented on this, I researched the math involved, and I put a bit more stock in the UK reviews due to UK diving more commonly employing planned decompression obligation than typical US / Caribbean diving, and opinions I found in their reviews. Also, the use of the Buhlmann based algorithm by Dive Rite in their offerings for many years weighed in significantly in my evaluation (although their more recent NiTek Duo owners manual has more weasel words about use for planned deco obligation diving than their older model like NiTek 3 had). This is simply an explanation of what I did, not an endorsement nor advisement for anyone else.

And for anectdotal stuff, it's a bit off topic but the most recent issue of Undercurrent had some interesting new anecdotal evidence on the benefits of diving Nitrox vs. diving air on apres-dive activities (no profile details were included). It's made me think about recording data from trips we take to more remote locations where Nitrox isn't available vs. trips where we use nitrox during the whole trip.

 

[Snowbear]

For the sake of this discussion, Why would someone take so much time to ascend when accepted opinion says 30 ft/min is no problem, even from depth? Arguing that even the 15 ft safety stop is optional on a ndl rec dive.

Yes, these are commonly accepted as safe standards.... which is why there is nothing wrong with the first profile in my previous post (even the last 10' is not a problem.... it was just a "normal" <30fpm ascent from the safety stop).

For the sake of discussion, this someone takes so much time to ascend because she has figured out how much better she feels after the dive, especially after 2, 3 or even 5 dives a day for several days in a row.

I also stopped getting those occassional post dive fatigue episodes and the old fracture site in my leg stopped aching after some of those same dives. Was I getting "sub-clinical" DCS before? I don't know, but I am positive that I'm not any more. So I will keep on running my own dives rather than trusting even the most "conservative" computer to "keep me safe"

 

[Potapko]

Yes, these are commonly accepted as safe standards.... which is why there is nothing wrong with the first profile in my previous post (even the last 10' is not a problem.... it was just a "normal" <30fpm ascent from the safety stop).

For the sake of discussion, this someone takes so much time to ascend because she has figured out how much better she feels after the dive, especially after 2, 3 or even 5 dives a day for several days in a row.

I also stopped getting those occassional post dive fatigue episodes and the old fracture site in my leg stopped aching after some of those same dives. Was I getting "sub-clinical" DCS before? I don't know, but I am positive that I'm not any more. So I will keep on running my own dives rather than trusting even the most "conservative" computer to "keep me safe"

Thank you again for your input and experience. After reading another article by Peter Bennet on Ascent speed, here, http://members.cox.net/bemcmillan/bennett1.pdf

I am seeing the wisdom in carefully watching my speed of ascent, as well as making some deeper stops. imo it seems that this is far more important than whether I am keeping time on a computer or a simple bottom timer.

What I am gaining from you more experienced divers is a greater awareness of what is involved in planning the dive as well as a better method of executing that plan.

Looking at my profiles the number one flag raised, BY MY COMPUTER, is that I am ascending too fast. At least once on every dive I'll get an alarm to slow down. This will change immediately!

thanks again,
Jeff

 

[DivesWithTurtles]

While not directly related to computers, regarding this discussion I've found the studies by A. Marroni of DAN-Europe regarding ascent rates and stops very interesting. What surprises me about these studies are the conclusions regarding relatively long deep stops to be considerably more influential in reducing bubbling than slow ascent rates. The studies even indicate that bubble grades are lower with a 60 ft/min (18 m/min) ascent rate than a 10 ft/min (3 m/min) rate (with two 5 minute stops on both profiles). The conclusions indicate that slow compartments, which continue to on-gas during slow ascents, primarily control the degree of bubbling.

See this

and this

and google this.

 

[Charlie99]

Looking at my profiles the number one flag raised, BY MY COMPUTER, is that I am ascending too fast. At least once on every dive I'll get an alarm to slow down. This will change immediately

Repeating what DivesWithTurtles said, but phrased differently ..........

What counts more is the average ascent rate. Don't worry so much about what is happening over a period of 10 or 20 seconds. Look more at what the dive profile looks like on the scale of a minute or two.

Think more in terms of total-time-to-ascend while keeping the average curve slower as you get near the surface. Another one of the Dan Europe articles specifically looked at the effect of instantaneous rate of ascent vs the overall average ascent rate or total time to ascend and found the effect of the instantaneous rate of ascent wasn't significant.

On a practical basis, I find the easiest way to get a good ascent curve is to decide when starting my ascent how long I will take. That, plus the current runtime on my computer tells me when I will surface. I then pick a couple other time/depth points that I will hit. If my ascent rate was faster than ideal, then I'll end up waiting at the stop until my prechosen time. For my typical dives, just choosing runtimes for getting out of the water, for leaving 25', and for leaving 40' are sufficient to control the dive profile. (Even for most of my dives to 130', there is signficant time at depths of 80' or less before starting the ascent. For a wreck dive where I'm starting ascent from 100' or deeper, then I'll add a 4th point a bit deeper than the 40' time).

A rather lengthy discussion about the best ascent profile from a minimum deco (aka NDL) dive has a lot of good info.

 

[Snowbear]

Thank you again for your input and experience. After reading another article by Peter Bennet on Ascent speed, here, http://members.cox.net/bemcmillan/bennett1.pdf

I am seeing the wisdom in carefully watching my speed of ascent, as well as making some deeper stops. imo it seems that this is far more important than whether I am keeping time on a computer or a simple bottom timer.

What I am gaining from you more experienced divers is a greater awareness of what is involved in planning the dive as well as a better method of executing that plan.

Looking at my profiles the number one flag raised, BY MY COMPUTER, is that I am ascending too fast. At least once on every dive I'll get an alarm to slow down. This will change immediately!

thanks again,
Jeff

By George I think he's got it

Merry Christmas and many safe and fun dives to you, Jeff

 

[DivesWithTurtles]

A rather lengthy discussion about the best ascent profile from a minimum deco (aka NDL) dive has a lot of good info.

Geez, Charlie, reading that thread gave me more frustration than education. I did agree with what you were saying there. And questions in that thread never did get answered.

Repeating what DivesWithTurtles said, but phrased differently ..........

What counts more is the average ascent rate. Don't worry so much about what is happening over a period of 10 or 20 seconds. Look more at what the dive profile looks like on the scale of a minute or two.

Yes and no. I agree with the concept of instantaneous ascent rates being less important, but as I understand the ramifications of the Marroni studies it's not a slow overall ascent that reduces bubbles, it's where and for how long stops are made and how quickly one gets to those stops that does. Per his studies, slowing the overall average doesn't do it. Note the considerably higher bubble scores on 10 ft/min profiles than 30 ft/min profiles despite considerably slower overall average ascent rate.

Bottom line, it appears to me, based on these studies only, is that emphasizing slow ascent rates may be over-done (is that sacrilegious?), and longer safety stops at deeper depths may be under-emphasized. (However, I don't believe that "slower is better" for the last 10 or 15 feet can be over-emphasized.)

Isn't it a shame that we can't strap a couple of hundred divers to a deco bar, bring them up with different ascent profiles, and Doppler test them to determine a truly cleanest method for ascents?