Why do computers rot the brain?

[Uncle Pug]

AquaTec once bubbled...
how do you know how long you can do this dive for and still remain within the NDL

Here is the deal Doug... the NDLs are not real... (I shouldn't have even used that term in my posts) and they are misunderstood by so many folks to be a threshold that puts you into deco when in reality all dives involve inert gas tissue loading to some degree... and the closer you get to the artificial convention called the NDL the more likely you are to have a problem if you are not careful.

Early on in this thread Dr. Deco posted this:

As I have discussed on several occasions on other SCUBA SOURCE forums, “limits” are a holdover term from the old HALDANE metastable-limit hypothesis. Tissue micronuclei were introduced into decompression in the 1940s by EN Harvey, and they have been gaining favor since the 1960s. In the concept of micronuclei, there are not any table limits . Instead, there is an increasing probability of a DCS problem developing as the nitrogen load increases in the body. It is not an all-or-nothing phenomenon.

nradov is right when he says that "the false concept of NDL" can lead divers into "doing less than the optimal deco."

 

[AquaTec]

i understand this well, it is more of a probability chart.

the same as saying that 1.6 Po2 will automaticly lead to a tox hit.
1.6 is just the point when the probability is very high


I understand that you are ongassing as Pn2 increases and off gassing as it decreases.

the principle behind tables and computers are that if you remain within thier guidlines then there is a high probability that you can make a direct ascent to the surface say at a rate of 60 feet/min [i uderstand that slower is better]

that is where the term NDL has come from, past that point then either a much slower ascent rate or deco stops would need to be included in the profile. the initials are meant to state "No Decompression Stop Required, Time Limit"

are you saying that you can never make a direct ascent to the surface say at a rate of 30 ft/min, that you MUST do stops along the way. or is their some point that you can make that direct ascent and then a spot where you need to do the slow ascent or deco stop

it is understood that a deco stop is simple a method of slowing your ascent, so i understand if the ascent is slow enough you would never need to stop. of course balancing slow ascent with further ongassing

keeping this in recreational terms, a recreational diver should alway be able to make a direct ascent to the surface.

 

[Uncle Pug]

You can make a direct ascent to the surface but the likelihood of incurring DCS gets greater as you approach and then cross what is conventionally called the NDL.

No... I am not saying that you must do stops... I am saying that you should do stops.... they are better even for the recreational diver who stays within the conventional NDL. You have probably seen me refer to minimum deco in other posts. These are 1 minute stops at 30,20,10 for all dives.

As the dive gets longer and/or deeper the stops become more important. The distinction between recreational *no stop* diving and *technical* diving is an artificial one. Recreational divers could benefit from making stops even on those dives that are not considered *technical*.

BTW... the stops are not merely a means of slowing the ascent and a very slow linear ascent is not as efficacious as an ascent with discreet stops in terms of off-gassing.

Dr. Deco's point about micronuclei is something I consider important as well.

 

[AquaTec]

You can make a direct ascent to the surface but the likelihood of incurring DCS gets greater as you approach and then cross what is conventionally called the NDL.

I agree with this, that is why there are tables/computers to track this fluctuating line. what do you do to stay on the "NDL" side of the dive.

No... I am not saying that you must do stops... I am saying that you should do stops.... they are better even for the recreational diver who stays within the conventional NDL. You have probably seen me refer to minimum deco in other posts. These are 1 minute stops at 30,20,10 for all dives.

yes this is a good theory, and i agree, although i question the benifitial difference between the 20 foot and 10 foot stop, but we will leave that untill another day. i agree there is more benifit to this type of "safety stop" as we agree it is not required

As the dive gets longer and/or deeper the stops become more important. The distinction between recreational *no stop* diving and *technical* diving is an artificial one. Recreational divers could benefit from making stops even on those dives that are not considered *technical*.

we agree that stops are benificail, I ask you if you agree that recreational diving should not require deco stops, should there always be direct access to the surface.....is there a time in your dive say to 100 feet that you would be "required" to perform these stops as opposed to have them be simply benificial

BTW... the stops are not merely a means of slowing the ascent and a very slow linear ascent is not as efficacious as an ascent with discreet stops in terms of off-gassing.

true i was just trying tio keep it simple, it takes two minutes for the blood to cycle through the body, so stopping at a point with the highest Po2 and allowing blood to cycle is deffinetly benificial, etc.


PUG now we are having a fun conversation, glad we got to this point

 

[nradov]

AquaTec once bubbled...

I agree with this, that is why there are tables/computers to track this fluctuating line. what do you do to stay on the "NDL" side of the dive.

There is no line, or at least no line that can be calculated in any sort of accurate way (don't confuse precision with accuracy). Understand that the models used to calculate that line were validated with a limited set of test subjects who were not even screened for shunts. So the data is suspect at best and shouldn't be taken too seriously.

Obviously if you have a serious problem that can't be sorted out underwater then go ahead and pop up to the surface, it beats drowning. On a shallow dive with a reasonable bottom time you're not likely to get seriously hurt, although there may be some subtle damage that leaves you feeling not so good afterwards. After a rapid ascent like that you shouldn't dive again for several hours at least to give the bubbles time to clear.

 

[madmole]

FYI taken from the DAN incident reports summary

<<The reported data did not allow to identify any significant correlation between age and gender and
DCI risk; similarly no difference in the incidence of DCS could be observed between Dive Table
Diving and Dive Computer Diving, whereas a significantly higher incidence of A.G.E. was
observed in Dive Table Divers with respect to Dive Computer Divers.>>

So table users, you are taking and extra risk over the computer users (it was something like 25%). Looks like tables rot the brain literally!!!!!

I'd hazard a guess its that a lot of divers are not good at ascent rates and computers act as a mediator that table divers dont have. Hence it probably reflects a skills shortcoming rather than any real difference between tables and computers

 

[The Iceni]

madmole once bubbled...
FYI taken from the DAN incident reports summary

. . . .no difference in the incidence of DCS could be observed between Dive Table Diving and Dive Computer Diving, whereas a significantly higher incidence of A.G.E. was
observed in Dive Table Divers with respect to Dive Computer Divers.. . .

So table users, you are taking and extra risk over the computer users (it was something like 25%).

FWIW, I think this is an extrapolation too far, madmole.

The current fashion (largely because of the poor reliability of trimix computers) is for technical divers to employ tables alone while recreational divers tend to do both. Technical diving, by definition almost, must be more hazardous.

Too many variables, perhaps?

Aquatec once bubbled
it is understood that a deco stop is simple a method of slowing your ascent, so i understand if the ascent is slow enough you would never need to stop. of course balancing slow ascent with further ongassing.

Hence my earlier references to a parabolic ascent, slowing more and more as the surface is approached. Here the rate of pressure change is kept constant.

There will be times when a deep stop will be counterproductive, particularly if it is of extended duration.

 

[madmole]

It was supposed to be tongue in cheek hence the double smilie's

(I will write out 1000 times "Remember American's dont understand sarcasm!!!)

Actually on the ascent rate. There is loads of research available on the DAN site that shows deep stops are very benificial on reducing dopler scores and that ascent rate is very poorly related to doppler scores

ie doppler score is related to total time to surface rather than ascent rate. Fast ascent rates with long stop give lower scores that very slow ascent rates

DCS II type symptoms are thought to be the product of overpressurisation of "fast" tissues (spinal colum etc), with a half life of around 12.5 mins. Thus ascents from 30m at 10m/min are only about a 1/3rd of a half life, therefore they allow those tissues to reach the surface while still fairly saturated. 5 min stops at say 15m and 6m bring the ascent up to around one half life and allow significant offgassing

As for the deep stops adding to the ongassing of the med/slow tissues. This is true, but they by definition are slower tissues and the ontake is very small. Also the tissues likely to still be ongassing at the deepest stop (ie 24m on say a 36m dive (where the VR3 tells you to stop first)) are not likely to be any where near saturation on the dives that even technical divers are currently doing and the very small amount of ongassing can easily be modelled and allowed for and is of less consequence (DCS 1 type) compared with the reduction of overpressures in the DCS II crytical fast tissues. If you model the tissue uptake then you'll be supprised at how small a decrease in depth is required to bring a tissue out of the ongassing area though

Those of us that swapped to using deep stops found that we felt much better. I used to think it normal to want to sleep after a dive to > 30 but now realise that I can be feeling alert and raring to go, by doing deeper stops and cutting down the shallow stops (playing with the mic helps here as well)

I use VR3 as my main computer with at least 1 other backup. I also carry a third computer in guage mode as a bottom timer to go with the backup worse case tables I carry (VPM and RGBM)

Personally I would suggest to many people to try stopping for 2 mins at say 10m intervals even from a 30m dive and see how they feel or making some attempt at Pyle stops on the way up. Deep stops are NOT just for those of use going over 50m or on mix

 

[joens]

I disagree with the statement based on DAN figures that table divers are more prone to accidents than computer divers .Have you read some of the stories of non computer accidents .Some of these divers were accidents waiting to happen .Just because someone doesnt use a computer DOES NOT MEAN THEY USE TABLES. I strongly believe that someone who plans their dives properly is more aware of what is happening in their body and why. They can always dive safer than one who relies on a computer . When I first learned to dive I was encouraged to go straight to computers . when I started to get into Tech diving I learned to use tables properly .I wish I had never used a computer .I would be a lot bettter diver if I had learned proper use of tables sooner.
Joens
P.S feel free to disagree with me but I will never rely on a computer again.

 

[AquaTec]

nradov once bubbled...


There is no line, or at least no line that can be calculated in any sort of accurate way (don't confuse precision with accuracy). Understand that the models used to calculate that line were validated with a limited set of test subjects who were not even screened for shunts. So the data is suspect at best and shouldn't be taken too seriously.

Obviously if you have a serious problem that can't be sorted out underwater then go ahead and pop up to the surface, it beats drowning. On a shallow dive with a reasonable bottom time you're not likely to get seriously hurt, although there may be some subtle damage that leaves you feeling not so good afterwards. After a rapid ascent like that you shouldn't dive again for several hours at least to give the bubbles time to clear.

My question here is.....at some point a dive becomes a dive where you can make a direct ascent to the surface, even if it is not an emergency....then at soem point a dive requires decompression......it concerns me that you guys have such a difficult time finding this point

here is an example of my line
100 feet for 20 minutes allows me a direct ascent to the surface
100 feet for 30 minutes requires a deco stop prior to surfacing