Descending too fast??

[sheck33]

The main 2 factors that determine N2 loading are depth and time. Why would descending faster imply a larger bottom time??? i dont rocket propel myself to the bottom and then sit there waiting for my buddy, that must be referring to PADI classes or something like that.
if anything, a slow descent with the same actual bottomtime would result in larger total nitrogen loading.

the statement : 'a fast descent results in larger N2 loading' is nonsense.

 

[Charlie99]

sheck33 once bubbled...
if anything, a slow descent with the same actual bottomtime would result in larger total nitrogen loading.

the statement : 'a fast descent results in larger N2 loading' is nonsense.

This depends on what table you are using. What table do you use?

The ones I use, such as NAUI, DCIEM, and PADI all include the time it take to descend as part of the "bottom time" or "actual bottom time".

 

[sheck33]

Charlie99 once bubbled...
This depends on what table you are using. What table do you use?

The ones I use, such as NAUI, DCIEM, and PADI all include the time it take to descend as part of the "bottom time" or "actual bottom time".

i was not referring to any tables.
the main N2 uptake takes place during 2 parts of the dive, the descent and the time spend on the bottom, lets ignore the fact that some compartments might still be on-gassing during the ascent. So the total N2 loading you have when you decide to start your ascent is :

N2 (descent) + N2 (bottom) = N2 total just before ascent starts.

lets say our BT is fixed at a fixed depth for simplicity but it really doesnt matter.

now, our variable is N2 descent, this depends on 2 factors, the depth we are going to and our descent rate.

at a fixed depth our N2 uptake depends only on the time we spend descending. from this it is obvious that the faster we descend the less N2 uptake will take place during this descent.
so our overall N2 uptake will be less with all other factors remaining constant if we descend faster.

this has nothing to do with what tables i use.

 

[Charlie99]

sheck33 once bubbled...


i was not referring to any tables. <snip>

N2 (descent) + N2 (bottom) = N2 total just before ascent starts.
<snip>
this has nothing to do with what tables i use.

The others had posted in reference to tables.

What is your defintion of "actual bottom time".? That is the key.

The tables use the definition of the total time actually spent underwater from the beginning of decent until leavingthe bottom for a direct continuous ascent to the surface or a safety stop.


If you decide to call the above "run time", then the statement that everyone is making is that, 'for a given runtime, you will have a higher N2 load if you do a fast descent.'

 

[sheck33]

Charlie99 once bubbled...
The others had posted in reference to tables.

What is your defintion of "actual bottom time".? That is the key.

The tables use the definition of the total time actually spent underwater from the beginning of decent until leavingthe bottom for a direct continuous ascent to the surface or a safety stop.


If you decide to call the above "run time", then the statement that everyone is making is that, 'for a given runtime, you will have a higher N2 load if you do a fast descent.'

i have purposely split descent and bottom time to make a point. in my reasoning above therefor i define 'bottom time' as 'time from reaching the desired depth until starting the ascent'

of course it is absolutely correct that including descent time in the 'bottom time' will result in larger N2 loading if the descent rate is higher.

my point was merely that this larger N2 uptake does then not take place during the actual descent phase of the dive.

also, these definitions are vague. on a run of the mill Alki dive i swim down the line to visit the octopus then to the I beams and or honeybear, all this with very gradual in- or decreases in depth, when does my ascent start?? as soon as i am shallower than my deepest point in the dive? as soon as i am 15 ft shallower than my deepest point? These definitions work fine for square profile dives, most dives are not though.

 

[Dr Deco]

Dear SCUBA SOURCE Readers:

“Bottom Time” as I understood it in this example referred to the standard definition which is the duration from leaving surface to the start of ascent. In a large measure, this is most easily seen with reference to tables and a square wave dive.

The whole concept dissolves away when using a computer since time and depth is broken into tiny increments. While it would seem self evident that a fast or slow ascent would not influence gas loading as tracked by the multicompartment model, it must be recalled that this is an untested hypothesis. I would believe it to be true, but it is untested.

When PADI produced their tables with the 60-minute residual nitrogen surface interval table and the reduced NDLs, they tested everything. Computer manufactures have tested virtually nothing. There is data that is added in retrospect, but not any prospective testing. Whether or not descent rates need to be tested is a moot point (I would suspect that it is probably not needed), but it has not been done, nevertheless. We do not see any evidence that problems are arising, but the divers themselves have been the guinea pigs.
Now that is a comforting thought, isn’t it? :scuba:
Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[Charlie99]

Dr Deco once bubbled...
While it would seem self evident that a fast or slow ascent would not influence gas loading as tracked by the multicompartment model, it must be recalled that this is an untested hypothesis.

Any comments or guesses on descent rate affecting the severity of narcosis? (look above to my post of yesterday for the specific questions)

 

[Dr Deco]

Dear Charlie99:
Narcosis :boozer:

It is my understanding that nitrogen narcosis comes on very rapidly. This is because nitrogen is only slightly soluble, and it equilibrates very rapidly. If this is the case, you will not the effects as you descend. Naturally, if you descend quickly to the 100 FSW level, you will experience the full effect of 100 fsw of nitrogen.

If you descend more slowly, the effect will come on at a reduced rate since you are shallower. The final effect at 100 FSW will be the same when you are at 100 feet irrespective of a fast or slow descent.

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[Saturation]

In no-deco diving to 130 even to 160' I'm quite certain the desent rate is of little consequence if you can tolerate the equalization.

I've heard some reports of enhanced narcosis from fast descent on air, but in no stop recreational depths using unassisted or FREE descents, its of little concern. Free descents depend on your buoyancy, and if a diver is weighed properly, the first 60' will be way under 60 fpm even with all the air removed from one's BC or wing.

The USN gives a rate of 75'/min but doesn't qualify why this is so.

Does anyone know?

There is a theoretical reason for limiting the descent rate, to simply make calculations for deco models possible.

http://www.decompression.org/maiken/VPM/RDPW/VPMech7/VPMech7.htm

However, practically speaking in recreational dives, its difficult to stick to a finite rate of FREE descent [as opposed to a powered descent by swimming or scooter] since its proportionate to one's buoyancy, which geometrically diminishes with every ATA of compression but braked with every breath one takes. FREE descent rates stablize depending on how many breaths one takes per minute, as each inhale causes some amount of positive buoyancy.

Free divers descend in excess of 100-300 fpm without issue, so it there is some physiologic concern, its related to breathing compressed gas.

Among the members of the AUE in S. Florida, they point their scooters DOWN and power descend, and claimed peak rates in excess of 200 fpm can be obtained. These divers do it regularly and consistently without issues. A descent video of one such dive I clocked the rate in excess of 100 fpm.

Most printed tables assume arrival at the bottom depth is instantaneous, as using a fixed descent rate that most divers likely will NOT do, puts unecessary complication in table calculations. In computer models, a fixed rate of descent can be used, and in many instances, it simply reduces one's actual bottom time or considers the dive a multilevel dive. For conservatism, many divers use instantaenous descent to calculate inert gas loading based on spending the entire bottom time at depth, not consumed by the descent. For example, a 30min at 80' means all 30' is spent at 80', even if it took 1-2minutes to descend at that depth. The presumed inert gas load is thus higher than what the diver actually experiences. Thus, there is built in conservatism in the instantaneous approach.

There is concern in rapid descents in deep dives past 300' to help limit the effects high pressure nervous syndrome. Its usually of concern around 600' at descent rates over 100 fpm. At these depths, any time spent here rapidly pushes one into inert gas saturation, so its only in saturation diving than descent rates become an issue.

 

[Dr Deco]

Dear Scuba Board Readers:

Descent Rates

While it is true that computer calculations put descent rates at instantaneous, the tests are performed at a definite, controlled rate. For the PADI Recreational Dive Planner, this was 60 ft/min. This, if you wish to use the tables (and their derivatives) under the conditions for which they were tested, you must not deviate from this rate.

Tables have a considerable amount of safety built into them, and I would thus not worry about it excessively. However, divers should recognize that testing is testing and they might want to follow the conditions.

It should also be remembered that most decompression meter algorithms are not tested at all and thus you are really on your own anyway.

Dr Deco :doctor:

Please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm