speed of descent

[simonk999]

This is a followup to an earlier thread, that I couldn't find. That original thread, as I remember, asked whether the speed of one's descent, presuming no equalization problems, mattered much.

While browsing the web for other things, I ran across this article from the Canadian diving mag that deals with exactly this question:

http://divermag.com/archives/nov2000/divedoctor_nov00.html

To summarize, it said that yes, it matters because a fast descent will probably mean that you're going to spend more time at depth (i.e. your avg depth will be deeper), thus loading Nitrogen for longer, thus affecting your dcs prognosis, thus affecting how you should ascend. Etc.

-Simon

 

[100days-a-year]

Depends greatly on the depth, the mixture and profile. Most deep divers will drop to the bottom at warp speed as thier computer models show this decreases inert gas loading.

Ask Dr Deco for a professional 2nd opinion.

 

[Dr Deco]

Dear Readers:

The actual fact is that it will not matter in the overall picture of gas loading, unless you are going to a very deep depth for a very short time. In this case, the descent time will be a significant fraction of the bottom time. Tables are generally calculated to reflect gas loads with a square-wave bottom time. That is, the descent is instantaneous. Real tables are tested however, with real descents at 60 feet per minute. My major concern would be with very active swimmer to get to the bottom and then having a short stay time. The former would build tissue micronuclei and the short time would not allow for their shrinkage.

These are really quite hypothetical however, for the profiles that recreational divers perform. I do not see the gas loads differing that much in practice. The real question, I believe, is how fast do you ASCEND and do you generate nuclei while on the boat (topside).

Remember that decompression (offgassing) does not end when you hit the surface.

Dr Deco

For those of you who are interested, this is a reminder of the [red]Decompression Physiology class[/red] I have at the Catalina Island station. For more information, connect on:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[Walter]

Narcosis hits harder when you descend rapidly. 75'/min is the recommended descent rate.

WWW™

 

[Iguana Don]

For those of you who are interested, this is a reminder of the [red]Decompression Physiology class[/red] I have at the Catalina Island station. For more information, connect on:
http://wrigley.usc.edu/hyperbaric/advdeco.htm


[/B]

I will be there selling the hottest item in the waters, The Dr Deco T-Shirt, along with Caps and the newest item, Dr.Deco Bubble Bath
New products are in the works, a soda pop called Dr. Deco's Nitrogen Fizz & Dr. Deco's Hyperbaric Bubble Gum, along with a champagne called DD's Micro Bubbly

ID

"Imagination is more important than knowledge".....Big Al E.

 

[jmsdiver]

Interestingly enough, I have never heard of a descent rate until now, though each of the posts strikes a thought: why is there not a descent rate taught in classes if there is a physiological reason for a slower rate? New divers do ask the question (and perhaps even seasoned divers) about descent rate and the general answer has been whatever speed one can handle in regards to airspace equalization. If it is true that deco-models have a set descent rate, then shouldn't the training agencies incorporate it into training so that divers will be safe?

 

[Walter]

I do teach a descent rate in my class, but then I teach lots of things left out of other classes.

WWW™

 

[Dr Deco]

Dear Readers:

DESCENT is calculated by table designs as being a “square wave,” that is, a zero time to the bottom depth. The tables are tested with the 60/min rate. It is true that, if one were going on the assumption that there is a “bends/no bends” point, then table should be dived as tested. As this column has indicated since its inception, this “limiting line” for DCS/no-DCS does not really exist for a population of divers; virtually all barophysiologists today would agree with this.

I believe that the reason that ASCENT rate is stressed today is that there is an increasing recognition that the Haldane model of the metastable state with its “bubbles/no-bubbles” basis is not correct. This was demonstrated by Doppler bubble detectors in the early 1970’s and was, in fact, my first scientific papers in the field of barophysiology. [MR Powell. Leg pain and gas bubbles in the rat following decompression from pressure: monitoring by ultrasound: Aerospace Med., 43, 168-172 (1972); MR Powell. Gas phase separation following decompression in asymptomatic rats: visual and ultrasound monitoring: Aerospace Med., 43, 1240-1244 (1972).] The ascent rate has a very big influence on the dissolved nitrogen elimination kinetics (= rates) from tissues into the blood.

Thus, you will hear about 10-to-1 comments regarding your surfacing rate as compared to you descent.

Dr Deco

 

[Green_Manelishi]

some of the deco software available (eg: GUE's DecoPlanner)
will allow you to input the planned descent rate.

 

[sky50960]

tables do have a descent speed witch is a quite important issue :

If descending too fast you average depth during descend will be greater than a fast descend causing more nitrogen uptake and increases DCI risk.

If descending slowly don’t forget to include descend time into you bottom time.

If descending fast try to begin ascend a few minutes earlier than expected.

The meaning is : stay conservative

If going down fast :
Increasing possibility to ear barotrauma ( difficulty to clear ears).
Increasing possibility of nitrogen narcosing
Increasing possibility oxygen toxicity

Be sure everybody who is diving along with you is able to perform a fast descend. Do not lose someone.

Fa