Decompression Stop Guidelines - What we have to do if got deco alert?

[boulderjohn]

What I mean is that on all (non-trivial) dives, absorbed N2 will be released during your ascent. This physiological phenomenon during the ascent is called decompression. The ascent needs to be such that excess N2 is less than your chosen limit.

For dives approaching NDL, you would exceed the limit if you teleported to the surface, but in reality you deco sufficiently during the ascent to be below the limit.

If you exceed NDL, you need to further slow the latter part of the ascent to prevent exceeding your limit while in the water, traditionally done with discrete successively shallower and longer stops. We traditionally call these dives deco dives, but in reality all (non-trivial) dives have nitrogen decompression occuring on the ascent.

A safety stop is a deco stop, it just isn't necessary to reduce N2 below the limit before surfacing.

As we use the terms, there is a difference between what is called a decompression dive and a no stop or NDL dive. Yes, all dives involve decompression, but in common scuba terminology, a decompression dive means a dive with required decompression.

And they are more different that that implies.

• Once you have required decompression, you must ascend at the prescribed rate to the prescribed stop. If you loiter along the way, you will acquire more required decompression time.
• If you begin an ascent while still within NDL limits, you can have quite a different experience while ascending, because it does not seem to matter how slowly you ascend, as long as you do not later violate NDLs. Divers ascending from 100 feet can do multi-level divers giving them a total dive time adding 30-40 minutes to the ascent without even reaching the need for a required safety stop.

 

[L13]

... Unworthy; your posts are usually quite clear, concise, and on point.

Thanks for the compliment! I guess I'm not at my best in this thread.

My point was that dives are on a continuum. Here is an example:

1)Take a dive to 20m for 25 min with your computer set to GF-high = 60%. This is a "deco" dive requiring a 3 min stop at 5m.

2)Now set your computer for Gf-high = 85% and do the same dive. It is now a NDL dive. you could ascend directly to the surface, but if you do your 3 min safety stop you will surface with exactly the same N2 supersaturation as you did on the previous dive (~60% << 85%).

Is a dive to 20m for 25 min a deco dive or not? Does a stop for 3 min change the answer? Does the diver doing dive 1) need to not dive for 24 hours, but not the diver doing dive 2)? Does a stop for 3 min change the answer?

 

[boulderjohn]

Thanks for the compliment! I guess I'm not at my best in this thread.

My point was that dives are on a continuum. Here is an example:

Take a dive to 20m for 25 min with your computer set to GF-high = 60%. This is a "deco" dive requiring a 3 min stop at 5m.

Now set your computer for Gf-high = 85% and do the same dive. It is now a NDL dive. you could ascend directly to the surface, but if you do your 3 min safety stop you will surface with exactly the same N2 supersaturation as you did on the previous dive (~60% << 85%).

Is a dive to 20m for 25 min a deco dive or not? Does a stop for 3 min change the answer?

Ah, so you are saying that if you artificially alter the algorithm, you have changed what is designated as a decompression dive into an NDL dive. That is true. That is also beside the point.

During a dive, there is a point in which a NDL dive becomes a decompression dive. That has nothing to do with the algorithms we use to try to predict when that moment occurs. Those algorithm are imperfect, because we don't know about decompression that precisely and because each individual is different on each individual dive.

So that means that different algorithms will differ in how they predict when an NDL dive becomes a decompression dive, but that does not negate the fact that the moment exists.

 

[tursiops]

My point was that dives are on a continuum.

Not really. There is a break-point when your saturation reaches the m-value in some compartment. At less saturation, you can come to the surface; after the break point you can't. Your GF-high puts some conservatism on that, but it doesn't change the dive saturation status into a continuum, it just means you are being a bit more risk-averse.

The fuzziness of what exactly *should* the m-value be is inherent in the decompression modeling, and in your individual physiology. These days, the former is getting good enough that the latter is probably dominating. thus, a GF-High of 100% is a really bad idea for most folks, given their physiology, and setting GH-High=70 or 80 is beginning to sound like a best-practice.

What you are calling a continuum is not so much the deco modeling as it is your sliding scale of risk-averseness, so to speak.

 

[inquis]

It's about exceeding a personal selected limit or not. Over the limit is "deco". Under the limit is "no deco". That's simply the prevailing understanding of the term. The vast minority use "decompression" as a synonym for "off-gassing".

@L13 , will you say you just completed a decompression dive during the SI on your next boat charter that has a "no decompression" policy?

 

[L13]

So that means that different algorithms will differ in how they predict when an NDL dive becomes a decompression dive, but that does not negate the fact that the moment exists.

The dive does not change, the algorithm does.

 

[L13]

@L13 , will you say you just completed a decompression dive during the SI on your next boat charter that has a "no decompression" policy?

I would avoid that by saying "I did not exceed my NDL."

 

[tursiops]

I would avoid that by saying "I did not exceed my NDL."

Clever.

 

[L13]

It's about exceeding a personal selected limit or not. Over the limit is "deco". Under the limit is "no deco". That's simply the prevailing understanding of the term. The vast minority use "decompression" as a synonym for "off-gassing".

If "decompression" is not "off-gassing" what is it? What are you doing at your stops if not "decompressing"?
How is it different from what you are doing at a "Safety Stop"?

Not really. There is a break-point when your saturation reaches the m-value in some compartment.

Actually not.

At the moment your computer says you have exceeded your NDL your tissues are not at the m-value limit, in fact it is likely all your tissues are subsaturated at that depth. But, at that moment, if you were to ascended directly to the surface with a particular ascent profile, they would be when you got there. Just by changing your maximum ascent rate, you can change whether a particular dive is NDL or not. by changing the ascent rate from 6m to the surface to 3m/min instead of the 9m/min I used in the "deco dive" 1) above, it again becomes an NDL dive.

Do you really think that a diver should not dive for 24 hours because their ascent rate from 6m to the surface was 4m/min(also a "deco" dive by your definition) instead of 3m/min? What if they planned on 3m/min, so at no time did their computer say they exceeded NDL, but actually did 4m/min?

 

[L13]

Dive theory should not be a "magical black box" with arbitrary inputs and outputs to dive professionals.