actual NDL calculations

[TSandM]

It means that, although the likelihood of DCS symptoms is higher in people who have higher grade bubbling, the correlation is extremely imperfect. There are people with Grade 4 bubbles and no symptoms, and there are people with lower grade bubbling and significant symptoms. Bubbles are what we can detect and measure; DCS remains a clinical syndrome of not entirely clear etiology.

 

[Thalassamania]

Exactly, as usual you put it much better than I do.

 

[NC Wreck Diver]

First of all, Doppler detectable bubbles have not been shown to actually have anything to do with DCS.

Second of all, the ascent rate is integral to the tables you choose to use. If you do not follow the advisable ascent rate you must deal with the difference time in the way the tables suggest, usually that is to add the discrepancy to bottom time.

I sincerely respect your opinion. Isn't SB great? We all get to follow the advise from fellow divers. I run with DAN's advice as my first go to information. But your's is always appreciated.

 

[wve]

... the ascent rate is integral to the tables you choose to use. If you do not follow the advisable ascent rate you must deal with the difference time in the way the tables suggest, usually that is to add the discrepancy to bottom time.

I don't know about other tables, but to clarify since the OP is a PADI diver:

Obviously, ascent rates matter, but they do not actually figure in to calculation of NDL limits with table use as far as PADI is concerned. Check this out:

From the PADI Open Water Diver Manual:

Ascent Rate - The proper speed for ascending, which is no faster than 18metres/60 feet per minute. A rate slower is acceptable, and appropriate.

Bottom Time - The time from the beginning of descent until the beginning of a direct, continuous ascent to the surface or safety stop.

So the first point is that Bottom Time -- which is used to calculate NDL limits -- actually ends just prior starting to the ascent. In other words, the ascent phase of the dive is irrelevant to NDL calculation when using tables.

Second, PADI teaches that one should ascend no faster than 60 ft/min, but going slower is even better. There is never any comment that one should "adjust" one's NDL calculations based upon slower ascent rates.

On the other hand, as the above definitions show, the assumption is that one is ascending directly to the safety stop or surface. Anything else becomes a multi-level dive -- in which case the rules change as has already been discussed.

 

[TSandM]

In other words, the ascent phase of the dive is irrelevant to NDL calculation when using tables.

wve, you are right that the time reflected in the NDL time is the descent plus bottom time. However, it is not correct to state that the ascent phase of the dive is irrelevant.

To use an extreme example: Suppose you do a dive on a wreck at 130 feet, and your NDL time is 8 minutes there. You spend your eight minutes, and then you begin to work your way up the wall next to the wreck. 15 minutes later, you have only gotten to 100 feet. You are ascending -- but you aren't ascending fast enough to avoid ongassing a whole bunch more nitrogen, and you are quite likely to be over your NDLs at this point.

A certain rate of ascent is necessary to consider it an ascent, rather than a multi-level continuation of the dive.

If you look at the Marroni studies, they showed that, for dives to 75 feet (IIRC), a 10 fpm steady ascent rate from the bottom was actually WORSE for bubble scores than an ascent performed at a higher rate, but incorporating shallow stops. There IS such a thing as too slow an ascent.

 

[peterbj7]

Many people misunderstand PADIs rationale and instructions because, as usual with PADI, they don't find the need to explain. They think that people should simply LEARN amd follow blindly.

The point is most certainly NOT that the ascent phase is irrelevant. Rather that PADI have factored in their standard ascent rate, which as it is unchanging can be disregarded for the purposes of computation. As TSandM states, an ascent rate can be too slow, a point which becomes obvious when you start considering diving with helium mixtures.

And although in the text PADI refers to ascent rates slower than 60 fpm being desirable this is not embodied in the tables, which assume 60 fpm ascents throughout.

 

[NC Wreck Diver]

Good post.............but even in an 'Extreme Example' of ascent, I would hope that it would not take a diver 15 minutes to come up from 130 to 100. My ascent rate from 130 to 100 would be 30 seconds, then another 35 seconds to 65 feet. A short 'deep stop' at 65 for two minutes, then the balance at 30 feet per minute (I may just slow down to 30 fsw per minute from 65 instead of 60), until I reach my safety stop. Typically, if I didn't violate my my NDL's too much, I'm typically clear by 15 feet. But I still hang for my safety stop, based on what my computers are reading. 130 would make me stop at 15 for 5 minutes, in a no decompression dive. But honestly, if I'm diving 130, I'm going to be in a deco dive (I know this is off the OP). I carry plenty of back gas and hang bottle(s).

If I saw a diver coming up from 130 to 100 in fifteen minutes, RED flags are going off. You are totally right, this is a multi-stage dive, not an ascent. I know your statement is only an example (I hope).

wve, you are right that the time reflected in the NDL time is the descent plus bottom time. However, it is not correct to state that the ascent phase of the dive is irrelevant.

To use an extreme example: Suppose you do a dive on a wreck at 130 feet, and your NDL time is 8 minutes there. You spend your eight minutes, and then you begin to work your way up the wall next to the wreck. 15 minutes later, you have only gotten to 100 feet. You are ascending -- but you aren't ascending fast enough to avoid ongassing a whole bunch more nitrogen, and you are quite likely to be over your NDLs at this point.

A certain rate of ascent is necessary to consider it an ascent, rather than a multi-level continuation of the dive.

If you look at the Marroni studies, they showed that, for dives to 75 feet (IIRC), a 10 fpm steady ascent rate from the bottom was actually WORSE for bubble scores than an ascent performed at a higher rate, but incorporating shallow stops. There IS such a thing as too slow an ascent.

 

[Thalassamania]

Ascent rate is integral to the calculation on no-D limits. The faster the assumed ascent rate the lower the no-D limit. You can not cut a table without having an assumed ascent rate as part of the calculation.

I don't know about other tables, but to clarify since the OP is a PADI diver:

Even being a PADI diver doesn't change the laws of physics or your physiology.

Obviously, ascent rates matter, but they do not actually figure in to calculation of NDL limits with table use as far as PADI is concerned. Check this out:

From the PADI Open Water Diver Manual:

Ascent Rate - The proper speed for ascending, which is no faster than 18metres/60 feet per minute. A rate slower is acceptable, and appropriate.

Bottom Time - The time from the beginning of descent until the beginning of a direct, continuous ascent to the surface or safety stop.

So the first point is that Bottom Time -- which is used to calculate NDL limits -- actually ends just prior starting to the ascent. In other words, the ascent phase of the dive is irrelevant to NDL calculation when using tables.

Second, PADI teaches that one should ascend no faster than 60 ft/min, but going slower is even better. There is never any comment that one should "adjust" one's NDL calculations based upon slower ascent rates.

On the other hand, as the above definitions show, the assumption is that one is ascending directly to the safety stop or surface. Anything else becomes a multi-level dive -- in which case the rules change as has already been discussed.

Bottom time runs from the begining of your descent to the start of your ASCENT AT THE PRESCRIBED RATE! If you ascend slower the time difference between the amount of time that it should have taken you and the time it did actually take you should be added to your bottom time. If this addition puts you over the line into a decompression status then a mandatory decompression stop is called for.

Good post.............but even in an 'Extreme Example' of ascent, I would hope that it would not take a diver 15 minutes to come up from 130 to 100. My ascent rate from 130 to 100 would be 30 seconds, then another 35 seconds to 65 feet. A short 'deep stop' at 65 for two minutes, then the balance at 30 feet per minute (I may just slow down to 30 fsw per minute from 65 instead of 60), until I reach my safety stop. Typically, if I didn't violate my my NDL's too much, I'm typically clear by 15 feet. But I still hang for my safety stop, based on what my computers are reading. 130 would make me stop at 15 for 5 minutes, in a no decompression dive. But honestly, if I'm diving 130, I'm going to be in a deco dive (I know this is off the OP). I carry plenty of back gas and hang bottle(s).

Using your example, on pure tables, you would need to add 5 minutes of bottom (plus the time of your safety stop) to you bottom time to make a proper decompression status/stop calculation.

If I saw a diver coming up from 130 to 100 in fifteen minutes, RED flags are going off. You are totally right, this is a multi-stage dive, not an ascent. I know your statement is only an example (I hope).

Any ascent that is not continuous and at the prescribed rate is a multi-stage dive and the only normal way (without repping up or using the wheel, etc.) to deal with that is to add the additional time to bottom time.

 

[wve]

wve, you are right that the time reflected in the NDL time is the descent plus bottom time. However, it is not correct to state that the ascent phase of the dive is irrelevant.

I did not mean to say that ascent time is irrelevant. To the contrary, I think it is extremely relevant, and one good reason to use a computer instead of relying upon the RDP.

All I was trying to say is that when using the RDP to plan the NDL for a dive, it only asks you for your planned maximum depth and planned bottom time -- which does NOT include the ascent.

I'm not saying that anyone should ignore ascent rate or the time it takes to ascend.

I am saying that whether you use a table, a computer, or dive-planning software, you need to make sure you know the definitions of the terms so that the dive actually mimics the plan, and that the log accurately describes the dive.

 

[Thalassamania]

If your ascent is other than a direct ascent to the surface at 60 fpm, the additional time used in the ascent must be added to your bottom time.