Ascent Rate - Deep Dives

[mccabejc]

I've always figured when it comes to ascents, slower is better. Well, except for deep dives, in which you may still be ongassing even while ascending, and your remaining NDL time may keep ticking down.

What I'm trying to figure is this: is there a goal, an ideal ascent rate? Of course, too fast (over 30ft/min) is bad, and my computer will throw in a mandatory stop if I exceed 33 fpm continuously or 39 fpm momentarily. But too slow is also bad, cause you may suddenly run out of NDL time.

So if you are deep, and need to start offgassing, is the goal to ascend at around 30fpm, or is slower really better?

 

[jonnythan]

Keep it around 30 fpm and you'll be fine. I think there are many better things to worry about than 28 fpm vs 33 fpm.

Slow it down after your safety stop(s)... I do the last 10 feet as slow as possible, but keep it at ~30 fpm until 10 feet to keep things on time. I generally do a 1 minute at each of 30 ft, 20 ft, and 10 ft.

 

[Doc Intrepid]

...But too slow (ascent) is also bad, cause you may suddenly run out of NDL time...

What he said.

With respect to the quoted statement, remember that for NDL recreational diving the measurement of TBT or total bottom time begins with the descent and ends with the beginning of the ascent (not the surfacing).

While you shouldn't be pushing your dive plan to the edge of the NDL limits for many reasons, the point is that your ascent time will not "run you out of NDL time". NDL refers to bottom time, which for NDL rec diving ends when you begin to ascend at the end of the dive.

FWIW.

 

[ChillyWaters]

What he said.

With respect to the quoted statement, remember that for NDL recreational diving the measurement of TBT or total bottom time begins with the descent and ends with the beginning of the ascent (not the surfacing).

While you shouldn't be pushing your dive plan to the edge of the NDL limits for many reasons, the point is that your ascent time will not "run you out of NDL time". NDL refers to bottom time, which for NDL rec diving ends when you begin to ascend at the end of the dive.

FWIW.

Yup, but still one should stress not going too slow. Because TBT is based on when you ascend, this assumes a certain rate of ascent. Going up at 5feet/minute breaks the assumption of the table, and may cause problems. But that said, if you aren't pushing the tables, this really shouldn't matter too much.

- ChillyWaters

 

[Scuba_Vixen]

Yup, but still one should stress not going too slow. Because TBT is based on when you ascend, this assumes a certain rate of ascent. Going up at 5feet/minute breaks the assumption of the table, and may cause problems. But that said, if you aren't pushing the tables, this really shouldn't matter too much.

- ChillyWaters

That is the key here.

Looking at deeper rec dives, ie: 100 feet for 20 mins or 120 feet for 10 mins, the gas transition point (offgassing starts) is about 65 feet. If you lolligag on the way up to that point, you're adding nitrogen faster than anticipated and it's the same as if you stayed longer at depth and then came up at 30 f/m. Once past that point, you can go as slow as you like, and you'll still continue to offgas.


Darlene

 

[Charlie99]

Once past that point, you can go as slow as you like, and you'll still continue to offgas.

Not really true. You will also continue to ONGASS in the slower tissues/compartments. The ongas/offgas transition point is different for each compartment.

For very slow ascent rates, you need to calculate the dive as if it were a multi-level dive, since that is what it has become.

 

[Charlie99]

With respect to the quoted statement, remember that for NDL recreational diving the measurement of TBT or total bottom time begins with the descent and ends with the beginning of the ascent (not the surfacing).

One needs to look at the specific table to see the rules. On a lot of tables, you will see an ascent rate of 30fpm or 60fpm called for, and the statement is often phrased as "bottom time ends with the direct ascent to the surface or safety stop" or something similar.

If one will be ascending significantly slower, on most tables, the ascent time gets added to the bottom time, just the same as one would do for a multi-level dive.

 

[Diver0001]

I've always figured when it comes to ascents, slower is better. Well, except for deep dives, in which you may still be ongassing even while ascending, and your remaining NDL time may keep ticking down.

What I'm trying to figure is this: is there a goal, an ideal ascent rate? Of course, too fast (over 30ft/min) is bad, and my computer will throw in a mandatory stop if I exceed 33 fpm continuously or 39 fpm momentarily. But too slow is also bad, cause you may suddenly run out of NDL time.

So if you are deep, and need to start offgassing, is the goal to ascend at around 30fpm, or is slower really better?

There are some important points to make about this:

1) If you ascend slowly you will continue to ongass from a deep ascent until you get to a certain depth. Your ascent to the shallow zone should be done at 10 metres per min to avoid additional ongassing. If you ascend any slower than 10 metres per min from a deep ascent then you run the chance of "running of of NDL". If you are already over the NDL then you will accumulate extra deco.

2) Ascending over the bottom (what may of us do during shore dives) will often give you slower than 10 metres per minute. In the case of where I live, a swimming ascent over the bottom can bring you up as slowly as 2 metres per min. The point here is to watch this and get a feeling for how fast an ascent over the bottom goes at various sites in your local area.

3) Slower is better once you get into the shallow zone. You should start your ascent from a deep dive at 10 metres per min, slow it to 3 metres per min when you get in the shallow zone (maybe 12 metres) and *arrest* your ascent at about 6 metres and ascend slowly from that point. From 6 metres to the surface you do not ongass in any significant (theoretical) compartment anymore and this part of the ascent should be done as slowly as it's practical to do. The reason we do safety stops in this zone is to arrest and slow the ascent towards the surface to allow for extra offgassing.

So that's just a long winded way of saying, start your ascent with a good tempo and slow down more and more as you get shallower and shallower.

4) It's proabably (theoretically) better for you to leave the bottom a little early (not right at the NDL) and ascend slowly than to run it up to the line and ascend with 10 metres per min.

5) Your tables, if you use them, are designed with a particular rate of ascent in mind. If you don't follow that ascent rate then your N2 load will be higher than you think. Over multiple dives this could become significant. This is in relation to Doc Intrepid's comment that the bottom time ends at the beginning of the ascent. That's partly true. The entire truth is that your bottom time ends at the beginning of an uninterrupted ascent done at the speed with with your tables were designed. It's not just a difference of nuance. It makes a difference in your N2 load.

6) Watching the ascent indicator on your computer can be misleading but it's better than nothing. What's much better is to watch your clock as you ascend and make a few little stops along the way.

Here's an example.

Say you're on a 30 metre dive and you start your ascent at 20 minutes (that's the NDL for air IIRC). That means that you should be at 20 metres at 21 minutes and 10 metres at 22 minutes. Follow?

What I would do is start my ascent and just swim at a good tempo from 30 metres to 20 metres. Once at 20 metres I would look at my bottom time. If it doesn't say 21 yet then I would stop and wait for it to flip to 21 and then start ascending again. If it already says 21 then I would keep going. Once at 12 meters (12 is a factor of 3, and it's the deep end of the shallow zone so you won't hit your ceiling here unless you botched the ascent beyond belief) I would stop again and look at my bottom time. If it didn't say 22 minutes then I would wait at 12 metres until my clock goes to 22. Once at 22 I would ascend to 9 metres and wait until it flips to 23, then ascend to 6 metres and wait until it says 24 and then start with my safety stop at either 6, 5 or 4,5 metres depending on which tables you're using. After the safety stop I would ascend slowly to the surface.

The reason I think that this is better than using your ascent indicator is that watching your clock makes you 100% sure that you didn't ascend too fast. Ascent indicators only give you a picture of a particular moment in time and are unable of warning you if your *total* ascent is too fast or too slow. In my mind they're completely usless but YYMV.

R..

 

[mccabejc]

While you shouldn't be pushing your dive plan to the edge of the NDL limits for many reasons, the point is that your ascent time will not "run you out of NDL time".

Not sure what you're getting at. If you are at depth, and ascend too slowly, you may still be ongassing, because you are still at depth and the pressures are relatively high. Therefore some compartments may still be increasing towards 100% saturation, and your NDL time may still be decreasing.

As a side note, concepts like that are why I'm such a big fan of playing around with computer dive simulators. You can do "what if" scenarios up the gazoo, and watch the compartments ongas and offgas, see how some compartments continue ongassing if you ascend too slowly, and see how the NDL time continues to decrease as you ascend. You can also see at what depth you start offgassing. Very cool. I think they can be an incredible learning tool (if used as one, of course).

For example, I just ran one where I drop down to 100ft., and hang out there for 10 minutes. I have my Suunto set for the most conservative setting, so at the end of 10 minutes I'm at 1 minute NDL left. If I then ascend slowly, and take 2 minutes to ascend 10 feet (5 fpm), it becomes a decompression dive at 90 ft. If I double that ascent rate (10fpm), I stay right on the edge of a decompression dive (99% saturation in a couple compartments) all the way up to 60ft.

All of that tells me that 10fpm is probably too slow, and 20-30fpm is probably a good target.

 

[Scuba_Vixen]

Not really true. You will also continue to ONGASS in the slower tissues/compartments. The ongas/offgas transition point is different for each compartment.

For very slow ascent rates, you need to calculate the dive as if it were a multi-level dive, since that is what it has become.

The "gas transition point" is the depth where the net effect overall is offgassing. In other words, you're lossing nitrogen faster than gaining it. That takes all the compartments into account.

Now if you STOP ascending, and start adding time at another depth, it may be another ball of wax. .... But that wasn't the premis. ... Slow but continuous ascent was what was originally posted.


Darlene