Ascent Rate - Deep Dives

[Charlie99]

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

OTOH, 10fpm is just fine as an ascent rate, provided you have the gas to meet your decompression option.

Initially, this thread was about what ascent rate the tables assumed. That isn't really the same as figuring out what is the best way to time and control and ascent. You continue to ongass the slower compartments while offgassing the faster compartments. Going too slow, loads you up excessively. Going to fast creates bubbles. Erik Baker's article on Deep Stops is a pretty good article explaining this balancing act.

Overall, though, if you play around with the simulator or any decompression program, you will find that in most cases the optimal ascent strategy is to have a variable ascent rate that gets slower and slower as you get closer to the surface. The easiest way to actually do this is by combining a reasonable ascent rate, such as 30fpm, with a series of stops.

When possible, I'll plan my dive such that after doing the deepest portion that I then go do a shallower dive for a while --- for example, go look at the bottom and propellor of a wreck, then the main deck, then the pilot house. Then my ascent effectively is starting from this shallower depth.

In those cases where there really isn't much between the bottom and the surface, then I'm balancing between 1) having enough gas for the ascent 2) having as safe an ascent/deco as possible and 3) Not getting too bored or impatient or wasting too much time on hanging mid-water vs. looking at whatever is down at the bottom.

My personal method is to use 30fpm ascent, 1 minute at 40', 1 minute or so extra in the 25' range, and then 3 minutes slowly going from 20' to 10'. On a practical basis, when I leave the bottom (or a the last level in a multiprofile dive) I decide at what runtime on my computer I will hit 40', 25/30', and the surface.

Having a preplanned ascent strategy that you use on most dives also helps in gas planning.

 

[Charlie99]

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.

OK. We both agree that if you stop ascending, then you are no longer offgassing. I also say that how slow you continue to ascend after reaching the transition point still matters.

My objection to your 1st post was the phrase at the end:

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.

 

[gcbryan]

Not sure what your 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.

For what it's worth I think you're using the term saturation incorrectly. If a compartment were 100% saturated it wouldn't increase no matter how much longer you stayed down (as in saturation diving where commercial divers stay down for a week go back into a chamber that is brought to the surface and decompress for days).

 

[mccabejc]

For what it's worth I think you're using the term saturation incorrectly. If a compartment were 100% saturated it wouldn't increase no matter how much longer you stayed down (as in saturation diving where commercial divers stay down for a week go back into a chamber that is brought to the surface and decompress for days).

I believe there are levels of gas saturation, and also with the right conditions, "supersaturation". Carbonated water is an example of a supersaturated solution of carbon dioxide gas in water. At the elevated pressure in the bottle, water can dissolve more carbon dioxide than at atmospheric pressure. Same thing with a diver at depth. Pressure is greater than normal atmospheric pressure, and therefore more N2 can be "ongassed" than would occur at normal atmospheric pressure, and as a result you can have levels of supersaturation (over 100% saturated).

 

[Doc Intrepid]

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?

(I think that the majority of responders are in violent agreement...sort of, and it sounds as if you've found an answer that works for you.) But to respond to Charlie99's comments, and yours, perhaps some definition of terms would be good.

C99 and the others are correct; the "ascent" is not when you begin to meander upwards slowly from your deepest point, examining this and that as your dive profile gradually works its way shallower and shallower. That is a multi-level dive. "Ascent" means you thumb the dive and commence a direct ascent to the surface.

For most recreational no-decompression limit diving, as recommended by most (not all) of the agencies, an ascent rate of 30 fpm is generally recommended. (It used to be 60 fpm twenty years ago. There are a number of threads on historical development.)

With respect to this sentence:

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

Now you (appear to be) talking about decompression diving. Different divers plan ascent rates differently. I recently did a dive on the St. Augustine which lies at ~250 fsw off Ocean City, MD. From 248' we ascended at a rate of approximately 25-30 fpm until we reached the first deep stop. Once we began deep stops, however, the rate of ascent slowed to ~10 ft per minute: ~30 seconds at the stop, and ~30 seconds ascending up to the next stop (the stops are 10' apart).

In an effort to be precise, then, if you're discussing recreational no-decompression diving, at the point you thumb the dive (versus 'begin to rise in the water column or along the wall') you ascend at a rate of 30 fpm and it does bupkis to your bottom time. (Unless something has changed recently, and I'm sure I'll be corrected if it has, the 3 minute safety stop you hold, which isn't a required stop but is rather a recommended stop, does NOT add to your bottom time - even though you've "stopped ascending" while you hold it.)

If you're discussing ascending along a dive profile OTHER THAN a no-decompression recreational dive, you ascend at whatever rate you've planned (20, 30, whatever) to the first deep stop, which is above the point at which offgassing begins. Once stops commence, most (but not all) divers slow their ascents significantly (10 fpm is common but not universal), to allow offgassing to occur.

Hope that is less confusing than my previous post.

Taking a wild guess at why you'd be interested in ascent rates and offgassing, here is an excellent article which discusses bubble formation and pysiological response to DCS tissue perfusion in much greater depth than I can. It's worth your time to review it:
http://www.immersed.com/Articles/PDFs/hit.pdf

Hope this helps,

Doc

 

[Scuba_Vixen]

OK. We both agree that if you stop ascending, then you are no longer offgassing. I also say that how slow you continue to ascend after reaching the transition point still matters.

My objection to your 1st post was the phrase at the end:

Quote:
Originally Posted by ScubVixen
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.


Once you ascend to the depth where the net effect is offgassing, you do need to continue the ascent in order to maintain the gradient that maintains that offgassing to ongassing ratio untill you reach about 30' or so (at which point ndl's are almost limitless). At rec depths and times, that ascent rate is pretty slow, and unless you continue the dive along a slowly sloping bottom, getting gradually shallower, then "ascending as slow as you like" (as in an open water ascent) is valid. If you're working your way shallower along a sloping bottom, that's more accurately analysed as a multi level dive, not as a slow ascent. Realistcally, you aren't likely to make an open water ascent that slow.

I think we basically agreed, it's a matter of what we considered "ascent" to include.


Darlene

 

[mccabejc]

Doc, the intent of my original post was this: a dive profile where I drop down to 100ft. hang out there for about 10 minutes, then see that my NDL time is down to only 1 minute left. Since I don't want to make it a mandatory decompression dive, I need to start heading toward the surface (I won't call it ascending, though I'm not sure I understand your distinction between thumbing and meandering). This will allow me to start to reverse the ongassing process. Given those parameters, there isn't a lot of room in my choice of ascent. If I go slower than 10fpm, I'm going to go into mandatory decompression as I head to the surface (based on the model I'm using). Therefore I need to go faster than 10 fpm, but slower than 30 fpm.

As you said, "an ascent rate of 30 fpm is generally recommended". However, as I recall, that is a "not to exceed" value. And that's the crux of my initial question. If 30+fpm is bad, and too slow (in this case, < 10fpm) may result in an unexpected mandatory decompression dive, it seems that 20-25 fpm is a pretty good goal, at least for the initial part of the "ascent".

 

[Diver0001]

though I'm not sure I understand your distinction between thumbing and meandering).

Thumbing is stopping what you're doing, changing your orientation from horizontal to vertical and going straight up at 10 metres per min. Meandering is anything else.

Often time divers who make a lot of shore dives don't make vertical ascents and continue to travel for long distances over the bottom before they really start going up.

R..

 

[mccabejc]

Thumbing is stopping what you're doing, changing your orientation from horizontal to vertical and going straight up at 10 metres per min. Meandering is anything else.

Often time divers who make a lot of shore dives don't make vertical ascents and continue to travel for long distances over the bottom before they really start going up.

R..

Right, I got that, but my point is that distinction doesn't really apply here. If you are at 100ft with 1min of NDL left, and don't want to do mandatory decomp, you have no choice but to head to the surface fast enough so that the dropping NDL doesn't catch up with you.

 

[jonnythan]

Thumbing is stopping what you're doing, changing your orientation from horizontal to vertical and going straight up at 10 metres per min. Meandering is anything else.

Why the switch from horizontal to vertical??