Modern research/thoughts on Ascent Rates

[Manatee Diver]

Since you're not taking on any inert gas what's the point of making such a slow ascent?

To prevent the formation or the growth of existing bubbles. The WKPP guys claim it works, I'm not sure if it applies anymore but I believe their DCS rate is near zero if it isn't actually zero.

I think there are many ways to skin a cat, I personally dive 60/80 but I rarely ascend at 80%, I wait for it to clear a bit more. And then do a slow ascent to the surface, not 1fpm but certainly a lot slower than 30fpm.

Oxygen is cheap and I can just watch the invasive South Florida T-Backs float overhead; besides I heard that the WiFi signal is poor in the chambers.

 

[EFX]

I wasn't thinking of bubble growth. However, a lot of the inert gas bubbles have been washed out by the O2. I read something about oxygen DCS. Has this happened? Is it true? Or, is it something theoretical?

 

[Manatee Diver]

I wasn't thinking of bubble growth. However, a lot of the inert gas bubbles have been washed out by the O2.

Yes but as you ascend you can have more form. Remember the whole thing about not holding your breath even from as little as 10ft as that is when you get the most air expansion?

Here is the ascent phase from a 182ft dive, the red line is your GF99 (your gradient factor at that exact moment). I didn't extend the dive because I was struggling a bit with trim.

This was after an extended swim at 50ft on nitrox, lowering TTS from like 30 minutes to 20 some minutes. So a fairly short deco by the WKPP standards.

I stop at around 30ft and switch to backgas to clean up my stage, and pick up my deco bottle. My GF99 goes from 0 to around 26% dropping a bit as I spent a couple of minutes there. I ascend to 20ft and it goes up to 34% dropping down to 1% during my 20 minutes floating there. Jumping back up to 75% as I ascend to the surface.

 

[Akimbo]

To prevent the formation or the growth of existing bubbles. The WKPP guys claim it works, I'm not sure if it applies anymore but I believe their DCS rate is near zero if it isn't actually zero.

The beauty of decompressing on pure Oxygen is 1) faster diluent gas removal and 2) no diluent is absorbed by fast tissues during decompression. I have used several company proprietary decompression tables with in-water O2 stops and ascents to the surface as fast as 60'/minute, and some with O2 stops as deep as 40' (surface supplied commercial diving).

A 1'/minute rate is pretty tedious to do in the water, especially in sloppy seas. A good downline on a decent size buoy and a heavy weight is mandatory — at least 10Kg or 22 Lbs of lead and 3-4x that in the buoy's buoyancy. The objective is to ride swells and decouple jerking from the boat. That rig has to drift in high currents so the diver's aren't working too hard on Oxygen, with a chase boat to follow them.

 

[boriss]

Using my spreadsheet I compared two dives, both to 250 ft for 20 min, at a descent rate of 60 fpm, using a GF of 60/85 on trimix 20/40. One dive used an ascent rate of 30 fpm for the entire dive. The second dive used an ascent rate of 120 fpm up to 100 ft which was the first stop and then 30 fpm between the stops and to the surface. It made no difference in the profiles. Both dives had the same stops, stop times, and total deco time.

For what it's worth here are the controlling TC's using Buhlmann 16 starting at the level segment at 250 ft (TC1 - TC16 = fastest to slowest):
1,3,3,4,4,4,4,4,4,4,4,5,5,6,6,6,6,7,7,9,9,11,11.

Since I'm not a technical diver I have nothing from experience to give you.

I cannot duplicate this in Subsurface. If I do 99 fpm ascent below 50% average depth (which is up to about 100 ft), there is a significant difference in total run time. 139 mins (fast initial ascent) vs 161 mins. The faster initial ascent had a deeper first stop (90 ft vs 80 ft), but it was reached 3 mins earlier.

Haven't heard of Subsurface, but decided to give it a try. It actually has a nice feature where you can select custom ascent rates for different portions for the dive. Thanks for mentioning it!!

On a dive to 250ft, I'm getting a 98min runtime with a 30ft/min ascent rate across the board:

Now, here's the same dive, but the bottom 25% portion has been changed out to 60ft/min ascent (represented as a red line) and the runtime drops by 6-minutes. Might be another couple of minutes if that ascent rate could be sustained all the way to the first deco stop. Doesn't seem like a lot, but that's over 6% of the runtime.

Full transparency, I've run several other scenarios, where the improvement is even better and in some cases where it was negligible.

But the question still stands: what should be considered a "safe ascent rate" for various portions of the dive? I know for a fact local technical divers often exceed 30ft/min to get from the deepest portion for the dive. I'm just trying to somehow quantify the hypothesis.

These concepts are important for our relatively deep dives, but I'd think even more important for much deeper dives, such as Richard Harris' 245m dive with 16-hours of deco. Imagine shaving 6% off of that dive, that's a full hour. If people haven't seen the dive, here it is:


Deco in the 7m habitat:


Thanks for all the responses so far, folks... I'm loving the thought-provoking posts.

 

[kensuf]

I can't find it anymore, I seem to remember reading a document by GI3 when he was running WKPP, that they used a 1ft per minute ascent rate after they cleared their oxygen deco at 20ft. As that is the time you will have the greatest pressure gradients.

Context of the dive is important, I'm reasonably confident that this protocol is only used on dives with significant deco. A person incurring 10 minutes of deco at Ginnie shouldn't be spending an additional 20 minutes surfacing.

 

[kensuf]

but I believe their DCS rate is near zero if it isn't actually zero.

Sorry, I was a WKPP member in the 90s. I assure you, the DCS rate was not zero.

Again, context on the dive is important. A dive with 10 hours of decompression is vastly different than a dive with 10 minutes.

 

[Superlyte27]

This is an academic discussion to get an understanding. If there's a ceiling, then we can't blow past that ceiling, but the question is how fast can we get to it?

For example, if you plan a 150m/500ft dive for 20-minutes, your first stop will be in the 75-100m (240-320ft) range. How fast can you get to that stop? If moving at 10m/min that's a 5-8-minute ascent, while you're picking up more deco obligation. If I go based on the calculation above, I'm suggesting it can be done in 1-2 minutes (say using a scooter, not uncontrollably just floating up quickly).

@Akimbo thank you! I understand the argument for simplicity, but I'm still curious what the relative limits should be. I wonder if it's safe to extrapolate those saturation table limits? They seem a lot more conservative, but then again, they have the time.

I'll read the rest of your links, thanks!

500’ to first deco stop of 300’? You can get there as fast as your scooter will take you. The bubble is too small to do damage at 300’

You asked for anecdotal data. This is a dive I did a few years ago. My first stop from 500’ was a 270’ I believe. I scootered straight up as fast as my scooter would take me.

When planning my dive, I looked at every piece of software I could find. None of them could plan a dive with a 200fpm descent or ascent. But, as a commercial diver , I knew my ascents were much faster at greater depths than the established 60’/min. I remember my first job. “We don’t care what you learned in school, now you’re going to learn the way to do it in the real world”

I don’t know. 10,000 plus dives. Almost all of them technical or commercial and I’ve never been bent. Maybe just really lucky? I guess I’ll keep doing what I’ve been doing. If it ain’t broke, don’t break it.

 

[Wibble]

The beauty of decompressing on pure Oxygen is 1) faster diluent gas removal and 2) no diluent is absorbed by fast tissues during decompression.

Aren't there some drawbacks to using pure oxygen for the 6m/20ft and above stops?

Such as:

• CNS and pulmonary oxygen loading is increased.
• Have to ensure you don't inadvertently drop below 1.6ATA (6m/20ft) as the PPO2 will be very high, rapidly increasing CNS
• Long deco hangs mean taking "air breaks" to give the lungs a rest - pulmonary oxygen toxicity

Given that a bottom phase PPO2 of 1.4 has been debated as being too high (have heard Dr Mitchell mention this), 1.6 seems almost excessive.

Reducing the oxygen content with a weaker mix (80%?) will mitigate this. As will ascending above 1.6ATA, but only when the ceiling's moved up.

 

[Superlyte27]

Aren't there some drawbacks to using pure oxygen for the 6m/20ft and above stops?

Such as:

• CNS and pulmonary oxygen loading is increased.
• Have to ensure you don't inadvertently drop below 1.6ATA (6m/20ft) as the PPO2 will be very high, rapidly increasing CNS
• Long deco hangs mean taking "air breaks" to give the lungs a rest - pulmonary oxygen toxicity

Given that a bottom phase PPO2 of 1.4 has been debated as being too high (have heard Dr Mitchell mention this), 1.6 seems almost excessive.

Reducing the oxygen content with a weaker mix (80%?) will mitigate this. As will ascending above 1.6ATA, but only when the ceiling's moved up.

It’s done on thousands of dives every year with no ill effects.