Safe Ascent Rate Guidance

[RobPNW]

When I read through different diving physiology discussions, I see reference to both 30 feet/minute and 60 feet/minute as the "maximum safe ascent speed".

Is this one of those things where the maximum is 60 feet/minute but 30 feet/minute is recommended to provide a margin of conservatism?

Or it one of those things where at one point, an authority on the subject changed it from 60 to 30 and some agencies haven't adopted the new standard?

What is your understanding? Or does it depend on the dive parameters?

Thanks,
Rob

 

[johndiver999]

Short answer: A long time ago 60 fpm was the standard and it was changed to 30. The best advice is probably to follow your computer which will help you to maintain the slower rate.

 

[RobPNW]

Short answer: A long time ago 60 fpm was the standard and it was changed to 30. The best advice is probably to follow your computer which will help you to maintain the slower rate.

That's what I figured. Thanks!

 

[tursiops]

Slightly longer answer: 60 ft/min was never based on anything more than a compromise between 10 ft/min (the rate of cranking up a hard-hat diver) and 100 ft/min (what the "new" scuba divers wanted to do). It was never science-based, but then became embedded in the tables being developed because the ascent rate is part of the deco algorithm. PADI has never ever wanted to say it was ever wrong about anything, even if new information arises, so it still maintains 60 ft/min as a max rate, but then says do what your computer says...which is almost always 30. We now know that -- especially near the surface -- 60 ft/min is stupid. From deep to mid depths, 60 is more tolerable. Physiologically, it ahs to do with the rate at which pressure is released on the body as you ascend, do 60 ft/min from 130-60 ft is not much of a fractional pressure drop, whereas 60 ft/min from 30 ft to the surface is a very large fractional pressure drop. One BIG reason that safety stops have become the standard is because it slows you down.

From DAN:
Ascent Rates: A Quick History
Historical guidelines as to rates of ascent are pertinent. In the 19th century, for example, the French physiologist Paul Bert in 1878 quoted rates of 3 feet per minute and the English physiologist John Scott Haldane in 1907 recommended ascent rates between 5 and 30 feet (1.5 and 9 meters) per minute. From 1920-1957, rates of 25 feet (7.5 meters) per minute were recommended.

Then in 1958, during the production of the U.S. Navy Diving Manual, the rate of ascent to be proposed came under review. Cdr. Francis Douglas Fane of the U.S. Navy West Coast Underwater Demolition Team wanted rates for his frogmen of 100 feet (30 meters) per minute or faster. The hardhat divers, on the other hand, considered this impractical for the heavily suited divers who were used to coming up a line at 10 feet (3 meters) per minute. Thus, a compromise was reached at 60 feet (18 meters) per minute, which was also a convenient 1 foot per second.

So from 1957 until 1993 the U.S. Navy tables have consistently advocated an ascent rate of 60 feet per minute, based on this purely empirical decision, with many recreational diving tables and even early computers following suit. In recent years this has been slowed to 30 feet per minute with a recommended safety stop for three to five minutes at 15-20 feet (4.5-6 meters). However, this still brings the diver quite rapidly to the surface, often after some 30-60 minutes at depth.

 

[Prometheanfire]

I am not a doctor. This is what I use personally, it kinda matches the 10M per minute that people use anyway so not much difference, but explains what I have going through my head for this.

I personally target a gas expansion rate (and use a simple idealized scale for general use). This is assuming no-deco and I still follow what the computer says. ~30% per minute. This means that gas expands from 100% to 130%.
At 30M depth it means ascending I can go to 18M (4 ATM - 4x.3 = 2.8ATM = 18M). The next minute I can go to ~10M (9.6 by math). The next minute brings me to 4M (more or less safety stop) and from there to the surface, but technically it's better to go even slower, by math it's 1M depth for that.

As I said, this is a target. It's probably easier to just think of it as you want to give gas time to escape as much as possible, the deeper you are the less depth actually compresses the already compressed gas (per M of depth), don't race to the surface. Try to go slow.

 

[Riles2226]

To simplify what Prometheanfire said, the shallower you are, the greater the percentage of pressure change. So, going from 10m - 5m results in much more expansion than going from 35m - 30m.

Many of the modern computers are doing a variant of what Prometheanfire is doing. They allow you to ascend a bit faster the deeper you are and slow you as you get closer and closer to the surface. For example, the ScubaPro M2 holds you to ~7m/min from 6-0m but allows ~20m/min from 50 - 44m. (There's a number of other rates in-between.) You'd need to look at the manual for your specific computer or one you're looking at to see what it does.

With this information, some people decide they'll just go SUPER slow. The science is showing that you can go too slow...especially from depth. When you're deep, as you ascend, you will reach a point where you are offgassing in faster compartments (they ongas faster) but you are still ongassing in the slower compartments. In recreational diving, the best guidance out there is generally to follow your dive computer and stay out of deco. Technical diving goes a bit deeper into Gradient Factors and all but even much of that is hotly debated and the research is still evolving there.

Keep in mind, though, that this is all theoretical...even your computer. It is based on a mathematical algorithm that does not know what is actually happening inside of your body. Along with looking at ascent rates, you also want to make sure that you are staying properly hydrated, are fit, don't have any injury that would impede bloodflow, etc. These are all factors that can contribute to you getting bent even when you follow all of the "rules" perfectly.

 

[Superlyte27]

I am a hard hat diver. We never did 10fpm. We were taught 60fpm, and usually were pulled up like a missile, because we had a very short window to get to the surface, get undressed and get back into he chamber and back down to depth.

We called it, “Bend em and Mend em”

 

[tursiops]

I am a hard hat diver. We never did 10fpm.

LOL. You weren't a hard hat diver back when the 60 fpm decision was made!

 

[Superlyte27]

LOL. You weren't a hard hat diver back when the 60 fpm decision was made!

I have no idea when that decision was made. I became a hard hat diver in 1998. The rule had been in place for at least a few decades at that point. I'm sure @Akimbo would know better.

 

[Akimbo]

The rule had been in place for at least a few decades at that point. I'm sure @Akimbo would know better.

I have been looking for a table showing the evolution of ascent rates and would appreciate it if anyone has something definitive. The easiest to document would be the US Navy's ascent rates but requires access to all the manuals.

I understand that the first diving manuals were pretty vague about ascent rates. I think it went to 25 FPM in the 1920s (1924 manual?), 60 FPM in the 1950s (1952 manual?), and 30 FPM in the 1988 manual -- but it could have been in the 1990s. There is a library at the US Navy Experimental Diving Unit in Panama City, Florida. This project is on my list if I ever visit NEDU.

I spoke with some Navy Divers who were there when it went from 25 FPM to 60 FPM. Everyone except the physiology geeks at NEDU hated 25 FPM because it was really hard to correlate between stopwatchs and pressure gauges. The UDT (Underwater Demolition Team, predecessor to the SEALs) was pushing for 100 FPM in the early 1950s for operational reasons, which led to the 60 FPM compromise. Divers and supervisors really liked it -- 1 second = 1'.

I expect that one day the simple bottom-to-surface ascent rate will go away. Computers will calculate the optimum ascent rate, which becomes slower at shallower depths. This reality is reflected in the slowing rates of saturation diving decompression tables.