Where does the 60ft/minute ascent rule come from?

[*dave*]

What's the scientific basis for PADI's(?) 60ft/minute ascent rule?
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60FPM came at the same time as the Navy tables, as I understand it. There really is no scientific or physiological basis behind it.

The Navy tables were made for working divers. They didn't swim up, they were lifted on a platform. Most platforms could be raised at 60fpm, so that's where the number came from.


That's the best explanation I've heard and I've heard it from a few sources.

 

[joe8mofo]

The 60 ft/min ascent rate was a compromise rate for the navy tables, some wanted faster, some slower

DAN Divers Alert Network

I think the new PADI rate is 30 ft a minute

Excellent read, everyone should read the above DAN article on the subject. It would seem that the greatest risk posed to Recreational diver's today is too short of a duration of time going from deep (100 ft) to the surface. As the spinal cord's half-life is 12.5 minutes, lets say we go to 100 ft for 20 minutes on air. We have filled our spinal tissue to its saturation limit of nitrogen. So lets go with the conservative 30ft/minute mark. So we have about 3:05 minutes ascent time to 15 feet and then a 3 minute safety stop. So we have only given 6:05 minutes of time for offgassing to occur from our spinal tissue; now as offgassing occurs at a slower rate than ongassing; do you feel comfortable knowing that you are now at the surface and your spinal tissue is at least 50% saturated with nitrogen? I definitely do not. So what can we do? We can do a deep stop for 3 minutes at 50 feet, which will get us to about 35% of a saturation rate for our spinal tissue. We can dive nitrox, say 34% and reduce the amount of ongassing which will occur to our spinal tissue; not to mention the reduced nitrogen content will allow quicker offgassing from tissues as we ascend and do a safety stop. We can additionally switch off to a mix of 40% nitrox at 50 ft and speed up offgassing.

 

[DennisS]

From the DAN site

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.

 

[muddiver]

From the DAN site

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.

That's what I was going to say. I knew I read it some where.

The newer 30'/min is based on dopler bubble experiments in the 80's. I think PADI/DSAT was also either involved or did some testing that supports the slower ascent rate.

 

[Mr Carcharodon]

No offense but... says who? And how do they know?

The whos are Schreiner, Buhlmann and Yount. Who respectively developed the instantaneous gas loading equation, modern dissolved gas models and modern bubble models. The idea to keep the gradient (difference between dissolved gas tissue pressure and ambient pressure) below tolerable levels. Dissolved gas models control gradients through "M" values, and bubble models indirectly do the same, but consider time as well.

The how do they know part is mostly known by doing studies decompressing animals. That is Michael Powell's forte (Dr. Deco in the decompression form) so you might get a better response if you posted there. I believe DAN also did an ascent rate study. But I am a bit vague on the details.

 

[boulderjohn]

Ancient history. 30 FPM is the new 60 FPM. Slower is always better.

No offense but... says who? And how do they know?

For dives where any tissue is close to it's Mvalue, a slow ascent is almost always better.

As stated earlier,t he 60fpm had little basis in science, although the general idea of it came from Bert, Haldane, et al. It was used universally for many years, during which time it was proven to be safe by the fact that people were pretty much doing OK with it. When PADI did the research for the RDP a quarter century ago, they used 60 FPM, which was still the standard then. Their extensive research supported it as a safe ascent rate.

Newer studies have shown that while it is a safe ascent rate, it is not the safest rate. Most notably a study published by DAN in 2004 compared ascent rates and safety stops. That study concluded that although 60 FPM was safe enough, 30 FPM was better. Better still was the use of safety stops--what was once just a "play it safe" practice was seen to have much more value than that.

Contrary to the "slower is always better" belief, 10 FPM was shown in the study to be worse than either the 60 FPM or 30 FPM ascent rate. A little thought will show why. During an ascent, the supersaturated fast tissues will be off-gassing, but the slower tissues will still be on-gassing. As the fast tissues approach equilibrium, their rate of off-gassing will slow to a crawl or even stop. You lose all the benefit of pushing the gradient if you do not ascend and push the gradient some more.

If you take the "slower is always" better approach to the extreme, you will see that at some point it is just an extension of bottom time. There has to be a happy medium, and 30 FPM appears to be it.

BTW, PADI really can't change to 30 FPM. According to the RDP, your bottom time ends when you begin your direct ascent to the surface. All the numbers on the RDP were determined based on empirical research with an ascent rate of 60 FPM. If they were to switch to 30 FPM, then all the numbers would be wrong. Since 60 FPM has been proven to be safe enough, they can stick with it. I would advise a diver who is ascending at anything short of the NDLs to use 30 FPM, though.

 

[Akimbo]

Fascinating story, but it sounds like you're saying the 60'/min ascent rule is about as scientifically valid as an old wives' tale? I can imagine the no swimming 30 minutes after eating tale having a similar basis.

It'd be nice to find some real evidence, since if one relies on the ZHL-16 model alone, you could safely ascend to 5' from 100' after 20 minutes, with no speed restriction. It drives me crazy to yield all of that premium diving time on an unjustified theory.

If your story is the extend of it, you're saying we have no idea why the ascent limit exists and why we follow it, but most people don't have the balls to ignore it.

I think you missed the point. Decompression procedures evolved from human testing. Tissue theories were evolved many years later and would have no basis without that body of work. Decompression theory is just that, theory. 60'/minute is scientifically valid only because there are massive amounts of quantitative data to confirm that 60'/minute coupled with the decompression procedures tested produced acceptably low hit rates. Other navies used different ascent rates at different times with equally good results.

Decompression ascent in saturation diving is continual and is "probably" closer to optimum. Releasing gas from a chamber at 3'/hour is practical; continually surfacing in the water at very slow and variable rates currently is not. It was a matter of choosing a rate that would be practical for divers to use at sea while experimentally testing decompression procedures on humans. Tissue theories came later and were mathematically modeled and tested against this body of data.

There are a lot of human variables, probably day to day. The vast majority of this empirical data is derived from subjects that are male, under 35 years old, in very good physical condition, have extensive history of decompression exposure, and consume more alcohol and caffeine than average. Data is just not available for the diverse worldwide recreational diving population.

The quality of chamber testing is considered the best because of well documented and controlled conditions. Data from military and commercial dives is second in quality because documentation is good, depth instruments are very accurate, and virtually all bottom time is at the same depth. Unfortunately, it is currently difficult to use data from recreational divers. I hope a standard for downloading data and registering personal profiles for recreational decompression computers will soon develop so that phase of research can begin.

This work started before my grandfather was born and will likely continue long after our grand children are retired. Our understanding of Hyperbaric medicine has a long way to go.

 

[Slamfire]

From the DAN site
Thus, a compromise was reached at 60 feet (18 meters) per minute, which was also a convenient 1 foot per second.

At some point I also thought it had something to do with not ascending faster than small bubbles. I remember many years ago looking at bubbles go up, trying to select the size of what a "small" bubble should be. "Yep, that's the size I'm looking for. That small bubble moved what looks like a foot in what seemed to be 1 second. From now on this size bubble will be my ascent speedometer."

 

[Gilldiver]

It is all far more basic and practical then scientific.

The Navy table are based for hard hat divers and were developed for Hard Hat divers who used a stage to get down to at least half of the working depth.

Up through the early 1950’s the ships used and the winches used with the stages were steam powered and relatively slow, they ran at just about 30 feet per minute. Then in the 1950’s newer ships with desiel engines and electric winches came into the fleet and the winches ran faster, at 60 feet per minute.

Eventually it was realized that 60 was pushing it and the rate was dropped back to 30.

 

[Thalassamania]

You can make a set of tables for almost any ascent rate (consider submarine escape at 300 to 600 FPM!). The ascent rate is part of the calculations for the table.

That said. Bennet's description closely matches Ed Thalman's (I asked Ed that same question once). The way Ed tells the story, Doug Fane wanted 100 FPM (or more), the hard hat guys objected because they couldn't raise a stage that quickly and because there'd be too little bottom time with lots of decompression. So they went down to the dock and put a dressed diver down on a stage and asked the crew to crank him up to the surface as fast as they were physically able. 60 fpm was how fast they could go, so that was the number used in the calculation.

BTW: When Ed told the story the winch was hand cranked.