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

[Thalassamania]

Thanks for the background.

 

[electric_diver]

I just found this reference in Sir Robert H. Davis' Deep Diving and Submarine Operations, Page 97:

The method was used as routine during the salvage of 250,000 dollars worth of silver from the Empress of Ireland in 1914, where several hundred dives we made at a depth of 170-190 feet.​

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Davis would be wrong about the depth of the Empress of Ireland. Port side rail is at 85 feet and the mud is at approximately 140 feet at low tide. A 50 foot tidal variation is just not realistic for Point-au-Pere.

See http://www.waterlevels.gc.ca/ for tide info at the Empress. Navigate to St.Lawrence, Zone2 and select Pointe-au-Pere. 5m tides seem to be the maximum and thats only 15 feet.

 

[Akimbo]

Davis would be wrong about the depth of the Empress of Ireland. Port side rail is at 85 feet and the mud is at approximately 140 feet at low tide. A 50 foot tidal variation is just not realistic for Point-au-Pere.

See Mares, courants et niveaux d'eau | Tides, Currents, and Water Levels for tide info at the Empress. Navigate to St.Lawrence, Zone2 and select Pointe-au-Pere. 5m tides seem to be the maximum and thats only 15 feet.

You're right, just wait till I tell some of by Brit friends! I Googled the Laurentic and Empress of Ireland to see if maybe Davis, the typist, or typesetter mixed them up. Both are around 140' to the bottom.

I would transcribe the whole paragraph except I really suck at it. Here are the next two sentences from where I stopped so see how you guys interpret it:

The divers came straight to the surface, were run up to 77 lbs. in the chamber, and then decompressed by the standard tables. There were no cases of compressed air illness.​

I assume 77 lbs is 77 PSI, which would be 173' of sea water. It actually makes more sense to me that the working depth was 140' and the starting depth for SurD on standard tables would be 170' — with no hits on "several hundred dives". If any of you have books on the Empress, and they describe diving operations in any detail, it would be interesting to know more about it. I wonder if RN Docs sanctioned it or they just tried it out since the divers were probably freezing their buns off during on-water stops.

 

[mbacarella]

Thank you (everyone) for your informative replies.

I guess my question finds its basis in how surprised I was at how opposed the ZHL model is with other dive safe recommendations.

Is there a currently accepted best practice model/algorithm I should be evaluating instead? Following a linear ascent rule drives the mathematician inside of me crazy.

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.

 

[mike_s]

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

how do they know.... they killed a bunch of goats testing it.

 

[Kingpatzer]

Slower is always better.

That may not be entirely true. I read some research paper out there on variable ascent rates that suggest some divers may benefit from ascending faster on portions of their ascent and slower on others. It's not something that's been fleshed out (or at least the paper I read didn't make it seem that way), and the idea may be wrong. But there's at least some evidence to support the idea.

 

[Thalassamania]

That may not be entirely true. There's some research out there on variable ascent rates that suggest some divers may benefit from ascending faster on portions of their ascent and slower on others.

That's true from a view point of optimum (fastest) decompression. From a prospective of minimized DCS risk, slower ultimately has less associated risk.

 

[rjack321]

Thank you (everyone) for your informative replies.

I guess my question finds its basis in how surprised I was at how opposed the ZHL model is with other dive safe recommendations.

Is there a currently accepted best practice model/algorithm I should be evaluating instead? Following a linear ascent rule drives the mathematician inside of me crazy.

You're obviously going to struggle with the "art" parts of deco.

30ft/min with a 10ft/min slowing (either as one 15ft stop or sliding through the ~30ft to 0 ft zone) is pragmatic and it works. Many of us start slowing down to ~10ft/min at roughly 1/2 our depth.

Gas and conditions permitting, the more you can shape your ascent like an exponential curve the better, always remembering you <could> surface directly. This is a function of the pressure changing most rapidly the closer you get to 1 ATA to have a similar pressure change over time that you did at (e.g.) 2.5 ATA you have to slow. (among other reasons).

 

[Kingpatzer]

That's true from a view point of optimum (fastest) decompression. From a prospective of minimized DCS risk, slower ultimately has less associated risk.

Here's a link to a prior discussion we had about this:

http://www.scubaboard.com/forums/ma...-research-deep-stops-recreational-divers.html

 

[Thalassamania]

Think about it this way: What is the DCS risk if your ascent from 60 feet to the surface took a week? Just about zero, yes?