Was "Mike Nelson" right in advising never to ascend faster than your largest bubbles?

[supergaijin]

The bubbles thing was taught as it was easy to visualise. Before computers it could be very difficult to gauge how fast you were ascending especially without visual reference ie. a blue water ascent.

Different agencies had different ascent rates- some were really slow, some extremely fast. Haldane had specific theories on tension ratios of Nitrogen which one could never exceed but also thought it was optimal to ascend at the fastest possible without breaking the ratio, resulting in rapid ascents from 200' to 100' and then slowing it down from there.

When the great minds of the agencies got together to hash it out, everyone had a different opinion- nobody was backing down so they took the average which was 60ft/min. Still very very fast. Doppler tests have since shown that a 30'/min ascent rate is optimal from most recreational depths.

No faster than the smallest bubbles is still taught and is the only meaningful way to judge ascent rates without computers for the vast majority of divers.

 

[GROBIOG]

The girl next to me asked...

-What if they're on ccr?

She's blond.

 

[GrumpyOldGuy]

and does any agency teach to disregard the computer and follow their guidelines?

Yes to disregarding the computer and following their guide lines, no to using the bubbles.

These agencies/groups are proponents of more conservative approaches including detailed, comprehensive planning and min deco rather than the fly-by-wire approach used by many rec divers.

 

[drbill]

Lloyd Bridges was taught by the Meistrell brothers of Body Glove and Dive N' Surf fame. The 60 fpm rate was the one used at the time (and when I started diving in the early 60s). Of course today a greater safety margin is suggested. I now ascend at a rate less than the flow of molasses on a cold day.

 

[dumpsterDiver]

It is a great rule and if you follow your SMALLEST bubbles you will be ascending considerably slower than 60 ft per minute. It is a great tool.

Even if your computer has not died, it is one more tool in the experienced diver's bag of tricks to help judge an ascent. Anytime you are ascending and you begin to rise into your own bubble stream, this is an immediate warning to exhale and to slow the ascent and get the bubbles above you. It could be a very important tool in an emergency, or when doing some other activity that has your hands full and you don't wish to have to stare at the computer.

 

[scjoe]

The 60 foot rule did not come from the agencies, it came from the US Navy. The commanders wanted 90-100 feet a minute to get people out of the water, but the tenders couldn't haul the hard hat guys up that fast, so 60 was adopted. It also happened to fit nicely with movement of a 60 second hand on a watch.

The 30 fpm standard is based on Doppler readings, but I don't believe there has been a proven link between those readings and incidents of DCS. In fact those readings are not measuring the bubbles that cause DCS according to the the bubble models. The DSAT PADI 60 fpm standard was established based on Doppler readings as well.

 

[Dr Deco]

Hi Scubalot:

The rule was actually "never faster than your smallest bubbles." This is actually faster than recommended today. Your gauge is your best tool for judging ascent rate.
The paper you quoted actually puts in some fine points in ascent rates. The simpler, basic equation has been known for more than half a century.

 

[jimlux]

The other problem (described in the report from the Smithsonian conference on diving from 20 odd years ago) is that when you have lots of bubbles, the water in the column around the bubbles starts moving upwards (just like in a fish tank, eh?) so the "free ascent rate" of bubbles number winds up being wrong (because they're ascending at 60 fpm relative to the water, but the water is moving up).

The other problem is that bubbles expand as they go up, and their ascent rate changes.

(when I learned to dive in 1974, we were taught the "don't ascend faster than your bubbles" thing.. because, hey, what gauges? you had no way to know how deep you were and as far as gas remaining.. that's what a J-valve is for)

 

[dumpsterDiver]

I got on the boat today and the computer would not boot up. No computer, no depth guage, one watch. Did 4 dives today in 75-85 ft, following bubbles.... and looking at the bottom and the surface when they were visible. 5-8 ft visibility in one nasty, mid-water layer. I think Mike Nelson would have been proud.. It does NOT bring you up too fast.. the rule is slower than your smallest bubbles... and when they expand..you pick NEW smallest bubbles..

 

[Gilldiver]

The 60 foot rule did not come from the agencies, it came from the US Navy. The commanders wanted 90-100 feet a minute to get people out of the water, but the tenders couldn't haul the hard hat guys up that fast, so 60 was adopted. It also happened to fit nicely with movement of a 60 second hand on a watch.

The 30 fpm standard is based on Doppler readings, but I don't believe there has been a proven link between those readings and incidents of DCS. In fact those readings are not measuring the bubbles that cause DCS according to the the bubble models. The DSAT PADI 60 fpm standard was established based on Doppler readings as well.

About half true - the original hard hat diving, which was rarely deeper than 100-130 feet till the 1930's was 30 ft/minute as that was the speed that the tenders could pull a diver up by hand to the dive stage which was at 20-30 feet. It was also just about the speed of the steam winches used at the time - so 30 ft/min was the standard - it was also easy to measure with a 60 second hand.

In the 1940's electric and hydraulic winches came into use and they could pull at 60-100 feet/min so the non-diving salvage officers advocated to speed up to that 60 to 100 feet/min. Safety wise, it made sense when near the surface as trying to hold a diver at 10' for 20-30 minutes when the ship draws 15-20' means that the diver got beat to hell by the ship.

So the practice was to get a diver to 30 or 20 feet, complete that stop, then pull them up on deck, strip off the hat and weights, and put them into the ship board chamber to complete the decompression. The time allowed between the 30' stop and back at pressure in a chamber was 5 minutes. So when a diver hit the deck, 2 or 3 tenders stripped them and literately tossed them into the chamber. When SCUBA came into the Navy, they kept the rates that worked for them and it just carried over.

So, the accent rates have a lot more to do with the speed of a ships winches than they do with diving science.