Navy Dive Tables

[Doc]

We (you, I, Nemrod, and others "of a certain age") were likely taught "Sixty for sixty" (60 feet deep for 60 minutes, with an ascent rate of no faster than 60 feet per minute).

This one wasn't trial and error as I had oversimplified before This "rule" was imagineered late one night over many beers after considering the hoist rate of the USN winch used to lift divers from the briny deep.

I got that gem from a guy who was there and still drinks beer every night. Still alive and diving, too.

I want to know, has the incidence of bends increased.....computer vs. tables and before and after the advent of computers? Hard to get to truth because one must dig through layers of BS and the diving equivalent of urban legend.

Not just hard: Impossible. Diving has changed in so many ways- it is no longer in the realm of extreme hobbyists who largely prided themselves on planning, precision, execution and debrief. Conversely: The collectibility of data has been enhanced (due to auto-logging dive computers), but there is very little empirical data graphing recreational dive profiles pre-Bend-O-Matic Era.

 

[captain]

My understanding is the original hoist rate was 30 feet per minute basically for no other reason than the mechanical ability of the hoist and that originally the tables were built around that assent rate. Along came scuba after WWII and its tactical use by UDT and the frogs didn't like 30 FPM, they wanted something faster and it became 60FPM.

Having started diving in the 1950's with the Navy tables and switching to the Edge computer in the 1986 I still mentally compare what the computer is telling me to my previous table experience. I rarely do a safety stop unless I feel it is wise to do so on a particularly long or deep dive.

 

[Akimbo]

Hoist Rates:
I have heard the winch hoist rate story before but don’t buy it. We used the same 1930s era air tuggers (pneumatic winch) on the stage during my training in Mark V Deep Sea gear. They could easily handle the 60'/minute standard of the day. Granted, that rate would have been less with smaller compressors and/or hoses.

It was pretty rare that divers rode the stage to the bottom anyway. They would usually exit the stage at their deepest stop. Running it to the bottom with divers was a nightmare for umbilical entanglement so it would often require a second diver on the stage to tend the working diver’s hose.

Ascent Rates
Unless I missed it, the first published table (both pages) in the July 1916 USN Diving Manual didn’t specify an ascent rate at all, only the duration of staged decompression stops. I believe the first ascent rate in the US Navy was 25'/minute and was introduced in the 1920s.

The way I heard the story from divers that were taught by instructors who were there (which makes this story third-hand), 25'/minute was determined by a medical officer at using a slide rule. That was roundly hated by everyone because it was such a PITA to control, especially at on a chamber where the tables were tested.

Image using a stop watch and a low-precision pressure gauge, or estimates of hose being pulled up, to control ascent. Estimating around 5"/second (0.416'/second) is sort of doable when hauling hose but is a nightmare when watching a gauge calibrated in Feet of Sea Water. It was even crazier because the earliest chambers used gauges calibrated in PSI. That is why ascent rates have changed back and forth between 30 and 60'/minute ever since.

Obviously 60'/minute is easiest to control using a stopwatch, but it is also much easier to estimate on SCUBA (open or closed circuit) because it is about the same as pencil eraser size bubbles. I have yet to find a 30'/minute size bubble.

Comparable Data:
I have to agree with Doc, you can’t compare data where the historic data doesn’t exist… at least outside the military. The hit rates have consistently gone down in the US Navy when comparing similar dives, but that doesn’t do much for recreational divers. The key component of the equation still isn’t available today for rec divers, the total number of dives.

 

[cocoajoe]

Had an instructor tell me navy tables were based on every dive being a deco dive. That was in response to a question I had about comparing navy table with some other NDL tables. True?

 

[EFX]

Had an instructor tell me navy tables were based on every dive being a deco dive. That was in response to a question I had about comparing navy table with some other NDL tables. True?

Comparing the DSAT PADI with the older Navy-based PADI tables it appears that the older tables accomodated a longer surface interval (up to 12 hours) to account for deeper longer dives which were typical for Navy hard-hat divers. See http://www.scubaboard.com/forums/ba...6-old-padi-dive-tables-vs-newer-padi-rdp.html for more information. The DSAT tables max SI is 6 hours which accomodates rec divers doing repetitive dives. The DSAT tables are more conservative for a single dive (60 ft for 55 min vs. 60 for 60) but are more liberal for SI's. The newer tables were good for rec divers because typically they weren't staying at depth long enough to accumulate any serious deco obligations that would warrent a longer SI. But, because of repetitive dives and residual N2 the tables had to be more conservative.

In regards to ascent rate according to Alex Brylske in his book "The Complete Diver" the 60 ft/min rate was a compromise between the hard-hat divers who wanted ascent rates limited to 25 ft/min and Navy frogmen who were routinely coming up at better than 100 ft/min.

 

[nimoh]

Had an instructor tell me navy tables were based on every dive being a deco dive. That was in response to a question I had about comparing navy table with some other NDL tables. True?

All dives are decompression dives. Dives within the "NDL" just means that the decompression obligation can be accomplished through a relatively slow ascent, and perhaps a stop at 15 feet to be extra safe.

 

[boulderjohn]

Recreational training agencies and their attorneys don’t know squat about decompression theory. All they care about is that their tables are defendable in court.

You should go to the Ask Dr. Decompression forum on ScubaBoard and tell Michael Powell that he doesn't know squat about decompression theory. You should also contact NASA and tell them to scrub all the knowledge his research on decompression gave them over the decades he worked for them because he does not know squat about it. Here is something of a blast from the past some might find interesting: http://www.scubaboard.com/forums/as...l-tested-dr-ray-rogers-dr-michael-powell.html

Comparing the DSAT PADI with the older Navy-based PADI tables it appears that the older tables accomodated a longer surface interval (up to 12 hours) to account for deeper longer dives which were typical for Navy hard-hat divers. See http://www.scubaboard.com/forums/ba...6-old-padi-dive-tables-vs-newer-padi-rdp.html for more information. The DSAT tables max SI is 6 hours which accomodates rec divers doing repetitive dives. The DSAT tables are more conservative for a single dive (60 ft for 55 min vs. 60 for 60) but are more liberal for SI's. The newer tables were good for rec divers because typically they weren't staying at depth long enough to accumulate any serious deco obligations that would warrent a longer SI. But, because of repetitive dives and residual N2 the tables had to be more conservative.

The U.S. Navy tables added a 120 minute compartment and made it the basis of surface intervals. That probably made sense given the kind of diving they did, and it didn't matter that much because the divers usually only did one dive per day. It was a real problem for recreational divers, though, because anyone wanting to do a second dive on a dive day had to wait a long time to get back in the water, even after relatively benign first dives. The PADI research led by Rogers (with help from Michael Powell, mentioned above), using the new science of Doppler bubble imaging, determined that for the kind of diving done by recreational divers, the 40 minute compartment could safely determine surface intervals. For the sake of increased safety, they made the first dives more conservative than the U.S. Navy tables, and they used the 60 minute compartment for surface intervals instead of the 40 minute compartment. Because it takes 6 iterations of compartment half times to wash out, that explains why the PADI RDP has a 6 hour washout (6 X 60) and the Navy tables had a 12 hour washout (6 X 120).

For an additional measure of safety, for multiple dives near the NDLs, they added extended minimum surface intervals (the WX and YZ rules).

 

[redacted]

All dives are decompression dives. Dives within the "NDL" just means that the decompression obligation can be accomplished through a relatively slow ascent, and perhaps a stop at 15 feet to be extra safe.

Your simple (and unnecessary) twist on the definition of decompression diving could leave lots of divers unable to buy insurance or paying more for it. What benefit from that definition would justify such a cost?

 

[EFX]

Your simple (and unnecessary) twist on the definition of decompression diving could leave lots of divers unable to buy insurance or paying more for it. What benefit from that definition would justify such a cost?

I wouldn't call it a "twist". The term "decompression" and "no decompression" are misnomers. They should be called respectively "staged decompression" and "no stop" dives. Nimoh's comment on safety stops (which are not mandatory) is unnecessary and confusing given the "no stop" dive term. I don't understand the insurance aspect. Can you elaborate?

 

[redacted]

I wouldn't call it a "twist". The term "decompression" and "no decompression" dives are misnomers. They should be called respectively "staged decompression" and "no stop" dives. Nimoh's comment on safety stops (which are not mandatory) is unnecessary and confusing based on the "no stop" dive definition. I don't understand the insurance aspect. Can you elaborate?

Some insurers (life) will ask in their questionnaires if the applicant is a scuba diver. While I understand some simply reject all scuba divers, other may reject or charge more based on the type of diving you do with "decompression diving" being a red flag for some.