"Riding your Computer Up" vs. "Lite Deco"

[boulderjohn]

..

I love this equivocation. OK, so we can't do two tank dives on navy tables. How about we call BS on their leading compartment? Let's use a shorter one, that works.

Second dive. Who is more aggressive, USN or sport?

I see both sides of this issue.

So PADI cloaks themselves in being far more conservative, yet the USN wouldn't touch that profile?

TYou seemed to miss the point that there was research involved. You write as if they just said, "OK, let's take a couple minutes off here, and do little of this and a little of that and see if it works." The research that led to the PADI tables was very thorough, and the results were peer reviewed. They did a lot of diving, and they tested the divers using doppler bubble imaging. ScubaBoard's Dr. Decompression, Michael Powell, was on the team that did the research, and his name is on the published documents. He was then primarily a decompression specialist working for NASA. If you want to check on the methodology, ask him about it. He does not come to ScubaBoard very much lately, but he might answer.

 

[Lorenzoid]

You don't think knowing the NDL is of practical use to the Navy?

No, just the opposite. As I said, I would imagine it's not uncommon for a Navy diver to dive to the limits (and beyond, into deco). I would think the Navy is all about pushing divers to their limits, so the limits at which too many divers are getting bent are exactly what they would want to know. Is this incorrect?

 

[stuartv]

No, just the opposite. As I said, I would imagine it's not uncommon for a Navy diver to dive to the limits (and beyond, into deco). I would think the Navy is all about pushing divers to their limits, so the limits at which too many divers are getting bent are exactly what they would want to know. Is this incorrect?

I totally agree with that. Your earlier post made it sound like you thought knowing NDLs was useless info to the Navy. I think I understand what you meant now.

Getting back to the original point of this little subplot.. Scubadada said something like the Navy table NDLs were tested to give 1% incidence of DCS. You said "so one out of every hundred typical Navy dives results in DCS?"

My point was that his point and your response are apples and oranges (I think). Your fruit nuggets are only apples if you assume that all the "typical" Navy dives are perfectly square profiles that exactly match the tables in all parameters - including exactly matching the tested profile's safety stop parameters (if they even do a safety stop). I don't know how the Navy dives, but it's hard to imagine that a large percentage of their dives are NDL dives that exactly match the table profiles. And it would have to be pushing 100% for your question to have an answer of "Yes."

SeewhutImean?

 

[Bob DBF]

A typical deep navy dive is going to be over the NDLs and way into deco, at about a constant depth, they do round to the next table though.

You are probably right, however they would would be using surface supplied air, not SCUBA in order to do extensive tasks and deep work because it is too inefficient on SCUBA.. The Navy SCUBA Divers I was most familiar with were aboad ship and diving was their second job. The would do hull inspections, mostly, and emergency underwater repairs if necessary. Never saw any repairs done and since the hull went underwater 20 something feet, I doubt if they ever got near NDLs.

No, just the opposite. As I said, I would imagine it's not uncommon for a Navy diver to dive to the limits (and beyond, into deco). I would think the Navy is all about pushing divers to their limits, so the limits at which too many divers are getting bent are exactly what they would want to know. Is this incorrect?

It is uncommon for a Navy diver to dive beyond the limits, unless they work for the NEDU. The Navy Experimental Diving Unit does the research which is used to set the limits for Navy divers. Unless there is a significant reason for diving past the established limits, it is not done. And when I mean significant, the call is made by the divers command, and if they made the wrong call it can cost them their career and/or brig (jail) time, along with the personnel in the chain of command that should have known better.

Navy Divers are an asset to the Navy, similar to a ship, you do not squander your assets needlessly.


Bob

 

[archer1960]

You are probably right, however they would would be using surface supplied air, not SCUBA in order to do extensive tasks and deep work because it is too inefficient on SCUBA.. The Navy SCUBA Divers I was most familiar with were aboad ship and diving was their second job. The would do hull inspections, mostly, and emergency underwater repairs if necessary. Never saw any repairs done and since the hull went underwater 20 something feet, I doubt if they ever got near NDLs.



It is uncommon for a Navy diver to dive beyond the limits, unless they work for the NEDU. The Navy Experimental Diving Unit does the research which is used to set the limits for Navy divers. Unless there is a significant reason for diving past the established limits, it is not done. And when I mean significant, the call is made by the divers command, and if they made the wrong call it can cost them their career and/or brig (jail) time, along with the personnel in the chain of command that should have known better.

Navy Divers are an asset to the Navy, similar to a ship, you do not squander your assets needlessly.


Bob

I understand what you're saying, but I think many will misinterpret what you say above to mean that Navy divers rarely go beyond NDL, which (I believe) is not what you mean. I think you mean that they won't go over NDLs without using proper deco procedures unless someone makes the explicit decision that they need to risk killing the divers.

 

[lowviz]

You seemed to miss the point that there was research involved. You write as if they just said, "OK, let's take a couple minutes off here, and do little of this and a little of that and see if it works." ...//...

Well yeah, that is called the hypothesis. They wanted to find a way to get a second dive in. Next, comes proper testing, then peer review. I have never called PADI or any agency out for improper procedure.

It appears that you missed this after that post:

... @boulderjohn Thank you for the Navy/PADI insight on the slower compartments. The PADI medium/deep sections look more evenly balanced (to me) in terms of loading as it appears PADI has nipped back on most of the first dive's longer exposures and had no need to do same in the deeper sections. ...

It appears that I need to keep restating my hypothesis: One can "safely" dive Navy NDL's by using a very conservative algorithm and any required deco that it generates. I am NOT making up deco stuff. I do have (what I consider to be) a reasonable limit after which all the accepted rules of hard deco are in force.

 

[scubadada]

TYou seemed to miss the point that there was research involved. You write as if they just said, "OK, let's take a couple minutes off here, and do little of this and a little of that and see if it works." The research that led to the PADI tables was very thorough, and the results were peer reviewed. They did a lot of diving, and they tested the divers using doppler bubble imaging. ScubaBoard's Dr. Decompression, Michael Powell, was on the team that did the research, and his name is on the published documents. He was then primarily a decompression specialist working for NASA. If you want to check on the methodology, ask him about it. He does not come to ScubaBoard very much lately, but he might answer.

This is a brief description of the derivation of the PADI Recreational Dive Planner:

The DSAT Recreational Dive Planner (PADI) model (1987)
Deco for Divers, Mark Powell, 2014, p 176
The M-values used for the RDP were adopted from the Doppler bubble testing and tested by Dr Merrill Spencer and tested by Dr Raymond E Rogers, Dr Michael R Powell, and the colleagues with Diving Science and Technology Corp, a corporate affiliate of PADI. The DSAT M-values were empirically verified with extensive hyperbaric chamber and in water diver testing and Doppler monitoring.

There is a lengthy description of the DSAT RDP in Lippmann and Mitchel Deeper into Diving, 2005, pp 283-306. This includes choice of the controlling tissue compartment as alluded to by @lowviz The USN tables use the 120 minute compartment while the DSAT RDP ended up using the 60 minute compartment, though the 40 minute compartment was considered. The extensive chamber and OW testing with Doppler studies for both single day and multi-day repetitive dives is discussed. Primary references are supplied.

 

[Bob DBF]

I understand what you're saying, but I think many will misinterpret what you say above to mean that Navy divers rarely go beyond NDL, which (I believe) is not what you mean. I think you mean that they won't go over NDLs without using proper deco procedures unless someone makes the explicit decision that they need to risk killing the divers.

Glad you cleared that up.

Also, the Navy would prefer to use Surface supplied divers for deco work as they have more flexibility in the operation and would have more facilities, like rescue chamber, backup divers, handlers, and equipment, to complete the job if it didn't go the way it was planned. Being on SCUBA within NDL is an advantage in small quick jobs, and is much better than bringing in surface supplied or relying on freedivers, which was done before SCUBA. The Navy SCUBA divers on board ship, that I was acquainted with, had no chamber, limited facilities and help, all of which was in addition to their primary job on board.


Bob

 

[Basking Ridge Diver]

Well yeah, that is called the hypothesis. They wanted to find a way to get a second dive in. Next, comes proper testing, then peer review. I have never called PADI or any agency out for improper procedure.
It appears that you missed this after that post:
It appears that I need to keep restating my hypothesis: One can "safely" dive Navy NDL's by using a very conservative algorithm and any required deco that it generates. I am NOT making up deco stuff. I do have (what I consider to be) a reasonable limit after which all the accepted rules of hard deco are in force.

I would go one step further - anyone in reasonable condition can dive the Navy NDL's for the first dive. Full Stop.

If you choose to dive the Navy Tables for a second dive - up your life insurance... That I think is the crux of what the second dive subplot is all about...

 

[archer1960]

Glad you cleared that up.

Also, the Navy would prefer to use Surface supplied divers for deco work as they have more flexibility in the operation and would have more facilities, like rescue chamber, backup divers, handlers, and equipment, to complete the job if it didn't go the way it was planned. Being on SCUBA within NDL is an advantage in small quick jobs, and is much better than bringing in surface supplied or relying on freedivers, which was done before SCUBA. The Navy SCUBA divers on board ship, that I was acquainted with, had no chamber, limited facilities and help, all of which was in addition to their primary job on board.


Bob

Also, keep in mind that there are two different groups of Navy divers (leaving out the SEALs): the Diving and Salvage community divers, and the shipboard divers. In a previous life, I knew people from both groups, and they operate very differently. The D&S guys routinely do deep work and deco as needed, either on SCUBA or on surface-supplied, whichever works better for the job at hand. The shipboard divers are (as you mention) essentially emergency divers, diving is not their primary duty, and will only be called upon when D&S guys aren't available. They won't generally have surface-supplied air available unless they're on a tender or carrier-sized ship, but will generally only be working on their own ship, so will be shallow. A typical job might be cleaning clogged sea water intakes, or pulling trash off the pit sword. I don't think they'd send them down to help recover a lost or stuck anchor; the ship would just slip the anchor and call the D&S guys to retrieve it.