More than "Advanced", but not really "Technical"

[DevonDiver]

Perhaps you can explain - what priorities a vessel commander might have during time of war, when the vessel is forced immobile due to diving operations and/or comes under threat under those circumstances?

 

[ianr33]

Im talking about the US Navy tables, and that they are very close to the agency modified tables in relation to no deco diving.

I'm no expert on the Navy tables but,I do know they give 25 minutes NDL at 100 feet whereas PADI only allows 20 minutes.

I don't consider 25 minutes to be "very close" to 20 minutes.

 

[mmadiver]

There are other prerogatives with the Navy tables... swift exit from the water being one of them, as a military-operational (not dive safety) concern.

Perhaps you can explain - what priorities a vessel commander might have during time of war, when the vessel is forced immobile due to diving operations and/or comes under threat under those circumstances?

You're implying that unique operational circumstances associated with military dive operations are somehow factored into Navy tables which is simply not true. Extenuating circumstances don't dictate how much deco you need to do. Whether you are diving a mine field in wartime or on a reef in peacetime, deco is deco. The development of the Navy's tables are based on human physiology and experimentation - all of which is documented and available via a google search. With regard to the Navy's Air tables, those were based on a compartment to optimize the typical hardhat air diving operations the Navy does (i.e. relatively shallow ships husbandry and salvage).

Somehow accounting for all the unique hazards of wartime diving, like clearing mine fields (see my avatar picture) in dive table development isn't realistic. Instead those hazards are addressed externally and mitigated in other ways.

 

[tracydr]

Don't Navy tables assume a 2% rate of DCS? In addition to being a table for young,fit divers, it also assumes a higher risk of DCS.
2/100 hit rate seems like a lot of risk for a recreational diver, who may or may not have reasonable access to a chamber.

 

[DevonDiver]

deco is deco.

Is it?

Why does it vary so much then? Depending on the table/algorithm used?

What you're forgetting is the issue of 'acceptable tolerances' - which is exactly why US Navy tables were mentioned in the first place. We all know that the tables were developed based on physiology and experimentation. However, so were other models. The reason that other (non-military/commercial) models give shorter no-stop and/or longer deco is that issue of 'acceptable tolerance'.

As you say, models can be optimized to suit certain parameters. Recreational tables are optimized to suit multiple, repetitive, short/shallow dives - with compartments selected to determine that outcome. Military tables favor singular dives and/or longer surface intervals - and they get you up quick. Quicker than any civilian-sold equivalent I'd hazard to guess.

Serious question, because perhaps I was given duff info before... but why do military surface supplied divers get pulled to the surface, risk DCI getting de-kitted on a stopwatch and deco in a chamber? I was under the impression this was to reduce the immobility of the vessel and get the diver out of the water ASAP. If 'optimal deco' and pure diver safety were the driving factors, why wouldn't the military replicate commercial diving ascents and procedures?

 

[mmadiver]

Serious question, because perhaps I was given duff info before... but why do military surface supplied divers get pulled to the surface, risk DCI getting de-kitted on a stopwatch and deco in a chamber? I was under the impression this was to reduce the immobility of the vessel and get the diver out of the water ASAP. If 'optimal deco' and pure diver safety were the driving factors, why wouldn't the military replicate commercial diving ascents and procedures?

There are a number of benefits of chamber decompression vs in water deco for surface supplied operations, as you noted reducing the immobility of the diving platform is one. Chamber deco may appear to be a dangerous practice, but in actuality is much safer than in water deco. Typically divers are lowered/raised via a stage, like this one:

If the diving platform/vessel is rolling in the sea state the stage will be moving up and down in the water column making it difficult to control depth and maintain accuracy in a deco profile - it can be a very bumpy ride. Pulling the divers out and putting them in the chamber allows 100% control of their table/schedule, they are no longer subjected to the elements, get to be nice and warm, eat and drink, and can be monitored by diving medical personnel. It also enables a much more efficient dive operation as the diving supervisor doesn't have to worry about managing deco divers while trying to direct the overall operation.

Commercial surface supplied divers do chamber deco as well. As I said it is much more efficient and on the commercial side time is money.

 

[boulderjohn]

I'm no expert on the Navy tables but,I do know they give 25 minutes NDL at 100 feet whereas PADI only allows 20 minutes.

I don't consider 25 minutes to be "very close" to 20 minutes.

Here is the explanation for this.

When the Navy tables were redone by Workmann, he added a 120 minute compartment to the theoretical tissues and decided (without a whole lot of research, BTW) that the 120 minute compartment would control the surface interval period. With that as the basis of pretty much all recreational diving, it meant that divers were out of the water sitting around a very long time before their next dive. PADI was concerned about this in large part because the people doing recreational dives were diving very differently from Navy divers, and they were thinking the 120 minute compartment might not be the correct choice. So they did a ton of very expensive research on actual divers using Doppler bubble imaging, and they determined that for the typical dives being done by recreational divers, the 40 minute compartment could safely determine the surface intervals. This was a big step, though. They decided to add a couple of levels of conservancy. They selected the 60 minute compartment for one, and they shortened the first dive bottom time for another. The difference cited above is an example.

The reason the PADI tables have shorter first dive NDLs than the Navy table is to ensure that the 60 minute compartment controls the surface interval and allows divers to get in a second dive more quickly. Navy divers usually did not do second dives.

 

[ianr33]

While I *think* I understand what you are saying, does it make any sense under the PADI system that a (possibly non existent! ) second dive shortens the length of a first dive ?

 

[DevonDiver]

Yes... the RDP is designed to better cope with repetitive dives. That's why it gives a shorter first dive than, for instance, the navy table. It penalizes less for subsequent repetitive dives - giving more credit for off-gassing over a shorter surface interval. This was calculated to be more appropriate to the patterns of recreational diving.

 

[lowviz]

...//... I don't consider 25 minutes to be "very close" to 20 minutes.

Along with the excellent explanation about the intended use of the Navy tables, seeing different times as being close to each other (or not) also depends very much on your depth and downtime.

We see time as linear but we are accumulating nitrogen at an ever increasing rate with depth. There is far, far less time for error at depth. The attached graph may help. look at how fast a deco obligation mounts up at 60 feet and compare that to 120 feet.