Getting bent on Haleakala

[LeadTurn_SD]

Could you explain how the tables show this? In the dives you describe, on the PADI tables you are in the Q-S range of pressure groups. If you instead did a 60 foot dive for 40 minutes or so, you would have the same range. All subsequent dives would be planned the same way for either first dive.

The PADI tables do show that a series of dives with at least one of them getting close to deco limits will have a different effect because of the loading of the slower tissues. There is a reason for that. When the tables were made, it was determined that the controlling department in typical recreational dives was the 40 minute compartment. To be conservative, they made the tables based on the 60 minute compartment. The 60 minute compartment clears in about 6 hours, and if you look at the maximum surface intervals on the tables, you can see that. On the other hand, a series of dives near the deco limits will bring the slower compartments more into play, so longer surface intervals are recommended. This was more like the Navy tables at the time, which were based on the 120 minute compartment.

Hi John,

My bad, I think I opened this can of worms, so let me try to explain, using acooper's example of a single shallow dive at 40 feet, and your example at 60 feet:

Using USN Table 9-7:

40 feet for 90 minutes = Group "J"

60 feet for 40 minutes = Group "H"

So, based on those repetitive groups for a single dive, and using those bottoms times, you will exit the water with a higher Group Designator on the shallower dive, and about four times the required surface interval at sea level before you can safely ascend to 4,000 feet, for example.

Whether an average vacation diver can really get that much bottom time from an Al80 is open to question, but I think if you read up and down a given Group Designator in Table 9-7 (and take into account bottom time vs air consumption), you'll often exit the water with just as high a Group Designator as a deeper dive; and if you start doing shallow repetitive dives with longer bottom times and short surface intervals you'll quickly equal the Group Designator of a deeper single deco dive.

The overall point I was trying to make (but not very well) regarding driving to altitude after diving is to be careful, even after dives most would consider low risk "shallow" dives.

Best wishes.

 

[boulderjohn]

Hi John,

My bad, I think I opened this can of worms, so let me try to explain, using acooper's example of a single shallow dive at 40 feet, and your example at 60 feet:

No, I insist it was my bad. I tunnel-visioned on to one point and didn't see the rest of the picture.

 

[purcellj]

This is just a general comment on what you mentioned, not directed at you purcellj, as I'm sure you know it already.

I'm FAR from an expert on anything to do with deco theory, but one of the things I learned many years ago was to be careful about making assumptions about the safety of "shallowish" dives in regards to nitrogen loading. Shallow depths will allow much longer "run times" due to reduced air consumption while still remaining within NDL's, and may in fact produce a greater nitrogen load than a "deep" dive with it's far shorter NDL and greater air consumption rate.

You do need to keep this in mind and refer to the tables if in doubt.

Just food for thought.

Best wishes.

My current computer will show compartment loading -- more interesting gimmick than practically useful. The interesting aspect to the tables on shallow dives is the very long bottom time allowed, even under repetitive situations.

For that given day we were pretty far from deco, so I wasn't too concerned. Especially with hours of SIT time. If/when you do repetitive deco diving, it's fascinating to see how slowly the NDL returns.

I do appreciate your concern.
John