Getting bent on Haleakala

[boulderjohn]

The tables would not be linear. Unlike water pressure, which is roughly linear, atmospheric pressure is not, the equation is an exponential. The last few thousand feet up the mountain sees a lot of pressure drop. There is only 60% of sea level pressure at the summit of Mauna Kea, about 620mbar.

That's true. That's why the altitude adjustment tables usually only go to 10,000 feet. After that the numbers really start to move.

In a conversation I had with DAN about this, they also said that is why leaving a dive site at 5,000 feet and ascending 3,000 feet does not have as much of an effect as a 3,000 foot ascent from sea level.

 

[purcellj]

A few years back we went to Big Island. We started with the telescope trip (13k alt.) then started diving.

A few weeks back in Maui we did go to the lavender farm (the girls idea, not mine) at 4k alt. on a dive day. While our runtimes were long, the depths were not. I had no concern going to 4k.

 

[bmrwizard99]

Many of our local dives here are in the Rocky Mountain lakes. We specifically cover altitude diving and on the drive back home, drive through a pass and people have not waited long enough and experienced symptoms so I'd definately suggest you consider a wait period before hitting Haleakela.

 

[LeadTurn_SD]

A few years back we went to Big Island. We started with the telescope trip (13k alt.) then started diving.

A few weeks back in Maui we did go to the lavender farm (the girls idea, not mine) at 4k alt. on a dive day. While our runtimes were long, the depths were not. I had no concern going to 4k.

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.

 

[Abidon]

We will either do Haleakala the first morning of the trip or at the very least, we'll only do one dive the morning before (24 hours +/-), and like Lead_TurnSD said - use Nitrox...

Doing it the first morning of your trip would be my recommendation, too. Seeing as there will be a 4-hour time difference from Colorado, you'll be waking up at 3-4 AM the first couple of days; whether you want to or not.

 

[acooper]

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.

I have often noted this, the tables are pretty clear that a long shallow dive results in a higher loading. More time for the nitrogen to penetrate tissues I suspect. I note this as Deb and I often do long shallow dives, 30-40ft for 90min+ is not unusual. On the other hand we usually do a single dive, the advantage of living 15min from the reef, no impetus to do multiple dives and pack in the bottom time.

But that is also a complication for heading to high elevation. Try to keep at least 48 hours in the schedule before heading to 13,600. And no diving on those weekends I am on-call, with possible, unexpected trips to the summit.

 

[boulderjohn]

I have often noted this, the tables are pretty clear that a long shallow dive results in a higher loading. More time for the nitrogen to penetrate tissues I suspect. .

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.

 

[StuartT]

One time on Maui we had done 2 dives. 1 at Molokini to 130 ft and the 2nd at the St. Anthony. Both on air and within ndl limits. About 4hours later we drove up to Kula which I think is around 3500 ft and I didn't feel any effects but my buddy who did the same profile got a pounding headache. We figured it was probably some dcs symptoms so we went back down to sea level where he was fine.

 

[acooper]

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.

With a single dive, and 40ft for 90+ minutes you can get into the S to U pressure groups while breathing standard air. A single dive at 80ft+ and you can not get into these pressure groups without breaking PADI no decompression limits. Seems clear to me.

My table shows a 60ft dive for 40min gets you into Q.

I have logged any number of shallow (30-40ft) dives lately with 90+ minutes bottom time, and a couple near 100min. Moving slowly while doing photography my Al80 lasts a good long time. Even with a short excursion to the base of the reef at 60-70ft then working my way back to the shallows allows a dive near 90min. Thus a good reason to spend a couple hours at sea level before driving home and up the hill.

 

[boulderjohn]

With a single dive, and 40ft for 90+ minutes you can get into the S to U pressure groups while breathing standard air. A single dive at 80ft+ and you can not get into these pressure groups without breaking PADI no decompression limits. Seems clear to me.

OK--I was comparing your original statement of 30-40, using 35, and then comparing it to 60. With your depth comparisons, that is true. I was reading it as comparing two dives that ended with the same pressure group.