How long to wait before Diving At Altitude?

[DandyDon]

I remember that one is supposed to wait after arrival at altitude before diving, but I forget how long? How are we supposed to figure that?

Plus - if I live at 3300 feet, and I'm traveling to 4600 feet to dive - with the 1600 foot increase happening over the last 2 or 3 hours of driving, does this affect the answer to the first question?

Thanks for the help, don

 

[IndigoBlue]

The best explanation and charts that I have seen available on this issue is The NOAA Diving Manual. Order a copy of it from Best Publishers and read the chapter.

 

[DandyDon]

The best explanation and charts that I have seen available on this issue is The NOAA Diving Manual. Order a copy of it from Best Publishers and read the chapter.

Got it at home - thanks! don

 

[MikeFerrara]

Yes the ascent prior to the dive matters.

The general recommendation is to figure 2 pressure groups per 1000 ft or wait 6 hours. As was already mentioned the NOAA manual has tables and "Technical Diving in Depth" explains how they work.

Some decompression softwares will calculate for altitude also.

There are other considerations that you should be aware of if you aren't already also.

 

[DandyDon]

The general recommendation is to figure 2 pressure groups per 1000 ft or wait 6 hours. As was already mentioned the NOAA manual has tables and "Technical Diving in Depth" explains how they work.

Okay, with a 1300 foot increase to 4600 feet, round up to 2000 and call that pressure group D...?

There are other considerations that you should be aware of if you aren't already also.

Got the Padi chart and the NOAA book. A 84 foot dive is planned as if it was a 100 foot, and the S-Stop is 12 feet rather than 15 feet. Is that what you're referring to?

thanks, don

 

[Pez de Diablo]

Here in the Rockies we teach the two pressure groups per thousand feet. So your first dive you would treat as a repetitive dive getting in the water as a D diver. Unless you spend 6 hours at the altitude you plan on diving at - think of this as your surface interval.

 

[MikeFerrara]

Yes
Some computers will take care of all of it for you so make sure you know how yours works at altitude if you use one.

 

[DandyDon]

Yes
Some computers will take care of all of it for you so make sure you know how yours works at altitude if you use one.

I like Oceanic computers, but they're no champs on this issue...

(1) The book says to turn them on only after arriving, so this will not help with the original question; and

(2) They'll read the alititude within 1,000 feet:

(a) 3,000-4,000;
(b) 4,000-5000; or
(c) 5,000 to 6000​

.

 

[DandyDon]

Got the Padi chart and the NOAA book. A 84 foot dive is planned as if it was a 100 foot, and the S-Stop is 12 feet rather than 15 feet. Is that what you're referring to?

Okay, I reviewed Altitude Diving in my PADI Adventures Book and in the NOAA Manual - what I should have done to start with - and I see both convert the 84 foot dive at nearly 5,000 feet to 110. (I think I got the 5% adjustment per 1,000 feet from another instructor - not PADI.)

If we were leaving sea level directly for 5000 feet by chopper, there would be a 10 group adjustment, but since we're only changing 1500 feet from 3300 to 4800, one might call that a Group C - degraded to Group A by the gradual altitude increase of the drive in...? :ditsy:

But instead - I'll follow the more conservative approach and call it a Group D - degrading to Group A with a 80 minute wait on arrival at Santa Rosa - while we check-in the hotel, make DeCaf for thermoses, eat lunch, organize gear. We'll then plan our dives for the next 4-1/2 hours with 3 minutes of Residual N2 carried over. :chillin6:

Have I got it right now...? :idea:

thanks, don

 

[DepartureDiver]

Hi Don. The repetitive group designation upon arrival at altitude will vary with the table you are using. I never saw you state you are using PADI's RDP, but by the posts form others, it appears that you are. PADI states to use 2 pressure groups every 1000' of altutide ascent. But last I knew, this also increases to 4 pressure groups when you start your ascent at some elevation (I forget what it is) above sea level. Now much of this appears to have just been based on coming up with some workable rules other than on any real scientific protocal. If you are using the Navy table (or a modified version therof), a pressure group of B is perfectly acceptable for the ascent you described. As far as the Oceanic computers go, they do not take into account any altitude ascent. They just make the typical corrections in allowable surface tensions based upon the standard ratio corrections (which in general are conservative). Now getting to your depth question, the typical correction tables do a double rounding ... which obviously penalizes you. If diving to 85 feet at 4800', you can treat this as an equivalent dive to 100' (not 110') in the ocean on air. As far as the safety stop depth goes, keep in mind it is only a safety stop. It's exact depth does not matter. Actually, if you are doing a "deep" dive, you should stage several safety stops and make your first one greater than 12 feet. There are several rules of thumb that can be used. Just choose what ever is easiest for you to remember. The one I recommend is to take your deepest depth and divide it by 3. You need not make any altitude corrections ... just use actual depth. In other words, if diving to 90', your first stop should be at 30' for 1 min, followed by another 1 min stop at 20' and then a 3 min stop at 15' (or 12 if you would prefer at altitude). I hope this helped.