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SurfLung
Master Diver
Posts: 1763
Joined: Sat Jan 01, 2011 2:03 pm
First Name: Eben
Location: Alexandria, MN
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High Altitude Decompression

Tue Jan 16, 2018 2:36 pm

High Altitude Decompression
- The airport where I live says we are at 1600 feet above sea level. My ZOOP diving computer has 3 altitude settings and the 1600 feet here as well as the 1400 feet altitude of Fortune Pond fall into ZOOP level 2 altitude.
- It makes a significant difference as I found out a couple of years ago diving in Cozumel with my ZOOP still set for level 2 (1000-5000 ft) when I was actually diving in Level 1 (Sea Level). The darned thing computed me up some stage decompression stops for the first dive and inadequate surface time for a 2nd dive gave me almost zero no deco time in only 30 feet depth. I just waited out the decompression stops in Dive 1 but Dive 2 it was obviously wrong so I omitted the 24 minutes of deco it computed and it went into error mode. This just to illustrate the point that NO-DECO times for Sea Level are not the same when diving at as little as 1400 ft Altitude.
- The standard No Decompression tables don't take altitude into consideration and I didn't see any fine print instructions about it. So I started looking for information. Kind of a fascinating topic and gets into the considerations of astronauts and space travel even.
- But I found my simplest solution in the Basic Scuba book by Fred Roberts. A formula credited to the famous E. R. Cross... You take the Sea Level pressure (14.7 psi) divided by the pressure at altitude 1400 (13.97 psi) which equals 1.05. Multiply that by the actual depth to get the deco depth. So at Fortune Pond, 1400 ft altitude, we go diving to 80 ft deep and we would use the no deco time for 80 x 1.05 = 84 ft. This you would round up to 90 feet. And it is a significant difference on the table:

No Deco Time for 80 feet is 30 minutes.
No Deco Time for 90 feet is 25 minutes. (1400 ft altitude)
These times are from the PADI No Deco Table

My ZOOP is a bit more conservative. I just looked up the instruction manual and found No Deco times as follows:
80 feet deep at Sea Level is 28 minutes
80 feet at level 2 is 23 minutes (1000-5000 ft altitude)
80 feet at level 3 is 19 minutes (5000-10,000 ft altitude)

So, altitude is definitely something to be aware of when diving deep at inland lakes around the country.
SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

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