Altitude/Nitrox

[George Scherman]

Just did my EAN check out dives the weekend at a Lake at 4000' above sea level. The instructor had us figure our starting pressure group, since most of us had gained 1000' to get to the dive sight, but he did not used an adjusted theoretical depth chart to plan our dives. We didn't go any deeper than 70 ft (yes, it was chilly) and no one suffered any ill effects. Is it unneccessary to use the altitude depth charts?

 

[Tod]

No, I believe it should still be necessary. The theoretical depths still apply due to the altitude - pressure is pressure and it has no concept of what is inside your air tank.

 

[knotical]

George,

Your instincts are spot on! Absolutely should have adjusted for altitude. Calculating your starting PG because you gained 1000’ is good, but you should also adjust to theoretical depth because you were at 4000’.

 

[do it easy]

Just did my EAN check out dives the weekend at a Lake at 4000' above sea level. The instructor had us figure our starting pressure group, since most of us had gained 1000' to get to the dive sight, but he did not used an adjusted theoretical depth chart to plan our dives. We didn't go any deeper than 70 ft (yes, it was chilly) and no one suffered any ill effects. Is it unneccessary to use the altitude depth charts?

Doesn't it depend on what type of depth guage you have. If I remember correctly, Bourdon tube gauges measure the theoretical depth, so no adjustment is necessary. A capillary gauge would need to be adjusted (i.e. charts). I might have this backwards, or flat out wrong...

 

[knotical]

Good point, do it. Actually it's the capillary that reads "theoretical" at altitude, but I see them so seldom now that I flat forgot about them.

 

[devilfish]

Doesn't it depend on what type of depth guage you have. If I remember correctly, Bourdon tube gauges measure the theoretical depth, so no adjustment is necessary. A capillary gauge would need to be adjusted (i.e. charts). I might have this backwards, or flat out wrong...

Capillary gauge does not need to be adjusted because it fills with ambient pressure at surface and therefore show precise depth of water. However it will not show ata's and that's why altitude dive tables should be used to calculate bottom time,(when not using a computer). If a diver is diving at an altititude of 8000 ft which is 1/2 atm and is diving to a depth of 34 ffw, the pressure is 3x of the surface and the diver is actually at 3 ata of sea level, and taking on gas at three times.

 

[Charlie99]

Doesn't it depend on what type of depth guage you have. If I remember correctly, Bourdon tube gauges measure the theoretical depth, so no adjustment is necessary. A capillary gauge would need to be adjusted (i.e. charts). I might have this backwards, or flat out wrong...

Backwards. The capillary gauge doesn't need to be corrected, the bourdon tubes and electronic depth gauges do.


As Knotical noted, capillary gauges aren't very common any more. A capillary gauge is easy to spot because the scale is expanded at shallow depths and very compressed and hard to read at deep depths. The bourdon tube is more or less linear -- i.e. 10' change in depth is about the same distance on the gauge, independent of depth.

 

[Tod]

I believe there are two issues:

1. The possible inaccuracy of your depth gauge; and
2. The affect of the difference in pressure changes between being under water and the surface.

In the second case, since there is less pressure (in atomosphers) at altitude, an increase in altitude affectively "deepens" your dive (as far as body tissues absorbing nitrogen are concerned). So EAN has another safety benefit at altitude.

However, as Do It Easy intended to note, if your pressure gauge cannot compensate for altitude this could further compound the second issue.

 

[Don Burke]

Just did my EAN check out dives the weekend at a Lake at 4000' above sea level. The instructor had us figure our starting pressure group, since most of us had gained 1000' to get to the dive sight, but he did not used an adjusted theoretical depth chart to plan our dives. We didn't go any deeper than 70 ft (yes, it was chilly) and no one suffered any ill effects. Is it unneccessary to use the altitude depth charts?

Is it possible the computer he was using made the adjustment? My Oceanic Data Plus does.

 

[roakey]

If a diver is diving at an altititude of 8000 ft which is 1/2 atm and is diving to a depth of 34 ffw...

0.5 ATA is about 15,000 ft, not 8000.

I live at 7000+ feet, and the funny thing I found when going through the PADI Nitrox course is that they consider .16 (or .15?) ATA of O2 hypoxic. When I calculated my ATA of O2 at my house, turns out PADI thinks I'm going to die there.

Roak