Conversion for Altitude

[Henryville]

Can someone help me with two questions:

- at what altitude is it worth adjusting for "effective depth"

- What is the formula for converting actual to effective depth so that standard tables can be used?

Thanks

 

[rmediver2002]

Equilization time:

When you travel to destination at altitude or a lower ambient pressure your body is off gassing, adjusting from the amount of nitrogen present at sea level .79 ATA to the amount present at your destination. For an altitude of 1200 m upon arrival you would a repetative group designator of "C" from the US Navy dive manual and RNT table. (your body will reach equilibrium with the amount of nitrogen at altitude within 12 hours)


Dive calculation:

When your body has equalized to the ambient pressure at altitude it will be exposed to a greater pressure change when decending on a dive, the wieght of the water does not change. (other than the difference between fresh and salt)

( at sea level 33 fsw will double the atmospheric pressure your body is exposed to / at 1200 m or 12.499 psi if going to 33 fsw or 34 ffw we still add an additional 14.7 psi of pressure "cross correcting allows us to compensate for the extra 2.201 psi)

You will need to adjust or "cross correct" to find the equivilent sea level depth of your dive, after cross correcting you can use the new depth in determination of your table and schedule on a standard air table. The sea level equivilent depth will always be deeper than the actual depth of the dive at altitude and will compensate for the increased decompression needed to offset the effects of diving at altitude.

You can use the following formula:

Depth at altitude X pressure at sea level in millibars (1013 mb) / pressure at altitude in millibars (861 mb [sample])

example 100 depth x 1013 mb / 861 mb = 117.65 or 118 FSW



If your depth gauge does not allow you to zero it at altitude you will need to add an additional 1 fsw to the measured depth for every 1000 feet of altitude before calculating...


A different calculation is required to determine decompression stop depth if needed.

Hope this is helpful..

 

[Walter]

There are charts for this in the NOAA Dive Manual.

 

[DepartureDiver]

You will need to adjust or "cross correct" to find the equivilent sea level depth of your dive, after cross correcting you can use the new depth in determination of your table and schedule on a standard air table.

Good advice here. FYI, they are called Cross corrections since E.R. Cross published them in Skin Diver Magazine many years ago (first in 1967 I believe), but he actually obtained the formula from another who he gave credit to. I prefer to call them a ratio correction since it is easy to remember. The amount of correction needed is debatable, but I believe any elevation above 1000' should be corrected for altitude. For a general rule of thumb, you can add 5% to your depth for each 1000' of elevation (good for altitudes up to about 13,500' ... which covers any lake you are going to find). So at 6000', a dive to 100' would be equivalent to a dive to 130'. If you do not have an altitude adjustable gauge, then 6' would have to be added to the gauge reading (i.e. 94' + 6' = 100 feet to be used for the conversion to calcualte the 130'). As pointed out, don't forget to take into consideration your repetitive group designation upon arrival at altitude from a lower altitude. For another tid bit of information, you can always dive nitrox and if the mix is rich enough, it will let you dive sea level tables at altitude. For example, a blend of 34% (or greater) at an altitude of 6000' will allow you to dive sea level tables without making a ratio correction, but you must still make the other adjustments such as taking into consideration your altitude ascent and whether you have an altitude adjustable depth gauge or not.

 

[Charlie99]

.. deleted post. Departure Diver already covered it.