Altitude dives

I have a unique opportunity to dive a lake in Spain next weekend, at 1,200 metres.
I will be using a Aladin Pro computer to assist me and diving on air.
Can I use the basic PADI RDP and add a given number of pressure groups to plan my dives? And how do I work out my starting point - given the altitude and my time since arrival at that altitude (surface interval since arriving)?

PS Anyone free next weekend for a dive in sunny Spain?

Regards,

Shaun

I shall start with the statement that you should only dive within your limits, training etc... and to do a dive like this you should do an altitude diving course.

When diving at altitude you can use a fairly simple formula to calculate the equivalent depth (although not simply adding a number). For example diving at 1200m to a depth of 20m would be equivalent to diving at 23m. Your guage would also be mis-reading so you would think you are at 18.6m.

If you want to do the same dive at the top of everest it would be the equivalent of a 60m dive although your guage would only read 13m.

You are likely to encounter severe thermoclines. You are also likely to be a long way from proper medical help so ultra-conservative diving is the order of the day.

There are factors to do with your approach and exit to the site (changes in altitude) that you need to consider also.

I would love to dive some of the lakes in the Pyranees. There was one we used to swim in as children that had a fairly in tact village in it. You could see some of the buildings when the lake was low in summer.

Equilization time:

At 1200 m or 39.37 feet above sea level the ambient pressure at the surface is 12.499 psi or .85 ATA.

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.

( 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)

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...

Jeff Lane

Shaun once bubbled...
Can I use the basic PADI RDP and add a given number of pressure groups to plan my dives?

Click to expand...

Can you?
I suppose you can, but it has nothing to do with altitude diving. I urge you to get the proper training before making dives at altitude. And while there is some good info in the posts above, I highly recommend you take a proper course before conducting the dives.
Rick