Decompression diving at altitude?

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Thanks everybody for your replies. It is not going to work out for this weekend anyway. Have to oversee rescue diver candidates in their surface work portions. Hopefully I'll get in a few fun dives. To respond to some of your questions: No we would not be diving 80's most likely would be using 120's with 72's staged at the deco stops. Have not decided on nitrox mix as yet but most likely 28%-30%. Altitude ascent rates of no more than 30 ft per minute. Stop depth adjustments work out to 17 and 9 for 20 and 10. Initial plan was starting with assumption of using the 140 ft tables with 30 min max bottom time. figuring nice slow descent of 3 min to depth of 90 ft leaving 27 min, where the platform is then 10 minutes out descending along the bottom to where we hope to find a depth of 118 ft actual (139for altitude adjust) mark and note location 2 minutes ,allowing 12 minutes back ascending along to 90 platform and up. This would give a 3 min margin even though we would be gradually ascending and then doing the required stops. As far as maintaining depth for deco that is not a problem. There are lines that we could use and my instructor has over 1500 dives as well as deco and blackwater experience. Me I just like to hang weightless and do skill drills or calculations while maintaining depth. good practice.
 
We do deco dives to depths of 150 ft in a lake at 4600 ft lake regularly.

If you use the ER Cross corrections to the US Navy tables you are essentially using a theoretical depth based on the lower atmospheric pressure at the surface. Since the atmospheric pressure is lower at altitude than at sea level, it takes fewer feet of water to increase the pressure by additional atmospheres.

What this means is that at 4000 ft if you dive to 130 ft, you will have a theoretical depth of 147 ft.

Now theoretically, your ascent rate is also decreased from 60ft per minute to produce the same rate of change in atmospheres in the same one minute period. This means at 4000 ft, you will need to reduce the ascent rate to 53 ft/min. Similarly, the deco stops are also theoretically shallower at 4000 ft with depths of 9, 18, 27, 35 and 44 ft rather than 10, 20, 30, 40 and 50 ft. In practice though most divers do the stops slightly deeper and don't mess with the deco stop depth correction as being a foot to a few feet deeper is not a problem.

If you use an old style capillary depth gauge, it will automatically be "calibrated" to the altitude as the air bubble will be at local atmospheric pressure. For the same reason it will also read the depth as theoretical depth and will also autmoatically adjust the stop depths as well.

With that being said, if you use US Navy tables for high altitude diving you are a bit of an idiot. The more conservative Buehlmann tables are available in a high altitude version that was specifically designed for alitude diving and they make more sense than using Cross corrections with either US Navy or recreational tables (SSI, old PADI tables, etc.) derived from modified US Navy tables.

Personally I prefer to use either DPlan or Palm VPM to cut my own deco schedules. The Bubble Gradient Model and Variable Permiability Model used respectively by those applications are more conservative and much safer for high altitude diving.

With DPlan for example a dive to 130 ft for 15 minutes at 4000 ft would require the following stops:

60 ft 1 min
50 ft 1 min
40 ft 1 min
30 ft 2 min
20 ft 4 min
10 ft 8 min

With Buehlmann tables for a 45 m dive for 15 minutes you would need:

6 M 3 min
4 M 3 min
2 M 6 min

With US Navy tables, you would only need:

10 ft 3 min

I can tell you from experience, that while I have never gotten clinically bent on US Navy tables, I basically feel excessively fatigued and achy after deco or repetetive diving with them. The US Navy did not normally do repetetive dives with them and deco dives were done with onboard deco chambers available so the higher hit rate of the tables in those applications was acceptable for navy purposes. They are not really designed for recreational repetetive or deco diving.

For the plan you listed above you could use a 120 ft for 30 min schedule with 32% in DPlan and it would want the following stops:

50 ft 1 min
40 ft 2 min
30 ft 3 min
20 ft 6 min
10 ft 12 min

Off gassing would start at about 60 ft and with a reasonable SAC rate of .6 for the working portion and .4 for the deco portion you would still need about 100 cu ft total.

If you use 50% for a deco mix, you would reduce the stops as follows:

50 ft 1 min
40 ft 1 min
30 ft 3 min
20 ft 4 min
10 ft 8 min

And would reduce the backgas used to about 83 cut ft and would need 11 cu ft of 50%.

So basically, with proper training ad equipment, you can use more conservative deco models to build in some additional safety and then use accellerated deco to reduce the deco time needed.

If it were me, whatever the gases or schedules used, I'd take the gas with in the event you cannot find your way back to the platform. 120 ft in black water over a silt bottom can be pretty challenging, the viz can quickly get too bad to read a compass and it is extremely easy to get separated from a buddy, especially when concentrating on a light and compass. I'd also take a long a bag and a reel to enable you to shoot an ascent line in the event you have to ascend somewhere else.
 
https://www.shearwater.com/products/perdix-ai/

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