Decompression plan vs. redundant computers

[TheCatFish]

Add legs to the dive in MultiDeco and then run your computer. Dive the plan. If you stay longer than your initial plan, but still within the parameters of your gas capacity, make note of your decompression schedule from your computer. As Wibble said, know what your maxiumum tolerable TTS is, and know how much deco you can do with the gas you have with you. Having two computers means that you don't have to as much work when it comes to your redundant planning but only having one computer is still okay as long as you have written down your schedules and you adhere to the stops. A little extra deco at the O2 zone never hurts either.

Also, assuming your computer is in working order or you are diving with redundant computer, it doesn't hurt to plan the dive with a conservative GF and then use your GF% and SURFGF to adhere to a "tolerable" GF schedule in case things go awry and you need to get out of the water sooner than you might like. I usually plan my dives on a 60/75GF but know that if I actually dive a 60/85 or a 60/90 that my body should tolerate it.

 

[bardass]

I use and teach a other method for decompression dives : you have to start from the end ...
The aim is to define the end of the dive and the volume of gas required to ensure this end.
without any computer, calculator or simulator ...

During the brief, you and your team choose the maximum amount of decompression stop time you want to achieve.
For example: 10 minutes.
It doesn't matter whether the deco stops are at 3, 6, 9 or 12 meters, the time-weighted average depth is 5 meters (for air and nitrox dives) and will be used for consumption calculations.
Then you choose the maximum depth for the dive, regardless of whether the dive is wreck or multilevel.
For example: 40m.

For simplicity, we use the following parameters:
linear ascent speed = 10m/minute
surface air consumption: 20l/minute

Now you need to calculate the minimum time required to ascend with maximum safety.
Minimum time is made up of the ascent time and the deco stops time (+ safety stops time if you want).

Ascent time is défined by :
Maximum depth of dive / speed + 1 minute of deceleration close to the surface (or more if you wish).
So if I dive to 40m, the ascent is 4 minutes + 1 minute of slowing down near the surface. That's 5 minutes.

If deco stops time you choosed is 10 minutes then :
Total Ascent Time (TAT or ASC time) is equal to 15 minutes.

How much gas do you need to ensure this Total Ascent Time ?
You must apply a ratio for each minute of Total Ascent Time
This ratio is :
with 15 liters block, you consume 3b per minute of TAT.
with 12 liters block, you consume 4b per minute of TAT
I ask you to believe me, I provide a document that mathematically explains this results.

So, with TAT = 15 minutes, you need : 15min x 3b/min = 45b of gas for his ascent + a safety margin that each diver is free to define (50b, 70b...).
This is the Take-Off Pressure. For example : 100b.

Durint the dive, you have to monitor two parameters : TAT (ASC Time) with his computer, and Take-Off Pressure with his gauge.
The first parameter reached means the end of the dive.

It doesn't matter if the profil of your dive has a maximum depth of 40m for 5 minutes, then 30m for 2 minutes, then 35m for 3 minutes then 20m for 4 minutes, the process is the same. With experience, it can even be refined.

Planning method in French :