Understanding Ratio Deco Air Tables
Before we look into the numbers we should review a few concepts: “ascent strategy” and “overall procedure”.
The strategy is the result of a combination of theory and practicality. The procedure accounts for Rock Bottom gas management for safety reason.
The overall procedure is consistence and compatible with all levels of training and depth. So divers applied the same set of skills and build upon them. You don’t need to unlearn skills!
And it’s easy to remember.
Normally, up to 40 meters we use Trimix 25/25 or EAN 32, but the reality is that in many places and with little budget, many scientific investigations are still limited to the use of air, so these tables were adapted for air.
As we already know, all dives are decompression dives and you do your decompression in the water and not in the boat. So we have two variables that we must manage that directly impacts the tissues on-gassing:
Depth- pressure and button time. Button time is count from the start up to the beginning of the ascent, for this consideration we can work with average depth.
The Minimum Decompression table is like follow:
- Gas: Air
- Set point: 30 meters/ 20 min NDL
- 3 meters shallower from the set point: add +5 min until 21 meters, add +10 min for shallower depths.
- 3 meters deeper from the set point: subtract – 5 min
- On a repetitive dive use:
- 60 min or more of Surface interval: same ascent rules
- Less than 60 min of surface interval: doble the shallow stops
|Depth||N.D.L||Ascent Profile- Time|
We work with average depth, but in case the diver is deeper than the average depth when he or she commenced the ascent, the maximum depth is use.
The first stop is prescribe at 50% of your average depth, you’ll spend 1 min there. Then you’ll make stops every 3 meters from there, spending 1 min in each stop including the travel time between stops. This is equal to the travel time from one stop to another plus the stop time.
Lets check an example:
Your average depth is 30 meters, so your first stop is at 15 meters. You’ll spend around 2 min to get there (1.7 min at 9 meter per second ascent velocity).
At 15 meters you have a min of time follow by 1 min. every 3 meters, which is a 5 min. total between 15 meters and 3 meters. For a grand total of 7 minutes of ascent time.
This procedure should be complied no matter if we did a 20 or 12 minutes of button time. It’s not affected by it. It’s always the same ascent procedure.
Gas Management for procedure base always on 2 divers:
Rock Bottom, Is the amount of gas that two divers needs so they can reach not only safe to the surface, but to complete the entire ascent profile.
So ask yourself: how much gas do I need to safely return and how long the ascent takes? If you are bellow that number you are no longer able to ascent properly.
How we calculate it:
- You should know your SAC rate
- Average depth
- Numbers of divers: 2
- Total ascent time
- Add 1 minute to resolve the emergency situation
A note on your breathing rate; when you are on an stress situation, your breathing rate goes up, so what ever your normal SAC rate you should add at least 5 liters per min. Same apply at resting, when you are doing the stops, you should subtract more or less 5 liter per minute as well.
For example, a diver has a SAC rate of 19 l. per minute, plus +/_ 5 I use 29 l. per minute. On rest the diver use 14 liters per min. (If you want to easy the calculations you can round your numbers to 15 l/min., 20 l/min. and 30 l/min.).
Continuing our example, if the diver is diving at 18 meters, the diver will add 1 min for the emergency, 1 min. of ascent up to 50% of the depth, plus 1 min every 3 meters for a total of 2 min. (6 meters and 3 meters stops). Which sum 5 min. total.
Now the calculations are as followed:
30 l. SAC. x 2 ATA. average depth ( around 10 meters) x 5 minutes total ascent= 300 l. for diver
2 divers X 300 l.= 600 l.
On the example, if the diver use a 11 l. cylinder, a 600 l correspond to a 54 bar of pressure of reserve gas.
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