catherine96821:
no kidding it is pretty cool. AND I can do the math at depth! They have been holding out on us folks. It took me how many years to stumble on this gem?
The 120 rule isn't a DIR thing. It's been around a lot longer than DIR.
Also, the GUE table changes things up a bit when you get to 60'. Here are the Minimum Decompression Tables for Air taught in my GUE classes.
130' - 5 min
120' - 10 min
110' - 15 min
100' - 20 min
90' - 25 min
80' - 30 min
70' - 35 min
60' - 50 min
50' - 60 min
40' - 170 min
One of the things that doesn't seem to have been discussed is why we choose the gasses we breath, and why we set the limits for those gasses. This comes into play with the question about Cold, and relates to the prevention of Narcosis, DCS, and CNS Toxicity.
The first rule is to limit your level of Narcosis to an Equivalent Narcotic Depth of 100'. By adding Helium to the gas mix, we can lower the END to within acceptable limits. For the math geeks out there, here are two equations to help with the process.
END = (1 - FHe) * ATA
MND = 4.0 / (1 - FHe)
FHe = Fraction of Helium, or the percentage of He in the gas
MND = Max Narcotic Depth (i.e. 100' = 4 ATA, or keep it below 100')
Second, we reduce the ppO2 of the gas we breath to stay within these limits.
ppO2
Working
1.4 Max
1.2 Average
Resting / Deco
1.6 Max
1.4 Average
In a basic Nitrox course, you learn that a ppO2 of 1.6 is max, and a ppO2 of 1.4 is recommended. These are basic limits that have changed over the years, and are designed to create a buffer between safe diving and Oxygen Toxicity. The reason behind this is that exertion, stress, cold, and a few other things increase your susceptibility to CNS toxicity. Thus giving yourself a bit more room on the ppO2 of the gas you're breathing will make the dive more conservative.
Here are the equations for the Oxygen calcs.
MOD = ppO2 / FO2
EAD = (1 - FO2) * ATA / .79
Some numbers on a few gas mixes.
21/35
MND = 170' END @ 160' = 90'
MOD = 1.4 @ 190' 1.2 @ 160'
25/25 - Not a GUE gas standard, but it fits their limits
MND = 144' END @ 130' = 90'
MOD = 1.4 @ 153' 1.2 @ 130'
EAD = 121' ~ 10' or 10%
30/30
MND = 157' END @ 100' = 60'
MOD = 1.4 @ 122' 1.2 @ 100'
EAD = 100' ~ 20' or 20%
Feel free to run the numbers on the other gas standards. The numbers will show what depths each of the gasses work at within the limits above. They will also show how flexible the gasses are in the event the dive plan changes.
18/45
15/55
10/70
That's it. That's all the hubub. The limits were set to keep things conservative in the event something goes wrong. If you're in a cave (wreck, deep dive, recreational dive, cold dive, etc) and something bad happens, your stress levels go up, and your breathing increases. That can change how the gasses we're breathing impact us, and setting limits for the gasses we breath a head of time while sticking to a plan, will make things safer for us in the unlikely event something bad happens.
Like everything, you decide whether these limits work for you or not.
I'll leave you with this, is it easier to come up with a standard gas that works with a standard decompression profile for specific depths, or would you prefer running the complicated math every time you get ready for a dive? Also, is the standard any different from running the numbers every time?
Food for thought.
~ Jason
