Using an SCR range for gas planning

[coldsmoke]

I'm not sure if Richard and I are saying the same thing in two different ways or perhaps we have a different understanding of the question. Just to clarify my answer:

Only plan for one failure.

Back gas only needs to cover the ascent to the first gas switch for two divers or the ascent for one diver to the second gas switch in the event that the deepest deco bottle is unavailable.

Said a different way, in a dive where there is bottom gas, a 70' bottle and a 20' bottle - my back gas should be enough to get two divers to the 70' stop, or enough to get me to my 20' stop in the event of a lost 70' bottle (which ever is greater), and my 70' bottle should be enough to get me to the surface in the event of a lost 20' bottle.

As I understand it - Minimum Gas is the amount of gas needed to complete your next ascent phase on your previous phase gas supply. Rock Bottom is the gas required to get two divers to the next gas supply.

Please correct any misunderstandings I may have.

Hunter

 

[Kevrumbo]

So
Each gas covers the loss of the next one
or
backgas should cover any 1 lost deco gas
?

And how is the amount of each 1 deco gas calculated? I mean is there any additional amount to calculated mandatory one?

Here you go vioch using that 54m example; see if this works per everyone's recommendations:

Dpth(m)/Time (min)
54 20
51 0.3
48 0.3
45 0.3
42 0.3
39 0.5 [Deep Stop 75%]
36 0.5
33 1
30 1
27 2 [Deep Stop 50%]
24 2
21 4 [Eanx 50 switch]
18 4
15 2
12 2
9 3
6 10 [Oxygen switch]
3 3
0 3

Tot Time: 59 min
Tank Rating: 22 L/bar
MGR/CGR Litres: 2288 L
Fill Pressure: 200 bar
Gas available: 2112 L
BGTotalsuppl: 4400 L
Eanx 50% Needed: 619 L
Oxygen Needed: 366 L
BG For Lost Deco: 3669 L

 

[Kevrumbo]

Here's CGR (contingency gas reserve) exercise using the above example for a deco dive to 54 meters on twin 11 L tanks:

Max SCR: 28.3 L/min*ATA

"Emergency/Mad-Minute" at 54m: consumes 181 litres [multiply Max SCR of 28.3 by 6.4 ATA equals 181 litres]

Litres needed ascending from 54m to 21m (Nitrox 50% deco stop) including deep stops: 963 L

Total CGR: 181 litres plus 963 liters multiplied by two divers gas sharing: 2288 L
Total CGR in bar pressure: 2288 litres divide by 22 litres per bar equals 104 bar

Total Backgas: 200 bar multiplied by 22 litres per bar equals 4400 litres. [a full twinset of 11 litre tanks]
Usable Backgas less MGR: 4400 litres minus 2288 litres equals 2112 litres.

So you have 2112 litres of gas for the mission requirements of your dive (20 minutes at 54m at SCR of 15 lpm consumes 1920 litres so you have enough gas to use without encroaching upon your CGR). . .

Lost Nitrox 50 Bottle: you need 619 L multiplied by two to cover the 21m -->9m range on backgas, which equals 1238 L; together with the backgas consumed from the bottom at 54m ascending to 21m --including the deep stops-- totaling 2430 L, you need a grand total of 3669 litres to cover this scenario. You start the dive with a full twinset of 4400 L, so you are covered (just don't have an emergency contingency where you have to gas share and lose your Nitrox 50 bottle!). . .