"Backing into" the gas plan

[Rick Murchison]

Note: For this example I am going to make some assumptions "typical" for recreational divers. The principles apply for any size tank and any SPG, but for the purposes of this example my diver is using a single AL 80/11L tank with a service pressure of 3000 psi/200 Bar.
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Before even starting a gas plan, you need to have some idea what your gas consumption rate is - your "SAC Rate" or "SCR" or "RMV." If you don't have any idea what your consumption rate is, see this thread before you read any further.
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I call this method “backing into” the gas plan because I’m going to start at the end of the dive and work towards the beginning…
The first thing to consider is how much gas you’d like to have back on the boat. In the US we usually say “500 psi/35 bar reserve,” so for now, let’s use that. (We’ll examine the efficacy of that number later)
Next, let’s see how much gas we need for an ascent and safety stop. For this plan we’ll use a SAC rate of 30 psi/2.0 bar per minute, and a dive site depth of 100 FSW/30MSW.
Let’s take a look at the safety stop as an event all by itself – our guidance is “3 to 5 minutes at 15 feet/4.5M.” I like to fudge on the safe (conservative) side when planning, especially if it makes the math easier, so I’ll plan on the five minute stop, and round the depth down to 1.5 ATA (16½ ft/5M). So for the safety stop I need to allow for 30x1.5x5=225 psi or 2x1.5x5=15 bar.
To ascend from 100’/30m at the recommended rate of no more than 30 feet /9 meters per minute, we’d need 200 seconds, or about 3 minutes 20 seconds. Our average depth for the ascent is 50 feet/15 meters, which is 2.5 ATA, so for the ascent we can expect to use another 30x2.5x3.33 = 250 psi or 2x2.5x3.33=16.7 bar.
Let’s also add that “one minute half stop” that seems like a good thing to do – so another minute at 50 feet/15 meters is another 75 psi/5 bar.
Add all those up, and we see that for the ascent, the half stop and a five minute safety stop we’ll use about 550 psi, or 36.7 bar, and that to arrive on the boat we’ll need to start our ascent with 1050 psi or 71.7 bar. I don’t know about y’all, but my eyes don’t read the SPG that well, so I’m going to plan on commencing my ascent with 1100 psi/ 75 bar.
This also tells me that if my buddy were to have a gas delivery system failure at the end of the dive, (and assuming we have similar SAC rates), that 500 psi/35 bar reserve would not be enough to get us both to the surface unless we cut out either the 1 minute half stop or part of the safety stop – and that more likely, since we’d be excited, it’d be a good idea to skip both. But… if we do make a direct ascent to the surface we should have plenty of gas to keep it to the recommended 30fpm/9mpm.
So, that 500 psi/35 bar reserve looks like a pretty good number to use, and we’ll stick with it for this dive. (It’s also apparent that for anything deeper, we’d best be tacking on a little more reserve…)
Now that we know how much gas we need to begin the ascent, let’s see what kind of dive we can expect to make. Subtracting our 1100 psi/75 bar from our starting gas of 3000 psi/200 bar, we have 1900 psi/125 bar for bottom time. At our planned depth the actual pressure is 4ATA, so we’ll use about 120 psi/8 bar per minute, and we can therefore expect our gas to last a bit over 15 minutes, and a 15 minute max bottom time is what we can plan for based on our gas supply (NDL allowing).
Piece of cake
Rick

 

[ReefHound]

This also tells me that if my buddy were to have a gas delivery system failure at the end of the dive, (and assuming we have similar SAC rates), that 500 psi/35 bar reserve would not be enough to get us both to the surface unless we cut out either the 1 minute half stop or part of the safety stop – and that more likely, since we’d be excited, it’d be a good idea to skip both.

I agree, if me or my buddy have a total loss of gas supply I'm skipping the deep stop to get nearer to the surface because a failure to the remaining gas supply means you both are going CESA. I would do the safety stop as gas permits though.

Your BT planning has to include your descent time, if you're breathing on the way down.

 

[Rick Murchison]

Your BT planning has to include your descent time, if you're breathing on the way down.

"Bottom time" does include descent time (standard definition).
Rick

 

[ReefHound]

"Bottom time" does include descent time (standard definition).
Rick

Yes it does, but it's amazing how often I hear people plan as if it doesn't.

 

[Cacia]

Rick, I like it that way.

 

[1_T_Submariner]

Rick, Nice write up....thanks

 

[Bill51]

"Bottom time" does include descent time (standard definition).
Rick

While bottom time does include the descent, do you figure the reduced air consumption during the descent as extra margin for error or does the energy and excitement of the beginning of the dive offset the reduced consumption at the shallower depths during the descent?

 

[Rick Murchison]

While bottom time does include the descent, do you figure the reduced air consumption during the descent as extra margin for error or does the energy and excitement of the beginning of the dive offset the reduced consumption at the shallower depths during the descent?

Yes!
Rick

 

[WesTexDiver]

Very nicely done, that looks like teachin material there, but a 30 SAC = Holy Gas Hog Batman!

 

[jpsexton]

Thanks Rick for posting these. Very nice write up! I just wished you'd done it a month or so ago when I was doing research for my AOW.

I created a Gas Planning chart during my class (actually my instructor jbd drew it out on a peice of paper and I just recreated it in the computer) that includes allot of this stuff if anyone is interested.