Basic gas budgeting

[dstrout]

OK, please bear with me - I'm trying to learn how gas budgeting works. For the most part, what I've gotten in training so far was "keep an eye on your computer, but you'll run out of gas on an AL80 before NDL anyway", and I'd like to do a bit better than that.

My two dream dives are the USS Blenny (70') and the John Morgan (110'). Assuming a 120 cu ft tank and a .6 SAC:

Blenny:
70' = 3.1 ATM = 1.86 cu ft / min
40 cu ft ( rule of thirds) / 1.86 cu ft per min = 21.5 mins out & 21.5 mins back = 43 mins total bottom time (including decent & ascent)
NDL for 70' = 45 mins
So for that dive, I'm very close to my NDL if I'm not careful

John Morgan:
110' = 4.3 ATM = 2.6 cu ft / min
40/ 2.6 = 15 out, 15 back = 30 bottom time
NDL at 110' is 15 mins total, so it would be really easy to blow past NDL

And, to run it backwards:
2.6 cu ft / min * 7.5 mins per "third of a tank" = 19.5 cu ft per "third" = 58.5 cu ft total, so an AL80 would be a fine tank for that dive.

Am I understanding the basic idea correctly?

thanks,
dave.

 

[Blackwood]

Gas and Bottom Time are related, but should be planned independently. Which ever one you 'have' the least of defines the dive parameters.

The most important gas quantity to me is how much I need to get back (be it 'out' 'up' or both).

 

[muddiver]

Here is a link to more information Rock Bottom and Gas Management for Recreational Divers

You don't appear to be taking into account having to leave bottom while supplying breathing gas for you and your buddy if he were to be out.

 

[LeadTurn_SD]

Here is another good one, by Rick Murchison, called "Backing Into The Gas Plan":

http://www.scubaboard.com/forums/basic-scuba-discussions/165474-backing-into-gas-plan.html

He works the problem in reverse, by having you first decide how much gas you want to climb back on the boat with, then works out the dive plan "backwards" from there.

Best wishes.

 

[Blackwood]

You don't appear to be taking into account having to leave bottom while supplying breathing gas for you and your buddy if he were to be out.

He's not doing it rigorously, but in many cases only using 1/3 of your gas on the bottom will leave sufficient reserves for two.

 

[Diver Tim]

He's not doing it rigorously, but in many cases only using 1/3 of your gas on the bottom will leave sufficient reserves for two.

The problem is the word "many" implies "not all..." I don't want to be on a dive where I used the rule of thirds for planning, but find out when I have a problem at depth that it was one of the dives where it didn't leave sufficient reserves for two. :shocked2:

 

[Codiak]

The problem is the word "many" implies "not all..." I don't want to be on a dive where I used the rule of thirds for planning, but find out when I have a problem at depth that it was one of the dives where it didn't leave sufficient reserves for two. :shocked2:

Then you'd better plan based on the the worst case SAC between you and your buddy.

Gas planning is going to vary based on the type of dive your doing.

 

[TSandM]

dstrout, yes, you have the basic idea down.

You have also learned that large tanks make it possible to end up with a decompression obligation, if you aren't careful.

Your last observation, that an Al80 would be sufficient for your 110 foot dive, is marginally accurate IF you can surface wherever you want off that wreck. You see, the gas reserve needed to get you and a buddy to the surface, making all appropriate stops, is almost 40 cu ft from that depth, which is half of your tank. Your out and back uses 38 cubic feet of gas, so if your buddy goes out of gas when you reach the upline, you will have one very empty tank when you get the two of you to the surface. I personally don't like to walk the line that closely.

If you HAVE to get back to the upline, and your buddy runs out of gas at the bow of the boat and you have to share gas to return to your ascent point, you do not have enough gas to do that.

There are many dives where an Al80 is just fine if everything goes well, but becomes a smaller and smaller tank as things go more and more awry.

 

[dstrout]

Great, thank you one and all. It'll be a while before I get to do either of those, but I enjoy learning in the meantime. I appreciate the help.