Let me know if I have this gas management stuff right

[SNorman]

To make sure I understand something that's mathematically based I usually write some computer code with my understanding and see if it spits out the right answers. In this case I'm using Excell. I made a table to calculate air consumption based on time at depth, and another table to calculate rock bottom PSI based on reading this: Rock Bottom and Gas Management for Recreational Divers.

Here's a screen shot of a sample dive of a one minute descent to 40', 30 minutes at depth, one minute ascent to 15', 5 minute safety stop, ascent to surface. Does this look right?

Playing around with the numbers, there were some depths that surprised me as to the amount of time available (less than I thought). This is really fun to plug numbers into.

 

[Jimmer]

Excellent setup. I have been procrastinating about making my own spreadsheet for rock bottom. I would suggest maybe setting it up so that you can punch in all the details of the planned dive, and have the spreadsheet tell you the required volume of gas, rather than always using an 80. You will find it wont take much depth before an 80 wont get you enough reserve gas for the time planned, or enough time for the reserve gas available.

Cheers,
Jim

 

[SNorman]

Yeah, with all the shaded cells you can punch in your own numbers. Tank size, RMV, etc. and it will use those for the calculations. You are correct, it *was* quickly evident to me that an 80CF tank is limiting!

 

[pjhansman]

I'm pretty new at this too, but the time seems a little short to me. I can do quite a bit longer at that average depth with an AL80....and my air consumption still needs improving.

Just my $0.02 worth.

 

[MXGratefulDiver]

Kudos for putting some thought into it ... that's the biggest hurdle right there ...

... Bob (Grateful Diver)

 

[SNorman]

I'm pretty new at this too, but the time seems a little short to me. I can do quite a bit longer at that average depth with an AL80....and my air consumption still needs improving.

Just my $0.02 worth.

All depends on what your RMV is. If it's lower, you get more time, also the numbers I plugged in show 700+ psi after the dive is done, you could go lower. Also, if you're spending more time shallower than 40' (shore dive, follow the bottom out to 40' and back) you'll use less air than a more square dive. Another 200psi is used for a 5 min safety stop which most people probably won't do after a dive to 40', so this is a very conservative plan.

The rock bottom number is enough air to reach the surface while sharing air with a distressed diver so it seems high at first.

If anybody wants to play with the file here it is: http://images.ihost.biz/GasManagement.xlsx

 

[Mr Carcharodon]

Hmmm...I think your rock bottom is a bit high, but it all depends on what assumptions you make. I assume 2x normal DAC rate for 2 minutes at max depth followed by a min deco type of ascent. My assumed baseline SAC is 0.6 cu ft/min so scaling to what you assumed you are still 20% high. The rest of it looks fine.

Once you have the spreadsheet worked out try to simplfy the process so you can do this in the water. For example 40 fsw is ~2.3 ATA. So your DAC will be ~1.6 cu ft/min, so 30 x 1.6~48 cu ft. Then you need a tank factor to convert volume to pressure. In this case 100 psig~2.5 cu ft. So you should consume ~2000 psi in 30 minutes. With a bit of practice you should be able to estimate to a few hundred psi, which is about as well as you can read the gauge anyway.

 

[SNorman]

I used a 2 CF/min rate simply because that's what the calculations at the link I posted above used, using your numbers it comes out to 540 psi (close to your calcs).

 

[Mr Carcharodon]

Oh right I have seen 1 cu ft min * 2 for two divers used, mostly I think because it is an easy number to multiply or divide by. 0.75 cu ft/min is also a common value. I used 0.6 cu ft/min since that is about typical for myself and my usual buddies. But if we swim a lot we can be closer to 0.7. Anyway 1 cu ft/min is easy and you can just take 25 or 30% off and then you can do it in your head. I think the spreadsheet looks good.

 

[TSandM]

Even in my technical classes, we used 2 cfm as a stressed rate for two divers. One hopes that experienced people, once the air-share is successfully established, will settle down toward a more normal consumption, but I think for brand new divers, the 2 cfm rate may not even be conservative enough in some cases. (Shoot, I've dived with new divers whose UNSTRESSED sac rate was more than 1 cfm!)