Gas Planning Example

[rx7diver]

All,

I computed the attached plan last night for practice and to show to a freshly minted diver. Then I thought it might be helpful to others.

Here's the skinny: Gas management plan for a 100 fsw air dive to the 25 min NDL limit. Diver is assumed to have a swimming RMV of 0.5 cuft/min, and will dive with "sufficient" reserves in case of emergency, descend at 60 fpm, stop briefly during descent at 10 fsw for a buddy/safety check, ascend at 30 fpm, and stop during ascent at 15 fsw for a 3 min safety stop. These calculations show that an Al 80 is insufficient, perhaps, for this dive, so assumes a PST HP 100 will be used. Dive will be turned either at 13:50 min:sec runtime or at 2,632 psig (whichever occurs first). Ascent will commence at 25:00 or 1,860 psig. Diver will surface at 31:20 with 1,640 psig remaining. Note: These calculations suggest that if the diver chooses instead to dive an Al 80 and carry his reserve gas in a pony, then the minimum size of this pony should be 30 cu ft.

Dive Safely (and apologies for my handwriting),

Ronald

 

[rongoodman]

I'm having problems reading your calculations, but most of the Rock Bottom examples I have read or worked out require about a 35 ft3 reserve for a 100' dive, which means you need to start your ascent at 1350 psig in a Al80, or 1225 in your HP100. Your numbers seem overly conservative, not that that's a bad thing, but I don't understand where they came from.

 

[rx7diver]

Rongoodman,

This is a simple example that assumes the things already listed, and, in addition, that the diver begins the dive with a full tank.

The first part of the calculations shows that the entire dive should consume about 54 cu ft of air, about 29 cu ft of which is consumed while the diver returns to the anchor line and subsequently ascends.

This suggests that the diver should carry 54 + 29 = 83 cu ft of air with him. Since this is more than the capacity of an Al 80 (77.4 cu ft @ 3,000 psig), this suggests using a larger tank. Here, I've chosen a PST HP 100 (102 cu ft @ 3,500 psig).

The rest of the calculation simply figures out how the gas usage ties to the SPG reading from the HP 100 as the dive progresses.

Okay. Now, I really don't know about "Rock Bottom." I think, though, the calculations for Rock Bottom differ from my simple calculations. For example, I think Rock Bottom assumes a pause at depth to get a tense situation under control. My approach doesn't include this kind of thing. Mine is a simple, straightforward approach that allows for the situation when a diver must return up the line he descended (say, because of current, or overhead boat traffic, etc.) while breathing off his reserve supply.

I would love to see this example worked out using the Rock Bottom approach so the two approaches can be compared/contrasted.

Ronald

P.S. When I get home tonight, I'll re-check my hand calculations. Could be I've made a computational mistake, though I don't believe so.

 

[gcbryan]

What table are you using that has a NDL for air of 25 minutes at 100 fsw?

 

[rongoodman]

Rock Bottom calculations address the amount of gas required to get two stressed divers to the surface(or the next breathable gas) including appropriate stops. Usual assumptions are 1 minute on the bottom to get things sorted out, and 1 cfm SAC rate for the stressed divers on the ascent, although you can use what you like. RB doesn't address how you use the remaining gas during the dive, which mainly depends on whether or not you have to return to the starting point to begin your ascent. Here's a more complete discussion.

 

[rx7diver]

What table are you using that has a NDL for air of 25 minutes at 100 fsw?

Gcbryan,

I memorized this (and the other rec NDL's a long time ago). This is on the submersible tables attached to my Scubapro BC. I think these are Nu-Way tables (or some such). I believe this 100 fsw-25 min NDL is consistent with the US Navy Standard Air tables and the NAUI tables (except the Navy tables use a 60 fpm ascent rate). Sorry, I committed these to memory a long time ago.

Hmmm. I'm getting so old, maybe I shouldn't trust my middle-aged memory anymore!

Ronald

 

[rx7diver]

Rock Bottom calculations address the amount of gas required to get two stressed divers to the surface(or the next breathable gas) including appropriate stops. Usual assumptions are 1 minute on the bottom to get things sorted out, and 1 cfm SAC rate for the stressed divers on the ascent, although you can use what you like. RB doesn't address how you use the remaining gas during the dive, which mainly depends on whether or not you have to return to the starting point to begin your ascent. Here's a more complete discussion.

Thanks for the reference link. Clearly, the Rock Bottom approach and the approach I've used in my example make different assumptions (and, perhaps, address slightly different problems), so the two solutions will differ.

Ronald

 

[Mr Carcharodon]

I think what you are saying is that you are going to turn the dive at half available gas. That allows you to ascend where you descended. In this case "available gas" is all the gas you have less reserve to get to the surface. I usually do rock bottom for reserve which assumes more stops than you did, and more time at depth to resolve problems. Even so I get a turn pressure of 2280 (assuming a 100 cu ft/3500 psig fill pressure tank). So I think your calculations are off.

 

[rx7diver]

Rongoodman,

...

P.S. When I get home tonight, I'll re-check my hand calculations. Could be I've made a computational mistake, though I don't believe so.

Rongoodman,

My old copy of Abyss 2.30.17 yielded approximately the same gas requirements as I calculated by hand. Abyss computed 55.8 cu ft, while I computed 54.2 cu ft by hand. The increase resulted because Abyss mandated a deco stop (even though its most liberal algorithm was selected)! See the attached pdf.

(Abyss used the simple Rule-of-Thirds to determine the reserve.)

Ronald

 

[gcbryan]

I think what you are saying is that you are going to turn the dive at half available gas. That allows you to ascend where you descended. In this case "available gas" is all the gas you have less reserve to get to the surface. I usually do rock bottom for reserve which assumes more stops than you did, and more time at depth to resolve problems. Even so I get a turn pressure of 2280 (assuming a 100 cu ft/3500 psig fill pressure tank). So I think your calculations are off.

I can't read the hand written figures very clearly but I'm wondering if the difference between your numbers and his is the following...

He is planning a dive for a specific time period and ascending...after all he is getting out of the water with approximately half a tank.

You are figuring turn iand Rock Bottom) based on available gas rather than a fixed NDL based dive...or do I need to read this thread a little more carefully

Also, can anyone else comment on the current NAUI NDL time for 100 fsw on air?