Gas Management Questions?

[waterpirate]

Gas planning and management are cornerstones of safety, but as stated above you can not mix and match the concepts. They are unique to each other.

What is your average run time and at what depth with the 130's?
Do you know what your sac rate is? Your buddy's?

Eric

 

[TSandM]

My guess is that guyharrisonphoto has adopted an ascent strategy that involves a serious of stops. A lot of us do this, and regard them as more or less mandatory -- mandatory in the sense that they WILL be done if they can be done, and would only be blown off if there were an extremely exigent reason for doing so (managing unconscious diver, for example).

Rock bottom IS conservative in some ways, but not in others. The assumptions about gas consumption can be way too optimistic, depending on who your buddy is. I have dived with novice divers whose unstressed SAC rate is about what my rock bottom calculations assume is stressed gas consumption. I'm quite sure my rock bottom would not suffice to rescue those folks, if they were at all frightened. Luckily, the dives I do with such divers are generally done with just plain silly amounts of reserve gas (me, an HP130, and a 60 foot max depth) so I don't have to worry about it.

I think the single biggest lesson from going through the gas management stuff is that it takes a lot more gas to get two people to the surface in an emergency than you think. And not all out of gas situations are the fault of sloppy diving (although many are). Freeflows and clogged dip tubes can ruin the day of even the most responsible diver.

 

[Hatul]

I'm trying to think how the Rock Bottom can be adopted to recreational multi-level diving, as having a single minimum pressure at the max depth just would not do it. We do multilevel dives and hang around at different depths. So what you need is a table with you on a slate of Rock Bottom psi vs depth at say 10 foot increments, so at each depth you make sure you're within the safe gas supply for the depth. If you find your tank pressure below the Rock Bottom it's a sign you need ascend to the next level and so on. The Rock Bottom is in fact the table version of the Remaining Air Time information you get with an AI computer, albeit in a different format.

The other way is to use an AI computer and use a method such as I outlined in a post earlier.

One other point, in the calculation of Rock Bottom since deep stops are considered controversial and not necessary I would skip that. After all Rock Bottom is supposed to be just that.

Adam

 

[aquaregia]

Consider whether you'd be as comfortable not having an octo as not maintaining rock bottom in your tanks. Choosing to share gas with less than rock bottom is choosing to bend or drown yourself.

Okay, perhaps that's a little excessive; "doing all your stops" and NWGD's "donkey's ass" routine are a bit different. I think it's important to have a plan, even if it's extreme or dangerous. It's better than not having a clue.

 

[aquaregia]

I'm trying to think how the Rock Bottom can be adopted to recreational multi-level diving, as having a single minimum pressure at the max depth just would not do it. We do multilevel dives and hang around at different depths. So what you need is a table with you on a slate of Rock Bottom psi vs depth at say 10 foot increments, so at each depth you make sure you're within the safe gas supply for the depth. If you find your tank pressure below the Rock Bottom it's a sign you need ascend to the next level and so on. The Rock Bottom is in fact the table version of the Remaining Air Time information you get with an AI computer, albeit in a different format.

I think it's a bit different. I calculate my RB based on two dudes with bad SAC rates who want desperately to get home that night. Pretty sure an AI computer would think I have a reasonable SAC. I only used one for one dive, but I'm also not sure whether it was assuming one or two divers for the remaining bottom time.

A "table" of remaining time isn't all that difficult. My RB volumes are roughly 40cf, 25cf and 15cf for 100, 66 and 33ft respectively, which are roughly half of AL80, a third of an AL80 and a fifth of an AL80 each, or 1500psi, 1000psi or 600psi. Everything's sort of ballparky and handwavy and I'm comfortable making guesses in between those areas.

 

[Mr Carcharodon]

Guy,I want to go back to the "individual rock bottom". It sounds like you are calculating how much gas it would take to get you to the surface, but that is not rock bottom. The core idea is your redundant gas is on you buddies back. Rock bottom is enough gas to get the both of you safely to the surface. There is nothing individual about it. A

 

[DevonDiver]

Guy,I want to go back to the "individual rock bottom".....There is nothing individual about it.

Post 5:

1) Calculating 'individual' rock bottom is just a means to the same end. That end being a minimum gas reserve to bring both divers to the surface. It's useful if two divers have very disparate air consumption - which would mean that calculating the rock bottom based on one diver's SAC and doubling it would be inaccurate. In this instance, both divers calculate an 'individual' rock bottom and add them together.

 

[DaleC]

Hatul, in practice I find I really only need to know rock bottoms for a couple/few depths, not every ten feet, as these are my usual way points (your depths may vary). I usually dive contours and ascend from this depth to that one to that one and then up.

You can use individual rock bottom calculations if you dive with a pony - though you need to use the highest consumers values. If you reserve that volume in your back gas and have a pony containing that much as well, both divers will have access to enough gas (assuming one failure).

 

[guyharrisonphoto]

Thank you all for your responses! I get the point that deviating from the rock bottom concept makes the dive much more dangerous if there is equipment failure or other issues. What I really learned from these (very polite) answers is to follow rock bottom for the particular depth (I mis-spoke when I said "dive") and you are much safer. That is what I plan to do.

In answer to one question, I am not doing planned deco dives at this time. I plan to get an advanced nitrox/deco procedures course next spring. But I am studying up on the concepts involved including reading up on deco (got Powell's book) and finding out about this concept of gas management.

For now, I plan to do some rock bottom calculations for some of my favorite reef and wreck dives that are pretty close to traditional square profiles. I think that would be a good education for me. So far, for 90' dives on steel 100s, beginning the ascent at 1000 psi gets us to the top (one three minute safety stop) with about 600 so we use that as our general guideline (our air consumption is very similar). I want to see how that compares with a more formally planned approach.

I have a learned a lot over the last couple of days. Don't be surprised if you see me back asking some more questions.

Thanks!!!

Oh, as for surfacing with 500 (or 300 or 200) psi, I did not intend to mix and match, but I thought that you wanted to plan to have at least some gas when you arrive at the surface?

 

[Mr Carcharodon]

In doing your calculations keep it simple. If you just figure out what your RB is for say 60 and 100 feet for a given tank then you have just two numbers to remember. Scaling for other depths or tanks will generally put you within 100 psi which is close enough. Keeping it simple enough to use every time is more important than precision.