Dive Comp for OW+AOW

[PerroneFord]

I see your dilemma. Based on this scenario with the numbers you have given, this would be a concern. However I am not convinced that your consumption rate is typical and it would seem from your statement that you may feel that way as well. I commend you on your ability to have a realistic look at you own limitations as a diver and adjust your behavior accordingly. Out of curiosity, what conclusions have you drawn from this, regarding the point when you personally need to turn the dive?

Matthew

Well, Iam no longer constrained by the bounds of NDL, so for me, all my dives are gas limited. I usually have a minimum of 160cuft of gas available, and often more than 300.

But in the rare circumstances I have on a single tank, I am at the ascent line (if we are using one) with half my gas + 200 psi for dives deeper than 60ft. At half my gas I am at half my max depth. And I am generally on my stop, if I am doing one, with enough gas to get myself, and my teammate out of the water with at least 300psi to spare. My dive planner is generally set for my worst breathing. I generally won't breathe as much as I have on my plan, but I'd rather err on the side of having more gas in the tanks, than to be caught short wishing I had some.

As for commending me for having a realistic view of my shortcomings, that is a requirement for diving in the environments I choose to dive in. You can't BS the cave. It WILL kill you if you allow it to. Margin for error is quite small.

 

[Ann Marie]

Question--why are you showing these students how to get their computers into nitrox mode?

It would seem to me that doing it for them only increases their dependence on you as an instructor and authority figure, instead of getting them to think for themselves. Personally, if someone had come to me asking how to get their computer into nitrox mode, I'd ask if they had read the manual for it yet. Chances are, they probably didn't. If they had, I'd get them to do everything they possibly could to find the answer for themselves (give them hints of where to look in the manual, if they don't have the manual I'd suggest they download it online, etc) instead of spoon-feeding them the answer.

Not jumping on your case...just something to think about.

I think you should re-read my post. I didn't say that I was setting their computers for them. I said that we were going over how to set the computers. I wanted to see them set it up themselves. I feel that is something that an instructor should do just as I had them analyze 2 different EAN cylinders.

If a student no longer had their computer manual and I did show them how to put their computer into EAN mode how is that coddling them? If I had to explain how to set it, I would still expect them to demonstrate their ability to. I didn't toss them a manual for an analox and tell them to figure out how to analyze a cylinder, I explained, demonstrated and then had them demonstrate.

By your way of thinking, I should expect a student to be able to clear a mask after watching a DVD or reading a manual. No need to even get in the pool....

The same goes for the story of the student who didn't realize his computer wasn't nitrox compatible--did he not read the product description when he bought it?! If he's that inattentive when shelling out money, how attentive will he be on the dive? Same as above--coddling students doesn't do them any good; it only increases their dependence instead of turning them into thinking divers. This guy learned the hard way, but he probably won't make that same mistake again.

This is the kind of thinking that makes divers feel like they've been taken advantage of when they buy equipment that may have been suggested only to find out it wasn't right for them.

 

[mjatkins]

But in the rare circumstances I have on a single tank, I am at the ascent line (if we are using one) with half my gas + 200 psi for dives deeper than 60ft. At half my gas I am at half my max depth.

Thanks for your openness. Just so I understand and don't jump to any undue conclusions, what size tank are you referring to in this example. If I am getting your math correct, this has you draining your tank at a rate of 200 psi per minute or faster in the 60 to 30 ft. depth range. I am basing this on your previous example specifying a 30 ft. per minute ascent rate.



Thanks,
Matthew

 

[Ann Marie]

I stand by my statement that if someone was pressing him to invest in a computer he should run the other way.

Pete,

Actually you said that if a instructor or LDS suggested buying a computer, he should run the other way. I was just offering a different point of view. I didn't see your statement as a personal attack, I actually saw it as a rather broad statement that I feel is misleading to a rookie. He is asking for advice and perhaps should have been given pro and cons on purchasing a computer now vs. later or at all instead of a broad statement that could make him feel insecure about the choice he made in a instructor or dive shop.

I hope that you didn't feel any anamosity in my posts, because there is none...really!

 

[PerroneFord]

I am not saying that am draining my tank that fast. I am simply specifying my TARGET. My *intent* is to be at the upline with half +200. My INTENT is to be at half depth with half my gas. I may get to half depth with half +175. And that's cool. But if I am not there with half my gas, I'm late, and I need to get moving.

 

[SparticleBrane]

I believe there's a difference between having someone find out how to do it themselves, and doing it for them. I read your post as you doing it for them, my mistake. Still, I'd much rather have the students find out how to do it for themselves instead of showing them. If they do it themselves they're much less likely to forget it when you're not there.
As an example, I used to do a lot of freelance computer work. I found that it's much more educational for me to get the user to figure out how to do xyz (and if that didn't work, show the person how to do it), than to just do it for them.

Personally I feel that a nitrox course should not cover how to use their computers for nitrox. Any monkey can read the manual and figure that out...and if not, should they be diving if they don't have the common sense to read a manual? And on that note, why isn't nitrox taught as part of the open water curriculum now?! *steps off soapbox*

This is the kind of thinking that makes divers feel like they've been taken advantage of when they buy equipment that may have been suggested only to find out it wasn't right for them.

If they know that they need a computer for nitrox, yet they were convinced by the salesman that they didn't want/need it...what does that say about them? Yes, they were taken advantage of, only because they let it happen, even though they knew that they needed the nitrox computer. That's their own stupid fault...

Just my own personal feelings...

 

[Ann Marie]

Where am I saying that I am doing it for them? This is where I am coming from; guiding them through something and doing it for them are two different things and I never said that I did it for them. Or even saying, let me see you set your computer.

Two of my students in that class own Cobras. Are you aware that if you dive in EAN mode then you have to wait 24 hours to change it to air? Should I not tell these students to keep their computers in EAN mode and when they dive air, just set it to 21%? Shouldn't taking a class help to cut down on the error-to-learning curve?

You are also making a lot of assumptions about the man and the computer. All of which are incorrect. This is not a new computer. He has been diving for a few years. Perhaps at the time, he didn't think he would be diving nitrox and hadn't thought anymore about it. Perhaps not, but who are you to judge him?:huh:

 

[SparticleBrane]

Where am I saying that I am doing it for them? This is where I am coming from; guiding them through something and doing it for them are two different things and I never said that I did it for them. Or even saying, let me see you set your computer.

Two of my students in that class own Cobras. Are you aware that if you dive in EAN mode then you have to wait 24 hours to change it to air? Should I not tell these students to keep their computers in EAN mode and when they dive air, just set it to 21%? Shouldn't taking a class help to cut down on the error-to-learning curve?

You are also making a lot of assumptions about the man and the computer. All of which are incorrect. This is not a new computer. He has been diving for a few years. Perhaps at the time, he didn't think he would be diving nitrox and hadn't thought anymore about it. Perhaps not, but who are you to judge him?:huh:

I don't dive a computer (keep it in gauge mode), but--
If I (a) dove a computer, and (b) dove a Cobra, you'd bet I would have read the manual front to back. If what you said isn't in the manual, then that manual sucks...

As to the other guy and his computer--he had it for several years and didn't know it couldn't do nitrox...?!

 

[mjatkins]

My *intent* is to be at the upline with half +200. My INTENT is to be at half depth with half my gas. I may get to half depth with half +175. And that's cool. But if I am not there with half my gas, I'm late, and I need to get moving.

Thanks again for this discussion, I came to this forum hoping to learn. Can you answer a couple of questions for me. I am not a cave diver, so perhaps that's where some of my confussion stems from.

This example we are discussing is with only a single tank. Do you personally do cave dives below 60 ft. using a single tank, or are these dives non overhead environment situations? I ask this because your logic seams to stem from your cave training based on your previous posts. If these are not overhead dives, and I have overestimated the speed with which you are going through your air, why would you apply the same constraints to such a different situation?

If this is a cave/overhead situation we are discussing, am I wrong in my belief that the standard thought is the rule of thirds?

Either way, to use your original example;

Dec to 80ft (1)
Level 80ft 23:24 (25)
Asc to 20ft (27)
Stop at 20ft 2:00 (29)
Surface (29)

68.2 cu ft TOTAL

At minute 25, if my low pressure hose ruptured, and I had to shut off my tank, I would have burned about 60 cuft of air or roughly 3/4 of my tank. Converted to PSI, that would be 2250 gone and 750 left.

So 2250 psi divided by 25 min = 90 psi per minute. 3000 psi divided by 2 + 200 = 1700 psi. So in this example you expect to use 1300 psi for descent and at depth. 1300 divided by 90 = 14.44 minutes (1 for descent, and a 13.44 minute dive). Or for a penetration we would say 6min.in, and 6 minutes out.

Is this a fair representation of what your "intention" would be?

Matthew

 

[PerroneFord]

Hey, now we are getting somewhere!

Yes, this discussion only deals with a single tank. I do not, nor would I ever enter a cave with a single tank unless it is a dire emergency. So yes, we are talknig about non-overhead environments real or virtual.

My logic stems from a desire to not put myself into a scenario where I "MUST* surface quickly to survive a dive. I had that mindset long before I went into a cave. Why do I apply the same constraints? Because I am a team diver. And the safety of my team is of paramount importance to me. Therefore, I won't do anything that might put myself or my team in harms way during a dive if I can possibly help it. This includes managing gas.

In OW recreational training, divers are taught that the surface is always an option. Thus the idea of CESA, and blowing off safety stops. In real or virtual overhead diving, the VERY first thing you must come to terms with is that the surface is NOT an option. In a cave, the surface simply doesn't exist. In OW deco diving, the surface exists, and if you're lucky you'll simply get bent going there. At worst, you'll die on the way. These are controllable factors. And you begin to change your mindset, and the way you approach your diving. The mantra now, is that you solve your problems underwater. You must because the surface does not exist. Your Reg freeflows, shut it down and fix it. Your BC bursts, fix the problem underwater then exit. Your finstrap breaks, try to fix it and swim out. Once you stop relying on the surface you become a LOT more efficient under the water, and you learn to plan your dives a LOT better.

Your math looks good to me. An 80ft dive on a single 80, would be about a 15 minute dive for me *IF* I was diving with one other diver. So, from this example, if we have two divers who are heavy breathers (and that happens in OW diving) at 80ft, they could be breathing 180psi per minute. Actually this is a low estimate as we are taught to use a 1.0 SAC rate instead of the .75 I believe my dive planner is set for.

In any event, when you look at what it would ACTUALLY take to get two divers sharing an AL80, to the surface from what is considered a mundane dive, it becomes clear that the AL80 is hugely inadequate for what we ask it to do. Now consider the problem that:

1. We have backed off the NDL quite a bit (5 minutes).

2. We are doing a fairly short dive. When you run the number for doing a dive at 60ft, staying out of the gray, it's a 47 minute dive. My planner says I need 100cuft of gas to reach NDL. On a square profle, I would be gas limited instead of NDL limited, so my computer would give me no guidance that it is time to ascend to keep myself and my teammate safe.

Thanks again for this discussion, I came to this forum hoping to learn. Can you answer a couple of questions for me. I am not a cave diver, so perhaps that's where some of my confussion stems from.

This example we are discussing is with only a single tank. Do you personally do cave dives below 60 ft. using a single tank, or are these dives non overhead environment situations? I ask this because your logic seams to stem from your cave training based on your previous posts. If these are not overhead dives, and I have overestimated the speed with which you are going through your air, why would you apply the same constraints to such a different situation?

If this is a cave/overhead situation we are discussing, am I wrong in my belief that the standard thought is the rule of thirds?

Either way, to use your original example;




So 2250 psi divided by 25 min = 90 psi per minute. 3000 psi divided by 2 + 200 = 1700 psi. So in this example you expect to use 1300 psi for descent and at depth. 1300 divided by 90 = 14.44 minutes (1 for descent, and a 13.44 minute dive). Or for a penetration we would say 6min.in, and 6 minutes out.

Is this a fair representation of what your "intention" would be?

Matthew