Genesis once bubbled...
To get any kind of accuracy on average depth you need a computer (even if its a small one) on your wrist. No way are you going to be able to keep track of that accurately without one.
If you work off max depth you're back to the PADI tables, basically, in terms of runtime.
Not if you use an "average running depth" instead of "max depth." But it depends on what you believe. DIR teaches an average running depth, which is a little complex to go into here.
Of course, all of this depends on what you believe as far as the bubble models and theories go, and where your body begins to offgas rather than continue to load.
Again, I'm not saying that this is the only way... I'm just saying that it's no less or more proven than the computers. And yes, it's quite easy to keep a "running average depth" in your head.
Watch... You're doing an 80' dive on a wreck, right? How long can you stay there? Rule of 120 says 40 minutes. First point... If you're running with a single AL80, then you're running out of air before you have to worry about deco anyway. In that case, you'd use the "turn pressure" to decide when to return to the boat. What pressure is "turn pressure?" Well... In class, another simple and easy-to-remember formula is taught... Turn pressure could be 750 psi, and it could be 1200 psi... Depends on your own circumstance.
Okay, so let's say you're diving doubles, and gas volume's not an issue. Okay, then... 40 minutes at 80' keeps you out of deco. What happens when you've averaged more like 75' for the duration of the dive? Well... Okay, then... Stay for 45 minutes, not 40. Simple.
At 40 minutes, begin your ascent. Ascent should be horizontal, facing your buddy (in case you two have any problems, such as OOA or whatever), and at 30 fpm. What's that translate to? Well, a little less than three minutes of ascention time, of course. Why not also do a deco stop at 20 feet? Sure. Spend a minute or two there. No biggie. Then do the same at ten feet. Great dive, you'll have time to do a couple of S-drills for a minute or two at your stops, and you're way within safety limits everywhere. See how easy? Why need a computer? Try putting that dive into your computer and tell me what you get. Do you think that would be a safe profile?
Okay, dive two... I'm not even looking at a table. I wait just over an hour at the surface (which is about how long it's going to take for me to get a little water and some corn chips and change out my tanks, not to mention chat about the last dive and make plans for the next dive), and then head back in. I think that I'd be at my max comfortable depth of 80' (assuming that I'm on the same wreck, but I could also do any other no-deco depth) for 30 minutes or less. This profile might be over the PADI table a bit... But not by much. And certainly not by more than the PADI tables are padded for the fat, smoking desk jockey with 3 dives under his belt this year. Again, I'm assuming a horizontal, 30 fpm ascent, with a couple of minutes spent at both 20' and 10'.
A little different profile, no? And still safe, isn't it? The whole thing was done in my head... No tables here.
Now, do I trust that over a computer? Frankly, yes... Although I still consider the computer a luxury. What happens when my "head" profile and my computer disagree? Well... Guess which one I'm going to believe?
Which leads to the question... Why even use a computer?
Well, I personally think they're really cool... And a very nice "backup brain" in case you make a mistake. I like them, despite what DIR teaches. Nonetheless, I can still understand the DIR philosophy.
I don't consider being crippled on my runtime "working". And if I have to take a computer with me to figure average depths, I'm back to the beginning - I have a computer on my wrist, which can fail.
Yeah, but it's one thing to have a computer failure that says that you had an average depth of 77.6', when you know in your head that it was somewhere between 70' and 80'... There's much less room for error there than there is if, say, you were doing a deco dive completely dependent on the computer.
Again, I'm not saying that computers are worthless... I'm simply trying to explain the philosophy. I see both sides.
Second, on a deco dive, you absolutely do need some form of backup. Whether its your buddy, hand-cut tables, or (preferrably) both. But both are "oh the water's brown!" things.
Agreed.
I think that the computer makes for a nice backup.
However you cut it, the point is to have redundancy... And the most redundant source of info that I can think of is the human brain. Personally, I like the idea of having a computer too as a redundant source, but that's probably because I haven't learned to trust myself yet with this stuff. I will probably develop that the more I dive.
See, rely on the brain, and your computer, the redundant source, is hardly ever used. Rely on your computer, and your brain, the redundant source, is hardly ever used. When having to switch to the redundant source, would you rather have your computer, hardly ever used to figure a profile, used, or would you rather have your brain, hardly ever used to figure a profile used?
My point: Use the brain. Computers, IMHO, are nice backups, and something that I'd like to have while I get really comfortable trusting my knowlege. If I use the computer only, then one day I'm going to be reliant on a brain which I never taught to do profiles properly. Scary.
As for no He multigas computers, oh yes there are. The Nitek He and the VR3 are two of them that I'm aware of, and I believe Abyss now has the Explorer He version, which is the first FULL (not folded) RGBM unit available on the market.
Agreed. Have you dived them? I think that if you had, you'd consider them "not" fully functional He computers. And you'd probably want to do your own math.
Yes, its conservative as it comes out of the box. But not OVERLY so. I've compared it against others, and particularly on multiple exposures, its not all that far off from even the "more liberal" computers out there. This is PARTICULARLY true if you drive it into deco, or if you're doing a lot of repetitive diving. In fact, my dive buddy, who has a different make, is often ordered out of the water before I am on repetitive exposures.
I haven't found that to be true at all. I've found that Suunto's "Suunto RGBM" is easily the most conservative of them all... To the point of being pretty rediculous. I have found little evidence to support that computers come anywhere close to estimating the same numbers. In one instance, I've seen a cheap Oceanic and an expensive Suunto disagree with each other by more than 20 minutes of deco time! Yes, they were on the same diver.
Furthermore, product reviews and even this board support that Vytecs and other Suuntos are considerably more conservative than other brands. In fact, it's been called the "most conservative" of any make, regardless of it's "conservative setting."
Now, having said that, I'm going to point out that I still am maintaining that I want a Vytec, and that most of the DIR instructors that I have seen have favored the Stinger, Vyper, and Mosquito... All Suunto models. They simply aren't using the computers, though.
Since I'm NOT 20 any more, a bit of conservatism is a good thing from my point of view. But the tables, as taught by the major agencies, are WAY conservative. More so than my computer by far, to the point that I find them annoying.
Yeah, it's that fat, smoking, coffee-addicted desk jockey that's at fault.
You'd be surprised to find that most of the current computers, particularly those that use RGBM, show offgassing starting at deeper depths than you'd expect from a computer. The Vytec shows you when offgassing has begun (you're no longer loading) if you're in a mandatory-deco situation.
Care to elaborate on that statement? It shows what?
It is a two-edge sword though. One thing that you have to realize about deco theory is that the most-overpressured tissue compartment (referenced to sea level) is the one that controls your dive. During your first dive this will typically be the "fastest" tissue compartment(s), but during subsequent ones in a given day it might not be, and once you start up it typically won't be the fastest compartment(s) that control when its ok for you to move up to the next stop. Not only that, but it is ENTIRELY possible to be offgassing THAT compartment, but ON-GASSING others (both faster and slower), depending on THEIR pressure gradients, all at the same time, depending on where you are in the water column. Worse, if you manage to ongas a SLOWER compartment you may actually EXTEND your deco by doing those stops even though "by the numbers" you are offgassing in net!
This is where the rubber hits the road with a computer on your wrist - no table can take this into account with any degree of accuracy if you are off even by a bit on your profile that is planned,
Agreed.
but a computer CAN, and DOES - in real time.
Sorry, I don't agree with that statement. Like the tables, computers can only make "best guesstimate"calculations, based on what research has been done to help whatever theory it is that they're using hold water. The computer can not take true tissue loading into account... Only an overall "best guesstimate." Remember, both tables and computers are developed from lots and lots of theory and observation... Then "dumbed down" for the worst case scenario. They may or may not apply to you, but likely will be a little conservative, which everyone, including the computer manufacturers and table producers, want to see.
If you bring your profile (from a bottom timer, or computer in gauge mode) to your deco computer (laptop on the boat, etc) to cut the second dive's (or third's, or N's) then you are still using a computer but doing the computations on the surface - losing accuracy, since they are all ex-post-facto and at best for your NEXT exposure.
Agreed. The point is, though, to not be completely dependent on a battery at depth.
Again, at least that's DIR's teachings on the subject. I believe in the theory, and I also like the idea of another device showing me "what's up."
Grab the Suunto Dive Manager, and look at some profiles sometime. You will be able to easily see how this plays out on a "real" profile, and it becomes obvious that this cannot possibly be handled "in your head" or with pre-planning with any degree of accuracy. You're guessing when cutting those tables.
I'm not denying that there's some degree of accuracy lost when doing tables. I wouldn't exactly call it "guessing," but I can see your point... There's a loss of accuracy there.
What you're assuming, though, in the above example, is that the Suunto dive manager is NOT "guessing." You're assuming that the dive planner is able to come out with the perfect dive profile, and that it's to be believed no matter what.
I'm telling you that there is no reason to believe that Suunto's dive alogarithms are any more or less accurate than say, Navy Dive Tables, PADI Dive Tables, or the "quick" DIR method. All of these ways are very close, but they're not the same... And in some cases, the numbers can be radically different.
There is no proof that the Suunto, or any computer's model, is any more or less accurate (or more accurately, "more applicable to you and your specific situation) than say, U.S. Navy Dive Tables. And yes, even when you model two perfect dive profiles, you will find that they come up with different numbers... Different deco obligations, and different times.
Your statement above is a perfect example of exactly what DIR warns against... Trusting the computer. It's already begun in you... You trust the computer, even though there's no proof to support that Suunto RGBM is sufficient to keep your body safe from DCS.
Of course, we know that historically, the Suunto model is a fair way of keeping one's body from DCS, but so are many other models, which can be dramatically different. What's to say that the Suunto model, even at it's most aggressive setting, isn't completely and totally "over the top" conservative?
If I'm a "real" buddy, I've shown my dive partner how my instrumentation works. He BETTER know, if he's expected to attempt a rescue if something goes wrong with me! And vice-versa.
Well, agreed... And as I've said before, I like computers, personally.
However, how often do you show your buddy how your instrumentation works? Once last year? Every dive? Next time you're with him, ask him to show you what your pressure is... See how long it takes him to find it. Now add a little panic underwater... Do you really think it'd be as effective as showing him one guage with the needle pointing to 2200?
I think this is a red herring - I can easily point to the appropriate number on my wrist. Its labelled "PSI" - pretty hard to miss Its also going to be the only three or four-digit number on the display, for most dives.
No offense, but it doesn't sound like you're practicing your S-drills very often. If you were, then you'd know that your buddy isn't taking any of that into consideration when you show him your guage.
Try it yourself... Without looking at the surface or asking your buddy, the next time you're diving, grab your buddy's guage and see how long it takes you to figure out how much pressure's in his tank. Then consider what that'd be like if you're in half-panic mode.
Wouldn't it just be easier to have the simplest, smallest guage... One that looks exactly like yours... Shown to you plain as day?
Now imagine you're 500' back in a cave when you experience this OOA... No doubt, you're looking for SIMPLE, man... I can completely understand why DIR teaches what it does.
Again, that said, I like computers too. But DIR does not support them.
Ah, but they're hypocrites. They're using a computer to plan their dives, and worse, they are doing it using incomplete and inaccurate information. You know the saying - "GIGO"!
I just showed you what they're doing to plan their dives. And I've seen many a DIR person whip out tables to plan a dive. Some use a computer, topside, where failure only results in them changing the batteries, not being stuck, dependent and clueless on their obligations.
I don't see hypocracy in that.
Yes, you should have a "bailout" or a "the water is brown" worst-case deco table cut, and have it with you, on a deco dive. Plan that one for as the absolute worst case possible with your gas supply - the longest and deepest you could be and still get back with the deco gas you have. If you used less at depth, you spend extra time decompressing. Oh well. That's the price of an equipment failure - you get to spend extra hang time.
At least you won't get bent that way.
Agreed. The perfect use for a computer. Backup. Works well in case the brain ever fails.
Fair enough. That's your right. And like I've said, I agree with a lot of your statements, and with almost all of them, to a degree. I don't think we're really that far off.
The SPG has two dynamic O-rings in it, which are under full tank pressure, and a tiny little spool that they ride on. Small-diameter, small-area seals, high pressurem dynamic seals = reasonably high failure risk. Plus, gauges can (and do sometimes) stick or just plain quit working. Been there, seen that.
If it fails, you will either have to abort or shut down the post. If you shut down the post you lose all knowledge of what's in the tank. It won't be an instant catastrophic loss, because all of the modern regulators I've seen have very small orifices in the HP ports, so the gas flow will be reasonable, but it will be real and something to pay attention to, and require your immediate attention. Either way the dive is over.
The transmitter, on the other hand, is a hard metal fitting screwed into the first stage. No hose, no dynamic O-rings, less to leak. Yes, it can go dead, but all of the current ones I know of give a warning in plenty of time (visually) before the power gets critically low. Yes, it can fail, as can anything else, but if it does its unlikely to leak out your gas supply - it will just stop working. If that happens, you abort - you knew where you were before it happened, so what have you lost?
Great example, and I agree with you. Up to the point where you say, "What have you lost?" Well, nothing if you're doing a recreational dive. If you've got deco obligations, you'd better know them, 'cause they're gone if your comptuter craps out.
Sure, not a problem if it's the transmitter... But it's a real possibility with the watch or guage. Flooding... Cracked seal... Cracked glass... Dead battery... And all of these things can happen witth s SPG or with a computer (well, except for the dead battery, which you've already mentioned has significant warning). The difference is, if you've got mandatory deco, and you've got a flooded SPG, it's time to abort the dive and go to deco. If you've got mandatory deco and you've got a flooded computer, you've got a real problem unless you've memorized your deco obligations (which isn't likely, since you've learned to dive the computer, not on your brain).
Which is easier? Oh, by far, the computer... And assuming that all alogarithms are the same (which they're not), no doubt the computer would be more accurate. Would the difference be significant? Well, it depends on how you define "significant." But I will tell you this... Rely on a brain and a plan, and you're unlikely to have a problem. Rely on a piece of equipment, and it's only a matter of time before you have a problem.
I like computers. They're real convienent. The real question is, would I put my life in the hands of one?
I don't see the difference in risk here.
Well, aside from the failure situation, which admittedly is unlikely in both the SPG and computer scenarios, I don't see much difference either.
Then why spend $1200 on one?
What I do see is that the wireless transmitters are accurate within a few psi, where the analog SPGs are often off by 100 psi or more, and are impossible to read to that level of accuracy. Is that a big deal? Not really - until the analog one is off <500> psi and you don't realize it!
Yeah, but I've heard of the transmitter type computers being off by as much as 800 psi... Something to do with "syncronizing" then before you get in the water. Someone else suggested that in a cave or wreck or other enclosure, radio signals could "bounce," causing variances in accuracy. I don't know how realistic that theory is.
It wasn't all that long ago that the way you knew you needed to come up was when your J-valved regulator started to breathe hard, you flipped the reserve and ascended.
Agreed. It was a bit before my time, but I've heard that. Scary, hunh?
And the truth behind DCS is that the statistical cases of DCS were low, even then... Now, they've fallen to even fewer numbers (although more cases are treated, hinting that we've developed better identification and treatment of DCS than before). So statistically speaking, anything we've done to help prevent DCS has been beneficial... Computer or not.
SPGs are relatively recent additions to diving. I don't see DIR making the argument that we ought to go back to reserve valves - even though they would eliminate several failure points!
Again, agreed... Which goes to show that DIR's not only about diving "religiously" or diving "blindly." They're not nutcases, Gen. Whether you believe all of it or only some of it, there's information to be gathered there, and it's coming from people who's skills are incredible... Which they should be, since we're talking thousands of dives every year...
