Does not compute but should I ?

[gzscuba]

Something struck me as odd about herman's dive, so I checked my tables the rules that PADI gave me. Somebody let me know if I'm screwed in the head.

According to the PADI rules, only the initial descent and the time at bottom matter for calculating the bottom time for the table. My instructor mentioned that once he hits his bottom time limit, he often takes it really slow on the ascent to get "free" additional dive time without incurring a deco obligation. I guess technically, all of Herman's time @ 20 feet could be considered deco time. So unless his initial descent and time @95 feet was more than 20 minutes (the NDL from my table) it seems like that dive should have been okay.
Please let me know if I'm screwed up, as I just got certified last year, and I generally don't have to be concerned with the NDL (I don't dive deep/long enough)

 

[SeaJay]

This thread already gets the "Most Combustible" award for the week.

Nice point, UP, about this stuff. I'm glad to see that you didn't take it upon yourself to try to teach someone how to do this over the internet. That would be doing a grave disservice, especially if you missed one of the finer points.

Herman: You said:

So Pug, are you telling me that you calculate the exact bottom time remaining, nitrogen loading, O2 loading and any possible deco time on the fly in your head during a dive?

Yes, that's exactly what he's saying. Freaky, isn't it? Yes, it's possible, but there's some real fine detail in there that needs to be taught and practiced. This should not be taught over the internet.

My advice? Take the DIR-F class closest to you and ask "the experts" about it. It's amazing.

Then decide if you need a computer. I bought one, but I only use some of the basic features on it (like the depth gauge, the built-in timer, and especially the logging function). Sure, I play around with the computer here and there, but I'm finding it somewhat annoying, as it's always trying to keep me out of deco (as if that's some sort of huge wall that I should neve cross). Why not just learn to deco properly and always make your own choices anyway?

 

[Uncle Pug]

gzscuba once bubbled...
I guess technically, all of Herman's time @ 20 feet could be considered deco time.

Recreational divers are not permitted to use the word *deco*.

Recreational divers are only allowed to do *safety* stops.

Oh... excuse me... you did say *technically*... yes you are right.

 

[SeaJay]

gzscuba once bubbled...
Something struck me as odd about herman's dive, so I checked my tables the rules that PADI gave me. Somebody let me know if I'm screwed in the head.

According to the PADI rules, only the initial descent and the time at bottom matter for calculating the bottom time for the table.

<snip>

So unless his initial descent and time @95 feet was more than 20 minutes (the NDL from my table) it seems like that dive should have been okay.

<snip>

Please let me know if I'm screwed up, as I just got certified last year, and I generally don't have to be concerned with the NDL (I don't dive deep/long enough)

Okay, that's not entirely correct, but your thought process is right.

There are several great places for you to get information about decompression... One of them is on my site, http://www.LCscuba.com . Go to Talk-->The Physics of Diving for a good, solid overview of what exactly is happening when a diver dives.

Your thought process is correct, but you may not have the actual numbers and times correct. Those things depend on many variables which vary from diver to diver, and from situation to situation. Thus, it's difficult to give a "catch all" yes or no to your question as to whether or not he was safe in the above profile.

The really scary thing is that he thinks that "if the computer says it's okay, then it must be okay." That's simply not true in every circumstance... Although most computers are very conservative in their calculations.

Think about this, then... In many cases, a diver would be OVERLY conservative using a computer. So while that diver might think that he's getting more accurate information and therefore longer dive time, the reality is that his dive time may be shortened by the computer in some cases too!

 

[gzscuba]

Uncle Pug once bubbled...

Recreational divers are not permitted to use the word *deco*.

Recreational divers are only allowed to do *safety* stops.

Oh... excuse me... you did say *technically*... yes you are right.

Sorry UP, my bad on the choice of words. I meant that the time at 20 ft would be a HUGE safety stop.

My instructor pointed out to us that the during the safety stop, you are decompressing. He made a distinction between voluntary decompression that you do during a safety stop and mandatory decompression in technical diving (or because you exceeded the tables). So I think of a safety stop as a primitive decompression stop. Of course, the caveat here is that I don't know anything about the depth and duration of decompression stops that technical divers take. Comments? Flames?

 

[Uncle Pug]

gzscuba once bubbled...
Sorry UP, my bad on the choice of words.

You're not bad at all.

 

[SeaJay]

gzscuba once bubbled...

So I think of a safety stop as a primitive decompression stop. Of course, the caveat here is that I don't know anything about the depth and duration of decompression stops that technical divers take. Comments? Flames?

Oh, that's just awesome. You are very, absolutely, and alarmingly correct.

There is no such thing as a no-deco dive. It's a misnomer. Every dive is a decompression dive. There is no such thing as a "technical" dive, UP's just givin' ya a hard time. There is no "higher order" of diving... It's all the same.

The tables which have been so drilled into all of us are simply standard models by which we can dive. You ARE decompressing during these dives; PADI, for example, says that decompressing at a constant rate of 60 ft/min will suffice for all of the numbers that they give you. Obviously, if you ascend at 30 ft/min instead, then your decompression obligation is reduced.

However, it's not linear... You may be able to rise at 60 ft/minute from 130' to 70', but need to go 30 ft/min from 70 on up... And even slower from 20' to 0'. Make sense?

 

[jonnythan]

Uncle Pug once bubbled...
I am not an expert on this and wouldn't attempt to explain it here anyway... consider my above comments a teaser to get you to take a GUE class and get the inside scoop from them.

I'm sick of this attitude. It basically says "you don't need a computer, do it in your head," but then reveals to you that the only way to learn this is a GUE class.

What trash. If one *needs* to take a GUE class in order to learn effective and safe computer-less diving, then just go ahead and buy the computer. Not everyone wants to put *that* much effort into diving, and a computer is a viable option for a lot of people.

Me, on the other hand.. I'm working on it and do plan on taking GUE classes in the future. That doesn't mean that every diver on earth should take GUE classes because they teach you not to rely on a computer.

 

[gzscuba]

SeaJay once bubbled...

<snipped>
The tables which have been so drilled into all of us are simply standard models by which we can dive. You ARE decompressing during these dives; PADI, for example, says that decompressing at a constant rate of 60 ft/min will suffice for all of the numbers that they give you. Obviously, if you ascend at 30 ft/min instead, then your decompression obligation is reduced.

However, it's not linear... You may be able to rise at 60 ft/minute from 130' to 70', but need to go 30 ft/min from 70 on up... And even slower from 20' to 0'. Make sense?

thinking....thinking....
(you know... I'd never thought about this...)
So do technical divers use variable ascent rates based on depth in addition to multiple decompression stops?
Seajay - It does make sense in a way. I don't remember how the solubility of N2 in water scales with pressure. I may have the sate my curiousity with a book on decompression theory.

Sorry to the original poster for hijacking the thread. In an attempt to bring the thread back on topic, I'll ask the following question:
Are there computer algorithms that can handle these variable ascent rate issues, or do they just assume some standard ascent rate? If I remember correctly, the Suunto computers whine at you if you ascend faster than 30 ft/min.

 

[SeaJay]

gzscuba once bubbled...


thinking....thinking....
(you know... I'd never thought about this...)

How very cool to actually "connect" with someone about this very-difficult-to-express topic.

So do technical divers use variable ascent rates based on depth in addition to multiple decompression stops?

Yes they do. Interesting, isn't it?

In fact, ALL divers use this technique. Think about this for a second...

Let's say that a diver ascends from 60 feet in one minute. It's easy to figure this out, right? That's 60 feet a minute.

Okay, that same diver takes two minutes to ascend from 120 feet... Still the same 60ft/min, right?

Okay, many recent studies show that 30ft/min is actually a better model... And for obvious reasons. That would make the 120 foot ascent actually a four minute ascent, right? Not so fast, Speedy... It's not that simple. Don't forget that you need breathing gas (air, whatever) for four minutes... Now you've got a situation where you need to specifically "turn" your dive around at a specific gas volume, right? Plus, don't forget that you may need to support your buddy if his system goes wrong... Next thing you know, you need to set higher limits on your "turn" pressure than you thought!

...Which is why your average "air integrated computer" isn't popular with these guys. What those numbers show you... Which can be very misleading... Is how much time you have, based on what YOUR air consumption rates are.

...But that's another debate for another time. Your specific question was about ascention rates and how varying depths may need varying ascention rates.

Let's say that you go to that 120 feet... According to PADI, they recommend a "safety stop" at 15 feet, right? 3 minutes... Well, they're saying that you should go from 120 to 15 feet in a little less than two minutes (remember, PADI is saying "60 ft/min") and then hang out at 15 feet for three minutes... Then surface. Guess what that's doing? It's simply a way to slow your ascent! Where? Right at the surface! Interesting how it all works out to be nearly the same, isn't it?

Now, here's another place where that computer stuff comes in... My Vyper, from the factory, is set to "beep" me if I exceed an ascention rate of 34 ft/min... Which means that it's ultra-conservative. Yet it still requires that safety stop at 15 feet... Three minutes. In other words, if I use the computer, I've got a three minute "deco obligation" that I shouldn't need. In other words, a wasted three minutes of dive time! In fact, if you're diving safely, you're actually wasting six minutes of gas... Three for you and three for your buddy! Six minutes is a lot of gas (air, whatever) when you've only got a scant few of them anyway because you've gone to 120 feet...

So the direct answer to your question is that yes, technical divers DO slow their rate of ascent as they reach shallower depths. So do all divers! They do it through a mandatory "safety" or "deco" stop.

Betcha didn't know you were already doing it, did ya?

Seajay - It does make sense in a way. I don't remember how the solubility of N2 in water scales with pressure. I may have the sate my curiousity with a book on decompression theory.

Yes, definitely check my site on the "partial pressure" theories regarding this. There's enough information to keep anyone busy for a long time, yet you don't have to have a degree in Physics to understand it.

Sorry to the original poster for hijacking the thread. In an attempt to bring the thread back on topic, I'll ask the following question:
Are there computer algorithms that can handle these variable ascent rate issues, or do they just assume some standard ascent rate? If I remember correctly, the Suunto computers whine at you if you ascend faster than 30 ft/min.

Yep, you're right. I believe it's 34 ft/min, to be exact.

And yes, all computers slow your rate of ascent as you surface. They do it through mandatory stops and deco obligations.

In a perfect world, a diver would be capable of swimming anywhere he wanted to at any rate, but as he neared the surface, he would slow his rate of ascent by a manner of squares. As he surfaced, his rate of ascention would decline more and more...

The problem is that it's really tough to ascend at 5 ft/min, so they just sorta "block it" and make you wait at certain points...