SeaJayInSC
New member
Me too. Too much stuff to do to get the time... But I'd love to watch it happen. Something about the pride of accomplishment...
entry level dive computer
- Thread starter TTU_Reefer
- Start date
Sorry. I generally like my DUO. The computer interface kit is rather expensive if you want to see your profile graphically. I really haven't dove many different computers so I like it because: It's easy to read, has Gauge mode, and it works. It's also fairly inexpensive. For me, it's really a glorified bottom timer. If you get it, I recommend the DSS bungie mount for it. I like the screen cover (sits flush unlike many others) for it too, but I'm a wreck diver, I dig alot, and it would get trashed w/o it.
PM away or start a new thread :bounce3:
Aaron
TTU_Reefer
New member
During finals week dive computer shopping IS NOT helping me haha
I really appreciate all the advice, more and more I read the more I wish I could dive each computer in question and see what I like
I really appreciate all the advice, more and more I read the more I wish I could dive each computer in question and see what I like
SeaJayInSC
New member
In a lot of places, you can.
Where are the divers there in Lubbock, TX? Do you have a local spring, quarry, or dive park? Gulf diving? My bet is that the local dive shops will let you dive a couple of them before buying...
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TTU_Reefer
New member
In a lot of places, you can.
Where are the divers there in Lubbock, TX? Do you have a local spring, quarry, or dive park? Gulf diving? My bet is that the local dive shops will let you dive a couple of them before buying...
balmorhea natural spring (max 20"ish) and Blue hole NM (84ft and colder than hell)
SeaJayInSC
New member
Hell isn't cold, my friend... I know. I used to ride my motorcycle there. (Hell, CA).
Perhaps one of the local dive shops at those places would be willing to let you test-drive some of their computers...
(Hell, CA).Perhaps one of the local dive shops at those places would be willing to let you test-drive some of their computers...
SeaJayInSC
New member
TTU_Reefer said:
Sure, man!
The Rule of 120 is an old Navy divers' addage. To be done correctly, there's some caveats, so you have to know the whole rule to be able to do it properly.
The bottom line is that an NDL can be calculated as depth + time = 120. For example, you can go to 60 feet for 60 minutes. 80 feet for 40 minutes. 20 feet for 100 minutes. Get it?
Here's the caveat: To do multiple dives, spend 1 hour of surface interval, then do the rule of 110. 70 feet for 40 minutes. 100 feet for 10 minutes, and so on.
The rule doesn't work if you always work it as the Rule of 120 (that's only for the first dive of the day) and it doesn't work if you don't spend an hour topside between dives.
If you compare your numbers to a PADI dive table, you'll notice that the Rule of 120 is actually conservative at the extremes (too short very deep and too short very shallow) but liberal in the middle of the table, around 60 or 70 feet.
Graphed out (time as the x axis and depth as the y axis) you'll notice that the PADI dive tables are a concave curve, whereas the Rule of 120 is a straight line.
Is it 100% accurate? No, although it's surprisingly close - and it'll give you a good starting point for knowing your NDL's right off the top of your head. For example, when someone says that they went to 80 feet for 55 minutes, you'll know right off that they exceeded their NDL's without having to look at a table first.
The Rule of 120 isn't the end-all of calculations, but it's a great starting point, and may save your butt the day your computer floods and you need to know how close you are to your limits, off the top of your head.
For what it's worth, there's two things you should know about ANY dive table - first, that it's a very exact number... Which gives the impression that dive physiology is a very exact science. It's not. People who stay within the limits still get bent, and people who exceed the limits don't get bent... There's just too many factors in too many people for it to be an exact science, so when a table tells you that it's okay to be at a certain depth for 27 minutes, don't believe it. Be back on the boat (or at least near the surface) in 20 or 25. The second thing to know about the PADI dive table is that it's rediculously conservative... That is, it's been developed for the cigarette-smoking, donut-eating desk jockey who's insanely overweight and more prone to DCI than the average Joe... So don't get bent out of shape (no pun intended) if your dives put you a minute or two over the tables - just consider it a mistake and don't do it again.
There are dozens of dive tables, and most of them disagree on exact times at exact depths. There are several different formulas (called "algorithms") that scientists use to create recommended NDLs, and they don't agree either. Some of them are conservative, some of them not, and others that are conservative at some depths and liberal in others. The study of them all can be completely mind-numbing!
So how do you really know when to come up? Well... Dive a lot, and your body will tell you. At the end of the day, do you find yourself tired and inexplicably fatigued? Dive more conservatively. Finding yourself slightly nauseated or feeling like you got some bad breathing gas? Dive more conservative. Feeing great? Perhaps a little more aggressive profile would be okay.
...And "diving more conservative" doesn't mean to dive less or get out of the water sooner - it means ascending slower... Taking the pressure off of your body slower. Go ahead and stop for a minute at 40' - then two at 30' - then four at 20' - then eight at 10'... Guess what? That's all that "technical" diving really is. You don't have to be past your NDLs to make the stops - they're ALWAYS good for you, physiologically, and that's all there is to it.
Why the stops? Well... Stopping at those depths is easier than ascending that slow, that's all. Ultimately, it would be physiologically best to ascend the whole time, at those rates... Passing through 40' over one minute, then passing through 30' over two minutes, then passing through 20' over four minutes, and passing through 10' over eight minutes... But most people don't have that kind of control, so it make sense to simply stop at those points for those amounts of time. Get it?
...So when PADI teaches you to ascend at 30 ft/min, that's actually a kind of "technical" skill. When they teach you to do a "safety stop" at 15 feet, that's actually a kind of "technical" skill, too. (The only difference between a PADI "safety stop" and a "technical" diver's "decompression stop" is the name.
) Scientifically, all they really want you to do is ascend at a slower rate. According to the profile I gave above, that's an ascent rate of 30 ft/minute up to 40 feet... Then slowing to 10 ft/min, then 5 ft/min, then 2.5 ft/min, and finally at 1.25 ft/min just before hitting the surface.Make sense?
Of course, it's easier to simply tell someone to stop and wait for a certain number of minutes at a certain depth... Although that's not really what your body needs, physiologically.
In fact, if you want to get really technical, your body isn't sensitive to depth - it's sensitve to pressure change over time. 30 ft/min as an ascent rate really isn't the key, nor is a reduced ascent rate as I demonstrate above... It's a percentage of pressure change over a given time that your body is sensitive to, and the more saturated your tissues are, the more important it is to release the pressure on your body slowly.
Did they teach you the concept of "atmospheres absolute" (ATA's)? In diver-lingo, they're commonly referred to as "addas." Ever heard the term? If so, what do you know about it?
In short, an "ATA" is the amount of air pressure - barometric pressure - on your body at sea level. Obviously, barometric pressure changes with the weather, but in dive physiology, it's considered to be 14.7 psi... Effectively, the weight of the atmosphere on your body when you're standing by the sea.
Every 33 feet of depth is equal to one ATA... So at 0 feet, you are experiencing one ATA. At 33, two ATAs, at 66 three ATAs, and at 99 four ATAs. Make sense?
When you ascend from 99 to 66 feet, that's a pressure change of four ATAs to three ATAs. Do it over one minute, and that's a pressure change of (minus) 25% over one minute. Still following?
Ascend from 66 to 33 feet over one minute, that's a pressure change of 33% over one minute. Ascend from 33 to zero, and you're talking about a pressure change of 50% over one minute, which is a lot!
When you dive, focus not on your ascent rate of 30 ft/minute or "stops" at certain depths for certain amounts of time, but instead on your percentage change of pressure over time... That is, don't exceed, say, a pressure change of more than 25% per minute. Think about that for a while and consider what that would mean at shallow depths - it would mean ascending VERY SLOWLY in very shallow waters.
...Which is physiologically correct, and truly what everyone is trying to teach, in very odd, abbreviated ways.
It's just that the concept of "pressure change over time" is a very difficult concept to grasp or teach.
...So what percentage of change over time is okay? Is 50% per minute okay? Is 25% the limit? The correct answer to that question is, "It depends." The longer or deeper you've been diving - that is, the more gas that's dissolved in your body - the lower the number has to be. Also, some people's bodies are more sensitive to pressure changes than others, so some people might be comfortable with a pressure change of 30% per minute, while others do better at 15% per minute. What's right for your body you'll only know if you go a little past your limits from time to time and end up a little dazed or spacey or just plain tired. While not a life-threatening situation (or even medically treatable), this is what hyperbaric technicians call "Type I DCS," and most divers will experience it from time to time. It's okay - a good night's sleep takes care of it.
...But if you can recognize it and know how to change your dive profile next time, you'll benefit.
...Which is where a graphing computer can come in so handy. You'll realize that you need to "flatten" the "tail" of the graph. You'll understand that when you see it.
(Interstingly, "stops" like "safety stops" or "decompression stops" appear as "stairs" or "steps" in the tail of the graphed profile, and tell a lot about the dive and the diver.)Think about all of this for a while, then consider how silly it is when a computer says you've got "7 minutes" to your NDL. Sorry, it's simply not that exact... Or that simple. And "7 minutes" until what, exactly? Type I DCS ("just tired"), or a helicopter flight to a chamber? And is this for you, or for the fat desk jockey guy?
...And what if you spend a long time going through the 40ft/30ft/20ft/10ft drill that I outlined above? Obviously, doing it is physiologically better for the diver than simply popping out of the water - yet with a gauge screaming at you to be out in "7 minutes," what's the prudent diver (who doesn't really understand all of this stuff) going to do? Does the computer give you CREDIT for that, or does that "decompression time" count AGAINST you? The reality is - it depends on the computer and whatever algorithm was used to formulate the time that it's giving you.
...So what's all of this mean? Well... The dive computer is a great tool in your mental toolbox - just like the "Rule of 120," but that's all it is - just a tool. THINK about your dive. Plan your dive. Stick to the plan. The computer may agree with you, and it may not - don't get blinded by the numbers that it gives you... That's a dangerous practice that'll leave you cutting your dive too short, popping out of the water too quick, making you physically exhausted at the end of the dive, or worse yet, dangerously close to a chamber ride.
...And the next time someone talks to you about an "NDL," (defined as a "no-decompression limit"), remember that there's really no such thing. EVERY dive is a decompression dive, and all dives have decompression limits. It's just that some dives allow for a 50% reduction in pressure per minute, while others allow only for a 10% (or less) reduction in pressure per minute. There is no such thing as a dive that allows for a 100% reduction in pressure per minute - so EVERY dive is, in some way, a "decompression dive."
Armed properly, there ARE no limits as to "how long you can stay down." There are only limits as to how much breathing gas you have with you and how slowly you must ascend to not reduce pressure too quickly.
Wanna go to 100' for two hours? No problem - simply take enough gas with you and ascend slowly enough, and you'll be better off than the diver who dove to 100' for only 15 minutes, then popped to the surface because "his computer told him to."
Hope that helps.
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