Basic computer questions

I am considering buying my first dive computer (most likely a Tusa IQ 700). I have never used a dive computer but anticipate that it would help me to dive safely. So far I have not dove on my own. I have always followed the lead of a dive master. I want to be able to dive on my own and feel that by using a dive computer I would feel more confident. But I do not know much about how they work and need some help to get a basic understanding of how they work.

I talked to a dive shop owner over the weekend and he said he has found that he can dive longer than the dive tables suggest and still be safe. He said the reason for this is because the dive tables assume that you were at your maximum depth for all of your bottom time whereas the dive computer knows exactly where you were and for how long so it is constantly recalculating your no-deco limits. For example he did a dive to 98’ maximum depth and his total bottom time was 55 minutes (he uses a 120 cubic foot tank) and he said his dive computer said he was safe. But if he had been just using the dive tables they would have told him he could only have a max bottom time of 25 minutes. I assume he was at 98’ for much less than 25 minutes. So he was able to more than double his bottom time by using the computer and he was still safe. That sounds to me like a great selling point for dive computers and it makes some sense to me but I wanted to find out more about this and to make sure that is the case (that you can generally have more bottom time using a dive computer than a dive table).

I also wonder if there is somewhere online that does comparison dives between using a dive computer versus a dive table to show likely scenarios and the difference between dive times. I would think that would be available by some of the dive computer companies to help sell their computers.

I downloaded the DiveRite Nitek Duo owner’s manual and have read some of it. I have a question about part of what it said. It said that the computer displays a variety of data including: date, time of day, current and maximum depths, no-decompression dive time elapsed and remaining, and water temperature. My question is about the no-decompression dive time remaining. My assumption is that for the computer to display no-deco time remaining that it must be figuring that with the diver staying at his/her present depth. For example you are at 50’ and it tells you that you have 20 more minutes of no-deco dive time remaining if you stay at 50’. But if you only stay at 50’ for 5 minutes and then ascend to 35’ would the computer recalculate your no-deco dive time remaining and add some extra minutes? Just trying to figure out how these things work.

How does the diver utilize the information that the screen is displaying? If it says I have 20 minutes of no-deco dive time remaining does that mean that in 20 minutes I need to be at the surface or that in 20 minutes I need to start ascending directly to the surface or that in 20 minutes I need to go up to 10’ and do a 10 minute safety stop and then ascend directly to the surface or what? The DiveRite manual also says the diver should monitor the screen for N2 and O2 loading. How would I utilize that info? Thanks, Rufous.

The dive computer in general gives you credit for not on-gassing as quickly at the shallower depths you pass through from deeper depths as you ascend from the deeper depths. In general, this is how most folks I've dealt with use a dive computer to arrive at different results than square profile tables would potentially allow.

I have a TUSA IQ-700 and I have been very happy with that model. If you wind up purchasing that model, and the PC download kit, I have the "trick" to how to perform drag and drop data transfer from the TUSA PC logbook to MS Excel if you're interested at that time. You can do lost of things with the data once it's in MS Excel - multiple overlays, regressions, statistical analysis, etc. if you happen to be a data freak (I am).

Here's how I interpret your last paragraph. If it says 20 minutes NDL time remaining at a particlular depth, you have 20 minutes calculated NDL time at that depth; you will have less than 20 minutes NDL time remaining if you descend to a deeper depth; you will have more than 20 minutes calculated NDL time remaining if you ascend to a shallower depth.

Now, there's a LOT of discussion on ascent rates and ascent rate philosophies, but in general there is a consensus that slowing your ascent rate as you progressively ascend to shallower depths to be more desireable. Since you've already downloaded a particular manual (and FYI I like the Nitek Duo manual better than the IQ-700 manual myself), take a look at what that manual shows as ascent rates for the three different depth ranges in that model's programming. You'll notice lower max speeds for the shallower depths than for the deeper depths. Pick a depth and then using say 40% of those max speeds, figure out how long it would take to ascend to a 15 foot safety stop. Do this for several different depths and it'll probably help answer your question better, and help illustrate there isn't really one way to answer your question.

If you exceed the NDL's and by doing so incur a decompression obligation, then the required safety stops and stop times will be displayed. If you don't exceed the calculated NDL's, those won't be displayed, but the three minute safety stop automatic countdown timer function will start once you ascend above 20 feet as described in the manual.

There is a dive simulator function in the TUSA PC Logbook software under "Tools". You can download the software for free and fiddle around with both the simulator function as well as some of the example logged dives that come with the software when it's downloaded - the link and serial number needed to use the software are available from the TUSA web site.

And don't neglect accounting for gas consumption and remaining gas - the SAC rate is one of the inputs for the dive simulator function.
Hope this helps!

I hate to over simplify... (Come to think of it... no I don't..) - but jump in the water, and don't let the big number hit zero. That's it!

Want to be conservative... don't let the big number hit 5.

If the number gets small, slowly head shallower, and it will get bigger. It's a simple and beautiful thing.

rufous:

. . . I have never used a dive computer but anticipate that it would help me to dive safely. . .

Click to expand...

Good questions, and I am a big fan of dive computers . . .

BUT . . .

A computer will NOT help you to dive safely! Being AWARE of your time, depth, NDL, and supply of breathing gas will help you to dive safely. A computer is a tool that displays some of this info, but so is a watch, SPG, and depth gauge.

In fact, a computer MAY help you to dive less safely, since you may start pushing the NDL once you have it displayed and updated according to your depth profile.

So my advice is buy the computer you like, but make some dives with some knowledgable experienced divers or take a good class in deep diving or Nitrox to make you aware of the more important issues, skills, and habits that will make you a safer diver.

Have fun,

theskull

I think you're asking great questions and are already figuring out the answers.

But...don't depend on the computer. You have to know enough about what's going on with nitrogen loading that you can dive confidently and safely without a computer. That means square profiles and repetitive dives, at least, just like in OW class, and, hopefully, squarish multilevel dives. It means being able to use your head to confirm the reasonableness of what the computer is reporting.

A big mistake would be to assume that a computer or even a table for a square dive will keep you safe. Computers and tables are based on experiments and experience. They're about history, other people, and animal experiments. Only by extrapolation and crossing fingers do they apply to YOU. If they work (and they do for most people, most of the time), great, but there are no guarantees. It's sort of like the hedges about advice that financial advisors give.

If you can accept that and stay fit, healthy and knowledgeable, then I think Larry's advice is great. Keep plugging away and you'll be fine.

OBTW, I'd suggest a requirement to overlay on your computer selection. It's a user interface (numbers, graphics, sounds, colors, whatever) that you can understand at a glance. Good luck!

EDIT
I think I came off sounding like a school marm, and I think that's because of your writing that you always follow a DM's lead and now want to dive on your own. If you don't already dive watching over your own skin, then putting a computer in charge instead of a DM won't make you any safer. The way you wrote that suggests that you might be putting the DM in charge of your life. But it's YOURS, not his. With or without a computer, even with or without a DM, you are on your own. Always.

There's another part to my reaction, too, and that's your short statement about wanting to dive on your own. I hope you don't mean diving without a buddy, or relying on a computer to qualify you for solo diving. That would be a big mistake.

Hmmm, maybe I shoulda been a school marm? Have a happy and SAFE new year!

As to your questions, I'd like to add:

1. O2 loading is only of consequence if you are diving on a gas mixture with extra oxygen (i.e. Nitorx).
2. You should only use the computer is only a backup for the tables. You should have a basic bottom time number from the tables. That way, if the computer fails, you still know roughly where you stand. (That holds true even if you immediately head for the surface in the event of a failure.)
3. Seriously consider getting an air integrated computer. It will account for gas consumption in telling you how long you can remain under water. In the beginning, your bottom time will probably be limited by your gas supply more than your no-deco time. I've got the Oceanic and am quite happy with it.

I agree that diving with a computer might make you a less safe diver, in as much as the intention is to allow you to dive longer (based on some established algorythm). Different computers use different models, and each seem to have their fans. I suppose a safer scenario than using a computer would be to not dive it to it's limits. (i.e. the previous suggestion to not let the big number get below 5.) Safer still to not let it get to 10, or 20, or even to not go diving.... A solid background in what the tables advise, should allow you to disregard your computer, and complete your dive based on the basic square profile dive... if your computer started giving readings that did not make sense.

I'd agree with the dive shop operator that advised that a computer can allow you to spend more time underwater, safely. If you were to dive to 98' (for 5 minutes for instance) to see some sight, then come back up to 60' (or even 30') for most of your dive, you'd skip a lot of "safe" bottom time, if you were diving the basic tables. If you were diving some small wreck (deep or shallow) so all your bottom time was at the same depth (as the tables assume... you should find the computer is no more "generous" with bottom time than the tables.

Diving with a computer you can make some "safe" dives, that would seem insane on basic tables. I recently made a 131 minute dive with a max depth of 45'. On any computer, or multi level dive planner, it was a "safe" dive. It was a snorkling reef with almost all there was to see at 10' - 15'. I dropped into a hole for a minute just to get the max depth to a bigger number, then began an extended safety stop for the next 2+ hours. 1 minute at 45' is pretty safe on any table I've seen, and my tables say I could spend well over 5 hours at 15'.... Yes it was a silly dive... but it's also an extreme example of what a computer can tell you.

Instead of just using the "common sense" guess that, "Well my one minute at 45' doesn't count. To heck with the tables." Could get you in a lot of trouble. Buying a computer is certainly easier than learning a multi-level dive planner, but learning the theory behind multi-level diving, can make diving a computer a lot safer.

Sooner or later, your computer will tell you something that isn't quite right. If the error makes you shorten your dive, perhaps not much lost. If it tells you something is safer than it should... you better hope you weren't diving your computer to it's "limit", or that you were as hydrated, rested, and healthy as it assumed.

If you wanted to see another computer company's software, SUUNTO also makes their computer interface available for free. http://www.suunto.com/suunto/main/a...T<>prd_id=845524442490167&bmUID=1136344177331 I use a SUUNTO Vyper, so I use the older version 1.6 software. In demo mode, you can use the software to see what your computer would say about dives to different depths, with different dive durations.

I also agree with a previous poster that I'd suggest you consider an air integrated computer. I wish I'd spent an extra $100 to get a Cobra instead of my Vyper.

Many people consider the SUUNTO computers to be on the conservative side. I've been on several dives where the divers with SUUNTO computers had to hang for deco stops, when all the other computers said 3 minutes at 15' was plenty.

rufous,

You are certainly asking the right questions. You have already looked at which model you are thinking of, so the comparison between models in these articles may not be of interest. However they do contain verbiage in the general writeup on general dive computer use. They will have similar verbiage, but interestingly as long ago as 1995 these folks concluded dive computers had improved safety (see the last 4 sentences of the 1995 article). But just like a motor vehicle, this is not true for all divers or drivers. Your individual diving and driving habits may vary. And of course, it's just their opinion.

http://www.divernet.com/equipment/computers2003/intro.shtml

http://www.divernet.com/gear/comps995.htm

The article in the Undercurrent May 2005 issue using DAN data was particularly interesting for me. I've posted a link to a thread where that info is discussed, as well as a number of posts in that thread regarding the Nitek Duo, which is quite similar to the TUSA IQ-700.

http://www.scubaboard.com/showthread.php?t=118821

There is no set of dive tables, nor dive compter that will give a 100% guarantee that DCs will not occur if one uses them. Each individual needs to take responsibility for their safety as others have posted here - I agree with that wholeheartedly.

There are also some recent magazine articles comparing dive profile results for different models. You can more directly compare say Suunto vs. TUSA quantitatively in the graphs in these articles.

http://www.scubadiving.com/gear/dive_computers/smart,_sleek_and_sexy/
http://dive.scubadiving.com/PDF/200505_divecomps_graphs_01.pdf
http://dive.scubadiving.com/PDF/200505_divecomps_graphs_02.pdf

http://www.scubadiving.com/gear/dive_computers/crunching_the_numbers/
http://dive.scubadiving.com/images/200408GR_computer_charts.gif
http://dive.scubadiving.com/images/200408GR_computer_table.gif

You may have already done this, if so, my apologies for redundancy of information.

Big Tuna, I really appreciate what you said and how you said it. I have recognized that I was blindly putting the DM in charge of my life and I do not want to do that again. I believe that I can learn while using a computer but I agree that I should not trust my life to it. No I do not ever plan to dive by myself, I meant diving without following the lead of someone else. My wife is my dive buddy and I want for us to dive safely most importantly of all. Next of course is to enjoy what we are doing or else we will not continue diving. Third if the use of a dive computer can give me longer safe bottom times than using the dive tables that assume I have spent all of my bottom time at max depth then that will be considered a very nice bonus.

Thanks to every one else who has helped to enlighten me and for the links. I greatly appreciate it and will continue to learn to be a safer and wiser diver. Rufous.