Computer vs tables

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I'm a newbie and don't do this. Does it make sense??

Thx

I'd encourage you to explore whether or not ascents should be counted towards bottom time.

If I sit down at 80 feet for 30 minutes and then begin my ascent (figure 30FPM), would you expect me to be ongassing, offgassing or both at minute 31? If both, which one am I doing more of? As I ascend, which tissues am I most concerned with (the fast or the slow)?
 
Years of study and development went into the 12 compartment algorythm in my computer. And, the computer took a lot of work to develop, too. It measures temp, averages depth, looks at ascent rate, PO2, and a bunch of other things.

I am not smart enough to perform all the calculations that the computer is constantly doing.... and dive at the same time. It does a more complete job than I can. That's why I bought the thing.
That 12 compartment model doesn't pay any attention to sawtooth profiles or rapid ascents. It also doesn't know your level of hydration, current aerobic fitness level, how late you caroused last night, or any of the many other things that affect your susceptibility to DCS.

What the computer does have going for it is that it doesn't get distracted by the whale shark swimming by, and doesn't get forgetful.

I treat it is the rather simplistic and dumb bookkeeper that it is. What it says is one of the things I take into account when I appropriate judgments about how long I want to stay at various depths, and what sort of ascent and deep stops I will do.

Both blindly rejecting computers and blindly following computers are sub-optimal in my opinion.

IMO, it's not a case of whether computers are good or bad, but rather the question should be WHAT IS THE BEST WAY TO USE A COMPUTER.
 
That 12 compartment model doesn't pay any attention to sawtooth profiles or rapid ascents.

Please explain this further. I never heard of the sawtooth profile. I always figured that if the computer samples every 15 seconds, it has the profile of the dive.

I do know that if you dive the tables for our drift dives, you get a lot less bottom time. Using tables for Saturday's dive would have given us a 40 minute dive and a second one of eight minutes bottom time. We got two of 50 minutes with computers. Less safety? Maybe an expert can tell us if cases of the bends has increased since we all started using computers.

I suppose that one could come up with thier own safe rules of thumb for diving. Would that be like conducting your own medical experiment?

Thanks / Stu.
 
Thanks everyone. This thread is really interesting to me.

Charlie99- I've enjoyed playing with your flat wheel before, but didn't quite understand where some of it was coming from, never having used the original myself. Now I think I do have a better idea, and will add it to my collection of models to philosophize with.
 
I'd encourage you to explore whether or not ascents should be counted towards bottom time.

If I sit down at 80 feet for 30 minutes and then begin my ascent (figure 30FPM), would you expect me to be ongassing, offgassing or both at minute 31? If both, which one am I doing more of? As I ascend, which tissues am I most concerned with (the fast or the slow)?

Hi Blackwood. Thanks for the advice. I misread the example, sorry about that. Here are my answers without looking anything up, so I'm probably wrong :):

#1. Both. Ascending you will be offgassing faster compartments and slower compartments will continue to ongas. At minute 31 you will be at 50FSW. I don't understand M values and pressure gradients, so I can't tell you the state of all the compartments at this moment in your dive.
#2. Offgassing. The faster compartments are "faster".
#3. Faster. Longer exposures will load slower compartments to a greater degree and require more time at depth to offgas.

The point of my deleted post was that the tables do consider exposures that tissues are subjected to during the dive the table is designed for, when assigning a pressure group, and that just because it's not included in how you count BT, doesn't mean that it's not accounted for elsewhere. Is this wrong? I guess since the slower compartments are slow, the ascent time doesn't matter for tissue loading.

My other point (in my deleted post) was that to use the table that has these exposures built into them for another purpose (square vs. multi level) is wrong, and if you calced it the way I showed (including ascent time and SS and descent time), you ended up in the same pressure group, which I thought was a funny coincidence. That wasn't clear in my post, which is why I deleted it. Anyway, I don't have the wheel; I use a computer! Would you plan a multilevel dive with the RDP?

I see your point though. Thx
 
Please explain this further. I never heard of the sawtooth profile. I always figured that if the computer samples every 15 seconds, it has the profile of the dive.
The limitation is not in the computer, but in the model. Simple dissolved gas models (also sometimes known as Haldanian, neo-Haldanian, or by the name of one specific dissolved gas model -- Buhlmann) don't care if you repeatedly ascend and descend. Those models also aren't affected by rapid ascents.

Real life and the dual phase models (also called bubble models. 2 examples are VPM and RGBM) show that this sort of repeated up and down profile has a higher risk of DCS. Real life has also shown, that although the dissolved gas models aren't affected by rapid ascents, that is another thing that increases the risk of DCS.

If you look at the depth vs. time plot of a dive with repeated up and down movements it looks like the teeth on a saw, hence the term "sawtooth profile".

Charlie Allen

p.s. The rate at which dive computers measure depth and update N2 loading calculations is independent of the setting for logging data. Even if your computer is only putting stuff into the logged data every 15 seconds or 30 seconds, it still samples depth about once per second and updates the deco calculations at about the same rate.
 
it's not about your computer having a problem or not. It's about being aware of what's going on during your dive. You seem a little experienced to be telling divers to rely solely on their computers.

David

As long as you yourself stick to NDLs you should be fine, no matter what you do, even if you use GUE-DIR methods (which I myself consider to be obsolete, but then you did not want to hear my opinions).

If someday you want to play in the deco realm, you should find a reliable software program for your PC and also 2 good computers to go with it, one for gauge mode, and the other for helium/trimix mode. DiveRite is coming out with a new such product soon. In the meantime, Nitek HE and VR3 are your main choices. These modern procedures should keep you out of the recompression chambre, unlike so many of your GUE-DIR friends on deco who claim to be diving with their heads.

For NDL, any single dive computer is fine, yes. If it fails, you can tell by looking up when you are to reach 15 fsw, and if you stay there for 3 mins, then surface, everything should be fine. A backup is good, of course, but few divers newly discovering dive computers nomally have access to a backup as well.

The smog, traffic, and crowds in Lost Angeles must be affecting reading comprehension there?:D
 
That wasn't clear in my post, which is why I deleted it. Anyway, I don't have the wheel; I use a computer! Would you plan a multilevel dive with the RDP?
Actually, I do sometimes use the RDP to plan a multilevel dive. A typical case is on a boat in a resort area and the DM is saying "OK, for this dive we are going to do xxx min at xx feet, then yy minutes at yy feet, and then the rest of the time above 40'.

The RDP is a good tool to do a sanity check on such a profile. My computer doesn't have a decent way of using it to pre-plan multilevel dives. It does a great job of tracking it realtime, but no way to plug in a profile ahead of time and see what it says.

In practice, the number of different multilevel profiles done with single tanks on dive boats is not all that many, and after a while I don't need to look at a table to validate a plan such as 15 @ 80', 20 at 60', then on up to 35' or so. Way too many divers, though, just sit there and accept the plan without thinking about it.

Of course, the other half of the plan is a quick calculation of whether or not it makes sense from a gas consumption perspective, but that's a different thread.
:D

===========================

You can also look at a proposed plan from the perspective of depth averaging. 15 minutes at 80' and then 20 minutes at 60' is 35 minutes at an average depth a bit shallower than 70'. That is close to but less than a standard NDL of 40 minutes at 70', and right at the limit of the "Min Deco" table on page 14 of the reference below.

If you want to look further into depth average and ratio deco,
http://www.txfreak.de/ratiodeco.pdf is the guideline.

DIR-diver.com - Average depth for deco? is an excellent article which helps to show what sort of profiles are valid for averaging, and discusses some possible "adjustments" for reverse profiles.

Both of those articles are primarily oriented towards dives with significant mandatory decompression and dives with minimal deco are kind of an afterthought, so it takes careful reading to see what does and does not apply to normal no-stop single tank diving.
 
Thx Charlie99. I'm not up to that yet.

What is a "flatwheel"?
 
Here is some informative basic stuff about dive computers if anyone is interested: Dive Computer Secrets Revealed - Scuba Diving Magazine

Someone mentioned they wondered if there were more cases of DCS/DCI from computers than tables. Does anyone have any info on any studies? I searched DAN, but I couldn't find anything.

Given the typically conservative nature of a square profile one would suspect there would be fewer with a table... However, I'd speculate that most divers do not remember how to use their tables properly for a repetitive dive, reducing the safety buffer... Then there is the more hits with computers than tables thing... I'd think there are more computers now than ever before. So I'd think there would be more hits with a computer now than ever before... Meaning this correlation would clearly not be causation... What I'd like to find would be some figures with the numbers behind them...

I remember seeing a recompression chamber with images of tables and of computers on the side of who needed treatment. As time went on there were more and more hits from computers. Again, of course, as time went on there were more and more computers...
 
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