Dive computers and DCS

[ScubaJill]

Others can do everything right and still get hit. I have an acquaintance friend in Florida with similar circumstances as myself. She knows if she goes deeper then 80 for more then 30 minutes, even on nitrox, she will likely rash. For me, the single greatest factor is multidiving and longer NDL's. N=2

As to reporting, I have no doubt that Type 1 DCS is under reported.

This is interesting. I know someone who got 3 Type 1 DCS (I'm assuming) hits in the first 15 or so dives below 100 feet. On one of those dives, we had actually dived as buddies and while he was occasionally a few feet below me, our dive profiles were almost identical. I was fine. That may have been the dive where we had to come up a minute or two before his NDL was reached, because of my Cressi Giotto. [;-)]

@uncfnp, any insight into your friend's condition?

 

[uncfnp]

This is interesting. I know someone who got 3 Type 1 DCS (I'm assuming) hits in the first 15 or so dives below 100 feet. On one of those dives, we had actually dived as buddies and while he was occasionally a few feet below me, our dive profiles were almost identical. I was fine. That may have been the dive where we had to come up a minute or two before his NDL was reached, because of my Cressi Giotto. [;-)]

@uncfnp, any insight into your friend's condition?

I can tell you her first episode was rash and lymphatic symptoms years ago and she was not a new diver when it happened. She was chambered. After that she started getting rashes after deeper and longer dives, even on nitrox. Mostly on her abdomen. Sh manages this by restricting her dives as I mentioned and only diving nitrox. She considered getting checked for PFO but has elected not to thus far.

 

[ScubaJill]

I can tell you her first episode was rash and lymphatic symptoms years ago and she was not a new diver when it happened. She was chambered. After that she started getting rashes after deeper and longer dives, even on nitrox. Mostly on her abdomen. Sh manages this by restricting her dives as I mentioned and only diving nitrox. She considered getting checked for PFO but has elected not to thus far.

Thanks for the additional info.

 

[dmaziuk]

So, your gist is that nobody dives to their NDL anyway (or at least, not on a regular basis), so worrying about which computer gives a longer NDL is a waste of time?

No, my gist is that "dive computers and DCS" is a waste of time unless you're diving your computer to its NDL all the time.

I said before, once my computer starts limiting my dives, I'll worry about computers and NDL.Until then I don't rate long NDL as one of the top deciding features when purchasing a new computer. That's an orthogonal (though somewhat related) argument.

Your diving pattern is great, but I would like to remind you that you are not only tech. trained, you went from OW to cave in a year. So are you now saying, with the benefit of 20-20 hindsight, that it was all a waste of time and money?

 

[redacted]

No, my gist is that "dive computers and DCS" is a waste of time unless you're diving your computer to its NDL all the time.

Dive computers are a waste unless dive tables are limiting your desired bottom time.

 

[dmaziuk]

After that she started getting rashes after deeper and longer dives, even on nitrox.

As a side note, is there an actual basis to "even on nitrox"? -- the way I see it, replacing a few percent of N2 with O2 doesn't make the gas "not bubble". O2 my have different solubility and whatever else relevant properties (not sure how different, being right next to nitrogen in the periodic table), but it's going to bubble just the same.

 

[stuartv]

No, my gist is that "dive computers and DCS" is a waste of time unless you're diving your computer to its NDL all the time.

And, among the general population of OW divers, that does not seem to be all that unusual. So, being dismissive of that scenario when discussing dive computers seems inappropriate.

I said before, once my computer starts limiting my dives, I'll worry about computers and NDL.Until then I don't rate long NDL as one of the top deciding features when purchasing a new computer. That's an orthogonal (though somewhat related) argument.

Your diving pattern is great, but I would like to remind you that you are not only tech. trained, you went from OW to cave in a year. So are you now saying, with the benefit of 20-20 hindsight, that it was all a waste of time and money?

I understand. But, you often seem to base your suggestions to new divers looking for a computer on your own particular diving pattern, with no recognition seemingly given to the fact that many new divers will have a different diving pattern than you do and will be limited by short NDLs, either immediately or in the not-too-distant future.

You may have noticed that I have never suggested that you use a different computer than what you're using. I have only suggested that people who posted questions about computers consider algorithm and NDL as one of the factors in their purchasing decision. And I have attempted to debunk statements by others that the algorithm and NDL don't matter. They may not matter to you, but they do matter in general, and I think it's inappropriate to do anything other than give people the relevant information and let them decide for themselves whether it matters to them.

For the record, I finished my first tech training approximately 18 months after I finished OW. I started my tech training after only about 10 months, but winter and schedules caused it to take a while to finish. I am now about to finish my second tech cert, which is for Helitrox. I have no cave training. And none of that really has anything to do with my appreciation for maximal NDLs on recreational dives. I was maxing my NDLs on NC wreck dives in my first 8 months after OW - before I even started tech training.

As a side note, is there an actual basis to "even on nitrox"? -- the way I see it, replacing a few percent of N2 with O2 doesn't make the gas "not bubble". O2 my have different solubility and whatever else relevant properties (not sure how different, being right next to nitrogen in the periodic table), but it's going to bubble just the same.

Using Nitrox gives longer NDLs (for the same risk of DCS - according to the theories, anyway). If I max my NDL on Nitrox that means that I would have maxed it even more easily if I were diving air. So, for divers who are not Nitrox certified, NDL could even more easily be a limiting factor.

 

[dumpsterDiver]

I am reading these responses with interest.

It seems computers can vary in two different ways: 1. they might "pay the piper" with different algorithms requiring different stops, etc. but are perhaps equally conservative in effect,, or 2. they may simply accept more risk than other computers, however minute.

Is there any data available on DCS when used with particular algorithms? I had always heard, for instance, that the Navy tables, created by and for young healthy male divers, bent more than a few of them.

My understanding is that a somebody can climb out of bed on a diving day and based upon his/her health, amount of sleep, and many other factors will already have x amount of DCS risk for diving that particular day, before even suiting up.

I guess I am wondering if one can adhere to a computer's algorithm, and still get bent, and if so, how often does this happen?

And, if it happened, could it have been prevented on a more conservative computer?

Thanks.

If you are doing one or two moderately conservative dives, the different computers may give you similar results, but if you are diving aggressively with multiple dives per day over 100 ft and coming close to the NDL limits (NOT that I recommend this) - then you may begin to see huge divergences in different computers. It is pretty well accepted that Suunto computers are "worthless" within the advanced scuba spearfishing community because you "can't get any time"..

This would seem to contradict the idea that all the computers are "safe". However, it really doesn't because the incidence of DCS is so low that it would be hard to make a statistical analysis showing the degree of "safeness".

One the other hand, if one computer says you can go to the surface and other is giving you a 12-15 minute hang - it is pretty obvious to me that one IS safer than the other.

 

[stuartv]

One the other hand, if one computer says you can go to the surface and other is giving you a 12-15 minute hang - it is pretty obvious to me that one IS safer than the other.

Stipulated. But, is the computer requiring the hang reducing your chance of DCS by 10%? Or by 0.1%?

And would you be okay with normally using a computer that regularly gives you a 15 minute hang just to reduce your chance of DCS by 0.1%? What if your buddies were all using the "no-hang" computer and getting straight out and never having a problem?

 

[uncfnp]

As a side note, is there an actual basis to "even on nitrox"? -- the way I see it, replacing a few percent of N2 with O2 doesn't make the gas "not bubble". O2 my have different solubility and whatever else relevant properties (not sure how different, being right next to nitrogen in the periodic table), but it's going to bubble just the same.

NDL is NDL.

NDL is based on nitrogen loading. The more nitrogen in the gas you are breathing the quicker the ongassing. So the significance of nitrox is not the added O2 (mod/oxtox is the limiting factor here) but in the lower percentage of nitrogen.

So if you lower the percentage of nitrogen in your breathing gas you can extend your NDL. But diving to the limit of NDL gives the same risk of DCS, air or nitrox.

The benefit of nitrox to the diver at risk is one of two options...

1. Dive nitrox but keep BT shorter (as if still diving air) for less nitrogen loading and corresponding lower DCS risk.

2. Dive nitrox and get longer NDL/bottom time then with air but with a comparable risk.