Wisdom of trusting one's dive computer?

[doctormike]

@doctormike you gave some really good info... Take er home and explain how / why one should choose their conservatism settings so I don't have to spend hours watching YouTube videos.

Is there a calculator anywhere for suggesting GFLo and GFHi settings based on whatever factors? (water temp, BMI, aversion to risk, etc...?)

You will find that experienced divers and instructors rarely if ever recommend specific gradient factors. They are by definition personal adjustments to a poorly understood risk. For recreational diving, trying to be precise about picking individual GFs really doesn't make sense.

Any modern dive computer will result in a reasonably minimal chance of DCS if used correctly. Most give you leeway to make general adjustments (like Shearwater's low, medium and high conservatism). Don't try to be more clever than the engineers by picking custom GFs if you aren't technically trained, there is no need for that.

Picking any algorithm and conservatism level is always a bright line through a grey area. While no dive computer can guarantee you won't get bent if you follow it, it can minimize that chance. Of course, if you are obese, dehydrated, hung over and working hard on a dive, set your conservatism to low and run your NDL right down to 0, omit your safety stop and make a relatively fast ascent over the last 15 feet, you certainly have a greater chance of getting an "undeserved" hit, even though you technically didn't bend your computer.

So unless you find that your computer is giving you ridiculously short NDLs, I would just set it to high conservatism and enjoy the dive...

 

[doctormike]

If you’ve been diving 30//70 and you’ve never gone into deco, and you’ve never had any symptoms of DCS, continue diving 30/70. The differences in the profile are minimal and it’s not worth the risk. It’s working for you, why change it?

Until such time that you really understand deco theory and GF’s, stick with what’s working. If you’re interested in learning, DO NOT go down the rabbit hole of YouTube videos to learn. Get a legit book like Deco For Divers and read it. It’s an excellent book and you’ll learn a lot.

This is exactly the advice that my instructor gave me. I dive 30/70, and I notice that lately a lot of people are getting shallow faster (using a bigger GFLo), so I asked him what he thought. He said - you have never been bent with 30/70, why change?

One thing thought. While I haven't been BENT with 30/70, I often feel tired after a deco dive, especially if I do two in a day. Sometimes with flu like symptoms. We really shouldn't think about decompression sickness as a binary disease where you either have it or you don't. Everybody bubbles, bubbles cause symptoms, possibly through inflammatory mediators. So the real question about getting shallow faster is what is the bigger driver of decompression stress? Is it the extra ongassing of slow compartments at a deeper stop (a reason to use a bigger GFLo). Or is it the extra overpressure on your first stop (a reason to use a smaller GFLo). Not sure I understand enough physiology to answer that question for my personal body.

We always try to be too clever and overanalyze things, but there are so many variables that aren't accounted for in dive computer algorithms, that a bit of excessive conservatism is not a bad idea.

 

[drbill]

As a marine biologist, I often have no dive plan as Mother Nature is not always predictable. I may descend thinking I'll be at a relatively shallow depth but during the dive spot something that takes me much deeper. I dive with redundant computers which gives me some additional security, especially if one should go haywire on me (like two of my Seiko manufactured ones did).

 

[johndiver999]

Some experienced divers get away with pushing the "relationship" in an aggressive manner. However, I suspect that these people have "experimented" with many hundreds (if not thousands) of dives and they begin to get a sense of what is too aggressive.

This theoretical pool of "experienced and aggressive" divers is not a random representation of the general population, because it is quite possible that quite a few divers, begin to realize that they feel bad, get sore, get super tired or even suffer from undeniable decompression sickness and they learn to back off and thereby remove themselves from the theoretical pool of aggressive divers. The remaining population, has self selected in some manner by possibly being less susceptible to DCS. You could simplistically assume that this sort of division might be correlated with the presence or absence of a PFO, but I bet it is more complex than that.

In any regard, I think there is a good bit of variability in susceptibility to deco stress. I know I have seen other people do things, that I would not do unless I was in an emergency (like casually blowing off 50% of the deco the computer wants on the last dive, for example).

So it makes sense to follow a computer carefully. IF a person is beginning to think of pushing the "relationship" (time, depth, # of repetitive dives, thermal stress, work load stress, doing actual deco, selecting aggressive settings etc.) - then they should proceed cautiously and NOT necessarily emulate what the guy sitting next to them does. Maybe this is somewhat of the reason why people may be hesitant to openly discuss conservative settings?

Put simply, the settings that work OK for skinny DM from Cozumel who does 600 dives a year might be different than the settings that a recently certified accountant who does 2 diving vacations a year might try.

 

[NAM001]

I have another concern with these computer chasers. Decompression is not an exact science and my own observation tells me that there are significant number of divers out there who experience mild decompression symptoms while staying within the NDL limits projected by their computers. Traditionally, tables would assume a square profile and therefore had a safety cushion built into them but computers were developed so that you may chase the NDL right to its very end. Now those NDL limits are not as precise as they appear. In reality, if you are feeling cold then your computer should show a different NDL and if you are dehydrated then your computer should show a different NDL. Computer has no way of knowing if you are dehydrated, obese, cold so obviously, when a hard limit is projected to a wide population, some people will be bent.

Technical divers deal with this dilemma by using gradient factors but recreational divers do not know enough to apply them. They are at the mercy of whatever limits the manufacturer has chosen to project at them. This is why if you were on a dive boat in North Carolina, a big percentage of people just fall into deep sleep on their way back from the dives. It should make people stop and think how could such a significant number of people on the same boat immediately have a "nap attack" in the middle of the day? Are they always this nappy at 2:00 pm when they are sitting at work? Then you look at a fishing boat where people have been out in the sea for the same time and no one is napping. Yet none of the sleepers on the dive boat will ever think that they are bent because their computers are telling them that they are safe. I have personally struggled to wake people up when the trip was over and they are all walking funny and complaining of headaches but it never occurs to them that this is what a mild decompression hit feels like because they trust their computers more than their own symptoms.

Furthermore, there is a significant percentage that thinks that breathing nitrox makes them feel so much more energetic after the dive. Now there is no scientific reason that an unbent individual will feel better when they breath a higher percentage of O2 so these are obviously the people who were getting mild decompression symptoms like fatigue and lethargy after the dive. Being on nitrox prevents them from getting bent so the comparative lack of fatigue that they experience is what they regard as higher energy levels associated to richer O2 mix.

My take is that while computers are very, very useful they are a supplement to your understanding of decompression. It is important to have a rough idea of how much time you have at a specific depth prior to jumping in Instead of jumping in and finding out. It is also important to realize that those limits on any given day may not apply to you and when you should not obey the computer and add your own layer of conservatism to the dive.

Good info. In rec diving there is a safety stop to cover these things you mention. Also in computer chasing you have conservative settings at your disposal to use in the same manor as you referenced using GF's. Unfortunately rec divers know little about how to use them, on the plus side many think max conservatism is best but then get bummed when the price they pay for that is loss of NDL. Untill they start teaching theory of tables again and some deco theory this will continue.

 

[NAM001]

Good post overall, but recreational divers do have conservatism options on most modern computers, which is a simplified way of doing what gradient factors do.

WHAT?????

 

[doctormike]

WHAT?????

"Good post overall, but recreational divers do have conservatism options on most modern computers, which is a simplified way of doing what gradient factors do."

 

[NAM001]

30/70 is just incredibly conservative for no stop dives. Sometimes advice on SB is just wrong.

I think the comment was directed at the capabilities of hte computer rather than advice regarding its use. the shearwaters have their own definable GF set also so you can make it ( for arguments sake) 50 or 60 if you want

 

[NAM001]

"Good post overall, but recreational divers do have conservatism options on most modern computers, which is a simplified way of doing what gradient factors do."

there are many more computers not on this list that have conservative settings in them. I have 2 aeris and they have them and are not o n this list. the settings are not necessary lables as gf's but they have low med hi or M0 M1 M2 etc and other names but they are there in every computer i have seen. not saying there are not a lot that do not have them just that these settings are COMMON in rec computers

 

[tursiops]

I think the comment was directed at the capabilities of hte computer rather than advice regarding its use. the shearwaters have their own definable GF set also so you can make it ( for arguments sake) 50 or 60 if you want

Are you suggesting there are some modern computers that do NOT allow setting some conservatism?