Thinking about getting advanced/nitrox...

[Michael Guerrero]

It does make a difference for recreational diving.

So you want to buy a new computer?

I read through your other thread and I disagree with you Stuart, but we can agree to disagree about it so as not to argue.

The algorithms used in dive computers are already more conservative than original Buhlmann tables. ZHL-16C was recommended for dive computers because it added additional conservatism over 16A and 16B. The real-time calculations at different depths were the concern in computers, which differ from diving tables and square profiles. Wikipedia actually covers the topic of choice well.

Anyway, the main point is that even though different computers will give you different numbers, to include for repetitive dives, they are all within conservative constraints relative to the original Buhlmann numbers, which themselves were created to reduce the probability of DCS to what was considered a tolerable amount (around 2% likelihood if I remember correctly).

If you want to know what diving with (maybe) a 2% probability of DCS run a table that gives you a dive with GF 100/100 using ZHL-16A and you'll probably be close to what Buhlmann's trials arrived at. Of course, individual physiology and differences on any given day will skew this some.

Anyway, safe diving.

 

[stuartv]

I read through your other thread and I disagree with you Stuart, but we can agree to disagree about it so as not to argue.

You originally said the algorithm the computer uses doesn't make any real difference.

I responded with the post which you quoted above - where I said it does make a difference for recreational diving.

Now you've said you disagree with me. There was only one sentence in my post, so I feel pretty good assuming that that is the statement you disagree with. I.e. you disagree that it makes a difference for recreational diving.

The thread I linked to contains, among other relevant things, links to lab reports where they put a variety of computers through a hyperbaric chamber to replicate 4 dives. The reports show that in some scenarios the different computers can tell the diver that he/she has NDL time left that varies by as much as 54 minutes.

I guess there really is nothing to argue about. What you've posted yields a clear conclusion: If you are in the water and your computer tells you you have 56 minutes of NDL left or if it tells you you have 2 minutes left, you feel that that is not a "real difference". I feel that it is a very real and huge difference. But, that is a subjective assessment and we are each entitled to our own opinion on whether 54 minutes is a "real difference". So, certainly no reason to argue. It is what it is and it's no skin off my nose that you don't think 54 minutes makes a real difference when you're doing recreational diving.

 

[dmaziuk]

... lab reports where they put a variety of computers through a hyperbaric chamber to replicate 4 dives. The reports show that in some scenarios the different computers can tell the diver that he/she has NDL time left that varies by as much as 54 minutes.

The point of that experiment was that they had a chamber in a nearby university that needed to put something into their annual report to the funding agencies.

If the computers are not identical, it follows that they should give different results. It further follows that you can come up with the set of inputs that will maximize those differences for a give pair of computers. You can also come up with inputs to minimize them. The fact that you can game them to as much as an hour difference is mildly amusing, but that's about it.

 

[stuartv]

The point of that experiment was that they had a chamber in a nearby university that needed to put something into their annual report to the funding agencies.

If the computers are not identical, it follows that they should give different results. It further follows that you can come up with the set of inputs that will maximize those differences for a give pair of computers. You can also come up with inputs to minimize them. The fact that you can game them to as much as an hour difference is mildly amusing, but that's about it.

So, you are saying that the results are completely meaningless and provide no useful information to consumers?

Do you think the dive profiles used in the testing were unrealistic?

Are you suggesting that they could have "gamed" the profiles a different way and the Most Liberal Computer and the Most Conservative Computer would have swapped positions?

Are you suggesting that if you took the most and least liberal computers tested and you went out and dived 4 dives a day for a month, and compared the NDLs you got along the way, there would be a random distribution of which computer gave more NDL on any given dive?

I'm trying to understand what makes the results completely useless for any purpose.

Maybe I just don't understand how computers work very well. I've only been developing software professionally for 32 years, so that is entirely possible. I could be entirely wrong. But, from what I have read, including but not limited to the reports I linked, I concluded that if I dived with the most and least liberal computers from that test, I would NOT see a random distribution of which computer gave me more NDL time. I concluded from that report that, statistically, I would see longer NDLs on the most liberal computer MOST of the time. How much longer would, of course, vary. But, I concluded that, in the right circumstances, it could be considerably longer. And even in the worst circumstances, it would almost never actually be less NDL by any significant (to me, of course - say 1 or 2 minutes) amount. I.e. I don't believe there would ever be any way to "game" the dive profiles to have the "most liberal" computer come up with 50 minutes less NDL than the "most conservative" computer.

Thus, I concluded that getting a more liberal computer would be pretty much a win-win. Sometimes, little or no benefit. Sometimes, big benefit. But almost never a drawback. In terms of allowed bottom time, of course.

How have I misunderstood the data or drawn inappropriate conclusions?

 

[dmaziuk]

So, you are saying that the results are completely meaningless and provide no useful information to consumers?

Correct.

Do you think the dive profiles used in the testing were unrealistic?

No, I think there exists an infinite number of very different and equally realistic dive profiles. The best I can figure they choose those 4 because someone else used them in some write-up before.

Are you suggesting that they could have "gamed" the profiles a different way and the Most Liberal Computer and the Most Conservative Computer would have swapped positions?

A complete swap is unlikely, esp. give the 54 minute difference, but coming within 2 minutes of each other: yes.

Are you suggesting that if you took the most and least liberal computers tested and you went out and dived 4 dives a day for a month, and compared the NDLs you got along the way, there would be a random distribution of which computer gave more NDL on any given dive?

Who said random? You're not getting the 54 minutes unless you dive those exact 4 profiles exactly the way they did it.

I'm trying to understand what makes the results completely useless for any purpose.

What makes them useless for my practical purposes is I'm on the 6th day of mostly 3-tank/day diving with the deepest one to 30m, diving the computer they IIRC drove to 2 min NDL: Cressi Leonardo. I'm going for 2 more tomorrow (last day ) and I don't expect to run out of NDL on either of them.

PS. point being, if their results were in any way representative my leonardo would've been giving me deco ascents since day before yesterday. If you're a software developer you're well aware of the GIGO principle. They set out to show those 4 profiles highlight the differences between different computers, they showed it. My problem with these kinds of "studies" is using graphs and numbers create an impression of legitimacy while tn reality they are just garbage out.

 

[stuartv]

Who said random? You're not getting the 54 minutes unless you dive those exact 4 profiles exactly the way they did it.

Who said 54 minutes (in the question you quoted)? I asked if your Leonardo and a liberal computer would yield a random distribution of which one gives more NDL on any given dive?

My own conclusion from the reports is that it would not be random and that the more liberal computer would yield a statistically significant result of giving more NDL a lot more often.

What makes them useless for my practical purposes is I'm on the 6th day of mostly 3-tank/day diving with the deepest one to 30m, diving the computer they IIRC drove to 2 min NDL: Cressi Leonardo. I'm going for 2 more tomorrow (last day ) and I don't expect to run out of NDL on either of them.

I didn't ask what makes them useless for your practical purposes. I asked what makes them useless for ANY purpose - which would include other people's practical purposes.

If your gas doesn't last long enough to get you to your NDL (or whatever portion of your NDL you feel is safe to dive), then obviously a more liberal computer is not going to help you. But, I don't have any trouble at all staying down right to my NDL - at least on the dives I've been doing over the last 9 months. And, from what I've read, I am not particularly unusual in this regard. IIRC, BoulderJohn posted that he's had brand new OW students that were good enough on gas consumption that they could last to their NDL.

It's fine if you want to tell people that a conservative computer doesn't limit you at all. But, telling them that it won't limit them doesn't seem like you're doing anything but trying to get other people to live with your limitations.

PS. point being, if their results were in any way representative my leonardo would've been giving me deco ascents since day before yesterday. If you're a software developer you're well aware of the GIGO principle. They set out to show those 4 profiles highlight the differences between different computers, they showed it. My problem with these kinds of "studies" is using graphs and numbers create an impression of legitimacy while tn reality they are just garbage out.

Yes, I am well familiar with GIGO. What is the "garbage in" in these lab reports? You don't expect a dive computer to behave the same way in the water as it would in a hyperbaric chamber that simulates the exact same profile? You think a dive this is one or 2 minutes longer or shorter or 1 or 2 feet shallower or deeper will have significantly different results on any given computer?

Why do you think those lab reports should imply that your computer would have you in deco? You dived the same profile and schedule as the lab tests and you exceeded the time that the lab report says you should have had for your NDL?


You realize these dive computers we're talking about are actually computers, right? They take input of pressure and time and breathing gas and they run a mathematical and table-based algorithm to give you a number - an NDL. Generally, they are very reliable and the results are very repeatable. And the results are generally continuous. You take any result and vary any of the inputs a little bit and the result is going to be pretty close to the previous result.

I can't see what the "garbage in" is. And I can't see how you can keep saying that the lab results don't allow anyone to draw ANY useful conclusions. These things do not have some random function built in. All I can see is that they aren't useful to you because you aren't able to stay down as long the NDL on your conservative computer.

 

[dmaziuk]

I can't see what the "garbage in" is. And I can't see how you can keep saying that the lab results don't allow anyone to draw ANY useful conclusions. These things do not have some random function built in. All I can see is that they aren't useful to you because you aren't able to stay down as long the NDL on your conservative computer.

Garbage in is throwing a bunch of different computers in a chamber on a few profiles chosen for no apparent reason, with no clearly stated goal. They'll calculate different NDL. Also, water wet, fire burns, film at eleven.

 

[rjack321]

I can't see what the "garbage in" is. And I can't see how you can keep saying that the lab results don't allow anyone to draw ANY useful conclusions. These things do not have some random function built in. All I can see is that they aren't useful to you because you aren't able to stay down as long the NDL on your conservative computer.

The garbage is the assumption you are making that somehow the aggressive computer is just as "safe" as the conservative one. And that there is a single answer or bright line between bent and not bent - and that its even possible to program a computer to faithfully replicate this. Sure some computers give you a ton of NDL time on repetitive dives. And some don't. The reality is that nobody actually knows the probability of DCS on any given dive - especially a repetitive dive. And nothing screws up a dive vacation (or a weekend) quite like a bunch of chamber rides.

Its quite easy to avoid being "limited" by conservative computers like the suuntos. Dive nitrox, get decent SIs, plan progressively shallower dives, and multilevel them with hefty amounts of time in the offgassing zone <30ft/10m

Doing five 100ft square profiles a day on air with 60 minute SIs is a recipe for getting bent on any computer.

 

[stuartv]

The garbage is the assumption you are making that somehow the aggressive computer is just as "safe" as the conservative one. And that there is a single answer or bright line between bent and not bent - and that its even possible to program a computer to faithfully replicate this. Sure some computers give you a ton of NDL time on repetitive dives. And some don't. The reality is that nobody actually knows the probability of DCS on any given dive - especially a repetitive dive. And nothing screws up a dive vacation (or a weekend) quite like a bunch of chamber rides.

Where did I (or anyone else) make an assumption that any computer is as safe as any other computer? All I am "assuming" is that these computers in the report run the stated algorithm and produce the results that they are supposed to, based on the algorithm.

As for nobody knowing the probably of DCS, I think that is patently false. I think (but I cannot back this statement up with supporting citations, so I admit I could be wrong) that there is a ton of research on these algorithms that would absolutely support predicting the probability of getting DCS on any given dive.

 

[rjack321]

Where did I (or anyone else) make an assumption that any computer is as safe as any other computer? All I am "assuming" is that these computers in the report run the stated algorithm and produce the results that they are supposed to, based on the algorithm.

Right here with words like "win-win"

Thus, I concluded that getting a more liberal computer would be pretty much a win-win. Sometimes, little or no benefit. Sometimes, big benefit. But almost never a drawback. In terms of allowed bottom time, of course.

How have I misunderstood the data or drawn inappropriate conclusions?

In thirty or more posts on this topic in multiple threads you have never once recognized that diving right up to the NDL on any computer and especially on an aggressive one is riskier and much more likely to put you in the chamber (or worse).

As for nobody knowing the probably of DCS, I think that is patently false. I think (but I cannot back this statement up with supporting citations, so I admit I could be wrong) that there is a ton of research on these algorithms that would absolutely support predicting the probability of getting DCS on any given dive.

You would be wrong. The Navy and DSAT tables both have some risk estimates behind them but neither have DCS probabilities from 5 dives a day or a 30 dives in a week type of situations.