Place of dive tables in modern diving

[kelemvor]

The wrong time is the wrong time. If I see it giving me such a short time while I'm planning, I'm going to distrust it the whole damn dive. What if they set things to 36 and you're diving air?

Not a good example. That is user error on your part rather than a result of repetitive use of the computer. If you don't know how to operate the computer then you shouldn't be using it. Just as you shouldn't be using a table if you don't know how to use the table (or wheel or whatever). At that point you might as well just follow the DM around and ascend whenever he says to do it.

 

[The Chairman]

Not a good example.

Sure it is. Hand me a PDC just before my dive and you are assuming that I know how to set it or even how to see that it is set properly. If they don't care that this PDC was already used today, then they probably don't care enough to show you how to use it and they also don't care to make sure it's set for air or whatever. If I loan you a PDC, and I've loaned them a number of times, I take the time to find out if they have already had a dive that day and that they know how to operate the particular model I'm handing them. I would never hand them a PDC that's been on a dive in the last 24 hours unless they were on the same dives with me and I was wearing it. Then it should be close enough.

 

[kelemvor]

Sure it is. Hand me a PDC just before my dive and you are assuming that I know how to set it or even how to see that it is set properly. If they don't care that this PDC was already used today, then they probably don't care enough to show you how to use it and they also don't care to make sure it's set for air or whatever. If I loan you a PDC, and I've loaned them a number of times, I take the time to find out if they have already had a dive that day and that they know how to operate the particular model I'm handing them. I would never hand them a PDC that's been on a dive in the last 24 hours unless they were on the same dives with me and I was wearing it. Then it should be close enough.

I guess I am of the opinion that it's on the person using the computer or table to know how to use it. The situation would be the same if your PDC broke and they handed you a table that you had no idea how to use. Either way, in order to dive safely, you have to know how to use the tool. Otherwise it's "trust me diving" at best. Probably worse than trust me diving because at least in trust-me the guy you're trusting might get it right.

I've only been diving for 5 years now, so lack your experience. I've also only dived 5 different DC's. However, they have all be easy enough to figure out how to dive - including checking the gas setting in a few minutes. The most difficult was definitely the sherwood wisdom they gave us in OW class.

 

[The Chairman]

I guess I am of the opinion that it's on the person using the computer or table to know how to use it.

That was my point. If they are negligent enough to send someone down with a PDC that's been dove on, then the negligence probably extends further than that. Most people diving air expect the PDC to be set for air. They probably assume that they won't have to do a thing.

 

[Neilwood]

All 8 cases I know of personally were on Ratio Deco. Those cases lead me to point out two interesting uses of logic related to the explanations of them.

1. The reason to use Ratio Deco, we were told, was because it is possible that the computer could make a mistake,so it is better to trust "the computer between the ears." In several of the cases I know, divers made mistakes, mistakes that were revealed because they had a computer in gauge more for a bottom timer and were able to check the log for the dive and find what they actually did rather than what they thought they did. When people make mistakes calculating average depths, miscount their deco times, etc., then it is their fault, not the fault of Ratio Deco. So apparently Ratio Deco is better than a computer because the human brain never makes mistakes, but when it does make mistakes, the brain is suddenly no longer an integral part of Ratio Deco.
2. All the cases I know of occurred at altitude--not quite 5,000 feet. We were told not to adjust Ratio Deco in any way for altitude, because altitude does not matter for decompression. Since that contradicts what everyone else believes, I asked how they knew it was safe to use RD at altitude without adjustment. I was told two reasons: 1) Andrew dives at Lake Tahoe without adjusting, and he is fine. 2) No one has ever been bent at altitude using RD. I responded that all the people in our group who had gotten bent were using RD at altitude. I was told those did not count, because there was some other reason for their being bent. I asked what those reasons were. They didn't know--maybe PFOs, maybe something else. How did they know it was not Ratio Deco? Because no one gets bent diving at altitude using RD, so therefore it had to be something else.

I find it interesting that when some people follow procedures that are different from everyone else and unsupported by any scientific studies, they are automatically assumed by some to be superior to everyone else. I was recently part of a FaceBook discussion in which PADI was mocked because it was still teaching traditional decompression approaches and not teaching this "more sophisticated" approach. When I pointed out that there was no science supporting it, I was removed from the discussion.

Sounds like one of the worst cases of normalisation of deviance that I have heard off. Pick any thing as the cause other than the one core idea. The reason for the belief that RD is safe to use at altitude might be something as simple as Andrew is one of those individuals who has a higher resistance to being bent, has developed some resistance to DCS or manages to stay just on the subclinical side of bent. Picking a methodology on the basis in item 2 seems like a huge gamble - you are going on the basis of a sample size of one who no-one verifiably knows how close he is to bent he actually is. This is especially true if (and I have no reason to doubt your views) any cases of DCS that do occur are effectively swept under the carpet due to "human error".

Surely the best situation is to do as much as humanly possible to minimise the chance and effect of human error. Pick an algorithm that has scientific basis (using informed sources to make your choice) and dive it conservatively (adjusting depending on physical factors such as age, fitness, altitude,cold, hydration as required).

Me, personally, I will stick to using a PDC and NDL diving. If my computer dies on me I will call the dive and ascend at a safe rate with my buddy (as indicated by his PDC) or, if he is somehow not available, at the rate of my bubbles and do a stop as near to 5m as I can for as long as I can. Then, depending on if I have a spare, I will probably sit a couple of dives until I am "clear" or carry on diving if I can work out my profile and residual gas (based on my buddies profile with a safety margin - I know where I have been in relation to him during the dive so it should be possible to get a reasonable "guesstimate").

 

[Neilwood]

That was my point. If they are negligent enough to send someone down with a PDC that's been dove on, then the negligence probably extends further than that. Most people diving air expect the PDC to be set for air. They probably assume that they won't have to do a thing.

If I am diving a rental computer (god knows why I would) I would not expect anything. I would be doubly aware during my checks of getting the set up right.

I wonder about the original computer though - most have a way to reset the memory for use by dive schools/ organisations such that they can be used multiple times per day by different people. The important issue though is knowing it has been dived that day which should be relatively easy to check.

 

[CT-Rich]

Okay, this is the core of table versus computer: Number of possible failure points. A computer can be incorrectly set prior to the dive, you can incorrectly interpret the information it provides, the battery could fail, it could flood or it could malfunction. All of those are possibilities.

The failure points for tables are different, you could forget to set your watch, you could fail to plan the dive correctly (your destination is 15ft deeper than anticipated), you incorrectly read the table either predive or you could fail to read it correctly at depth, you can fail to make correct assumptions about the dive profile you are actually following versus the plan. All the rounding up that occurs is to create a margin of error that will keep you out of trouble.

The number of potential human errors is enormous. Airliners use computers to do most of the flying these days because the technology guiding the aircraft is so much more reliable than a human could ever be. Which would rather rely on, the stall indicator or the experienced touch of the pilot? The pilot is there for a reason, but all that technology makes the plane safer.

Edit: If you enjoy using table and know how use them correctly, have at it. But IMHO, you are not diving safer, you are diving in a way that you like or feel comfortable. If you have any doubts about the ability of a rational person using flawed judgement, look at who is currently in charge of the USA.

 

[Storker]

If I am diving a rental computer (god knows why I would) I would not expect anything.

And this is why I try to make room in my luggage for some of my own gear if I'm diving abroad. After packing my camera gear, I usually am able to stuff in my own mask, reg set and computer. Those are, IMO, the most critical parts of my equipment and those parts I really prefer to have and know their service history and previous performance.

 

[kelemvor]

A computer can be incorrectly set prior to the dive, you can incorrectly interpret the information it provides, the battery could fail, it could flood or it could malfunction.

I think every one of those are also potential failure points of the watch.

 

[The Chairman]

The number of potential human errors is enormous.

The number of actual human errors is ginormous. Some are worried that a PDC might fail, but to err is human. We are defined by the fact that we are going to screw up. We are our weakest link and our worst enemy. The irony is that so many refuse to trust the more trustworthy of the two. Hubris? Luddism? Technophobia? Bad at choosing? There are many reasons for this, but it's obvious to me that the PDC is far more reliable than my brain at depth.