DIR and computers

[String]

Decompression methods are more based on statistics than hard draw out science - i know the tables i followed are like they are because they killed a sufficiently small number of goats under certain conditions.

Whenever you pick a table to follow you are taking a chance that statistically you are unlikely to suffer following it.

A dive computer does this, a human with the 120 rule does this.

However a dive computer is far more accurate with its maths and time snap shots that it adheres more closely to the tables than a human.

Its extra precision.

There is nothing wrong with that provided you dont rely solely on it as it could fail. Using a computer whilst knowing where you are on the tables is in my view a good idea.

So while doing a multilevel dive, by all means use a computer to stay nearer to the appropriate decompression model but just be prepared to use "worst case scenario" tables if needed.
Doing everything in your head makes unneccesarily rough approximations meaning you will deviate further from the intended model.

 

[Diver0001]

MHK once bubbled...


<snip>

That being said, when you look at our approach a DIR diver need only remember two numbers[ ie; 120 and 20%] <snip>

To that end, once we accept the premise that you only need two numbers, then all a diver needs to do is take 5 minute snap shots of his dive to compute average depth.. Take your average depth, apply the 20% rule and the 120 rule and away you go.. It's really quit simple and I suspect people make more of it then it needs to be in part because they were never taught it in the first place, and secondly it would diminish sales of computers..

Hope that helps..

I just had an "ah-ha" moment. Thanks for that. I'm familiar with the 120 rule. I used to use this before we had computers except I used max depth instead of avg depth. Is the 20% just 20% extra on the depth to be on the safe side? Now I also understand the preoccupation about not being narced.... LOL

Do you keep using the avg depth once you go into deco or do you apply the deco-table to match your max depth? Do you plan your deep stops ahead of time or is there a "trick" to working them out on the fly (1/2 max depth, 1/2 avg depth or somehting along these lines....)?

R..

 

[cornfed]

Jonathan once bubbled...
What about the benefit to dive accident analysis the computer gives you?

In this respect, they provide nothing new. In "gauge" mode you can still record and download your profile. And if that's all your interested there are cheaper alternatives then using a computer in gauge mode.

if our computer registered .1 deeper than the planned max depth then that dive did not count towards training requirements

Pedagogically there isn't anything wrong with this. However, if you believe this invalidates the tables you cut, then you're not understanding what Mike is trying to tell you.

 

[cornfed]

String once bubbled...

a dive computer is far more accurate with its maths and time snap shots that it adheres more closely to the tables than a human.

Its extra precision.

You didn't read the whole thread before posting, did you?

 

[DA Aquamaster]

The 120 rule makes sense as a backup if your computer takes a dump on you but I have to agree it lacks precision and, the way MHK has desribed it, assumes NDL's are linear. They aren't and averaging depth is effective at best to maybe 20'-30' spreads in depths. This in my opinion limits it usefulness in comparison witha computer.

I agree that offgassing is not an exact science, but in my opinion this is an argument for increased conservatisim and more accurate sampling to minimize the variables involved.

The 120 number is also based on Navy tables that were normed on physically fit male divers with an average age of 23. They also assume a relatively low hit rate that was determined acceptable for navy diving where in most cases a chamber is aboard ship. They also included provisions for repetetive diving but were not really intended for it and the hit rate on square profile repetetive dives is approx 4%. The argument that the Navy folks are pretty bright and have been using them since '58 is valid only if you take into consideration exactly how the Navy has been using them and operate accordingly.

The Navy tables are safe for rec diving but only when used with a number of substantial saftey factors added to the dive in both planning and diving phases. Most training agencies have long since moved to more conservative tables for good reason. Selling 120 as "the" number is pretty ill advised.

My preferred plan for computer failure is to plan the dive and use contingency planning to figure the deco required if the computer fails at various points in the dive. If the computer fails I can still dive the plan and ascend normally referring to the stop depths and times taped to the top of my fin. And again, with frequent reference to the computer and a fair amount of expereince with past dives, I am always goig to know my deco status anyway at the moment I look at my computer and discover it has stepped out for a beer and be able to terminitate the dive safely.

A backup depth gauge is not a bad idea and is normally available strapped to your buddy. But in solo situations, (intentional or otherwise) a low profile an inexpensive capillary depth guage works well and has the advantage of being very sensitive at shallow stop depths.

But I do agree completely with the philosophy that a computer is not a replacement for a brain during the dive and that divers should fully understand how their computer works, how it can fail and have a plan for what they should do when it does.

 

[Big-t-2538]

Diver0001 once bubbled...


Do you keep using the avg depth once you go into deco or do you apply the deco-table to match your max depth? Do you plan your deep stops ahead of time or is there a "trick" to working them out on the fly (1/2 max depth, 1/2 avg depth or somehting along these lines....)?

R..

Well, the "trick" as I understand it to deep stops is 80% of your averave max ATAs.

So you are at lets say 100' for an average depth....that's 4 ATAs. Your first stop should be at 3.2 ATA's or 73'.....so the question is how do you get 73' so quickly? Simple 80% of your ATAs will be at 80% of your max depth minus 7 feet. I can draw you out the two equations and simplify them if you like, or you can just try this for an array of depths and ATAs and see if I'm right.

Anyway, that's sort of beyond the scope of recreational diving. On the dives I was doing in the Straits last August, I was keeping track of my average depth once we hit the wreck. Then we did a stop for 1 minute at 80% ATAs (again all recreational no deco stuff....just taking extra precautions here) then a nice slow ascent (20ish fpm) to 30' 1 minute there, a minute at 20, and a minute at 10. Then maybe a few extra minutes waiting at about 10' for the ladder on the boat to clear so we could just surface and hop on.

 

[Custer]

DA Aquamaster once bubbled...
The 120 rule makes sense as a backup if your computer takes a dump on you but I have to agree it lacks precision and, the way MHK has desribed it, assumes NDL's are linear. They aren't and averaging depth is effective at best to maybe 20'-30' spreads in depths. This in my opinion limits it usefulness in comparison witha computer.

Yup.

DA Aquamaster once bubbled...
I agree that offgassing is not an exact science, but in my opinion this is an argument for increased conservatisim and more accurate sampling to minimize the variables involved.

The 120 number is also based on Navy tables that were normed on physically fit male divers with an average age of 23. They also assume a relatively low hit rate that was determined acceptable for navy diving where in most cases a chamber is aboard ship. They also included provisions for repetetive diving but were not really intended for it and the hit rate on square profile repetetive dives is approx 4%. The argument that the Navy folks are pretty bright and have been using them since '58 is valid only if you take into consideration exactly how the Navy has been using them and operate accordingly.

Just like PO2s, IMO.

DA Aquamaster once bubbled...
The Navy tables are safe for rec diving but only when used with a number of substantial saftey factors added to the dive in both planning and diving phases. Most training agencies have long since moved to more conservative tables for good reason. Selling 120 as "the" number is pretty ill advised.

Within the DIR assumption, of course, is a reasonable or better level of fitness.

DA Aquamaster once bubbled...
My preferred plan for computer failure is to plan the dive and use contingency planning to figure the deco required if the computer fails at various points in the dive. If the computer fails I can still dive the plan and ascend normally referring to the stop depths and times taped to the top of my fin. And again, with frequent reference to the computer and a fair amount of expereince with past dives, I am always goig to know my deco status anyway at the moment I look at my computer and discover it has stepped out for a beer and be able to terminitate the dive safely.

My dive plan would cover bottom time.

Once on the surface, I'd just square profile it on a table, and continue to dive.

DA Aquamaster once bubbled...
A backup depth gauge is not a bad idea and is normally available strapped to your buddy. But in solo situations, (intentional or otherwise) a low profile an inexpensive capillary depth guage works well and has the advantage of being very sensitive at shallow stop depths.

My backup is an incremented spool line.

Anyone know of a smaller than average SPG?

DA Aquamaster once bubbled...
But I do agree completely with the philosophy that a computer is not a replacement for a brain during the dive and that divers should fully understand how their computer works, how it can fail and have a plan for what they should do when it does.

Absolutely.

And if my computer ever failed, I'd have a new one the next dive.

 

[HammerNoMore]

Custer once bubbled...
Dive Rite 2-gas computers can be had for around $300.

Minor correction here. MHK was discussing helium, not the ability to switch gasses. The Nitek He cannot be had for $300.

James

 

[jviehe]

Ok, heres what I got out of it:

1. computers fail
2. if you rely on the computer, when it fails, you dont know your math
3. you rely on the computer and so dont plan your dive well

As I see it, computers/tables/120 has one function. To keep you from getting DCS. That said, all of these methods will accomplish that goal. Rapid ascent is the #1 cause of DCS according to DAN. Now, i think theres a myth out there as to computer failure. I think any of us can give plenty of examples of the thousands of dives completed with no computer failures. If you check your battery, get it serviced, and keep it in good condition (including not buying used), as with any gear it will function correctly almost all the time. Those times when it does fail, abort the dive, just like with any piece of equipment. Or use 120/tables as a backup, its up to you.

Second, yes relying on a computer happens, and in fact I think this is a good thing, as it reduces task loading. A computer is only doing the math that you do in your head with the 120 rule. I would also argue, that humans are more likely to make a mathmatical mistake than the computer. If you dont believe me, multiply 29375 x 9276456 15 times in a row in short hand, and tell me how many different answers you get.

Now, I would have to agree that dive planning is somewhat lessened when using a computer. But so what, diving is easy. When I dive I tell my computer what mix I'm using, and it tell me how long I can stay down, and how deep I can go. I then dive till I'm low on air or time, then come up. In almost all cases I end up with more bottom time and/or less SI. I have flexibility, no worries, and no DCS.

For those who dont trust the algorithms, remember that they are based on 40+ years of lab and real world tests. Think of how many others have your computer, and how many dives have been logged on all the thousands of computers out there. Look at DAN statistics and try to find a pattern of injury due to the computer messing up.

Now, computers arnt for all situtations, and people do need to learn to use them, just like tables or any other equipment. Redundancy in computers for tech situtations is probably a neccesity. But, in rec. situations, I beleieve they allow people to get in the water quicker, and out of the water safer, and isnt bottom time what its all about? Planning your dive isnt the fun part. Doing math under water isnt the fun part. Whatever allows you to focus on the fun part more is what I'll always be for.

My name is Jon and I use a Sherwood Logic.

PS. the 20% thing refers to figuring EAD i think, adding 20% fsw when on 32% nitrox. I could be mistaken.

 

[ENikS]

The dive plan is as good as its calculations are.
Humans are prone to errors and being underwater does not help to improve cognitive abilities. Sometimes it takes only one error in your calculations to have disastrous or possibly deadly results. Even a small hit could be disastrous if you have no access to professional help. It may not happened to you ever but what if…
Computers do not require anyone to turn off their brain, one can dive any method or algorithm but rather they provide backup mechanism which could point out any deviations. As long as calculations agree (to certain degree) with computer readings everything is OK but if there are considerable differences its time to start checking math or equipment.
Basically it gives you redundancy for main computer between your ears.

Eugene