I've been thinking about safety

[KrisB]

You know, much of the discussion on this forum of SB centers around the conservative/liberal bias of different computers... with little regard for safety.

It seems worth considering that the safest computer is the one that gives the least amount of bottom time... there is no question there.

So, the question becomes not how conservative the computer is, but how safe it is -- one that forces you to go up at about the same time the PADI tables would is unarguably safer than one that lets you stay down for another 25 minutes.

Just something I've been considering,

 

[MoonWrasse]

You know, much of the discussion on this forum of SB centers around the conservative/liberal bias of different computers... with little regard for safety.

It seems worth considering that the safest computer is the one that gives the least amount of bottom time... there is no question there.

So, the question becomes not how conservative the computer is, but how safe it is -- one that forces you to go up at about the same time the PADI tables would is unarguably safer than one that lets you stay down for another 25 minutes.

Just something I've been considering,

I would think that the more "conservative" computer would generally be the "safer" one, however that's a simplistic view of the term "safety". You have to consider other aspects, such as ascent rate, repetitive dive interval for example. Also remember that the computer only measures time and events - never your actual N or physiological state (how much sleep you had, how many beers the night before, smoker, etc).

Just my 2 cents

 

[Dive-aholic]

True, but there are some computers that will make you surface before the PADI tables call for it. This is also only if you descend to a particular depth and stay at exact depth for the entire dive. Not many dives work out this way, so there isn't a computer that will be exactly on the tables. We could record our exact profile and figure it out through a series of calculations when to ascend, but why do that? Just buy a computer and it will do it for you.

Computers allow you to surface later than PADI tables because they account for your depth at 15-60 second intervals (depending on the computer). This allows the algorithm used by the computer to calculate your N2 uptake and off gas for a customized dive. It's like having a customized table for each dive.

 

[KrisB]

I agree, MoonWrasse -- but often enough, an individual diver may not take those considerations into effect when calculating their NDL from tables!

Dive-aholic: among the most conservative of computers is the Suunto line. I compared the tables in the back of the Cobra manual with my PADI tables -- the result is that unless you have the "personal adjustment" or "altitude" settings turned on, you will be allowed the same amount of time with the Cobra as with the PADI tables, if you dive a square profile.

Keep in mind -- the tables don't guarantee no-bends diving, either!

 

[BigJetDriver]

(1)You know, much of the discussion on this forum of SB centers around the conservative/liberal bias of different computers... with little regard for safety.

(2) It seems worth considering that the safest computer is the one that gives the least amount of bottom time... there is no question there.

(3) So, the question becomes not how conservative the computer is, but how safe it is -- one that forces you to go up at about the same time the PADI tables would is unarguably safer than one that lets you stay down for another 25 minutes.

Just something I've been considering,

Kris,

(1) Actually, there is a very high regard for safety here on this Board. In regard to wet-computers, the discussions take into account the inherent safety of the various computers. If they were found NOT to be safe, they would be yanked off the market immediately.

(2) There IS a question with your second assumption. Various decompression models have varying statistical safety factors. Not by much, but there are variances. They are still way in the safe zone, in any case.

It must be remembered that ALL diving is decompression diving. If you limit your bottom time to the point where it is safe to immediately surface at any point within that time frame, you are doing "No decompression stop required" diving, or what we call "recreational scuba"

(3) Actually, the PADI tables are not the most conservative. Tables put out by DCIEM (now DRDC) in Canada are more conservative. It could be argued, additionally, that tables based on Buhlmann's later algorithims are safer, as well.

In short, it is just not as simple as what you are proposing. (Sigh...I sometimes wish it were!! :11: ) It is ALWAYS time/dose related, i.e. how long, how deep, and what fraction of inert gas. In addition, however, there are a lot of other factors that each decompression model must take into account.

Cheers!

 

[miketsp]

Kris,
(1) Actually, there is a very high regard for safety here on this Board. In regard to wet-computers, the discussions take into account the inherent safety of the various computers. If they were found NOT to be safe, they would be yanked off the market immediately.
..snip..

Unfortunately not true:
http://www.sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/2003/05/25/MN309974.DTL

 

[Diver0001]

You know, much of the discussion on this forum of SB centers around the conservative/liberal bias of different computers... with little regard for safety.

It seems worth considering that the safest computer is the one that gives the least amount of bottom time... there is no question there.

Is it really? If a particular (hypothetical) computer gives divers who uses it correctly a DCS instance of 0.000001% is that any safer (read significantly better) than one that gives you 0.0000001%? The PADI tables gave a DCS incidence of ZERO in testing. On that basis we could conclude that safer than the PADI RDP is impossible......

Compared to the model, heavy lifting may have an impact at the third point after the decimal and maybe fast ascents at two points after the decimal.... (see where this is going?). There are many other contributing factors.

The piont is thta depth and bottom time are only two things in the weigh scales of DCS risk. At some point other factors have more impact (much more impact) than the model itself. So if a computer is keeping you safe by being "extra conservative" then what's really happening is the diver is applying a solution to the wrong problem but still getting a result..... (in other words, he/she is throwing technology at a training issue like inability to control ascent speed, for example).

So, the question becomes not how conservative the computer is, but how safe it is -- one that forces you to go up at about the same time the PADI tables would is unarguably safer than one that lets you stay down for another 25 minutes.

Just something I've been considering,

Well...as I alluded to above, the real question becomes this:

Now that we have a good model, how do we take additional measures to limit the DCS risk of other factors which are suddendly showing themselves to be the critical elements?

Do we have a complete picture of the other factors?

Do we know the impact they have on your safety?

Can we avoid or should we mitigate the risk?

Do we know what can and/or should be done to mitigate?

and so on..... See what I'm getting at?

In that light a more conservative computer isn't necessarily "safer"

R..

 

[miketsp]

You know, much of the discussion on this forum of SB centers around the conservative/liberal bias of different computers... with little regard for safety.

It seems worth considering that the safest computer is the one that gives the least amount of bottom time... there is no question there.

So, the question becomes not how conservative the computer is, but how safe it is -- one that forces you to go up at about the same time the PADI tables would is unarguably safer than one that lets you stay down for another 25 minutes.

Just something I've been considering,

I don't really see where you want to go with this. The ultimate conclusion of your 2nd paragraph is that it's safer not to dive.

Anyway depending on the algorithm it is quite possible that a computer that gives you less bottom time than another when diving a smooth profile is actually less "safe" than the other if the other uses an algorithm that will penalise you heavily if you do a jagged profile with a lot of sudden short range ascents or a very short surface interval. The penalty for an overshot stop may be significantly different between the 2 models.
A single square dive evaluation against tables is not what makes a computer safe or not.

Edit. PS. I just saw that Diver0001 made a post on similar lines while I was typing.

 

[scubatoys]

We got a product in our store once... it was a tank weight... and on the strap in big bold letters is said:

Caution! To avoid scuba related death - do not dive.​

I've got one of those pool alarms on our pool... if anyone jumps in it blows an alarm in the house so we can make sure no little kids fall in. I guess if we use these as dive computers... that would be the best computer as it is the most conservative... alarms if it gets wet at all.

Or... maybe I'll stick to my approach. Use the Oceanic / Aeris algos, which are built on Padi tables - and based on Dan tests, most all your bends cases are from too fast ascents, not staying too long anyway... Then if I want to be more conservative, I'll just set it to beep at me when the nitrogen bar hits the yellow instead of the red.

I don't understand why people think if a computer says you can do a 60 foot dive for 57 minutes, that means you have to stay that long. I had a car that did 150, but I didn't drive that fast (all the time anyway).

But everyone should understand the differences and choose the one that makes them comfortable. Me... give me liberal, and if I want, I'll back down from the allowable times. I've done many a dive on a live aboard while other divers were looking at their computer saying they had to sit this one out..

There are more divers diving with Padi algo's either on table or aeris/oceanic/sherwood/genesis computers than any other... and the last Dan stat I saw was a bends case is about a 1 in 28000 dive experience.... and how many of those 1 in 28,000 were dehydrated, came up too fast, etc...