Mares Puck 4 runs Buhlmann with GF

[Etmutt]

I don't see it that way. Let's say diver gets bent. Doc gets hold of it's dive computer, now he/she needs correct software to read it and interpret dive profile. Shearwater needs few button presses to show tissue saturation in real time. I see it as I see pilot's job. Pilots are not paid big money for when everything runs smoothly, they are paid when for when everything goes to sh#t.

Ok this needs debunking.

So taking a step back what are makes a computer "safe"?

1- it is working and is solide enough that it won't break mid-dive
On this point, beside few black ship, all the computer out there are safe

2- the model used is safe
Well, nonetheless all the buhlmann prophets out there; there is simply no proof that the rgbm is not on par or better or worst than another model. While selecting his computer one might want to have look to the deeply safe lab data and select the one that suits your desired safety level

3 - in case of accident, it give the needed data
in case you are bent, no doctor will even give a s*** on the saturation level estimated by a computer. You are bent, you have symptoms, they will need the depth, at worst they will need a quick look at the profile, to help them select the navy table they will use.

 

[Angelo Farina]

The issue is when you didn’t really miss a deco stop, but the computer has bricked itself anyway. Even if you did, what would be the point of staying out of the water for 2 or 3 days or longer, assuming you were asymptomatic?

If the computer bricks not having missed any deco stops, then its software is faulty and should be fixed.
If the computer bricks after missing (or shortening) deco stops, then it is doing what it was programmed for, no fault there.
The point of staying out for water for a couple of days is for allowing the body to get rid completely of any residual nitrogen load, so the calculations can be started again from a zero-load situation.
Most DCS episodes happen on repetitive dives, when the residual nitrogen load, for any reason, is larger than expected.
After missed deco stops the body is in an "unplanned" situation, outside the range of the model used by the computer. Nitrogen could be distributed in different tissues in amounts which are not correctly evaluated by the algorithm, even if programmed on a Shearwater computer. You cannot trust those nitrogen levels provided by the algorithm.
At this point, is it better to still compute and show saturation values which are almost certainly wrong and misleading, or is it better to obscure the resulting values and admit that the computer cannot provide realistic information on what's happening inside the diver's body?
I find the second option more honest.
Unfortunately I see many colleague engineers trusting more their mathematical models than reality....

 

[rongoodman]

If you don't trust the algorithm your computer is using, why use it? I can't think of any reason to believe that the calculations would be wrong just because you surfaced with the leading compartment showing a gas load above the M value*GF Hi. Only the slower compartments would need more than a day to clear, and they aren't the ones causing issues for recreational divers anyway. I would be willing to bet that the number of computers locked due to user error far outnumbers those reacting to actual omitted decompression and if that is the case, the "solution" seems to be an overreaction.

 

[rongoodman]

2- the model used is safe
Well, nonetheless all the buhlmann prophets out there; there is simply no proof that the rgbm is not on par or better or worst than another model. While selecting his computer one might want to have look to the deeply safe lab data and select the one that suits your desired safety level

The issue isn't whether RGBM is safer than Buhlmann or any other model, but whether RGBM is the model actually being used. For the most part, that is impossible to know. Even Suunto only claims to implement "full RGBM" on some of their computers for a subset of dives.

 

[Centrals]

I would be willing to bet that the number of locked computers due to user error far outnumbers those reacting to actual omitted decompression and if that is the case, the "solution" seems to be an overreaction.

I will take your bet!
Omitting decompression is also user error!

 

[JMarc]

Unfortunately I see many colleague engineers trusting more their mathematical models than reality

Sadly the unquestioning confiance on model seems to be well spread even outside engineer circles.

Precision and accuracy wise, models used for desaturation computation are more alike subway maps than those used for orienteering competitions. I would recommend against putting too much faith in them.

 

[Divin'Papaw]

I dive off charter boats in Florida fairly frequently. I've never seen dive staff check any computers. However, every year I am asked several times why a computer does not seem to be working correctly. The most common answer is that they are locked out in violation gauge mode for skipping a deco stop. The divers generally do not have the slightest idea what I am saying.

This generally does not occur after the first dive of the day, but after the second, with a 45 min surface interval or so. These are generally the 3rd and 4th dive in the afternoon charter. In general, I would say that the divers continue to dive, relying on their buddy's computer for NDL data. I don't remember anyone skipping dives.

These are generally with RGBM computers, Cressi, Mares. and Suunto, but occasionally with PZ+ when their buddy was diving DSAT or Buhlmann.

I see the exact same thing. It always amazes me at the number of people who will do a dive right at the edge of NDL (or over) with a device that they are clueless about how it works and what it is really telling them. You see it quite regularly on the charters in Palm Beach County.

I always like the buddy pair who are amazed that one of their computers is giving them very different NDLs than the other buddy's computer without any appreciation for how each computer is set for conservatism, what algorithm is being used, or the fact that one may spend much of the dive 5-10' deeper than the other. They always blame the 'stupid computer'. Ummm, the computer isn't the stupid one.

 

[JMarc]

I can't think of any reason to believe that the calculations would be wrong just because you surfaced with the leading compartment showing a gas load above the M value*GF Hi.

There are two aspects in a model used for establishing a decompression procedure (or preventing the need of deco stops). One is having variables trying to describe the state of your body (loads in compartments, maximal load during the dive for GFlow applicability if you are using a GFlow /= GFhigh, bubbles related variables with a bubble model, ...) Then there are the limits you apply on those variables in order to prevent DCS (MValue on the loads, limits on bubbles related variables, ...)

It is not questioned that you can continue the computations when the variables become out of bounds. You'll get some values. What is questioned is the possibility to use the previous limits to prevent DCS in the new situation (assuming that you haven't already been hit and aren't just waiting for the symptoms to develop).

In other words, what is second guessed is the accuracy of the models themselves when their inputs stops to be in the validated range. There are already enough reasons to think that models are neither accurate nor uniformly accurate when the inputs are within the expected range (and I'm thinking of influence of aspects which are visible in the inputs, like repetitive dives, not of aspects which are not).


One may wonder if it is better to continue to show the results of the computation as best info available or to lock the computer out as a way to be clear that it is now out of its operating envelop. I'm of no strong opinion on that matter, although those operating on the assumptions that the numbers provided have a strong relevancy are making me very understanding for those holding for the lock-it-out option.

 

[admikar]

Sadly the unquestioning confiance on model seems to be well spread even outside engineer circles.

Precision and accuracy wise, models used for desaturation computation are more alike subway maps than those used for orienteering competitions. I would recommend against putting too much faith in them.

We are not talking about skipping 30 minutes of deco here. These rec computers will lock you out for 30 seconds of missed deco. You can't really argue that you are in grave danger for skipping 1 minute of deco on compputer that is set to high conservatism.
No one said there is no risk in diving.
If you don't want to accept any risk you should not dive. End of story.

 

[Centrals]

Read the manual and fully understand the consequence of missing deco stop.
There is nothing to argue about if the computer stays lock out for missing deco stop. It is your OWN fault.
Have I ever lock out my computer? Yes, because I used it as a bottom timer for deco dive. The worst was the continuous sounding of the alarm(I had to wrap it with couple of socks and left it inside the wardrobe). Good experience but never again.