Tec Dive computers and dive plans

[lermontov]

Just to go a bit off topic, I am truly amazed as I read this thread because I have been reading threads on ScubaBoard about planning tech dives for many years. If you haven't been around that long and have lots of spare time on your hands, seek out a thread on this topic from, say, 7-8 years ago. You will see a very different tone and a very different consensus. A few of the same people might be involved, and you will see some of them (including me) with a different mindset. The participants in this thread have been nearly unanimous in a way that would have been hooted down years ago.

can you elaborate ? what significant mindset haas changed? what has stayed the same?

 

[MaxTorque]

I doubt it's about off-gassing during ascent, my guess would be the next 3m up is "too high": your stop is not at M-value, it's at the nearest x*3m below. Unless of course you have the exact same profile with slower ascent and the first stop 3m up.

Buhlmann in Decompression - Decompression sickness wrote that 3m separation is likely an artifact of practical difficulty of holding the exact depth in open water, and that in a chamber one could go for 0.1 atm spacing instead, or even "ride" the M-value line to maximize decompression efficiency.

Sorry, i should have been clearer with my description, yes, because the stops are descretised and you can't ever be "above the line" you end up below it on the first stop, and then wait there till the next stop step effectively just falls below the line, so from then on your stops effectively start right up against the line, with the time at each stop proportional to how long it takes to off-gas by 3 meters (0.3 bar reduction in saturation). But in the same case, if you look at the 1st calculated stop depth before you leave the bottom on the ascent, effectively an instant ascent line with no off-gassing, then that line intercepts the limiting value at the current saturation, in effect it picks the shallowest possible stop available at the moment you left the bottom.


It's easy to switch the model into continuous decompression mode, where it simply sits on the ceiling at the limit value without any descretisation of stop depths, and yes, here, you simply ascend, hit the limiting M value, and your rate of ascent is then controlled by that line, You are, in the model, effectively continuously ascending, but the rate can be very very slow, almost certainly impractically slow, hence the adoption of descrete stops. Descrete stopping has the effect of adding a bit of conservatism to your tissue tensions (ie max saturation) but also slightly inceases deco time because your average depth is a bit deeper than it would be with the continuous profile, that rises to stay at right that limiting saturation point.

 

[Wibble]

No

there’s a ceiling and there’s stops. The ceiling constantly moves (upwards when on deco) and is expressed as metres/feet on a proper computer.
Stops are expressed in 3m increments and will be below the ceiling.

You therefore could ascend above the stop but must remain below the ceiling.

But we don’t do that as it is far easier and arguably safer to remain at a stop for longer than attempt to follow the curve. In any case it’ll make no practical difference to the TTS.

 

[KenGordon]

can you elaborate ? what significant mindset haas changed? what has stayed the same?

In the past you’d be flamed for not using a written plan, for using a computer to give stops rather than the written plan and a timer/depth gauge. Read the baker’s dozen of computer faults for a hint as to attitudes, although that was an extreme case.

This idea of using TTS and a couple of computers is a bit like recreational dives jumping in and swimming about until they get to 50 bar and then going up. It is how it really happens rather than what is taught.

 

[rongoodman]

In the past you’d be flamed for not using a written plan, for using a computer to give stops rather than the written plan and a timer/depth gauge. Read the baker’s dozen of computer faults for a hint as to attitudes, although that was an extreme case.

It's only fair to point out that that article was written long before the availability of PDCs with enough computing power to run the same software that divers trusted when run on desktop or larger computers.

 

[boulderjohn]

In the past you’d be flamed for not using a written plan, for using a computer to give stops rather than the written plan and a timer/depth gauge. Read the baker’s dozen of computer faults for a hint as to attitudes, although that was an extreme case.

This idea of using TTS and a couple of computers is a bit like recreational dives jumping in and swimming about until they get to 50 bar and then going up. It is how it really happens rather than what is taught.

Exactly. You would have seen the phrase "Plan your dive and dive your plan!" written emphatically many times. An article from Shearwater extolling flexibility could not have existed.

 

[MaxTorque]

This is what i am talking about really, to make use of the increase in performance and capability for modern platforms, and the now universal access to easy trasfer of data between those platforms. Thanks to things like bluetooth and USB, sync'ing a dive computer to some pc software is really easy these days! Hell, you could tow a SMB with a sat-modem on it and stream your dive in real time to the internet if you really wanted for not a lot of cost these days! That would have been impossible at virtually any price really not that long ago

Basically, to consider a holistic approach to planning and deco !

 

[lostsheep]

Read the baker’s dozen of computer faults for a hint as to attitudes, although that was an extreme case.
.

I don't think ANY of the reasons listed in the bakers dozen applies to someone who uses dive software in advance and then runs redundant shearwaters. Which of the 13 talking points is relevant today? #5?

BAUE Newsletters

 

[boulderjohn]

I don't think ANY of the reasons listed in the bakers dozen applies to someone who uses dive software in advance and then runs redundant shearwaters. Which of the 13 talking points is relevant today? #5?

Perhaps you were missing the point. The reference to those 13 points was a reference to the dominant train of thought only a few years ago in an attempt to explain the comparison of that era to this one.

 

[dmaziuk]

This idea of using TTS and a couple of computers is a bit like recreational dives jumping in and swimming about until they get to 50 bar and then going up. It is how it really happens rather than what is taught.

Well... in all fairness we do try to swim against the current until half tank, or decide whether to go left or right first if we don't see any current. We plan.