Tec Dive computers and dive plans

[MaxTorque]

Another question for our deco guru's!

Once a critical tissue compartment saturation exceeds the chosen surfacing limit M value we have moved from a No Stop profile to a manditory Decompression profile. (I don't really like the term "no decompression limit" (NDL) because all dives involve decompression, it's just for an NDL dive you don't have to stop to achieve that off-gassing)

At this point, your dive computer swaps from displaying NDL time remaining, to indicating TTS (Time To Surface). My question is what algorythm is used to estimate this value? Obviously at any given moment we know the super saturation of all of the tissue compartments, so we know how far over the limiting saturation they are, but i am wondering what estimate of inert gradient is used to effectively estimate the time it takes to off-gas those tissues back down to below the limit value? In effect (for a constant breathing gas mix), at what depth does it consider the off-gassing to occur? It would i guess make some sense to consider the current ceiling depth as this depth cannot be breached. This also means that as the diver ascends towards that ceiling, off-gassing, and the ceiling rises, the TTS also falls as both the tissue saturation falls but the availabel off-gassing gradients increase?

 

[rjack321]

i am wondering what estimate of inert gradient is used to effectively estimate the time it takes to off-gas those tissues back down to below the limit value?

The gradient depends on the inspired gas vs the inert you are eliminating. The time is derived from half life of each gas in the mixture, inerts act independently of one another. Buhlmann published these 100 years ago, there have been a couple attempts to refine them but they are not massively different.

 

[lostsheep]

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.

I get that, I probably should have expanded on my thoughts.

I know a decent number of people around me who still don't like computers and I think their reasons are no longer valid.

 

[dmaziuk]

... your dive computer swaps from displaying NDL time remaining, to indicating TTS (Time To Surface). My question is what algorythm is used to estimate this value?

Tehre's two parts to it:

1. in ZH-L on- and off-gassing are symmetrical. The same Schreiner's equation is used to compute tissue loading.
2. The method of scientific guess: just like the NDL, you current TTS is based on things that haven't happened yet and is subject to change without notice.

 

[MaxTorque]

The gradient depends on the inspired gas vs the inert you are eliminating. The time is derived from half life of each gas in the mixture, inerts act independently of one another. Buhlmann published these 100 years ago, there have been a couple attempts to refine them but they are not massively different.

I understand that, that's the easy bit! The question is what gradient does the calc assume ie at what depth will the off gasing / deco occur. It cannot occur shallower than the ceiling as mentioned, so i suspect this is the gradient chosen for the TTS calc?

For example, lets say one of the fast compartments is fully saturated as we have been at 20m for some time, if we are breathing air (79% N2), then at this point, that compartment has an ppN2 of 2.37 bar abs (3 bar abs x 0.79).
If we said, arbitrarily, that the surfacing super saturation limit for that compartment was say 2.0 bar abs, then clearly we have 0.37 bar abs to off-gas.

And clearly, if we stay at 20m, then the TTS is just infinite because we are not off-gassing, we must ascend to start that process. In this example, the ceiling calculated will be set at a depth of 3.7m, because that matches the maximum saturation gradient. In this case, would TTS, therefore be estimated assuming we ascended to that depth?

ie TTS assumes the fastest we can off-gas is AT the maximum staturation allowed by the surfacing M a value limit, (2bar in our example) so we efefctively calculate TTS based of off-gassing 0.37 bar with a 2 bar gradient at whatever half-time is allocated to the particular tissue compartment in question?

 

[Wibble]

I know a decent number of people around me who still don't like computers and I think their reasons are no longer valid.

They’re relevant to those divers.

This has been an excellent thread that challenges our thinking and training from even recently. Planning the dive is done using software such as MultiDeco; tables aren’t used nowadays except for introducing planning (in my experience).

Pragmatically, creating a 'slate' with the dive plan is basically pointless when they bare little resemblance to the actual dive (just how many assumptions do they make — descent speed, ascent speed, actual depth — compared with the worn computerS which constantly monitor the dive. Modern computers, OK Shearwater computers, allow for adjustment whilst in flight and provide all that information to the diver (e.g. surface GF) should the dive plan need to be changed.

In short, slates are overly restrictive, overly conservative wasteful of expensive gas and specific only to the planned dive NOT the actual dive. How on earth do you know in advance how much time you’re going to spend on the bottom of the wreck as opposed to rummaging inside or topside?

Aside from the first couple of trimix dives I did after qualifying, I’ve dived every subsequent dive (hundreds of deco dives) using MultiDeco to plan it, but the computerS to run the dive. Of course I know the approximate dive plan, but the computers know the details.

One last point, I do not trust myself to not screw up the plan transposition from screen to slate - especially a deeper one. I absolutely trust the Shearwaters to provide my decompression profile.

Even ratio deco proponents rely upon bottom timers == computers

 

[J-Vo]

I understand that, that's the easy bit! The question is what gradient does the calc assume ie at what depth will the off gasing / deco occur. It cannot occur shallower than the ceiling as mentioned, so i suspect this is the gradient chosen for the TTS calc?

For TTS the PDC assumes you follow the currently calculated deco profile perfectly and use the most optimal gas you have programmed into the PDC at every point of the profile. Essentially the PDC has to re-forecast the dive profile and off gassing all the way to the surface every time it updates the current tissue compartment loading.

Now that I think of it, it would be pretty cool to be able to set your TTS calculation to "Cave Mode", so it calculates a revers trip of the profile you already did and adds any required deco.

 

[boulderjohn]

Now that I think of it, it would be pretty cool to be able to set your TTS calculation to "Cave Mode", so it calculates a revers trip of the profile you already did and adds any required deco.

How would it calculate the difference between entering into the flow and exiting with the flow or entering with the flow and exiting against it?

 

[J-Vo]

How would it calculate the difference between entering into the flow and exiting with the flow or entering with the flow and exiting against it?

Obviously there would be certain assumptions just like with the current version of the TTS calculation. I agree it would probably get some people in a lot of trouble.

I suppose the biggest issue is how would it know you turned around, and that pretty much kills the concept.

 

[rjack321]

And clearly, if we stay at 20m, then the TTS is just infinite because we are not off-gassing, we must ascend to start that process. In this example, the ceiling calculated will be set at a depth of 3.7m, because that matches the maximum saturation gradient. In this case, would TTS, therefore be estimated assuming we ascended to that depth?

There is no such thing as an infinite TSS. The computer assumes you want to exit someday and just continually adds ascent time. Eventually you get into saturation diving. All your compartments are full, there is no more on gassing happening - and therefore the TSS plateaus and does not increase.

In some (very old) computers TSS was calculated based on what you were breathing at the moment. Modern computers assume you ascend right now at the prescribed rate and make all all required stops and make gas switches at the required depths. That is the TSS - its essentially a bare minimum to ascend to the surface at any moment in time.