Poll: GTR to include stops when one gas is active/on for Shearwater’s AI Teric & Perdix computers?

[hammet]

Sit's hopefully not an exhaustive amount of brainpower required to find a solution / simple way to handle the feature during a SS. From reading the link from @CWK's post below they blank out GTR during a deco stop. Crude, but effective!

And in terms of safety including stops is safer than not including known ones

The point I'm trying to make is that GTR calculations during a safety stop would have to make an ASSUMPTION that you are going to stay at an precise depth for the calculation to be accurate. Say you are at 7 feet during your stop. Does it assume your going to be at 7 feet the whole time remaining or does it assume you will drop down to 15? What if you then drop too far and go to 27 feet? That's about a 50 percent difference in air consumption if you do the math.

 

[npole]

The point I'm trying to make is that GTR calculations during a safety stop would have to make an ASSUMPTION that you are going to stay at an precise depth for the calculation to be accurate. Say you are at 7 feet during your stop. Does it assume your going to be at 7 feet the whole time remaining or does it assume you will drop down to 15? What if you then drop too far and go to 27 feet? That's about a 50 percent difference in air consumption if you do the math.

This is already happening with the TTS and even with the deco countdown timer (in fact those minutes/seconds aren't real seconds, they can go slower or faster according to the difference between your real depth and the calculated one).

 

[hammet]

This is already happening with the TTS and even with the deco countdown timer (in fact those minutes/seconds aren't real seconds, they can go slower or faster according to the difference between your real depth and the calculated one).

What assumption is the TTS countdown making other than a value for a safe ascent rate?

 

[CWK]

I agree moving one's GF away from the M-line is more conservative, and that a SS is also adding in conservatism. Yes, very broadly speaking one could advocate using no SS and a very conservative GF, or a very aggressive GF and a very long SS. I would prefer not to get into the detail of that as I'll take this thread into a 'GF in Rec debate' lasting pages, but these two forms of conservatism exist on a computer for use together, and one shouldn't have to turn off SS to have GTR work properly when it can be done properly (pick any Pelagic AI Computer).

I would like to note that a SS countdown is not part of OC Tec even though it may be in OC Rec. Any change to GTR would have to take OC Tec into consideration.

 

[scubadada]

@CWK I believe most divers do a safety stop when doing no stop dives, regardless if they are diving DSAT, Cressi RGBM, or Buhlmann ZH-L16C with GF at a conservative or liberal setting. So, do you make a direct ascent to the surface on all your no stop dives?

I would prefer that Shearwater include the SS in the GTR calculation, but it is not a big deal, this is a small amount of gas. I have been diving Oceanic AI computers for 16 years, this is not difficult to implement. In fact, I dive an 8 year old VT3. It is able to calculate air time remaining, based on end pressure chosen, including the SS or any deco stops, using any of the 3 possible transmitters/cylinders.

I think the real problem here is that these are technical computers that have been adapted to recreational use as a second thought. The technical bias overrides the recreational use/function. This would not prevent me from buying a Teric today if my trusty VT3 died, I already have the transmitter. Thank you Shearwater

 

[npole]

What assumption is the TTS countdown making other than a value for a safe ascent rate?

None I believe, it takes in consideration a 9m/min ascent rate and all the deco stops executed exactly at the indicated level, if you ascent slower or if you perform your deco stops at different depths (even within the acceptable range) it will adjust the TTS according, it is always updated in real time, so you may have a TTS of 10 minutes then you start to end your dive, but your dive could really end in 12 minutes because you went up slower than 9m/s and/or because you performed your deco stop at 4 meters instead of 3 meters in example.

 

[hammet]

None I believe, it takes in consideration a 9m/min ascent rate and all the deco stops executed exactly at the indicated level, if you ascent slower or if you perform your deco stops at different depths (even within the acceptable range) it will adjust the TTS according, it is always updated in real time, so you may have a TTS of 10 minutes then you start to end your dive, but your dive could really end in 12 minutes because you went up slower than 9m/s and/or because you performed your deco stop at 4 meters instead of 3 meters in example.

Exactly. Because the TTS just needs to know that you have your computer set for a 3 for 5 minute stop and add that to the ascent time.
When GTR is calcuated, the computer needs only to know:
1. Your current depth to ascend from
2. The accepted rate of a safe accent
3. Your current SAC rate

However...
At a safety or deco stop, the computer once again needs to know what depth you are at to calculate your GTR. But as I stated above, what value for your depth is it going to use? Is it going to use the set value... say 15 feet even though you might be at a higher or lower depth? Or is it going to use your actual depth? Well, if you are hovering at 7 feet, it might say, for instance that you have 5 minutes of gas time. But of course, you don't want to be at 7 feet... so you drop down to 15 feet. Now you're GTR might say 4 minutes because you are deeper and breathing under greater water pressure. So, the value of your GTR at he 7 foot depth is actually misleading. Since you didn't plan to stay at 7 feet, you never really had 5minutes of air.

 

[npole]

Why are you making this so unnecessary complex?
They are both estimates, the GTR is nothing else than the TTS * SAC ...stop, the SAC at depth is interpolated by your air consumption to the ATA.

None of the two are precise (because of the reasons above), you won't blindly trust to your TTS, neither you would trust your GTR, they are indicators so you have an additional value to compare with your diving plan and decompression.

The GTR can be wrong, so the TTS.. but the TTS is actually shown, the GTR is removed from the screen as soon you hit the deco. We are asking to have the option to show it as well: I love the tools where it's me to decide how they are configured and not someone else.

 

[hammet]

Because TTS gives a value that can never mislead the diver into staying a given depth longer than he has air for.
It's a little nitpick yes.... but I think that's why SW says, screw it... we're not gonna bother putting safety stops into the calculation for GTR: It complicates the algorithm and it relies on certain assumptions of what depth to use for the calcuation. It's easy enough just to add 100psi to your reserve settings. (Of course your alarms are going to go off while you violate your reserve while chilling at your safety stop because you went below 600psi).

 

[hammet]

Just to add.... I'm not a technical diver, strictly rec.. in fact i can count on one finger the times I went into deco mode. But with the proliferation of AI dive computers with GTR calculations growing, I wonder if more and more divers will begin to use GTR rather than PSI to measure how much time they left (assuming they are above NDLs). It would seem more intuitive to a casual diver... "I have this much time left to dive before I have to come up."
Instructors will ask their students to read their GTR back to them and not PSI perhaps?