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

[Jay]

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

Why change? I’ve listed some reasons below, but the most succinct one is: once it’s known a SS will be performed, then it’s also known that GTR will not put you on the surface with the Reserve Pressure remaining, and being on the surface at the Reserve Pressure is the goal of GTR.

Why only when one gas is active/on? This mostly limits the change to Rec users and makes the implementation simpler.

If this proposal has enough support (votes), then it’s possible the change might be implemented by Shearwater. Given this idea was one of the few not implemented from the Perdix AI wishlist, and is wished for in other posts, then I think it won’t happen unless it’s by popular request.

If you do vote no, could you please explain why; I’d prefer to perfect any flaws rather than drop the concept.

Please vote! (even if it doesn’t directly affect you).

p.s. For Sidemount divers, further below I’ve floated an idea of how this feature might be of some benefit.

p.p.s. Voting doesn’t bump a poll. The occasional bump is welcome (hopefully not against the rules). On the number of votes 'needed'; I don’t have a clue.

Reasons to vote YES!

•If a Rec user did got into deco, and their GTS is positive, then they instantly know they have enough gas to perform those stops. They don’t know that from the current GTR and the typical Rec diver mayn’t be best placed to correctly compute the gas used at various depths/stops (or even know their SAC).

•It’s a valid argument that users should know their SAC and how to compute the amount of gas typically used for stops. What is more ideal (IMO) is that this occurs, and also the computer computes it. i.e. If the computer can do it accurately, then it should.

•You could bump your Reserve Pressure Reserve up by the estimated difference. Also true, but I would say two 'wrongs' don’t make a right.

•Your Reserve Pressure wanrning won’t activate on every SS as it now would now (if you ran your GTR to close to zero before starting an ascent).

•If GTR was designed from scratch, it would include stops, rather than being an apparent logical afterthought. Shearwater’s moto is "Powerful Simple Reliable” and including known stops certainly keeps things simple.

•re SW's definition of GTR, and by definition it currently does what it should. It’s currently defined as “the time in minutes, that can be spent at the current depth and SAC rate until a direct ascent to the surface would result in surfacing with the reserve tank pressure”. Obviously the new definition/use of the would delete the word direct and ascent would be defined as what you’ve told the computer to do (SS on, off, Adapt mode) and to comply with any deco obligation (when only one gas is on/active).

•How much gas are we talking about? Using the data from @Scubadada’s poll, the average and upper quartile numbers for a 3 & 5 min SS using an AL80 are: 6.2 & 10.3bar (avg, 3&5min) and 8.1-13.4bar (upper quartile) (I used the mid-points of each range). Whilst the numbers are not massive, they are a reasonable proportion of reserve gas. To my thinking, if I want 50bar as Reserve, then I want all 50 of those bars. I’d also suspect the non-SB diver might typically be high end of those %'s.

•Finally why? Why not? The status-quo should have some compelling merit!

Implementation: a negative GTR would be a Critical Warning.


Sidemount

For GTR Sidemount mode, it would assume that T1 and T2 are both connected to the same-sized tanks filled with the same gas. So, it would apply the current measured SAC to both tank pressures to calculate the total GTR (copied from @Stuartv’s earlier post from Perdix AI wishlist). T1/2 would display T.SM (/whatever named) being the sum of all active tanks.

 

[CWK]

TTS includes stops. Just compare GTR to TTS.

 

[Jay]

TTS includes stops. Just compare GTR to TTS.

Good point ... however, I'ld prefer that TTS and GTR both include stops for consistency.

I note you said compare and not subtract. I suspect that difference might not be at the forefront of a typical Rec diver's mind, nor adjusting for it accurately on a dive.

I'm not sure if you're aware but SW said that they do plan to make position of TTS customisable in Rec mode: post 219 The Shearwater Teric Announced

As a typical Rec diver I wouldn't use TTS, so if it's customisable off the screen I'd probably do that. It's much easier to compute than any of the above calcs involving GTR.

Thanks for voting and commenting.

 

[CWK]

Good point ... however, I'ld prefer that TTS and GTR both include stops for consistency.

Note that even Rec divers may carry more than one bottle of gas, and in OC Rec mode, Shearwater not only allows more than one bottle of the same gas but also 3 different gases.

KISS?

 

[Jay]

Note that even Rec divers may carry more than one bottle of gas, and in OC Rec mode, Shearwater not only allows more than one bottle of the same gas but also 3 different gases.

KISS?

I'm not sure I've got your point, so apologies if this is not what you were getting at: SW also say they don't recommend using GTR for multi-tank and gas diving. The proposal above merely does edge into that for sidemount. Here's SW's link: How is GTR calculated for multi-tank dives or decompression dives? - Shearwater Research

My understanding is that GTR is not used by Tec divers. So the proposal merely creates a more accurate GTR for Rec divers (as opposed the a slightly aggressive one), and only affects Rec divers. So yes I would say it's KISS.

Please do let me know if I've understood your point correctly as my aim is to keep all divers happy.

 

[npole]

I already asked Shearwater, they said that it is a decision (intentional) because the computer couldn't know how many tanks you may carry and if you have stages the GTR will mislead you. So it is a "safety" decision.
I questioned this decision, telling them that I have a brain, and eventually making this feature optionally available in the advanced settings will make both the parties happy, because I hate the GTR going off as soon you hit a deco stop. They said they will look at it (I don't count on it... ).

I then learned to use the TTS and comparing it with my SAC and gas left... however I still find a unnecessary operation that the computer could calculate for me, hence the proposed solution above.

 

[hammet]

I think the problem with including safety stops in GTR would be how it is calculated DURING the stop.
Before, the stop, it is easy to apply your current SAC rate to an specific time and depth (say 3:00 and 15 feet or whatever your settings are) to calculate how much gas time is required.
However, once you are at the stop, you may no longer be at 15 feet. You could be higher or lower.
Say you are at 11 feet instead of 15. Should the computer recalculate your GTR at 11 feet or assume you will drop down to 15?
The problem is that the simple formula that SW uses to calculate GTR as stated in the manual has to change to a much more complex one once the safety stop is reached.
Easiest solution: set your reserve at about 600psi instead of 500psi.

 

[CWK]

I'm not sure I've got your point, so apologies if this is not what you were getting at: SW also say they don't recommend using GTR for multi-tank and gas diving. The proposal above merely does edge into that for sidemount. Here's SW's link: How is GTR calculated for multi-tank dives or decompression dives? - Shearwater Research

My understanding is that GTR is not used by Tec divers. So the proposal merely creates a more accurate GTR for Rec divers (as opposed the a slightly aggressive one), and only affects Rec divers. So yes I would say it's KISS.

Please do let me know if I've understood your point correctly as my aim is to keep all divers happy.

Since I am now seated at my laptop (as opposed to using my phone) and it is more convenient to write a longer post.

If you dive 100/100 you would be diving at M-value. Most, if not all, divers would dial their GF back to be conservative. Lets say you dive 35/75. The difference between 35/75 is and 100/100 is effectively a safety margin that is expressed in GF. It is possible express this safety margin as a time at 5m, but that time may be different depending on actual dive profile. In effect doing a 3min safety stop at 5m is similar to doing 35/75 instead of doing 100/100. The one difference is that 35/75 gives a quantitative indication of where you are in relation to M-value.

Let us say you dive to NDL with a GF of 35/85 and do a 3 min safety stop at 5m. You know that the 3 min safety stop takes you further from 35/85 but where? Is it to 35/75, or 35/65, or 35/55? You don't have a clear idea where you are.

Shearwater squares GTR with NDL. See How is Gas Time Remaining (GTR) calculated? - Shearwater Research

In OC tech mode, you set your safety in terms of a GF and that is it. Shearwater does not add on a 3 min safety stop on top of the GF in tech mode because the any GF lower than 100/100 is the safety margin that you have chosen. It is similar to being able to set your recreational safety stop to 1min, or 2min, or 5min. It is a more precise theoretical means of letting a diver set his safety margin. It gives you more control than just doing a 3 min safety stop.

NDL is calculated based on the GF that you choose, therefore what you want (for Shearwater to include safety stop in their GTR calculation) is already there. It is there in terms of GF. If you understand this, just set your safety margin in terms of GF.

I do mostly recreational diving although I am tech trained. My computer is set permanently on OC Tec. The GTR currently gives me the correct calculation. It is just that the safety is set in terms of GF instead of a safety stop.


There is really no need to add a 3 minute safety stop on top of a GF that you have chosen. So in terms of KISS, I think keep it as it is. Divers should just understand GF if they want to use a GF based computer.

 

[Jay]

I already asked Shearwater, they said that it is a decision (intentional) because the computer couldn't know how many tanks you may carry and if you have stages the GTR will mislead you. So it is a "safety" decision.

The problem is that the simple formula that SW uses to calculate GTR as stated in the manual has to change to a much more complex one once the safety stop is reached.

Sure SW have a point for multi-gas setup complexity and safety in that regard, but for single gas, or even the proposed SM mode, it'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

 

[Jay]

Since I am now seated at my laptop (as opposed to using my phone) and it is more convenient to write a longer post.

If you dive 100/100 you would be diving at M-value. Most, if not all, divers would dial their GF back to be conservative. Lets say you dive 35/75. The difference between 35/75 is and 100/100 is effectively a safety margin that is expressed in GF. It is possible express this safety margin as a time at 5m, but that time may be different depending on actual dive profile. In effect doing a 3min safety stop at 5m is similar to doing 35/75 instead of doing 100/100. The one difference is that 35/75 gives a quantitative indication of where you are in relation to M-value.

Let us say you dive to NDL with a GF of 35/85 and do a 3 min safety stop at 5m. You know that the 3 min safety stop takes you further from 35/85 but where? Is it to 35/75, or 35/65, or 35/55? You don't have a clear idea where you are.

Shearwater squares GTR with NDL. See How is Gas Time Remaining (GTR) calculated? - Shearwater Research

In OC tech mode, you set your safety in terms of a GF and that is it. Shearwater does not add on a 3 min safety stop on top of the GF in tech mode because the any GF lower than 100/100 is the safety margin that you have chosen. It is similar to being able to set your recreational safety stop to 1min, or 2min, or 5min. It is a more precise theoretical means of letting a diver set his safety margin. It gives you more control than just doing a 3 min safety stop.

NDL is calculated based on the GF that you choose, therefore what you want (for Shearwater to include safety stop in their GTR calculation) is already there. It is there in terms of GF. If you understand this, just set your safety margin in terms of GF.

I do mostly recreational diving although I am tech trained. My computer is set permanently on OC Tec. The GTR currently gives me the correct calculation. It is just that the safety is set in terms of GF instead of a safety stop.


There is really no need to add a 3 minute safety stop on top of a GF that you have chosen. So in terms of KISS, I think keep it as it is. Divers should just understand GF if they want to use a GF based computer.

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).