After re reading your comment I think I misunderstood it.
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Shearwater Perdix AI dive log: starting/ending tank pressure?
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After re reading your comment I think I misunderstood it.
OP
Sorry I thought it wasnt showing on the desktop or that only sensor#1 would show on the desktop but not #2.
In a Shearwater computer, you can have multiple gases defined. They are all either Enabled or Disabled. The ones that are Enabled are the ones that the computer includes and assumes you will use, when it calculates your ascent profile and TTS.
Only 1 gas at a time will have a status of Active. That is the one that you are breathing. Or, the one that you have told the computer you are breathing, anyway.
If you are diving with deco gases but only your back gas enabled, then the computer would be calculating your ascent and deco on the assumption that you will do all your deco on only your back gas. You CAN dive a Shearwater that way, but I am skeptical that you or almost anyone else is really doing that. Are you? Why? You CAN switch a Shearwater to a gas that is Disabled. But, why would you want it to be calculating your TTS based on a deco plan that is not what you are really going to do? The normal way to use a Shearwater is to have all the gases that you are actually carrying defined in the computer and marked as Enabled. When you ascend to a depth where you will switch and you start the switch on the computer, it will default to the richest gas that is Enabled that has an MOD equal to or greater than your current depth. Carrying gases and having them marked as Disabled in the computer doesn't make sense for the vast majority of scenarios.
What *I* want GTR to do is tell me when I have to leave the bottom in order to ensure I can get all the way to the surface, doing all stops that the computer will prompt for (optional or mandatory), and arriving at the surface with some amount of gas in reserve which I configure in the computer's settings.
If it worked the way you described, then it would be useless for multi-gas dives (unless they added a bunch of stuff for the diver to enter the tank size of each defined gas). It would have no way to know how long any gas will last except the ones you have actually breathed from, so it could not predict GTR for deco gases (since it hasn't yet measured your SAC on each deco gas and it does not know the tank volume for each deco gas).
So, if GTR worked as you described, where it just tells you the time remaining until your next gas source, then it would be useless for multi-gas dives because you would know you have enough gas to get to your next switch but you would have no way to know (from the computer) whether you have sufficient volume of the next gas. You might need 50 cu-ft of O2 and only be carrying an AL40, and you would have no way to know that. Thus, GTR would not get you safely to the surface. I.e. it would be useless.
And all of that is why, in lieu of Shearwater adding programming to let you enter all your cylinder sizes - which they have previously posted that they tested and found to be too hard to use - I would request that they simply change GTR to only operate when there is only 1 gas Enabled. And to always display GTR (in that case), even after you have incurred mandatory deco stops.
You are right. I did not know what you considered that enabled gas. defined gasses or gtr gasses or Ai gasses. all different settings. the GTR is done in a mode that can not be accessed while submerged. Again you are right while on deco you have no reliable GTR value because it is assigned to your back gas. I never suggested carrying gasses not enabled inthe computer. I suggested that when switching to those other gasses that the GTR does not follow the gas change. It is fixed to one tank sensor. OIt would be reasonable (guessing) that when changing the AI PSI sensor to another tank that the GTR data will not display. The display allows both PSI sensors to be seen at the same time with the GTR of the surface assigned tank.
Aslo I suggested that when diving sidemount that the gtr should probably be assigned to the last tank you will be breathing on. To do other wise will leave you with (exagerated) a near empty tank left with a 2 min GTR and a full tank on the right. You would breath down the right tank prior to finishing the left tank in its rotation cycle. Also itisfoolishto have a sinsor on a deco tank cause you are doing a planned timed stop as opposed to tooling around waiting for GTR to say time to head up. You essentially forgo the GTR system while doing deco and return to it when deco is done.
I dont think our 2 view points are very different.
I was just responding to where you said this:
Anyway.... since GTR cannot tell you when you need to leave the bottom to ensure getting to the surface safely when you are carrying multiple gases*, that is why I think it would be better to just not display GTR at all (when you have multiple gases enabled). In other words, do it right or don't do it at all.
For sidemount (with no deco gases), you would only have 1 gas enabled. But, you could have 2 transmitters - one on each tank (as I do). For that, it seems to me that the computer firmware could be "smart" enough to recognize this situation from the behavior of the two transmitters - one moves consistently within a reasonable SAC range, then it stops and the other moves for a while in a similar reasonable SAC range. The computer could recognize that behavior and predict GTR based on the combined volume of gas available. At the start of the dive, where you haven't breathed off the second tank yet, it would just show GTR based on the first tank. After switching to the second tank and breathing it for a couple of minutes, the computer could recognize the situation and the GTR would suddenly go up by somewhere a bit less than double what it was showing. Anyone using a Perdix AI and diving sidemount with 2 transmitters could/should be able to understand this behavior. And, after the first tank switch, the GTR should be pretty accurate for the rest of the dive anyway. Or, the firmware could simply have an additional setting for "Sidemount" to TELL the computer to provide GTR based on the assumption that both transmitters are on tanks of the same gas that are the same size. So, it could constantly calculate SAC for whichever transmitter is currently showing the biggest delta in pressure and then do the GTR calculation based on that measured SAC and each tank's current pressure, summed.
If you're like me and change between BM and SM by just swapping one reg hose on your reg set, then you could end up diving BM doubles with the same two transmitters in use simultaneously. In that case, both transmitters would constantly show the same pressure and should be reading the same SAC. The computer could also be "smart" enough to recognize that and just pick one transmitter or the other to base GTR on and simply ignore the other transmitter. I mean, obviously you are not actually breathing two gases at the same time.
Of course, for either SM or BM doubles, even with the firmware working as I just described, the diver would have the option to just set the Perdix AI to only monitor one transmitter. And, for SM, as you say, they would probably want to just monitor the transmitter of the last tank they're planning to use.
* Again, predicated on not enhancing the computer to allow the diver to configure tank size for each defined gas.
The shearwater book defines GTR as the time remaining for DIRECT ascent at 33 ft per minute to the surface with no stops. mandatory or safety.. Some thing I have not thought about is that the GTR is measured using the last few minutes of gas consumption. So it would be reasonable to say that as you head up your GTR will inrease but you may not see that increase of a useful value because from 100 ft a 33 ft / sec means a steady ascent to the surface in 3 minutes. Now TTS I would have to look at to see what it is based on in regards to stops asociated with the ascent. I am guessing (with out benifit or re reading the book ) that it is not a direct ascent value. Whether deco is included in it , I am not sure.
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I think one of us may be misunderstanding exactly how the Shearwater GTR works.
Yes, if you head up your GTR should increase. But, I'm not sure if it's for the reason you think. Maybe that's because your post is unclear to me regarding exactly what you think, regarding your statement that I bolded.
To be clear, though, regarding how I believe it works:
Say you're at 99' depth and the computer measures your last couple of minutes and sees that you dropped 100 psi per minute, so it tells you your GTR is 30 minutes. Then you ascend directly to 33'. It is now going to tell you that you have roughly 60 minutes GTR left. That is because it factors current depth into the GTR. So, it knows that if you breathe 100 psi per minute at 99', and you continue to breathe the same, you'll breathe 50 psi per minute at 33'. It's not going to calculate GTR at 33' based on still using 100 psi per minute just because that's how much you used in the preceding 2 minutes or whatever. It will factor in the depth, too. I think. I could be wrong.
So, yes, the manual says GTR is based on a direct ascent, with no stops, at 33'/min.
But, all the other AI computers with which I am familiar actually also factor in any gas you will use if you make any stops. And the computer knows whether it's going to prompt for a safety stop. So, other computers include the gas you will use when you're hovering there during your safety stop.
I would like the Shearwater GTR to be just as sophisticated (and useful) as the GTR display on my Oceanic Atom computer.
And if it can factor in gas at a safety stop, it could surely also factor in gas at any mandatory stops that it will call for.
The way GTR works on my Oceanic computer is simple. I set the Reserve setting to 500 psi. That tells the computer I want to arrive back at the surface with 500 psi left.
If I then do my dive and stay down somewhere until my computer counts down to exactly 0 GTR, then begin my ascent, and if I ascend at precisely 33' per minute, and I continue to breathe just as I have been, and I do every stop the computer prompts me to do, then I will arrive at the surface with 500 psi left. And it actually works!
Unfortunately (in my opinion), the Perdix AI does not work as well. If I set the reserve on the Perdix to 500 and I start my ascent when it says GTR is 0, and I do all the stops it prompts for, I will arrive at the surface with less than 500 psi.
The Perdix AI manual says that safety stop time is not included in the GTR calculation to simplify the meaning of GTR. It then goes right on to explain how to do the math to figure out how much gas you will use at your safety stop, based on your SAC, so that you can add that gas into your Reserve setting. How is that SIMPLER? It's tell the user to do math. That's what computers are for. If I have it set to prompt for a safety stop, then you do the math for me and include it in the GTR. If I don't, then don't include it. The way it works now, every time you change tank size that you're using, you would need to figure out your SAC for that size tank and do the math again, to figure out how many PSI you need to include in your Reserve for that size tank. I mean, if you were being really anal about it. Otherwise, just include an extra 100 psi and be done.
But, really, there is just no reason I can even imagine to not let the computer do the math for me, based on the stops that it KNOWS it is going to prompt for.
It's all explained in the manual, people doesn't read it fully and that's the cause of misunderstandings. It could be improved? Probably yes.. but in the meantime a tool should be used within its parameters.
GTR is only intended to be used in OC REC (with no deco), no other options are offered atm. It will give you the time to surface with no stops but with the reserve (set by you) left in the tank. If you set your reserve to 40bar and you have 40bar left in your tank, your GTR will be ZERO, simply as that.
If you get trapped in any deco (you shouldn't if you're doing REC) the GTR will disappear (it will change to ---). The reason is due to the fact that the value would be not reliable as is, for that kind of usage, since the additional setup required. Shearwater experimented it already and in the end, because of the many mistakes experienced by their testers, decided to not risk it and leave that function disable in the presence of any deco stop.
If you're hitting any deco, a much more valuable data you should read is the TTS, that take in account all the stops, and you can do your own calculation based on the gas (or gasses) you have left.
I would love if SW will experiment more on this point, to offer a more reliable GTR even in the presence of deco (it's a common situation if you do repetitive deep dives, even in REC), but in the meantime, if the value is completely off, I prefer to not have it at all.
GTR is only intended to be used in OC REC (with no deco), no other options are offered atm. It will give you the time to surface with no stops but with the reserve (set by you) left in the tank. If you set your reserve to 40bar and you have 40bar left in your tank, your GTR will be ZERO, simply as that.
If you get trapped in any deco (you shouldn't if you're doing REC) the GTR will disappear (it will change to ---). The reason is due to the fact that the value would be not reliable as is, for that kind of usage, since the additional setup required. Shearwater experimented it already and in the end, because of the many mistakes experienced by their testers, decided to not risk it and leave that function disable in the presence of any deco stop.
If you're hitting any deco, a much more valuable data you should read is the TTS, that take in account all the stops, and you can do your own calculation based on the gas (or gasses) you have left.
I would love if SW will experiment more on this point, to offer a more reliable GTR even in the presence of deco (it's a common situation if you do repetitive deep dives, even in REC), but in the meantime, if the value is completely off, I prefer to not have it at all.
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