Shearwater Perdix AI dive log: starting/ending tank pressure?

[NAM001]

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:

.

I agree with the GTR statement and your depth example. Perhaps the TTS is the info you are looking for. I think it takes into account all your required stops.
I agree with your GTR assesment in you para of the manual discussion on GTR.
Perhaps you can set the onboard psi 100 psi higher and treat teh GTR as ttime remaining to get to the safety stop instead of onboard.

I think we are both on the same page.

 

[NAM001]

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.

good info on gtr and deco

 

[scubadada]

...I would like the Shearwater GTR to be just as sophisticated (and useful) as the GTR display on my Oceanic Atom computer...

Or, my 7 1/2 year old VT3, exactly

 

[stuartv]

I agree with the GTR statement and your depth example. Perhaps the TTS is the info you are looking for. I think it takes into account all your required stops.
I agree with your GTR assesment in you para of the manual discussion on GTR.
Perhaps you can set the onboard psi 100 psi higher and treat teh GTR as ttime remaining to get to the safety stop instead of onboard.

I think we are both on the same page.

I know what TTS is, and it doesn't tell you anything about your gas situation.

 

[NAM001]

I know what TTS is, and it doesn't tell you anything about your gas situation.

Doesn't TTS take in to consideration all stops when it calculates time?

 

[npole]

Doesn't TTS take in to consideration all stops when it calculates time?

Yes it does, but it doesn't care how much gas left you have, so if you have a mandatory deco, your GTR will disappear and you are left instantly "blind". But in reality you are not blind, you still have your tank pressure and your SAC (air consumption per minute), so with a rapid calculation you will know what's your GTR including the stops. But I can understand that for someone seeing that number disappearing could be alarming (that is probably what they wanted to emphatize).

Shearwater said that GTR isn't a reliable value in certain cases (mostly due to the fact that it's based on the SAC, that is per se "variable") and so it shouldn't be used with a diving with decompression, that cannot rely on not much accurate values. They listed the example of using multiple tanks with different gases in a usual diving with deco.

However since we're talking about REC dives, and since it's proven that accumulating deco in a REC diving is very common when doing multiple dives, and due to the fact that we're talking about single tank dives; I find the solution to completely disable the GTR as soon the deco is hit, doesn't make much sense... the manual invites you to do the mental calculation based on the SAC (as I wrote above), but why I should do it mentally when I have a one thousand computer on my arm that can do it for me? I hope that SW will re-enable back the value (eventually it can change color when there's deco) or by adding an option to turn it on/off leaving the decision to the final user.

 

[charlier]

However since we're talking about REC dives, and since it's proven that accumulating deco in a REC diving is very common when doing multiple dives, and due to the fact that we're talking about single tank dives.

Full disclosure, I am feeling a bit ornery at the moment. My reality in the Banda Sea, with mutliple deep rec dives over a period of 10-12 days on OC, single tank with Nitrox 28/29, deco is not common. After a few days diving with my group, I can easily calculate their SAC and remaining gas pressure throughout their dive. Now, back to my evening cocktail.

 

[npole]

Full disclosure, I am feeling a bit ornery at the moment. My reality in the Banda Sea, with mutliple deep rec dives over a period of 10-12 days on OC, single tank with Nitrox 28/29, deco is not common. After a few days diving with my group, I can easily calculate their SAC and remaining gas pressure throughout their dive. Now, back to my evening cocktail.

There's the tables, if you respect them intentionally limiting (as depth and time) your dives you won't get any deco, that's your "reality" and what the REC courses tells you to do, so it's the "right" choice. However often the reality fight with the theory and you'll find yourself deeper because you can, or extend your dive because you have plenty of air (for the dive.. and for the deco..) even if you're "instructed" to not accumulating any deco: I'm a thinking diver not an automa. I'm my repeated dives there's always someone with a deco stop, especially if it's a 3-tanks dive. Also, not everyone uses (or likes to use) the nitrox.

However, we're not talking about what we believe should be done or not, but about the fact that the user manual suggest to do your own calculation under the water, that is a no-sense after have bought a 1500 dollars computer. I understand their concerns perfectly, they should make it optional and let the user to decide.

 

[NAM001]

Yes it does, but it doesn't care how much gas left you have, so if you have a mandatory deco, your GTR will disappear and you are left instantly "blind". But in reality you are not blind, you still have your tank pressure and your SAC (air consumption per minute), so with a rapid calculation you will know what's your GTR including the stops. But I can understand that for someone seeing that number disappearing could be alarming (that is probably what they wanted to emphatize).

Shearwater said that GTR isn't a reliable value in certain cases (mostly due to the fact that it's based on the SAC, that is per se "variable") and so it shouldn't be used with a diving with decompression, that cannot rely on not much accurate values. They listed the example of using multiple tanks with different gases in a usual diving with deco.

However since we're talking about REC dives, and since it's proven that accumulating deco in a REC diving is very common when doing multiple dives, and due to the fact that we're talking about single tank dives; I find the solution to completely disable the GTR as soon the deco is hit, doesn't make much sense... the manual invites you to do the mental calculation based on the SAC (as I wrote above), but why I should do it mentally when I have a one thousand computer on my arm that can do it for me? I hope that SW will re-enable back the value (eventually it can change color when there's deco) or by adding an option to turn it on/off leaving the decision to the final user.

Yes i agree, as long as one understands the difference between gtr and tts all is well. They have 2 different functions. Yes once you start tanking about deco the GTR applicaton goes down the toilet. You switch to another gas and you no longer are using the back gas and you would now have eternity as GTR is concerned. I look at gtr as a single tank function. One has to understand just what they mean and the constraints of both to use them correctly. I have a petrel 1 and 2 and preditor so all that i have to deal with is TTS, and to be honest i dont use it. I just brain calc it since i seldom if ever exceed NDL.