New Shearwater AI transmitter - the Swift

[ATJ]

One other thing is that the gas consumption at the beginning of a dive can be off based on the initial inflation of the BC while descending. It takes awhile for that to average out of my ongoing consumption monitoring(a little calculation that I do in the background of my thoughts, stupid restless brain syndrome) but I have a pretty good idea at this point of how much it matters and for how long.

Yep. Which is why I wait until I am "steady state" before I pay much attention to GTR. Once I get to steady state, I find it useful.

 

[ATJ]

For various reasons SAC is often higher at the beginning of the dive. This tends to offset the error induced by the ideal gas assumption for real dives. The result is the GTR prediction may be more accurate than it would seem at first glance.

Actually, the reverse is true. Ideal gases compress linearly, real gases do not. With higher pressures (in the range of our tanks), the higher the pressure, the less the real gases compress. This means that from a full tank, the rate of change in pressure is higher than would be expected and so the SAC is actually higher. I showed the numbers earlier in this thread.

 

[ATJ]

Edit: I just heard back from PPS and they do not correct for deviations from the ideal gas law

Morales (Pelagic Pressure Systems)

Aug 3, 2021, 11:02 MST

Hi,
Please accept our apology for the misunderstanding. Pelagic Pressure Systems air time remaining algorithms do not correct for nonlinear compression of gas at higher pressures.

Kind regards,
Support team

Wow! That really surprises me. So now I don't understand why GTR on Oceanic computers is so accurate throughout my dives even when using 300bar tanks.

 

[ATJ]

Nope. Unless the computer becomes clairvoyant and understand the whole dive profile, GTR is nonsense.

That's not what GTR is representing. It is not trying to predict the whole dive profile.

All that it is saying is that based on your current breathing rate and depth, this is how much air you have if you stay at around the same breathing rate and depth.

It basically serves the exact same function as the "distance to empty" that almost every modern car displays. The computer in the car looks at the rate at which fuel is being used over the last n minutes and estimates how far you can travel under the same conditions before the fuel tank is "empty". It is also not clairvoyant about what route you are going to take: how many hills, how fast you will drive, whether you will be stuck in traffic, etc.

 

[AJ]

That's not what GTR is representing. It is not trying to predict the whole dive profile.

All that it is saying is that based on your current breathing rate and depth, this is how much air you have if you stay at around the same breathing rate and depth.

After reading your previous replies, I understand why GTR is useful to you.

To me GTR not useful at all. I have to make my own dive plan and monitor/recalculate while diving because my dive profile is way different from yours. No computer can ever do that planning for me. Therefore remaining gas and time to surface (not computer TTS, but brain calculated Rock Bottom/TTS) are critical to me. This is why GTR is of no use to me.

 

[Wibble]

You are making too many assumptions about me and my dives!

First of all, I am not receiving too much information and I am far from overloaded. In fact, I have selected just the information I need and it is perfect.

Second, as I have said a number of times already, the GTR from the Oceanic computers is accurate. I have done hundreds of dives with Oceanic computers have have found these data to be extremely useful on my dives.

In fact, for most of my dives at my regular dive sites I really only concentrate on three data: GTR (on the Oceanic), elapsed dive time and NDL. Almost all my dives are non-deco and my most frequently dived site I usually dive with EAN25% and really don't have to worry about NDL. On my last dive at that site, my NDL never got below 30 minutes!

For me, GTR gives me more information that my tank pressure or SAC. About the only time I look at the tank pressure is if the GTR is higher or lower than I expect at that time of the dive and I check the tank pressure ton understand why it is not what I expected - which is usually a short burst of exertion.


Clearly, your diving is not even close to my diving. For a large number of my dives I stay with in a narrow depth band for the whole dive (other than descent at the start of the dive and ascent at the end of the dive). Sure, there are small changes in depth (plus or minus 2 metres) but these changes make little difference to the GTR and what changes they do make are obvious and understandable. My workload barely changes on a dive. Certainly, if there is current or surge I use more air but that those conditions tend to be consistent for my dives and so, again, the GTR is consistent and understandable.


Again, you are making assumptions about me and my diving.

For me and my diving TTS gives me very little value at all. As most of my dives are non-deco, all that TTS tells me is how quickly I could surface right now. Why would I care about knowing how quickly I could surface when I'm 10 minutes into a 2 hour dive? TTS also doesn't give any indication how long my gas is going to last.

Perhaps, the only time TTS MAY be of value is that if I went into deco, however, I would rather know what my deco commitment (what depth and how long) is rather than how quickly I could surface. Interestingly, TTS would be useful in conjunction with GTR as you could make sure you have enough gas to surface as slow as you need to surface. i.e. you have a problem if TTS is greater than GTR.

It's not a personal "attack" (which you've definitely not said).

It's a philosophical difference in approach to information processing and usage during a dive -- my "in the red & blue corner" post earlier.

I like minimal information and to simplify things. My plan for a dive is to set the max dive time and the minimum gas to return safely to the surface with ample reserves. Computer gives me my dive time, SPG / AI gives me my gas. When either reaches the limit I ascend to the surface.

From your post, it appears that you use the Gas Time Remaining as a proxy for minimum gas. I do hope that you also check the actual gas pressure remaining.

I stand by my comment about dives being far from constant with variable depths and breathing rates.


We can agree to differ in our approaches; I'll stick with my traditional approach, you can stick with your technological approach. We both like diving.

 

[hedonist222]

I like minimal information and to simplify things. My plan for a dive is to set the max dive time and the minimum gas to return safely to the surface with ample reserves. Computer gives me my dive time, SPG / AI gives me my gas. When either reaches the limit I ascend to the surface.


I stand by my comment about dives being far from constant with variable depths and breathing rates.

Maybe off topic but I find auxiliary information useful - such as SurgGF.
On occasion I've decided to quit the (solo) dive a bit earlier than planned.
I use the SurfGF to determine how long my safety stop could be.

 

[inquis]

This means that from a full tank, the rate of change in pressure is higher than would be expected and so the SAC is actually higher.

Yes, I got turned around, thank you for setting me straight. With that corrected understanding, though, I'm not so sure about the magnitude of your conclusion:

Using the data from Subsurface, @300bar I have 3,197L of gas, @270bar it is 2,945L., @80bar it is 967L and @60 it is 717L.

On a per volume liter basis, this is 9.38 bar per 100 L to start and 8.37 bar/100L at the end. Assuming your volumetric consumption rate is the same, the pressure consumption rate (bar/min) -- and therefore SAC -- is only 12% higher at the start vs. the end. I think other factors must be contributing to the 50-100% higher SAC that is displayed on your computer.

 

[Wibble]

Maybe off topic but I find auxiliary information useful - such as SurgGF.
On occasion I've decided to quit the (solo) dive a bit earlier than planned.
I use the SurfGF to determine how long my safety stop could be.

SurfGF's definitely useful in the excrement/ventilator interface. If you've not got enough gas (bailout), or if it's getting very rough so you need to stop your deco short, or if there's a real emergency... If SurfGF says you're below 100, then you can choose to go.

On the other hand there's the GF99 which I still don't understand! OK, it's the current percentage of ambient pressure to M-value. But it's most definitely a number that requires brain cells to use it.

 

[stuartv]

SurfGF's definitely useful in the excrement/ventilator interface. If you've not got enough gas (bailout), or if it's getting very rough so you need to stop your deco short, or if there's a real emergency... If SurfGF says you're below 100, then you can choose to go.

On the other hand there's the GF99 which I still don't understand! OK, it's the current percentage of ambient pressure to M-value. But it's most definitely a number that requires brain cells to use it.

I find the SurfGF is useful pretty much only when I'm at my final stop and waiting for SurfGF to drop before I make my final ascent to the surface.

GF99 doesn't require any more brain cells to use, I don't think. GF99 is what I would look at during an ascent TO my final stop. Basically, you don't want GF99 to ever exceed whatever you have chosen as your GF Hi setting. It's that simple. If you're doing deco stops, then it gets more nuanced by factoring in your GF Lo setting. But, for NDL dives, it's simple keep GF99 at or below your GF Hi setting.

If you do everything exactly by the book, ascending at 30 fpm, ascending exactly when the computer says you can, etc.., then when you get to the surface, your GF99, SurfGF, and your GF Hi will all be the same number.

What GF99 allows you to do is, in an emergency, you can decide you want to use a higher GF Hi. Say, you're diving GF30/70, but you have an emergency and want to ascend as fast as you'd be allowed with a GF Hi setting of 95. You can use GF99 to do that without actually changing your computer. Just watch GF99 as you go up and don't let it go higher than 95.

Tech note: What I just described will not give you EXACTLY the same ascent as changing your computer to GF Hi of 95. It will be very close. The difference is that if you change your computer, it will let you ascend to a 10' increment that keeps your GF99 below 95. I.e. if you are at 30' and your GF99 would exceed 95 at 25', then it will hold you at 30' until you can ascend to 20' without having your GF99 exceed 95.

If, instead, you ride the GF99 setting up, keeping it at or just below 95 the whole way then, in the previous example, you would ascend above 30' sooner than in the previous example. But, only by a short amount of time.

Lastly, even this explanation is somewhat simplified, as it still does not factor in GF Lo. But, for NDL dives it is (as far as I know) correct.