Subsurface

[dmaziuk]

I thought that it kept multiple data samples in memory rather than just using the overall time and volume used.

It does, or at least it should. I don't have AI so when I type starting pressure, end pressure, and tank size, I get the overall time & volume. But if yours is AI, you should get the real(-ish)-time samples. And be able to correlate spikes in your gas use with dive's events like having fin against a surge or getting into a thermocline.

PS and we had to bang rocks to make ones and zeros. In the snow.

 

[GlennL]

It does, or at least it should. I don't have AI so when I type starting pressure, end pressure, and tank size, I get the overall time & volume. But if yours is AI, you should get the real(-ish)-time samples. And be able to correlate spikes in your gas use with dive's events like having fin against a surge or getting into a thermocline.

PS and we had to bang rocks to make ones and zeros. In the snow.

This is interesting, because I can differentiate periods of excessive consumption. So I wonder if subsurface uses just start and end pressure along with average depth, or if it averages the data samples.
I know on my computer you can adjust the sampling rate, I currently have it set for every 30 seconds.

 

[dmaziuk]

Does it show your gas consumption in the pop-up on the graph? You could look at the file, its a b*tch to read, being xml, but you should be able to see if it has gas pressure recorded in the sample tags.

 

[stuartv]

Subsurface does show momentary RMV (labelled SAC) in the popup window when you mouse over the dive profile.

And, now that I think about it, since it does do that, and it definitely could tell which cylinder you were using at that point, it at least COULD display the actual SAC at the same time.

 

[dmaziuk]

Subsurface does show momentary RMV (labelled SAC) in the popup window when you mouse over the dive profile.

And, now that I think about it, since it does do that, and it definitely could tell which cylinder you were using at that point, it at least COULD display the actual SAC at the same time.

It knows pressure drop over time. It needs to also know some kind of volume to tell what the cylinder was.

 

[stuartv]

It knows pressure drop over time. It needs to also know some kind of volume to tell what the cylinder was.

Huh? Have you used Subsurface?

My post that you quoted said it shows RMV. Thus, it MUST know the cylinder size. You set the cylinder size in the Cylinders panel of the Equipment tab. Otherwise, it won't show an RMV.

And my point was that, since it does show momentary RMV, using the logged tank pressures from your AI (this whole discussion being predicated on having AI), it could certainly show momentary SAC.

My earlier posts were explaining about not showing aggregate SAC. It could easily do that for a single tank dive. But, to do that for any dive involving multiple cylinders, it would have to calculate the average depth over just the portion of the dive where each individual cylinder was in use (since SAC is cylinder-specific but RMV is not). I don't think it has code to calculate that (average depth during just a portion of the dive). So, displaying SAC for a whole dive (versus RMV, which it does do, but has labeled as SAC), is only "easy" if it's a single tank dive. And the Subsurface developers don't seem to be too fond of coding in exceptions - e.g. "Subsurface will do XYZ if it was a single tank dive, but not if there was more than one cylinder defined for the dive."

 

[dmaziuk]

And my point was that, since it does show momentary RMV, using the logged tank pressures from your AI (this whole discussion being predicated on having AI), it could certainly show momentary SAC.

Yeah, I missed that bit. Without knowing tank size it could show momentary "pressure per minute" gas use, knowing tank size it could show either "pressure per minute" or "volume per minute" gas use. Moving along.

 

[GlennL]

This is interesting. Since I don't enter a tank size until after downloading dives, what basis does an ai computer use to give ATR during the dive?

 

[tursiops]

This is interesting. Since I don't enter a tank size until after downloading dives, what basis does an ai computer use to give ATR during the dive?

The computer knows your psi/minute, which it calculates every 30s. It knows what you've set as the minimum psi you want to have at the end. It calculates psi remaining divided by psi/min and gets minutes. It only is a valid estimate if you stay at the same depth breathing at the same rate. But, if you get shallower, it is a conservative estimate.

 

[stuartv]

What @tursiops said. It's telling you how long IF you stay at the same depth and IF you continue to breathe the same.

Different manufacturers do things differently.

For Oceanic computers, I think the computer looks at your consumption over the last 90 seconds to determine what your current consumption rate is and then uses that for its calculations.

Also, for Oceanic, the ATR is based on allowing time to get to the surface, including any safety or deco stops. So, if you have your reserve set to 500 psi, and you start your ascent when the ATR gets to 0, then you will arrive at the surface with 500 psi. That is, of course, assuming your breathing stays the same the whole time and that you ascend at precisely 30 feet per minute.

Other computers do ATR differently. I don't remember which brand, but for one that I looked at, when ATR gets to 0, you will be at your reserve pressure at your current depth. I.e. it does not build ascent time into its calculation.

Which means that if your computer displays ATR, you should make sure you read the manual to understand exactly what it is telling you. If it works the second way I described, then you would want to calculate for yourself what tank pressure is where you need to START your ascent and set the computer's reserve setting to that. With the first method I described (i.e. Oceanic), you just need to set the reserve to what you want to END at, on the surface.

And, just to throw it out there, the Perdix AI GTR calculation doesn't work exactly like either of the methods I already described. So, again, if you have AI and you ever look at the ATR/GTR display, read the manual to make sure you know exactly what it is telling you.