No compensation for ideal gas law deviations

[scubadada]

The OP says that Shearwater does not compensate for gas non-compressibility. I.e. Shearwater is being simplistic (and, thus, less accurate). I don't know if Oceanic does compensate for non-compressibility, but if they do, then that would explain a scenario where, early in the dive, Shearwater shows a shorter GTR and Oceanic shows a greater GTR.

The difference, at 3000 psi, is relatively small. But, the small difference is being extrapolated over the whole rest of the tank. So, if you are consistently seeing a lower GTR on Shearwater and bigger on Oceanic, I would strongly suspect that that is why.

I doubt Oceanic takes this into account. The Oceanic ATR has been available since at least the late 1990s.

 

[ATJ]

I doubt Oceanic takes this into account. The Oceanic ATR has been available since at least the late 1990s.

Interesting. I often dive with 300 bar tanks and I still find the Oceanic to be quite accurate even at the start of the dive when my tank is between 250 and 300 bar.

 

[Angelo Farina]

Can you provide a link? I'm not finding anything very useful. Thanks!

Here an example worked out, and the data tables required for evaluating the compressibility factor of air at any pressure and temperature:
https://www.enggcyclopedia.com/2011/09/air-compressibility-factor-calculation-sample-problem/
From the table provided, here the values of Z at ambient temperature (300 K) and various pressures in bar:

So, up to 150 bar you can consider air as a perfect gas. At 200 bar, you are "loosing" 3% of your tank capacity. At 300 bar, your are "loosing" almost 11% of the nominal tank capacity.

 

[Storker]

At 200 bar, you are "loosing" 3% of your tank capacity. At 300 bar, your are "loosing" almost 11% of the nominal tank capacity.

3% is hardly - if at all - noticeable for practical purposes. 10% is noticeable, and besides, a simple number to use in mental math.

My 10L 300 bar tank thus holds 2700 surface liters, while the 15L 200 bar tank I had used to hold 3000 surface liters.

 

[Storker]

@scubadada Post #1 may answer a question you had about why the Shearwater GTR is so off in the early part of a dive.

Hi Stuart,

Thanks for the post. Nearly all my dives are with AL80s, 3000psi, 207 bar. I don't think this relatively small difference would make a significant change in the GTR calculation. I didn't know that SW accounted for variations in the ideal gas law compared to Oceanic, is that right?

It could be as simple as the tank cooling off after you've jumped in. What are typical topside and water temps for you?

My tank is usually at its rated pressure topside when I'm setting up the gear. During summer, it's often some 10-20 bar lower when I've paddled out from shore and start venting my wing. During winter, I have to leave my tank in the trunk overnight and top it up in the morning to have it at rated pressure at the site. On the other hand, the pressure is pretty similar when I'm venting my wing. At Expedition Bjurälven, where they can have down to -30°C topside on the site, they may well see an increase in tank pressure during the first minutes of the dive.

PV=nRT. Don't forget the T

 

[tursiops]

Here an example worked out, and the data tables required for evaluating the compressibility factor of air at any pressure and temperature:
https://www.enggcyclopedia.com/2011/09/air-compressibility-factor-calculation-sample-problem/
From the table provided, here the values of Z at ambient temperature (300 K) and various pressures in bar:
View attachment 620495
So, up to 150 bar you can consider air as a perfect gas. At 200 bar, you are "loosing" 3% of your tank capacity. At 300 bar, your are "loosing" almost 11% of the nominal tank capacity.

thanks

 

[stuartv]

I doubt Oceanic takes this into account. The Oceanic ATR has been available since at least the late 1990s.

This science of compressibility of gases is not remotely new. Even Subsurface, the free dive logging software, applies a compressibility factor when it is doing gas calculations.

I can't remember if you have said whether Shearwater consistently shows a lower GTR than the Oceanic at the beginning, or if you just said that they are different.

If the Shearwater is consistently showing a lower GTR, then I would think it just about has to be that Oceanic is accounting for non-compressibility in their calculation (as any quality GTR algorithm should).

I believe you have a good RMV, maybe you could test the theory by going for a dive with both computers where your starting tank pressure is something below 2000. Say, 1500psi. As shown above, compressibility makes pretty much no difference at that pressure. If Oceanic is compensating for compressibility in their calculation, then that dive should show pretty much matching GTRs between Shearwater and Oceanic right from the beginning.

It could be as simple as the tank cooling off after you've jumped in. What are typical topside and water temps for you?

I don't think that is a factor when he is comparing 2 computers being worn side by side on the same dive.

 

[scubadada]

...I can't remember if you have said whether Shearwater consistently shows a lower GTR than the Oceanic at the beginning, or if you just said that they are different.

If the Shearwater is consistently showing a lower GTR, then I would think it just about has to be that Oceanic is accounting for non-compressibility in their calculation (as any quality GTR algorithm should).

I believe you have a good RMV, maybe you could test the theory by going for a dive with both computers where your starting tank pressure is something below 2000. Say, 1500psi. As shown above, compressibility makes pretty much no difference at that pressure. If Oceanic is compensating for compressibility in their calculation, then that dive should show pretty much matching GTRs between Shearwater and Oceanic right from the beginning...

Thanks @stuartv

Yes, the SW GTR is always lower at the start than the Oceanic ATR. However, it is often quite significantly lower, more that one would expect when starting with a tank pressure around 3000 psi. So, I will have to do some research and record some values. This will require that I go diving, you do what you have to do

 

[jborg]

Apart from any gas law deviations and initial extra gas use it might simply be a difference in algorithm with safety margins added before your rate for the dive is well established. For example, Suunto documents that their GTR algorithm will quickly decrease the displayed time when consumption rate increases, and conversely only increase the displayed time quite slowly over time -- in order to avoid an over optimistic estimate. Not the same thing, but an example of tweaking the algorithm to avoid being overly optimistic when data is lacking.

 

[scubadada]

Apart from any gas law deviations and initial extra gas use it might simply be a difference in algorithm with safety margins added before your rate for the dive is well established. For example, Suunto documents that their GTR algorithm will quickly decrease the displayed time when consumption rate increases, and conversely only increase the displayed time quite slowly over time -- in order to avoid an over optimistic estimate. Not the same thing, but an example of tweaking the algorithm to avoid being overly optimistic when data is lacking.

ATR/GTR has nothing to do with the decompression algorithm, only gas consumption and what it will take to get to the surface with the reserve gas pressure you chose. Oceanic ATR takes into account the ascent and whatever stops you may have, safety or deco. I believe the ascent rate for DSAT is 60 ft/min, same as the PADI RDP, for which DSAT was developed. Shearwater GTR does not take a safety stop into account, it is a direct ascent to the surface. GTR disappears if you enter deco. I believe the ascent rate is 33 ft/min.

If any of the above is inaccurate, feel free to correct, I did not go back and verify the ascent rates, dredged up from memory

Edit: sorry @jborg, looks like I probably misinterpreted your comments about the deco algorithm, no criticism of you here.

My Oceanic checks gas pressure once per second and updates the ATR based on the last 90 seconds. My Shearwater calculates SAC based on the last 2 minutes and uses that with current tank pressure to make the GTR calculation. The first 30 seconds of each dive is discarded in the calculation, see pp 52-3 Teric owner's manual