Perdix AI SAC above 3000 PSI

[vinsanity]

The Perdix AI manual includes the following warning:

No compensation for ideal gas law deviations: Note that all SAC and GTR calculations assume that the ideal gas law is valid. This is a good approximation up to about 3000 PSI (207 Bar). Above this pressure, the change in gas compressibility as pressure increases becomes a noticeable factor. This is mainly an issue for European divers using 300 Bar cylinders. The end result is early in the dive, when pressures are above 3000 PSI/207 bar, the SAC is over-estimated, resulting in under-estimation of GTR (although this is the good way to err, as it is more conservative). As the dive progresses and pressure drops, this problem rectifies itself and the numbers become more accurate.

So when diving with a 3400 PSI tank, the live indication of the SAC & GTR will be off for the beginning of the dive. What about the dive log data when imported into something like Subsurface? I assume the raw PSI data that is logged is accurate, just some calculations done by the computer in real time are inaccurate, correct?

 

[jborg]

Yep. SAC rate will appear worse than it in fact is at the beginning of the dive, but processing the data afterwards can produce other conclusions. I’m fine with getting a more conservative GTR etc at the start of a dive.

Though I don’t usually notice this much with 3400 PSI (232 bar) tanks, it’s more noticeable at 300 bar.

 

[vinsanity]

Yep. SAC rate will appear worse than it in fact is at the beginning of the dive, but processing the data afterwards can produce other conclusions. I’m fine with getting a more conservative GTR etc at the start of a dive.

Though I don’t usually notice this much with 3400 PSI (232 bar) tanks, it’s more noticeable at 300 bar.

Thanks. The error should worse as the pressure goes up, and they specifically called out 300 bar (4300 psi) as having noticable error. I wondered if that means the the top end of a standard 3442 psi HP steel is less accurate than below 3000 psi, but minimal enough error that it's not really noticable.

 

[jborg]

ISTR the Z factor for air being around 1.05 at ~230 bar and around 1.10 at 300 bar. This doesn't seem huge but of course unless we empty the tank a lot of the consumed gas is in the higher range. The effect is enough that for a dive where I started at 280 bar and ended at 130 bar, MacDive shows 15.7 L/min (it doesn't compensate) and Subsurface instead says 12.8 L/min.