No compensation for ideal gas law deviations

[scubadada]

I think you have that backwards. Yes, SW does not compensate. If Oceanic does, then Shearwater would be consistently lower (in the early part of the dive), not Oceanic.

That is why the SW documentation that you quoted earlier says in the early part of a dive the SW computer will overestimate SAC and underestimate GTR.

Your observation is that your SW is consistently underestimating GTR (compared to the Oceanic). That is consistent with the theory that the Oceanic IS compensating for non-ideal gas behavior.

Thanks, you're right, my mistake . I dive AL80s, generally filled a bit over 3000 psi, I would not expect a large effect, but may approach 5% at the beginning of the dive. I will contact Oceanic/Huish and see if they can give me information regarding the ideal gas law

 

[ATJ]

I think you have that backwards. Yes, SW does not compensate. If Oceanic does, then Shearwater would be consistently lower (in the early part of the dive), not Oceanic.

That is why the SW documentation that you quoted earlier says in the early part of a dive the SW computer will overestimate SAC and underestimate GTR.

Your observation is that your SW is consistently underestimating GTR (compared to the Oceanic). That is consistent with the theory that the Oceanic IS compensating for non-ideal gas behavior.

I agree. Because SW does not take the non-ideal gas behaviour into consideration, it will appear that you are going through your gas more quickly at the start of the dive.

For example, if I was diving with a 12L 300 bar tank and I started with 300 bar and had my end pressure set to 50 bar. At 250 bar, SW would think I have used 50 bar of 250 bar and so 20% of my tank. In reality (using the numbers from Subsurface), I would have used 430L out of 2599L or 16.5% of my tank. If Oceanic is compensating, it would give me a longer GTR - which it does.

Ooops - didn't see @scubadada reply.