Are all Cu. Ft created equal?

[RJP]

Are the electronic versions of the SPG such as those built into AI dive computers a lot more accurate at the low end than the mechanical SPG?

There's also the classic "accuracy versus precision" conundrum.

A digital spg might be very accurate, but typically cannot provide an adequate level of precision to rely on low end measurements. For instance if an SPG is +/- 100psi it might accurately tell you how much air is in the tank at 3442psi, but when it tells you 100 it could be 200 or it could be 0.

For any device with a numeric readout - especially dive computers - never assume that just because it SHOWS all the decimal places that the device can accurately/precisely DETECT that level of information.

My rule of thumb with any SPG/computer is to round down to the next lowest 500psi increment. So at 3442psi, I read it as 3000psi. At 2499 I read it as 2000. At 1000 I assume it's only got 500psi. At 500psi I regard each additional breath as a pleasant surprise!

 

[dumpsterDiver]

The comments about the accuracy of a gage are true. One sure way to tell you are getting really low is to take a super fast breath, if there is any resistance at the end of the breath, you definitely need to be heading up. If you quitely sip your air, you can almost completely drain a tank without noticing it.

I typically dive a 125 or 149 cu-ft tank and there is a lot of air left with 500 lbs. I can easily do a slow ascent and some deco on 500 lbs.

I need to be careful when I infrequently dive an 80, because there really is much less air in the last 500 psi.