Curiousity question about my Luna and my SPG

[LowDrag]

I just recently bought a Luna but I am have noticed something that I don't understand. I have a ScubaPro SPG that I use as a backup which I have checked at the shop (where I bought it) against their gauge they use for checking filled tanks. It reads almosty identical to the shops pressure gauge. Enter my Luna... I have done two dives with it since I bought it and both times the pressure reading from my Luna to my SPG are different. Yesterday's dive ended with 1500 PSI according to my SPG but my Luna says I ended with 1365 PSI. Being a newer diver I am not sure what is going on here. Can anyone give me some insight as to what I need to doplease?Any ideas or help would be great...

Thanks!!!

 

[dmoore19]

At 1500 psi the tolerance is +/-150#

http://www.scubapro.com/media/267298/scubapro_instruments.pdf

I don't think it is anything to get concerned with. I would probably use the Luna pressure reading.

 

[ontdiver]

I agree with dmoore19. I have a Luna and an Oxycheq SPG, and always defer to the Luna for a true pressure reading.

 

[LowDrag]

So....out of nothing more than curiousity why would there be a difference between the two? If the SGP is reading nearly the same as the shop gauge then why is the Luna's reading different. I will follow the advice you have given so I am not questioning your replies just curious.

Thanks guys!!!

 

[JahJahwarrior]

When building tools to measure something, you generally have a tradeoff between accuracy, precision and durability. The most accurate and precise gauges wouldn't last long being thumped around on boats and underwater, or atleast, not at the price point of an SPG!

If you look at a traditional SPG the gauge line itself is so thick, it could easily cover 25-50psi, and gauge to gauge a difference of a hundred PSI is to be expected at a minimum. But they last for years and years and years in what is, for a gauge, a harsh environment.

Enter digital SPG's which show very precise numbers, probably to their detriment. If you were reading it off a traditional gauge, you'd have rounded to 1360 or 1370 atleast, and maybe even just said you had 1400. But the digital gauge is more precise and gives that exact number, even if it's not accurate.

Long story short, when you calculate your "rock bottom" pressures, it's never a dumb idea to pad the turn pressure a bit and head up early. The last PSI are the hardest to suck out, and gauges can be off by more than 100psi.