SPG and DIVE watch disagree

[Dan]

Interestingly enough, I ran a tank down to nearly empty the other day (on purpose), and the SPG & transmitter both converged at the very end. The transmitter was actually reading slightly more air at that point (88psi) than the SPG (which was at 0).

SPG uses Bourdon tube (150 years old technology) and the tube can get less springy as it gets old, hence the % error would increase with time.

 

[2New2Scuba]

SPG uses Bourdon tube (150 years old technology) and the tube can get less springy as it gets old, hence the % error would increase with time.

Thanks all for the knowledge and advice.

 

[dmaziuk]

Thanks all for the knowledge and advice.

The tube's "spring" is also not linear: it tends to be less accurate at the ends of its operating range. Normal expectation is that it's good mid-range and less accurate at max and min pressures -- but a smart move would be to calibrate the SPG to be more accurate where it matters: when the gas is running low.

The crystal pressure sensor in the transmitter has its own operating quirks but is generally expected to provide better accuracy over much wider dynamic range, so all things being equal, I'd take its reading over the SPG's.

 

[clownfishsydney]

Part of the difference may be because AI computer tank pressure is normally adjusted to show the pressure as if the temperature is 20C. Thus, if the air temperature before you get in is 30C, then there will be a big difference. Same if the water is warm.

It is likely the AI computer pressure is correct rather than the very old combo you are using. Just remember the difference between both if the sh!t hits the fan.

 

[jborg]

Part of the difference may be because AI computer tank pressure is normally adjusted to show the pressure as if the temperature is 20C.

Huh? Surely, the sensor reports the actual measured pressure, not some adjusted theoretical value? Certainly the sensors I've used have seemed to show the actual pressure, both in hot and cold circumstances.

 

[Pearlman]

My Apeks SPG and the Sunnto transmitter vary by ~3-6 bar with the transmitter showing less of the two. I rely on the computer by default but insist on having both - saved me last week when my transmitter turned out to be dead and required a battery replacement.

 

[Belzelbub]

Huh? Surely, the sensor reports the actual measured pressure, not some adjusted theoretical value? Certainly the sensors I've used have seemed to show the actual pressure, both in hot and cold circumstances.

Yeah, I'd be surprised if the pressure was temperature compensated. It certainly doesn't appear to be as I often see a very rapid drop in pressure when my tank that's been sitting in the sun on the deck of the boat is submerged and begins cooling.

 

[mr_v]

These SPGs aren't very accurate at the ends of the reading spectrum. DGX has a disclaimer at the bottom of the page that provides some interesting info.

Pressure Gauges (SPG)

Submersible Pressure Gauges (SPGs) and Instrument Consoles

www.divegearexpress.com

 

[MidOH]

None of these need perfect accuracy.

200 is a Non-issue.

 

[Bloody Marvelous]

No two SPG's are going to give the exact same reading. Same is true for transmitters.
They'll be in the general ballpark with a maximum accepted deviation.

Over time, the reading becomes less accurate. The bourdon tube in an analog SPG will stretch, giving incorrect readings, at which time you'll need to replace the whole SPG. And with a transmitter, the sensor can give incorrect readings due to stretching of the membrane, or compression of a spring, at which time that part will need to be replaced.

If you're finding both SPG's give wildly differing readings, try with a new one to see which one is in need of replacement or servicing. Likely the one with the highest reading will be the one that needs replacing/servicing. If you're diving with both, use the lowest of the two readings as the truth. Even if that one is off, you run the least risk of running out of gas.