Ananlog SPG vs AI Computer

eelnoraa · Apr 16, 2010

Which one is more accurate? I have been using AL80 usually filled to 3300PSI tank, at this pressure, SPG and AI reading agree. My SPG is backup, so I hardly look at it.

Yesterday, I use a LP steel tank. Not sure what is the correct fill pressure. But my SPG read 2500psi, but AI read 2200PSI. I think at lower pressure, one of these device is not reading correct.

So, which one is usually more accurate? I will borrow another SPG for compariosn tomorrow, but let's say if the analog is off, I know it can be adjusted. But if the AI is off, can I adjust or calibrate it myself?

Matt S. · Apr 16, 2010

I have been told that the mechanisms in AI transmitters are of a different nature than those in SPGs, and are inherently more accurate. If the AI is in error I doubt you can calibrate it yourself though.

Bubbletrubble · Apr 16, 2010

eelnoraa:
Which one is more accurate? I have been using AL80 usually filled to 3300PSI tank, at this pressure, SPG and AI reading agree. My SPG is backup, so I hardly look at it.

Yesterday, I use a LP steel tank. Not sure what is the correct fill pressure. But my SPG read 2500psi, but AI read 2200PSI. I think at lower pressure, one of these device is not reading correct.

So, which one is usually more accurate? I will borrow another SPG for compariosn tomorrow, but let's say if the analog is off, I know it can be adjusted. But if the AI is off, can I adjust or calibrate it myself?

Have you considered comparing your analog SPG and AI computer to someone else's analog SPG or AI computer? Check the tank pressure at the fill station. Most shops will have a pressure checker available. Alternatively, you could borrow your buddy's reg setup to read the pressure of your tank. I call this consensus gathering.

It's not a true way to measure accuracy...but it should tell you which gauge is "off."

It's silly for someone on the Internet to speculate as to the accuracy of either of your instruments. Compare your gauges to a known accurate standard if you really want the answer to your question. FWIW, the designs of both analog SPGs and electronic AI computers are accurate and precise enough for scuba application. Although it's possible that depending on construction the electronic AI will be more accurate, you should known that a computer may display incorrect data when the battery is low. You don't have this issue with an analog SPG.

Charlie59 · Apr 16, 2010

In either case I would always go with the lower reading when diving.

Not to high jack the thread, my Luna AI failed while diving in Cozumel last week. Glad I had a spg in reserve. Will keep it on my 1st stage.

OldNSalty · Apr 16, 2010

Charlie59:
In either case I would always go with the lower reading when diving.

But the higher reading could give you infinitely more bottom time.

eelnoraa · Apr 16, 2010

Bubbletrubble:
Have you considered comparing your analog SPG and AI computer to someone else's analog SPG or AI computer? Check the tank pressure at the fill station. Most shops will have a pressure checker available. Alternatively, you could borrow your buddy's reg setup to read the pressure of your tank. I call this consensus gathering. It's not a true way to measure accuracy...but it should tell you which gauge is "off."

It's silly for someone on the Internet to speculate as to the accuracy of either of your instruments. Compare your gauges to a known accurate standard if you really want the answer to your question. FWIW, the designs of both analog SPGs and electronic AI computers are accurate and precise enough for scuba application. Although it's possible that depending on construction the electronic AI will be more accurate, you should known that a computer may display incorrect data when the battery is low. You don't have this issue with an analog SPG.

Yes, I have considered comparing reading with other people. In fact, I will do it tomorrow. I just, in the mean time, want some general information about the analog gauges vs AI computer.

elan · Apr 17, 2010

Digital are usually more accurate. It is hard to get good accuracy on a small face of an SPG

reefduffer · Apr 17, 2010

You might be interested in having your LDS (assuming this is a full-service shop) do a gauge check on your computer and/or SPG.

Last year I bought a used Suunto Cobra from another SB user. I took it to my LDS and they put it on their gear and calibrated the tank pressure reading in 500 psi steps down from 3000, and then put it in a pressure vessel and calibrated the depth in 10 foot steps down to 130 feet. They charged me $15 for all of that, including a written report.

Maybe the most comforting $15 I've spent.

FWIW, all readings were within 1% of the shop's instruments. At that point it's hard to say which is more accurate. Great for me, but not really my point here, which is that it won't cost you much to know.

fnfalman · Apr 17, 2010

Both of my AI computers are pretty much dead on with each other when I test them on my tanks. We're talking about difference of maybe 10-psi. When I test the tanks with the SPG and the pressure gauge testers, those things are off by maybe 100 to 150-psi.

kanonfodr · Apr 17, 2010

My AI is normally 100-150 psi off from my SPG, no biggie. At the point where it really matters, you should be trusting the lower number to be safe.

BTW, any measurement instrumentation is only as "accurate" as it's calibration is. In other words, your AI reading down to the single PSI may seem to be more accurate than your SPG, but it may not readily be so. You need to compare readings from a benchmark pressure to determine accuracy.

Also, knowing down to the single PSI is really not required unless you are REALLY trying to stretch your gas. But there are much better options for extending your dive time.

Peace,
Greg

Ananlog SPG vs AI Computer

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