Indicated air pressure on AI computer

[rk288]

First, we're neighbors. I live in Bulverde.

But to the question at hand, I bet you never get a resolution, unless you find a CALIBRATED tank pressure gauge, use it on a tank, then put your AI computer on the tank and compare the result. Forget all the gas science stuff, all the temperature variance stuff, all that crap. Just get a device you can use as a standard, use it and then compare what your AI reading is.

I'm highly suspicious of the "experienced" diver's attempt to get you to chill. I'm equally uncomfortable with incongruous instrument indications. I'd be concerned enough to try to get some kind of resolution. You need to know if your tank pressure readings are right or not.

Where I do get a comfort level is when I have my TWO AI transmitters attached to my reg 1st stage, and pressurize. They read within 2psi of each other, and, in a perfect world, they'd be equal. But that's good enough for me.

So, CriticalMass is correct...

pv=nRT => P1/T1=P2/T2 direct proportionality.

Do you know anyone with a certified analog test guage with the proper fittings to do the analysis? If you do, do the test, record your results and contact the AI computer manufacturer with your findings. The ambient temperature where you are standing or swimming while doing the test should have maybe a minute effect on the data. It's still a closed system, i.e. constant volume. It would be interesting to know their response. The algorithm can't be that complex. Maybe contact the mfr tech support with the original question which seems to me to be well founded. I'd like to know...Isn't science wonderful?

 

[Thalassamania]

Of course it does. But what's the probability that both sensors on devices that cost over $1K and are fairly new would be wrong? Pretty small, IMO.

You are missing the point. All that you show with that test is that both pressure transducers agree. That is likely in any case. If there were a problem in anything other than the temperature sensor or the pressure transducer (temperature is required to calibrate the pressure transducer) that test will not show it.

 

[Crush]

Forget all the gas science stuff, all the temperature variance stuff, all that crap.

Wrong. Not if you take the measurements under all the same conditions.

In my "daily life", at any given time and place, all those factors will be CONSTANT.

If you think that this is all crap, I hope that you don't:

• ride in any vehicle with an internal combustion engine
• use air conditioning
• use refrigerated products
• consume frozen good, including ice
• like cold beer

 

[fnfalman]

SPGs are not exactly precision instruments, and neither are AI pressure transducers.

That's why the recommended practice is to keep about 500-psi in the tank (indicated) by end of dive and at the surface so that an error of a couple hundred PSIs on the wrong side wouldn't put the diver in danger.

Anyway, FWIW, my AI computers; Sherwood Wisdom 2 (hosed console) and Aeris Elite T3 (wireless wrist) have pressure reading differences of 20-psi with the Elite T3 being 20-psi higher than the Wisdom 2.

 

[Saudi-Diver]

I went away from AI computers because I got a wee bit dependent on the remaining bottom time function and as a result stopped doing proper dive planning. Saying that I all ways had a spg attached to the HP port as well as my pressure transducer and I cannot remember any large variations between AI readings and the SPG even though air temperatures in Southern Red Sea were in the high 40 C and water temperatures in the mid 30 C s sigh those were the days.

 

[CriticalMass]

If you think that this is all crap, I hope that you don't:

• ride in any vehicle with an internal combustion engine
• use air conditioning
• use refrigerated products
• consume frozen good, including ice
• like cold beer

I understand the physics involved with gas temperatures and pressures.

The original issue, quoting from the OP, was:

my wife and I as well as several others using AI computers (not all the same brand) found the Nitrox pressure reading on our computers to be frequently 200-300# lower than the pressure shown on the pressure/O2 gauges used by the dive op when we checked the Nitrox percentage/air pressure.

Since he didn't state that the dive op's reading was done on a warm, post-fill tank, my assumption is that the tanks had been filled for some time, and were at essentially the same temperature and at one ATA of surface pressure when the tank pressure was taken using the dive op's device and the AI devices. In such a case, no temp/pressure factors come into play.

If that's the case, we're now full circle back to my statement that all the scientific postulating and posturing is nothing more than mental masturbation, since all we're seeing is a difference between the dive op's pressure gauge and the various AI devices involved. It's not more complicated than that, and over-thinking it only serves to obfuscate.

I would go even further and call the dive op's device suspect for two reasons. One, it get's a lot of use and gets banged around a lot, which can lead to instrument error. How do we know the last time he had it serviced/calibrated? Second, a whole gaggle of AI devices all read higher than the one dive op device. That points to the dive op device as the culprit by virtue of simple probability theory. I happen to know for a fact that one LDS I use has an analog pressure gauge in use at their fill station which has a marking on it's face showing where a full tank should read, since it reads several hundred pounds low at full pressure. This ain't rocket science, folks.

 

[reo]

Please, after you get to the surface, run it down to 100 PSI and check it every 100 along the way. This will be interesting.

We had to end the dive early, someone blew through their air rather quickly. I did check my computer vs the mechanical gauge, and it was consistently about 50 psi higher than the mechanical guage at several checks during the dive. The water temp was 55 degrees F, the computer literature says that air pressure displayed is normalized to read what the pressure would be at 68 degrees F, so it appears to agree with the mechanical gauge all of the time. I ended with about 1500 PSI showing on the gauge and the computer (the computer was a tad higher).

From what I can tell so far, the computer displays the actual pressure. It calculates remaining air time based on consumption rate down to a minimum pressure set by the user. I set my reserve to 400 psi, so the computer tells me how much time I have until I reach 400 PSI.

I didn't see your post until this morning after my dive, so I didn't take it down to 100 psi, but I'll try it soon.

Ron

 

[doctormike]

Hey, late to the party, but I'll throw in my anecdotal data, why not...?

I would be very interested to hear the results of the "science experiment". As they say "knowledge is power", and I can use all the knowledge I can get.
Yesterday 03:51 PM

I recently switched from an AI Suunto Cobra to a wireless Suunto Vyper Air. Of course, I added a brass and glass SPG (the wireless system is worthless on night dives when my camera strobes are lighting up the place). So for my last 30 dives or so, I have been tracking the difference between the digital sensor in the transmitter and the analog sensor in the SPG.

I found exactly what the OP was told - the digital reading is a couple of hundred PSI below the analog, and they come together around 1000 PSI. I'm inclined to believe that the digital gague software is deliberately conservative to avoid OOA, etc... but who knows. I just found this effect to be consistant with every dive so far...

SPGs are not exactly precision instruments, and neither are AI pressure transducers.

That's why the recommended practice is to keep about 500-psi in the tank (indicated) by end of dive and at the surface so that an error of a couple hundred PSIs on the wrong side wouldn't put the diver in danger

I thought that was to keep moisture out of the tank and prevent corrosion. But I guess it couldn't hurt to have extra air, right?

 

[fnfalman]

I thought that was to keep moisture out of the tank and prevent corrosion. But I guess it couldn't hurt to have extra air, right?

I can't speak for other dive charters but most dive boats in Southern California recommended for divers to surface with 500-psi in their tanks for safety factory.

As far as keeping moisture out of the tank and keep the corrosion down, I doubt that having some pressurized air in the tanks would help much. I understand that you wanted positive pressure so that outside air/water can't get into the tank and cause corrosion. The problem is that a lot of dive boats' air compressors don't deliver dry air, or during the refill process, water gets forced in anyway. I kept my tanks filled to hilt with diving air pressure and the innards got corroded anyway.

 

[NAM001]

Novice diver here and could not find an answer on the search function to a question that came up during a recent trip to Bonaire. On this trip to Bonaire, my wife and I as well as several others using AI computers (not all the same brand) found the Nitrox pressure reading on our computers to be frequently 200-300# lower than the pressure shown on the pressure/O2 gauges used by the dive op when we checked the Nitrox percentage/air pressure. One of the experienced divers told us not to worry, that AI computers frequently showed a couple of hundred pounds low until you got down to @1000 psi at which time the pressure reading would be correct. I had never heard of that issue before, which is not surprising due to my low experience level, especially using an integrated computer. However, since I always err on the side of caution, I planned my wife's and my dives with the assumption that what we saw on the computer's gauge is what we had, regardless of what the pressure gauge showed at the OP, and regardless of what the "experienced" diver told us. Since my wife's and my gauge read the same for a given tank, it did not seem logical to me that there would be 200+ pounds more air than indicated. BTW our computers are Atmos AIs.
Thanks for any comments

i would think since both you and your wifes read the same , the gage from the shop is bad reading high. i had an ai and it read 280 low i sent it in and they fixed it. of course both ai's could be off the same. check it against some other sources.