Indicated air pressure on AI computer

[Altamira]

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

 

[Cleavitt]

It's more likely that the mechanical gauge that the dive op provided was not accurate. Their gauge likely gets a lot of abuse. Digital pressure sensors are generally very accurate througout their entire working range so I would disregard what the 'experienced diver' told you. Some mechanical gauges are considered more or less accurate at different points in their range. Maybe he was confused by that and applied it to digital gauges instead.

 

[Thalassamania]

Not knowing which gauges are which in this case makes this all speculation, but here goes: the AI computers that I am familiar with do not "ramp off" to zero, they ramp off to somewhere between 300 and 500 PSI, that is to say their "air time remaining" is really "air time remaining till X psi." It maybe that they will, in fact, read zero when you still have a few hundred PSI in the tank. If I were you I'd experiment, I'd breathe a tank down till the AI computer says zero and then I'd mechanically gauge it.

 

[Crush]

Not knowing which gauges are which in this case makes this all speculation, but here goes: the AI computers that I am familiar with do not "ramp off" to zero, they ramp off to somewhere between 300 and 500 PSI, that is to say their "air time remaining" is really "air time remaining till X psi." It maybe that they will, in fact, read zero when you still have a few hundred PSI in the tank. If I were you I'd experiment, I'd breathe a tank down till the AI computer says zero and then I'd mechanically gauge it.

Thalassamania - I have never heard of AI computers ramping off to a non-zero pressure. I am not questioning the accuracy of your post, but it confuses me - I can see an AI computer giving you less "safe time in dive" than you might otherwise think because it is factoring in some set "hit the surface with XXX psi," an ascent rate, and a safety stop. I fail to see the reason for it to approach a non-zero pressure and display that pressure as zero... Why would this be?

 

[GrumpyOldGuy]

If you are comparing the measurement at the time of fill vs what you see a few hours later, part of the issue may be the gas was a bit warm in the shop and has cooled down.

Also, I found that my SPGs tend to measure a little low, I suspect the manufactures may calibrate them a little low so the tolerance is all biased to one side in order to reduce legal liability. I don't know this to be a fact, just a thought.

 

[Thalassamania]

Thalassamania - I have never heard of AI computers ramping off to a non-zero pressure. I am not questioning the accuracy of your post, but it confuses me - I can see an AI computer giving you less "safe time in dive" than you might otherwise think because it is factoring in some set "hit the surface with XXX psi," an ascent rate, and a safety stop. I fail to see the reason for it to approach a non-zero pressure and display that pressure as zero... Why would this be?

I know that is was true of both the ORCA PHEONIX and the EON (the only two that I owned), if it is true of others, well ... I do not know. I failed to see the reason also and told the designers that I thought this to be both strange and counter productive. This may not be be true of "more modern" AI computers.

 

[Saudi-Diver]

Does any one know if AI dive computers calculate tank pressure at a standard temperature. That would cause a big difference between an Analogue gauge and a AI read out in a hot environment?

 

[boulderjohn]

If you are comparing the measurement at the time of fill vs what you see a few hours later, part of the issue may be the gas was a bit warm in the shop and has cooled down.

And if you are comparing it after you have been in the water for a few minutes, the difference will be even greater.

This is because of Charles' Law. The warmer a tank is, the higher the pressure reading will be. If you want a rule of thumb, for every 1 degree drop in temperature, you can estimate a 5 PSI drop in pressure. It is not uncommon for an AL 80 to lose 200 PSI between filling the tank (which heats the gas due to compression) and actually using it.

 

[boulderjohn]

Does any one know if AI dive computers calculate tank pressure at a standard temperature. That would cause a big difference between an Analogue gauge and a AI read out in a hot environment?

Any gauge can only measure the pressure coming out of the tank at whatever temperature it may be.

 

[GrumpyOldGuy]

Any gauge can only measure the pressure coming out of the tank at whatever temperature it may be.

I think he meant could it compensate for the temperature difference. The problem of course is that the temperature inside the tank is unknown, the AI computer measures ambient of the surroundings of the computer, which may or may not reflect the tank temperature.