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I see no point to it. I could be missing something, but I find no reason to prefer a calculated pressure adjusted to 20C (assuming that the air in my tank will remain constant and at water temperature, which it will not) over the true pressure in my tank.
This seems so silly and arbitrary to me that I must be missing something. What is it?
I see no point to it. I could be missing something, but I find no reason to prefer a calculated pressure adjusted to 20C (assuming that the air in my tank will remain constant and at water temperature, which it will not) over the true pressure in my tank.
This seems so silly and arbitrary to me that I must be missing something. What is it?
I'm not possitive, but I *think* that normalization to 20'C is for calculating SAC rate only. I seem to remember a thread about this in the past. It made it seem as though peoples SAC rates were slightly better than they actually were. The user found the descrepancy after they calculated their SAC using the raw data from their computer and the standard formula. It didn't match the computer's SAC calculation. It was only for one brand of dive computers. I'll try to find the thread.
I would hope that it's not used by the computer to display actual tank pressure because that would be very misleading and just wrong.
EDIT: Couldn't find the thread, but it was about SAC rate with Uwatec computers. However, after looking at the graph that Jax posted, it seems as though the manufactuer is saying that calculation applies to the actual pressure display as well. Very strange.
I'm not possitive, but I *think* that normalization to 20'C is for calculating SAC rate only. I seem to remember a thread about this in the past. It made it seem as though peoples SAC rates were slightly better than they actually were. The user found the descrepancy after they calculated their SAC using the raw data from their computer and the standard formula. It didn't match the computer's SAC calculation. It was only for one brand of dive computers. I'll try to find the thread.
I would hope that it's not used by the computer to display actual tank pressure because that would be very misleading and just wrong.
If the water is 0 C and the computer "warms" the air to 20 C then my 3000 psi fill would show up as about 3200 psi. As the tank approaches zero pressure the relative error of 7% would remain the same. At a displayed pressure of 320 psi I would really have 300 psi.
Wait...take any pressure measuring device; hook it up to any pressure vessel with a long hose...now, change the temperature of the gas in the pressure vessel...
pv=nRT
The pressure will change in direct proportion to the temperature regardless of the temperature of the gas in a vessel with a fixed volume.
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 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.
All that shows is that the sensor's agree with each other. Pressure transducers, when temperature corrected, are rather accurate, that's not where the problem might be and that's all that you are testing. Not to unduly alarm you but that gives you no check on possible errors in the processing unit or in the software itself .... which is what was being discussed.
Wait...take any pressure measuring device; hook it up to any pressure vessel with a long hose...now, change the temperature of the gas in the pressure vessel...
pv=nRT
The pressure will change in direct proportion to the temperature regardless of the temperature of the gas in a vessel with a fixed volume.
Yes. However, after measuring the pressure the computer in question (and I am assuming that this computer does work the way others have reported that it works) applies P1 / T1 = P2 / T2 to convert from P1 (actual pressure) at T1 (temperature the computer measures, which is water, not tank, temperature) to P2 (at 293 K), which is what it displays, at T2 = 293 K.
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.
You guys are trying to make this into a science fair project. The fact is that the OP can just measure the pressure at any time at home with a calibrated gauge and compare that to what his AI reads. It's not any more complex than that.
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