Temperature Compensated Pressure Transducer

[ancanclin]

Forgive me if this has been asked before, but I have what is likely a basic question. I am using a Galileo Sol computer (air integrated), and the manual notes that the pressure transducer on this computer, unlike other (earlier model) UWATEC computers, is not compensated for temperature: "Unlike other UWATEC gas integrated dive computers, this value is not temperature compensated. Temperature compensation is still performed to ensure a correct RBT (remaining bottom time) calculation, but the uncompensated value is displayed..."
My question is, what are the ramifications (good or bad) of displaying a value that is (or is not) compensated for temperature, and why would ScubaPro choose to display an uncompensated value, but compensate it for the purposes of the computer's calculations. Thanks in advance for any insight you may have.
Craig

 

[SteveAD]

DOT regulations define the service pressure of cylinders at 72 degF. An al 80 filled exactly to spec will have 3159 psi at 100 degF or 2773psi at 32 degF.

I am not positive that this is the case, but temp compensated pressure might mean that it shows what the pressure would be at 72 deg F (reading ambient temp and doing the calculations).

Assuming this is what is meant, temp compensation doesn't really give you any benefit.

 

[don Francisco]

I'm not sure if temperature compensation refers to changes in the accuracy of the transducer or to the change in tank pressure with temperature change, I'll assume the latter here.

Assume that your tank has 3,000psi on the surface at 90 degrees on a hot summer day and you submerge it into 40 degree water. Without any air loss the pressure will drop almost 10% as it cools by 10% in absolute degrees (degrees F. plus 460). It doesn't happen instantly, so it would create the illusion of faster air consumption early in a dive.

However as a practical matter the air is only available to you based on the actual current pressure in the tank, so that's what ultimately counts. (Unless you have a means of rewarming it, to eke the last bit out)

 

[RJP]

DOT regulations define the service pressure of cylinders at 72 degF. An al 80 filled exactly to spec will have 3159 psi at 100 degF or 2773psi at 32 degF.

I am not positive that this is the case, but temp compensated pressure might mean that it shows what the pressure would be at 72 deg F (reading ambient temp and doing the calculations).

Assuming this is what is meant, temp compensation doesn't really give you any benefit.

I may be missing the boat entirely here, but the tank "knows" what temperature it is at, no? That temperature is already "calculated" into the pressure of the tank that your computer is reading merely by reading the pressure.

Consider this. You get a hot fill and put a pressure gauge on it - it reads 3000PSI. You drive to the site, put your gear on, get in the water, and check your pressure and it now says 2700PSI. No one needed a computer to "compensate" for the temperature. Charles' law already did that by "changing" the pressure in the tank.

 

[SteveAD]

I may be missing the boat entirely here, but the tank "knows" what temperature it is at, no? That temperature is already "calculated" into the pressure of the tank that your computer is reading merely by reading the pressure.

Consider this. You get a hot fill and put a pressure gauge on it - it reads 3000PSI. You drive to the site, put your gear on, get in the water, and check your pressure and it now says 2700PSI. No one needed a computer to "compensate" for the temperature. Charles' law already did that by "changing" the pressure in the tank.

You missed my point. I might be wrong, but my thought would be that a temp compensated system would read 3159psi @ 100degF or 2773psi @ 32degF as 3000psi @72 degF. I never said this would be a good thing, just that it was my only idea of what temp compensation might be.

 

[don Francisco]

As the instructions said, it displayed current, uncompensated gauge pressure, which would make sense since that's what you need to know, as opposed to what the pressure would be under different conditions.

I think (just reading between the lines) that what they're referring is compensating for temperature whan measuring changes in tank pressure so to have a more accurate estimate of your true air consumption rate, and the other calculations based on that.

As I said, that's just a guess as to what they might be thinking, only their hairdresser knows for sure.

 

[RJP]

As the instructions said, it displayed current, uncompensated gauge pressure, which would make sense since that's what you need to know, as opposed to what the pressure would be under different conditions.

I think (just reading between the lines) that what they're referring is compensating for temperature whan measuring changes in tank pressure so to have a more accurate estimate of your true air consumption rate, and the other calculations based on that.

As I said, that's just a guess as to what they might be thinking, only their hairdresser knows for sure.

I guess theoretically that makes sense, but would only be practically useful if the water temperature changed SIGNIFICANTLY during the dive (I'm not talking about a 5deg thermocline, but a good 20-30 degree swing.) And even then it would require the computer to ANTICIPATE temperature changes later in the dive in order for the "compensation" to be meaningful.

Lastly, the pressure differences that would occur based on the change in water temperatures during a dive would likely be smaller than the level of accuracy that the computer can reliably report.

 

[eponym]

I am using a Galileo Sol computer (air integrated), and the manual notes that the pressure transducer on this computer, unlike other (earlier model) UWATEC computers, is not compensated for temperature: "Unlike other UWATEC gas integrated dive computers, this value is not temperature compensated. Temperature compensation is still performed to ensure a correct RBT (remaining bottom time) calculation, but the uncompensated value is displayed..."

Craig, interesting question. And certainly the Galileo Sol is different from SPro/Uwatec's earlier models.

As I understand it, the Galileo Sol remembers any thermoclines you passed on the way down and assumes you'll pass them again on the way up. So when calculating your remaining bottom time (RBT) and taking breathing rate into account, it also allows for changes in tank pressure due to changes in water temp. It would have to be one heck of a thermocline, and fairly deep, for it to modify your tank pressure enough to show up as an additional minute or two. I have been in some high mountain lakes where the thermocline is around 30 feet or lower and pretty severe, so that would probably be an instance. Anyway, that's how I interpret the manual:

"The RBT (remaining bottom time) is the time you can spend at the current depth and still have enough gas supply to make a safe ascent and reach the surface with the tank reserve. The RBT calculation is based on your current breathing rate, accounts for any existing and upcoming decompression obligation and for any temperature gradient in the water."​

My question is, what are the ramifications (good or bad) of displaying a value that is (or is not) compensated for temperature, and why would ScubaPro choose to display an uncompensated value, but compensate it for the purposes of the computer's calculations. Thanks in advance for any insight you may have.
Craig

I'd say that an uncompensated pressure display is preferable, since you want to know what you actually have at this time and depth and temp. The compensation for purposes of RBT is theoretical (as is the RBT value). Breathing rate will change, you could avoid the thermocline, any number of variables will change during your ascent and affect the RBT.

Hope this helps,
Bryan

PS. I think RJP, posting while I was composing, hit the nail on the head.