I dive with two computers, each displays different tank pressures ?

[KDAD]

Being able to calibrate something is a benefit. Calibration is a way to identify inaccuracies and to correct them. A sensor that cannot be calibrated and is inaccurate is not that useful.

All devices that measure something can get "off" over time that is why periodic calibration and adjustment as needed is recommended or even required.

As far as an analog SPG goes, your guess is as good as mine. Manufacturers dont exactly list their specifications. They may not be accurate (or linear, another important attribute) or they may drift over time due to getting banged around and other factors.

 

[DiveNav]

...... It seems that most pressure transducers are non-linear, ......

Based on what?
I am looking at the specs of an high pressure sensor (5000 psi) and it says
"Accuracy (combined linearity, hysteresis & repeatability): +/-0.25 %Span"

That seems quite good to me

 

[gr8jab]

Based on what?
I am looking at the specs of an high pressure sensor (5000 psi) and it says
"Accuracy (combined linearity, hysteresis & repeatability): +/-0.25 %Span"

That seems quite good to me

Yes. It does. I do admit that I am not familiar with the specific devices used for scuba. I do have a lot of experience in other applications with transducers. I wonder what the trade offs are for this specific application. Price v size v quality v accuracy.

 

[RonR]

That’s actually pretty comparable specs to what we use- all kinds of electronic sensors are getting amazingly precise and less expensive all the time. But other factors enter in, amplification losses, resolution of A to D conversion, signal paths for very tiny voltages, temperature response and stability, all of which will decrease accuracy somewhat. I think the level of precision we see with electronic sensors would be difficult to discern on a typical SPG even if it were as accurate.

 

[Jax]

If they "are accurate" why do the really good ones have a way for the user to calibrate them? If the really good ones can get "off" and need to be recalibrated every so often, why do you think the ones we use on our reg sets wouldn't also get "off" over time?

They are pressure sensors. Atmospheric pressure changes. Calibration takes that into effect.

Try going from 6000' to sea level.

 

[KDAD]

All pressure sesnors, and any device which measures what it is intended to measure, has inaccuracies. Calibration only assesses that inaccuracy. A good device has the means to adjust its output based upon the calibration and make it more accurate.

A pressure sesnor is supposed to measure across its intended range. They have a certain accuracy across that range but that accuracy may be stated at full scale (e.g. 3000 psi).

Dive Navy found a sensor which has a stated accuracy of Accuracy (combined linearity, hysteresis & repeatability): +/-0.25 %Span. Which is quite good. So at 5000 psi that the sensor range is it should be accurate to 7.5 psi or 0.25%. As presure drops to 1000 psi, it will be accurate to 7.5 psi or 0.75 %. Still accurate enough for our purposes.

As the sensor is connected to a high pressure port which is essentially a closed system and atmospheric pressure should not have an effect on the output. Has anyone found that a tank filled to 3000 psi at sea level is no longer 3000 psi at a mile high?

 

[dmaziuk]

As the sensor is connected to a high pressure port which is essentially a closed system and atmospheric pressure should not have an effect on the output. Has anyone found that a tank filled to 3000 psi at sea level is no longer 3000 psi at a mile high?

Of course if that ever becomes an issue, in a digital system you couple it with ambient pressure sensor and compensate in firmware. Also for temperature. Not that we need that level of a**l retentiveness for our purposes.