Accuracy of data presented in mV or PO2

Meng_Tze

In a different thread the discussion changed to the supposed accuracy issues of data acquisition and signal processing. Some comments were geared to the same data being processed twice (mV readout and subsequent offset for PO2) and there being issues with that.

The question I have is how accurate does this data need to be? The cells themselves have an error, the gas analyzer has an error, the 'voltmeter' has an error, the ADC has an error etc, etc. Is diving a Rebreather requiring a set up that can match a lab's instrumentation.

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David Pye

I don't personally think accuracy is the problem, the issue is about processing.

If something has a CPU (e.g. VR3, shearwater, HH) etc, it is far more likely to be able to hang, or have the memory locations of where it stores the ppO2 readings etc (or various bits of the software that reads/writes the PPO2 values to screen) mashed up as a result of a programming or hardware error. By contrast, this can't really happen with a KISS display (the little panel voltmeters) as they don't have that level of complexity.

David

Meng_Tze

But surely as long as the data is presented, what does it matter if it is processed three times? You have primary handset, secondary and maybe a HUD. That is three representations of that data.

I think people are looking at accuracy without calling it out specifically. If the processor hangs, looses bits or anything and represents that on a screen, how does that relate to the information needed? Can it still be relied upon?

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Dsix36

This is all relating to my hammerhead controls.

If the processor hangs, it will only effect 1 handset. The other will still be accurate and may be relied upon.

I believe it is more important that the processor can "calibrate" from the MV reading and convert to PO2 for the display. I think this is done just because of the fact that the O2 cells will deteriorate over time.

IMHO all the modern CPU's are quite capable of doing this. The accuracy of the MV is not very important at all. As long as the CPU will convert it to the proper PO2 readout.

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David Pye

I'm not looking at accuracy, I am specifically not worried about error margins etc. I'm wanting to know if the data displayed on the screen actually relates to what the current input from the cells is.

If the processor is somehow no longer able to get accurate information from the ADC, then the mV and ppO2 readings may appear to correlate, but the information is garbage.

Meng_Tze

?

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lizardland

Hi,

my point in the original thread, which may have gotten lost, isn't so much the accuracy of the data but the fact that you don't have anyway of verifying it on a single computer based system. You can't use the mV display to verify the PO2 display because they are based on the same signal. You can do flushes but you either have to suspect there may be a problem or be flushing every 10min or so (or whatever the time period you are comfortable with a PO2 varying over). Or simply bite the bullet and accept that what is being shown is correct.

I don't doubt the accuracy of the display but my own feeling is that the more complicated the system then the more chance of some kind of interference along the way, be it software, salt water intrusion, broken wires, etc. A triple display has three distinct circuits, not ideal but provides a bit more comparison for me.

Cheers,

Stuart

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Dsix36

That is why we have a SECONDARY handset.

To verify that the Primary is accurate.

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diverklondike

Personally I think the bigger question to ask is does the pp02 that reads on your wrist match the ppo2 that you inhale out of the mouth piece? I think there is a lot more variance to this that we realize depending on where the o2 is injected in relationship to the cells etc..

Meng_Tze

This is indeed a core question but is somewhat relegated by all the errors and accuracy margins we put in the entire sequence before diving.

We assume purity of gases delivered to us, we analyze this with O2 cells that have their own variances, we calibrate through O2 cells in our systems with their respective margins, then we dive the system. When diving we have a PO2 read out based on calibration, injection position etc... And we do a calculation based on a depth reading (dil/O2 flush) through a transducer that has it's own margin. What we calculate is not the same as what we see on our handset, is not the same as actual PO2.

The question is what is that margin of error and is it trivial? I do not think it needs to be (nor that it is) that accurate. Fact of the matter is that even the limits we have with O2 exposure is based on a broad set of assumptions.

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