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Thread: Calibration of Cells - Single point - 2 point -Air or O2?

  1. #61
    RBW Member Dubaifalk is an unknown quantity at this point Dubaifalk's Avatar
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    Re: Calibration of Cells - Single point - 2 point -Air or O2?

    Regarding calibration of O2 cells:

    We have to remember that the cells are measuring partial pressure of the oxygen. Therefore if the ambient pressure is less than standard, less than 1013 hPa or 760 mmHg, the cell will not show 100% if you flush it with pure oxygen at sea level.

    Formula: ambient pressure/1013*oxygen%=cell output reading

    For example; low pressure day 990 hPa the cell will only show 97.7 % if we flush it with 100% pure oxygen.

    990/1013*100=97.7


    Or if we calibrate a O2 analyzer for Nitrox in ambient air and pressure in a low pressure day, 990hPa, it should read 20.5% in normal air at sea level and not 21%;

    990/1013*21=20.5


    This has also be taken into account calibrating cells at high altitude where the ambient pressure is lower than on sea level.

    Approximately 11hPa lower per 100m above sea level.


    And off course the cell will show higher at higher ambient pressure. On a 1022 hPa day, flushed with 100% O2, it will show:

    1022/1013*100=100.9




    The Shearwater Petrel for example does show the ambient pressure to help us do that calculation
    Last edited by Dubaifalk; 12th November 2014 at 12:55. Reason: Uppdate of facts

  2. #62
    RBW Member uwxplorer is on a distinguished road uwxplorer is on a distinguished road uwxplorer's Avatar
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    Re: Re: Calibration of Cells - Single point - 2 point -Air or O2?

    Quote Originally Posted by Dubaifalk  View Original Post
    The Shearwater Petrel for example does show the ambient pressure to help us do that calculation
    Not only that, but it will allow you to use that ambient pressure to calibrate the reading:

    System Setup->Display Setup->Altitude->Auto

    (by opposition to SeaLvl).

  3. #63
    RBW Member Andy T is just really nice Andy T is just really nice Andy T is just really nice Andy T is just really nice Andy T is just really nice Andy T is just really nice Andy T is just really nice Andy T is just really nice Andy T is just really nice Andy T is just really nice Andy T's Avatar
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    Re: Calibration of Cells - Single point - 2 point -Air or O2?

    Quote Originally Posted by Mike  View Original Post
    You should do some more diving.
    Perhaps he should do some more reading and less diving - we don't need another statistic.

  4. #64
    Dave Tomblin wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc's Avatar
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    Re: Calibration of Cells - Single point - 2 point -Air or O2?

    The problem we keep running into with the calibration argument is we are not really comparing apples to apples.
    I believe the best calibration is to use a partial pressure as close to the operating range as possible. The problem is the availability of O2 at 1.3 ATA during the assembly stage of the rebreather dive.
    There are devices that do that maybe some more reliable than others but they are not universally any more accurate than the cells you are trying to calibrate.
    Next would be pure O2 at 1ATA which is readily available to anyone who is filling CCR tanks (probably) but there is the potential for cells to be calibrated with an unknown gas since we are still relying on the same failure prone cells to verify our pure O2.
    Next we have air. Although air is further removed from our operational parameters of our rebreather it is an easily verifyable gas and pretty stable throughout the globe under conditions divers tend to operate.

    The problem with 2 point calibration is it assumes the high point is PO2 1ATA which may or may not be the case and relies on a test using another unreliable cell so the errors can easily be compounded.

    My practice it to verify in two points, air and O2 but if there is a unexplained variance it is the O2 that come into question because I know the air is going to be stable.

    One exception occured when a shop operator in Mexico was using tank air to calibrate his O2 tester. As he was adjusting his reading off the tank to 21% I picked up a piece of duct tape lying beside the tank that read 32%. I retested on another tank and his reading was way off. It turned out the tank he was calibrating from contained nitrox.

    The only gas I truly trust is atmospheric air but it is far enough away from the operating range of CCR that the cell linearity comes into question.
    Last edited by wedivebc; 13th November 2014 at 17:01.
    Cheers,

    Dave....

    www.wedivebc.com

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