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MV reading check

Discussion in 'Rebreather Diving' started by lermontov, Jul 30, 2020.

  1. Jack Hammer

    Jack Hammer Solo Diver

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    I have a JJ. Closing the DSV will change your calibration. It needs to be open for the gas to flow. The O2 injects next to the sensors and flows over them. The O2 bottle itself is analyzed before attaching to the unit and rechecked prior to diving. Breathing moisture is irrelevant as calibration is part of assembly/setup, you are not breathing the loop.
     
    rjack321 likes this.
  2. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

    # of Dives: 500 - 999
    Location: Napa, California
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    And do your mV compared with air confirm that you're actually hitting 100% on JJ cal, as opposed to 98%?
     
  3. Jack Hammer

    Jack Hammer Solo Diver

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    It's usually pretty close, within a few points.
     
    rsingler likes this.
  4. broncobowsher

    broncobowsher Solo Diver

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    For the rEvo that is also true, but it isn't the flowing over the sensor, it is the pressure built up in the counterlung. Since the pressure transducer for the calibration is outside the loop, the gas pressure on the sensor needs to be the same. Open the DSV equalizes the pressure.
     
    rsingler likes this.
  5. helodriver87

    helodriver87 DIR Practitioner

    # of Dives: 500 - 999
    Location: Alabama
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    I don't think I've ever seen mine calibrate more than 2mV off from predicted. I also built a spreadsheet to predict calibration results at a different altitude so you can quickly calibrate on site if anyone wants it. That aspect of it should be unit agnostic.
     
    rsingler likes this.
  6. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

    # of Dives: 500 - 999
    Location: Napa, California
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    I think that leaves the question open, then.
    Predicted mV @98% O2 is ~1mV lower than at 100%. Is it the cell? Or is it the O2 content of the calibration gas after dilution with a whiff of residual loop air? If the 1-2 mV difference is consistent across all three cells, and lower than predicted, that suggests that the loop isn't reaching 100% O2.

    My spreadsheet is attached.

    That sounds great! Do you multiply cell predicted results by the relative fraction of mb at alt vs mb at seal level?
     

    Attached Files:

  7. helodriver87

    helodriver87 DIR Practitioner

    # of Dives: 500 - 999
    Location: Alabama
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    That's how I'd interpret it. Seems like the .98 calibration FO2 is intended to compensate for the method used by the rEvo. The JJ doesn't do that since it calibrates by injecting O2 essentially direct to the sensors. No real residual inerts or water vapor to be concerned with.

    I basically generated a table of ATA for a given elevation (assuming standard temperature conditions) and drew a best fit line up to 9900 ft. That's in the ballpark of where the relationship stops being close enough to linear. Turned that into an excel function that outputs ATA when you input elevation in meters. Then I used a function to calculate percent change between assembly site and dive site and apply it to your assembly site calibration result to predict on site calibration readings. Also added a function that allows you to convert mV air readings at altitude to a sea level equivalent so if you're traveling back and forth, you can continue to track cell linearity.

    I've been using it for the past few months and it seems to be tracking perfectly. If anyone else is doing any altitude diving with meaningful changes between assembly and dive, I'd be curious to get some feedback on how accurate it is.
     

    Attached Files:

    lermontov and rsingler like this.
  8. JahJahwarrior

    JahJahwarrior ScubaBoard Supporter Staff Member ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: West Palm Beach, Fl
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    Like most of you, I've also written spreadsheets and charts to laminate to use during my checklists.

    I like how your chart takes into account the fact that most mv readings are in the middle of the acceptable range, instead of using the same division between points across the entire range. Makes for a less congested chart.

    My printed chart only includes air cal, o2 cal, and 1.6 20' flush. We've had some great discussions here lately about cell limiting, checking for cell limiting, and the best way to get a 1.6 verification, which might mean doing an o2 flush as you are ascending from 30 to 20.

    Also enjoyed the chart of mv impacts of temperature and humidity. I don't worry too much about humidity impact because I'm assuming the air in the tanks is not humid, but it does make me wonder if there is a lagging impact on the cell, ie, could the cell face take longer to reduce humidity the same way a wet blanket will take a while to dry when you put it in a lower humidity room.

    Fortunately I have never had to deal with altitude effects at all, but someday I might.

    In addition to the charts, I threw the math into a calculator at the very bottom of the gas planning page here. Doesn't help at florida caves where cell signal is sparse, but it was a fun exercise.
    Gas Planning
     
  9. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Port Orchard, WA
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    Same
    But I analyze the fO2 in the supply bottle (although this isn't mandatory). Then I measure the fO2 of the gas leaving the head so I know that the sensor is seeing 100%. Then I calibrate.

    The 98% thing is a shearwater kludge for units that can't be verified to have 100% in them.
     
  10. JahJahwarrior

    JahJahwarrior ScubaBoard Supporter Staff Member ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: West Palm Beach, Fl
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    Agreed on the analyzing, all tanks get analyzed for o2 and co!

    O2ptima caps make it easy to flush the head, QD one side and mushroom on the other, no nooks and crannies.
     
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