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Thread: Carbon Monoxide Alarm Went Off... False Positive?

  1. #1
    SJ's Avatar
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    Carbon Monoxide Alarm Went Off... False Positive?

    Got the skywagon back from a mega annual which included a new bladder and a bunch of other stuff, but one of the BIG expenses was replacing both exhaust manifolds with brand new as the old ones were no longer serviceable. Bear in mind I replaced both exhausts last annual due to CO when the heat was pulled. So now that everything is new, we flew down to K81 for a little BBQ picnic from the on-field restaurant

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    Laura even brought flowers for the table.

    Now completely stuffed we get back in the plane with a great tailwind and a few minutes into the flight I hear "carbon monoxide warning" over my headsets from my Sentry ADSB gadget. I didn't even know it did that (through Foreflight), but it's a pretty cool feature because I NEVER look at the little light where I have it placed.

    Now our minds are racing. We are not running the heater anyway, but we open all the vents and the warning and light goes away. Close them, then it comes back on. Back downtown we go.

    Today, we went out to test it again with three different CO detectors. We tried ever configuration of heat, cabin air, carb heat, and flight direction and speed (downwind, upwind, slow, flaps etc) could never get the Sentry to go off again. Our highest reading on the flight was 14 or so, which ain't much. Most of the time it was 4 or 5. The high dollar Fluke tester did not agree with the one behind it at lower levels, but was in sync above ten. The cabin heat knob made zero difference nor did the cabin air.

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    So I guess the question is, what do you think? Was it a false positive? The Sentry might have been in the sun while we were having lunch - although it was not a hot day. I of course looked at all the exhaust connections and they look great.

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    Your thoughts are appreciated!

    sj
    "Often Mistaken, but Never in Doubt"
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  2. #2

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    Are your door and window seals in good condition? No holes in the firewall?
    Do any of these electronic devices get calibrated at the factory? Fluke is usually pretty good stuff. I am not surprised by two different devices that measure the same thing not agreeing with each other. Take digital levels - they never perfectly agree, at least not mine.

  3. #3
    SJ's Avatar
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    Quote Originally Posted by 180_jeff View Post
    Are your door and window seals in good condition? No holes in the firewall?
    As good as any 65 year old plane, I would think I would have gotten high readings today as well if that were the case.

    sj
    "Often Mistaken, but Never in Doubt"
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  4. #4
    wireweinie's Avatar
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    Oil burning off the exhaust parts after maintenance? Head/tail wind helping exhaust enter from doors or aft compartments?

    Web
    Life's tough . . . wear a cup.
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  5. #5
    SJ's Avatar
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    Quote Originally Posted by wireweinie View Post
    Oil burning off the exhaust parts after maintenance? Head/tail wind helping exhaust enter from doors or aft compartments?

    Web
    The plane had flown more than an hour closer to two hours after the work by the time this happened. I wonder about the tailwind, but we flew this plane hundreds of hours last year and never had a peep from one of these gadgets.

    sj
    "Often Mistaken, but Never in Doubt"
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  6. #6

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    Quote Originally Posted by SJ View Post
    As good as any 65 year old plane, I would think I would have gotten high readings today as well if that were the case.

    sj
    Yeah, that is puzzling. On your door seals - I doubt that those are original. If so, maybe consider something like this:

    https://www.aircraftdoorseals.com/cessna/cessna-180/

  7. #7
    wireweinie's Avatar
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    The flowers!

    Web
    Life's tough . . . wear a cup.
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  8. #8

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    Texas Skyways solved my intermittent problem.

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    Last edited by OLDCROWE; 04-12-2020 at 08:11 AM.
    Remember, These are the Good old Days!

  9. #9
    Eddie Foy's Avatar
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    A tailwind in flight is not going to affect the airflow around the airframe. Unless you do a downwind turn!
    "Put out my hand and touched the face of God!"
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  10. #10

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    Because you had work done on the exhaust I'd be inclined to decowl it and and check all the nuts and bolts for tight and look for exhaust leak feathers. Question is would I fly it another hour or not to magnify the feathers.
    You can't get there from here. You have to go over yonder and start from there.
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  11. #11
    SJ's Avatar
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    Quote Originally Posted by OLDCROWE View Post
    Attachment 48108
    Texas Skyways solved my intermittent problem.
    Kirby, I've a very different exhaust system on the 55', duals coming out of the cowl flaps.

    sj
    "Often Mistaken, but Never in Doubt"
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  12. #12
    www.SkupTech.com mike mcs repair's Avatar
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    Where did the noisy one take it’s reading from? Behind the panel?


    Sent from my iPhone using SuperCub.Org

  13. #13

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    Quote Originally Posted by SJ View Post
    Kirby, I've a very different exhaust system on the 55', duals coming out of the cowl flaps.

    sj
    Can you extend them further into the slip stream?
    Remember, These are the Good old Days!

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    How old is the Sentry?

    My two year old Sporty’s CO detector started giving intermittent alarms. I couldn’t reproduce the problem, so I replaced the CO detector. No issues since.

    I’ve read in several places that when CO detectors begin to fail, they produce false positives. My experience has been the same.
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  15. #15

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    Sooo I it is late and Jane had a great day flying and let me open 2 bottles of whine!!! But I have some input take it for what it is worth. If I remember you have an early 180. The Cessna 180 from the very first on has had issue with this problem. Do a search on it, include pencil lead and it should show up. I have a 1959 180C, we found that if you close the fresh air door you get a CO2 buildup. So the question is did you close the fresh air. If you usually just pull the heat than you have positive pressure into the cabin. But if you close the fresh air you have only wing vents for positive pressure and if they are closed CO2 buildup is very possible.
    DENNY
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  16. #16
    SJ's Avatar
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    Denny, we tried every combination of air vents and cabin air and heat controls.

    I have over 1000 hours in this plane, never had this before except when the muffler went bad then it came through only when the heat was on.

    sj

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  17. #17
    SJ's Avatar
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    Ok, I did some testing this morning...

    "Often Mistaken, but Never in Doubt"
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  18. #18
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    I have a suspicion....
    did your BBQ picnic include beans (the musical fruit) ?
    Cessna Skywagon-- accept no substitute!
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