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Our Cessna 172P is equipped with a digital CO-meter rather than the traditional Quantum eye carbon monoxide detector. Today I was flying along in winter conditions, and naturally used cabin heat. During the flight, I heard some occasional short beeps, and couldn't figure out what it was. I looked at the CO-meter which has a red alarm light, but that light was definitely off. On the ground, after shutting down the engine, I heard another short beep, and it definitely came from the CO-meter! The digital readout was showing 10ppm. During post-flight, with the door open, the readout dropped to 0 ppm. I routinely check the CO-meter battery during preflight, so I'm pretty sure the beep is not a reminder for a dead battery.

According to these sources: http://www.coheadquarters.com/ZerotoMillion1.htm , http://library.thinkquest.org/10121/textonly/exposure.htm and wikipedia 10ppm is nothing to be concerned about with exposure over a two-hour flight.

Does anyone else have a digital CO-meter and can tell if a reading of 10ppm is normal with cabin heat? Or should I be concerned our exhaust or heat exchanger is faulty?

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    $\begingroup$ Hi Phillip, what kind of CO meter is it? The manual for it should have information about when you need to be concerned.... $\endgroup$
    – Lnafziger
    Commented Jan 12, 2014 at 21:00
  • $\begingroup$ Thanks for all answers. I told maintenance to check at the last 100hr inspection. Since then, I haven't had that beeping anymore. $\endgroup$
    – Philipp
    Commented Mar 4, 2014 at 18:36

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The short answer is you should be concerned.

Although 10PPM of carbon monoxide doesn't pose an immediate health risk for short term exposure the fact that the level is rising when you turn on the cabin heat & falling when you ventilate the cabin (by opening the door) indicates you may have an exhaust leak, and those tend to get worse over time.
You should have your shop open the heat muff & inspect the exhaust system for cracks/leaks before your next flight (or if that isn't possible avoid (or limit) use of cabin heat & defrost & monitor the CO meter as part of your scan until the exhaust system can be checked).

You should also check your CO meter: electronic CO meters have a finite life, just like chemical spot detectors (though they generally last several years) - your detector's manual will specify a replacement interval. As the sensors age they may give inaccurate readings.

A note about ground operations - Electronic CO detectors are sensitive enough to occasionally generate transient alarms during ground operations due to exhaust gasses entering the cabin. This can happen during summer or winter.
As long as the levels are low and the alarm clears when you're moving (taxi or during climb-out) this is probably not a problem, but you should know what is "normal" for your aircraft.

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Even though voretaq7's answer is correct, I just want to add that there is also a chance that the sensor is broken.

You should know that the sensor is made of a ceramic plate that has to be heated in order to get a readout, and then it is cooled down again. Also it has to be calibrated before using, to get an absolute measurement instead of a relative measurement.

Have you ever left the sensor in a very cold/wet or very hot place? Or did you leave it in the aircraft on the ground during a sunny day? The ceramic plate can easily be damaged by exposing it to the conditions mentioned above. You might want to test it in another plane and if it still shows the same output, the chances are high that the sensor is broken.

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    $\begingroup$ Indeed, one of the things to be concerned about here is an inaccurate sensor - Both electrochemical and semiconductor/ceramic sensors can fail if "abused" as you describe. In this case the sensor is behaving in a manner consistent with detecting carbon monoxide (it's beeping when cabin heat is on, and during ground ops, and the level drops when the cabin is ventilated), but if the sensor has been abused or is past it's lifespan the reading may be inaccurate, and you can't easily tell if it's reading low or high. $\endgroup$
    – voretaq7
    Commented Jan 13, 2014 at 19:31

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