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Testo NOx filter on CO sensor question

I am surprised to see very low CO readings on my testo 330 while testing oil burners using #2 fuel. I compare the co readings against my bacharach and I get a higher CO reading.

After speaking with a Testo service tech on the phone, he explained to me that Testo has a NOx filter on their CO sensor to get a "true" CO measurement. He is sending me some documentation to that effect but I wonder if anyone else has gone through this same thing.

I was hoping to get a discussion going about this, as well as the bennefits of NOx measurement.

I am pretty ignorant about the NOx thing but in starting my internet research to remedy that, I figured this would be a good start.

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  • I found this on the EPA website,..


    My wonderings are more trade specific,..
    We know the health issues of CO,.. and as stated on the epa site NOx contributes to respiratory issues,..

    My questions are to what degree is NOx in fluegas a health concern?

    Is this something I should be measuring? and What levels are acceptable?

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  • Jim Bergmann_2
    Jim Bergmann_2 Member Posts: 79
    Low CO


    Many manufacturers of combustion testing equipment do not filter out the Nitric Oxide (NO) from the combustion gas sample. NO is an acid gas which is a cross-interferent to all electrochemical CO sensors. (A cross-interferrent will add “false CO” to the reading proportional to the amount of NO present. EG. 100 ppm of NO gas will show on an unfiltered CO sensor as an additional 25 to 50 PPM CO.) All Testo stack gas analyzers incorporate replaceable NO filters that remove NOx gas from the CO sample to provide an accurate CO reading. Such filters are not important when measuring ambient CO as NO rapidly converts to NO2 in ambient air. NO2 is not a cross-interferrent.

    The equipment you are using is specifically designed to measure low levels of CO and incorporates a NO filter. In fact all testo combustion analyzers do. Many other brands of combustion analyzers still manufactured and methods used for low-level CO detection have inherent error in instrumentation or the measurement process. While Testo is a leader in this technology, other manufacturers are following suit. Industry has still not caught up to technology often leaving the technician in the dark when using instrumentation and making an informed decision.

    It does make you wonder how many false high CO readings are the results of a poor measurement. I keep a bottle of CO/NOx mix just to demo the effect.

    High levels at light off of CO on non-filtered combustion analyzers can be the result of prompt NOx produced at light off. Quite simply, if your Testo is calibrated, and is not measuring CO, it is not there.

    Prompt NOx is produced by the formation first of intermediate hydrogen cyanide (HCN) via the reaction of nitrogen radicals and hydrocarbons in the fuel, followed by the oxidation of the HCN to NO. The formation of prompt NOx has weak temperature dependence and a short lifetime of several microseconds. It is only significant in very fuel-rich flames, which are inherently low-NOx emitters.” What you are seeing during poor burner performance is often a combination of CO and NOx.

    Remember, combustion is not an art, it's science. A quick incorrect measurement is just that.


    Jim Bergmann

    Testo HVAC/R Technical Specialist
  • GW
    GW Member Posts: 4,676
  • Jim Bergmann_2
    Jim Bergmann_2 Member Posts: 79
    An age old arguement

    Before we even start we may as well just agree to disagree.
  • S Ebels
    S Ebels Member Posts: 2,322

    Is some choice info. I have experienced the identical scenario when comparing readings from my Testo 325's to other analyzers and always wondered why.

    Thank You Jim for the explanation and......

    Thank You Norm for the question

  • Thank you VERY much Jim for the informative response and for the combustion guide.

    Excellent info Thank you!

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  • Mark Hunt
    Mark Hunt Member Posts: 4,909
    Actually Jim

    the TSI unit I have has an NO sensor as well. It spooked me when I first started using it because of the low CO readings I was getting. Jim Davis explained the NO sensor the same way you did.

    I still want to see spikes if they occur at start-up or shut-down. My Bacharach and TSI units see them.

    Mark H

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