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I am currently working on a project measuring gas composition of car exhaust. In order to dilute the gas concentration down to something electrochemical sensors can read, I diluted the sample with pure helium.

The following is true for these sensors:

Importance of Oxygen. The reactions at the sensing electrode (anode) for some gases are as follows: \begin{align} \ce{CO + H2O &-> CO2 + 2H+ + 2e-} \\ \ce{H2S + 4H2O &-> H2SO4 + 8H+ + 8e-} \\ \ce{NO + 2H2O &-> HNO3 + 3H+ + 3e-} \\ \ce{H2 &-> 2H+ + 2e-} \\ \ce{2HCN + Au &-> HAu(CN)2 + H+ + e-} \end{align} Simultaneously, the reactions at the counter electrode (cathode) need oxygen molecules to complete the process: $$\ce{O2 + 4H+ + 4e- -> 2H2O}$$ An inadequate supply of oxygen to complete the reaction will shorten the life of the sensors, hence the sensors will not operate properly.

Does this mean that doing what I intend is going to affect their performance and damage them?

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Aiming for dilution of the exhaust gases, you need a gas that is inert, i.e. does not react at these conditions with neither the gases present, nor with the metals present in in the catalytic converter (if there is such device installed) and electrodes of the sensors.

Under this restraint, using a nobel gas was a good idea. Yet instead of helium, that you still may use to spot leaks in the tubings, I would opt for argon, which is much cheaper per litre. Note that at ambient conditions, its density as gas is higher than the one of helium and of air and has a slightly lesser thermic conductance.

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    $\begingroup$ Yet still, based on the spec sheet data above, that won't help with the bit about needing oxygen on the cathode. What is wrong with using dry air to dilute the effluent? $\endgroup$
    – Jon Custer
    Jun 7, 2017 at 22:19
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    $\begingroup$ Oxygen may be either still present (Diesel engine, p. 6 at volkspage.net/technik/ssp/ssp/SSP_230.pdf) or may be added intentionally (and then monitored) by the design of the test engine. At the other side, using dry air may to dilute the exhaust gases may render the analysis more complex: i) the then added oxygen may offer some combustion of previously not combusted material, ii) nitrogen may react at elevated temperatures to yield $\ce{NOx}$. $\endgroup$
    – Buttonwood
    Jun 7, 2017 at 22:43
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    $\begingroup$ It would seem that a deeper reading of the spec sheet (and a call to their application engineer) might be in order for the OP. $\endgroup$
    – Jon Custer
    Jun 7, 2017 at 22:45
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    $\begingroup$ @JonCuster I have read the sensor datasheets and they do not seem to provide any informaiton. Even the extract I added wasn't from a datasheet, but from a book. An when it comes to contacting them, I do not think that is possible since most of them are chinese copies. $\endgroup$
    – wizzzz1
    Jun 8, 2017 at 8:15
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    $\begingroup$ @Erol From the spare information provided (OP), and as you say, the data sheet available, the experiment may provide an answer. Do you aim for $\ce{O2}$? If so, modern cars perform oxygen monitoring regardless if based on a Diesel, or Otto engine; which may be seen as "there are sensors that by design may used on both". A statement "The [$\lambda{}$] sensor does not actually measure oxygen concentration, but rather the difference between the amount of oxygen in the exhaust gas and the amount of oxygen in air." at en.wikipedia.org/wiki/Oxygen_sensor for some of them supports this. $\endgroup$
    – Buttonwood
    Jun 8, 2017 at 12:06

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