# What is the effect of Helium-based surroundings (99% Helium) on electrochemical gas sensors?

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?

• 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 $\ce{NOx}$. Jun 7 '17 at 22:43
• @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. Jun 8 '17 at 12:06