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The AQS’s new NO2 analyser module can not only be calibrated directly from gases on site, but it has also shown to have such a stable calibration that this may only be required every 1 or possibly every 2 years. This means that, not only can you be confident that the data is reliable long term. It also means you do not have to factor in the cost of routine “LSO” visits to site.

How is this possible? Unlike other apparently similar systems, the AQS has a flow through system which switches sample air with internally generated zero air so it is, in effect, being zero checked continuously. This together with the stability of the sensor used gives it its unrivalled measurement stability.

To prove this we ran our reference AQS1 (number 847) next to an MCERTS reference chemiluminescent NOx analyser last year as we described in our earlier case study ( ) carried out for three months between April and July 2020. This shows how the two continued to agree over the whole period to a remarkable degree. More recently (March 2021) we have had the same AQS1 NO2 independently checked against reference gases and this has shown the NO2 values are still correct when operating on the factory calibrations from January 2020. Such calibration stability is unheard of.

Calibration March 2021


Calibration was carried out using GPT (Gas Phase Titration). This is the same equipment as is used to check the calibration and converter efficiency of chemiluminescent NOx analysers and as such this type of equipment is already used on site.

NO2 (Nitrogen Dioxide) was generated by mixing NO (Nitric Oxide) with O3 (Ozone) in a balance of zero air. In the test below calibration gases were fed to both the AQS1 and an MCERTS reference chemiluminescent NOx analyser. Gases were also fed to a reference. To start with NO from a certified high concentration gas standard cylinder was first diluted with zero scrubbed ambient air to create a 250 ppb standard. Neither analyser responded. The ozonater was then switched on and titrated with the NO to create an NO2 250 ppb standard. The same was then used to generate a 90 ppb standard. The NO was then switched off and the analysers tested at 90 and 250 ppb. The results are shown below.



The calibration data proves how stable the AQS1 is over a long period of time (more than 12 months) without any adjustment. It also shows that the NO2 module in the AQS1 is extremely selective and has no response to either NO or O3. Data from the AQS1 was not only close to the traditional “reference” analyser but proved to be far more stable than would be expected from a chemiluminescnet NOx analyser. It is also small enough to be easily moved and installed without an air-conditioner. Indeed it can be carried in one hand and can even be installed on a lamp-post.


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