I am using Ozone analyzer Thermo Scientific model 49i. It is always giving values in negative. But when we compare the values with another analyzer (Teledyne model 400 A, installed at same place under same conditions) it gives similar trend of ozone but with negative values. Using Zero air generator when zero air is passed it shows zero but during O3 span it goes to very low values in negative.
I agree, is seems that the reference cell and the sample are not connected in the right
way... or electrical connection are inverted...
Check your installation guide or re-calibrate both of your instruments...
It can also be that if your values are small enough usual uncertainty of device can turn actual positive value to negative.
For example if you measure 0.1 and uncertainty in this region is 0.2 then according rules of metrology sometimes it shows -0.1
Do not worry, it is just rules of metrology.
IN EU rules of reporting values of Air Quality measurements there is even written that keep negative values since they give information about 1) drift of 0-value, 2) stability of instrument, 3) stability of environment (if the room temperature is not accurate enough drift naturally occurs) etc
Thank you very much Velijo Kimmel. should the data be reported in negative or we should simly report the same value as positive?
The above answers are a good place to start. Another possibility depends on how the air is getting into the instrument. If the mounting is such that the wind blows on the exhaust of the instrument more strongly than whatever pumping arrangement is getting the air in, you'll get a negative reading.
Never turn automatically measured values into something else. If in check-list you discover some 0-drift or other reasons for correction (for example a new calibration ) usually it is better to keep measured values. Otherwise you will loose important information about above mentioned aspects.
IN EU there recommended to keep values as they are - certainly after usual check-list (0-drift, calibration etc)
The swapping of cells won't matter, as the SOVs inside instrument itself alternates between sample and reference cells. It might be the electrical connections.
But then there is another issue:
The values are very less in the span!. Have you checked the intensity of cell A and cell B? Probably the lamp intensity has gone down to very low values or the detector is aged. Please do that.
You can also check the instrument operation range and calibration factors. You can download the manual from the link
https://tools.thermofisher.com/content/sfs/manuals/EPM-manual-Model%2049i.pdf
Thank you Vinod and all others who contributed. We have got the problem solved. it was really swap of ref and sample. actually someone made the detectors connection on wrong port as there was nothing indicated on pins and ports so it was placed by mistake on opposite places so we changed it and now the values are OK i.e. Positive +ve
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