The measurement of gasses flowing in conduits is quite complex with lots of 'physics' involved.
Most such instrumentation is not 'direct read' and must be 'corrected' for pressure, approx molecular weight of the gas mixture, and the approx. velocity of the gases in the dead center (not along the walls) of the conduit being monitored.
Also too, the 'sensor' portion usually should be mounted 'dead center' in the center of any piping.
Your 'manual' should have all that information.