Are you calibrating your analyzer as required?
Oxygen analyzers are required to be calibrated each fill day with a specialty calibration gas that has a COA like our calibration gases. The FDA does not allow USP oxygen to be used to calibrate analyzers. (i.e. you can't use the oxygen you get to fill your tanks to calibrate your analyzer!)

For Servomex Analyzers:
Oxygen Specialty Calibration Gas, E cylinder
SG-100-1
Nitrogen Specialty Calibration Gas, E cylinder
SG-200

For Oxygraf Analyzers:
Oxy/Air Mix Calibration Gas, E Cylinder
SG-400