Calibrating the Model 42i NO NO2 Nox Analyzer with Zero Air or Nitrogen

You need to isolate and determine where the elevated background/zero is coming from as it could be external contamination in your zero air or your sample line/umbilical. The elevated background could also be electronic noise from the PMT (Photomultiplier Tube) or your input board may require calibration. See the Model 42i NO-NO2-NOx Analyzer
  Troubleshooting High/Low background errors technical bulletin. 

1. Isolate external contamination
   Disconnect sample line and see if concentrations drop.
2. Isolate internal contamination
   If you disconnect the sample line and the NO/NOx Concentrations stays high this could be internal Contamination and the reaction chamber may need cleaning.
3. Measure and determine the electronic noise level
   Measure the dark current/noise from the PMT, should be a very low level.
4. Calibrate input board for any offsets
   Perform an automatic input board calibration from the service menu.
5. Clean your reaction chamber if contaminated
   If this process determines your Model 42i NO-NO2-NOx Analyzer is contaminated, use this cleaning procedure to clean your reaction chamber, you may also need to replace some Teflon tubing inside the analyzer as well, depending on the contamination level. See the Model 42i NO-NO2-NOx Analyzer Reaction Chamber Cleaning Procedure technical bulletin. 

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