An interesting conversation arose after last week's discussion on calibration accuracy. What happens when a variety of solvents are used in a process? 

A note in NFPA 86 section 11.6.10.6 states “Where a variety of solvents are used, the solvent to which the controller is least sensitive shall be the primary calibration reference.”

Reading error can be in either a safe or unsafe direction. A reading that is lower than the actual concentration is unsafe because the hazard is greater than displayed. A reading that is higher than actual is considered a “safe” error because the hazard is lower than displayed. But even “safe” errors should be minimized because they produce false alarms and too-early process shutdown.

Only the flame temperature sensor can be calibrated so that all cross-calibration errors are absolutely minimized and in a “safe” direction.

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