We've discussed the 6 essentials of meeting NFPA 86, this week let's take a closer look at the third monitoring requirement: calibration accuracy.
Section 126.96.36.199 of NFPA 86 requires calibration to be valid “for the application and solvents used.” If a variety of solvents is used, cross calibrations must be accurate or the sensor must be recalibrated whenever solvents are changed. Calibrations must be made using known concentrations of test gas mixtures.
Of the sensor choices, catalytic, infrared and flame ionization all have large calibration correction factors for varying solvent types. Annex E states that the calibration response of a catalytic sensor “does vary significantly for different solvents.” Similar language is used to describe the response of infrared and flame ionization sensors.The Annex adds that infrared is recommended only when monitoring single solvent atmospheres.
It is also important to understand that flame ionization detectors (FID) should not be used except in single solvent applications. The FID technology is based on measuring ionized carbon: this method makes it very difficult to convert the response into a meaningful indication of flammability when measuring more than one solvent.
These three sensor types therefore require recalibration whenever solvent formulations are changed. Annex E adds that “the use of relative response data in making field calibration checks is not recommended.” Recalibration calls for zero and span checks using known concentrations of test gas mixtures. Also: “The user should understand how the instrument responds to vapors for which the instrument is not calibrated.”
Only the flame temperature sensor has a very small change in response when reading varying solvents. This response, called Universal Calibration, means that the flame temperature sensor can read varying solvent formulations without recalibration.