Continuous emissions monitoring systems (CEMS) require calibration during compliance audits (Relative Accuracy Test Audits or RATA's). For a number of years, the U.S. Environmental Protection Agency had mandated that these calibrations be conducted using EPA Protocol 1 Gases in an undiluted form to check the analyzer performance. Appendix M has now changed this requirement and allows the use of dilution systems.

It is important to outline EPA Method 205 in order to detail the cost and time savings which can be realized by using the method in the field. The method specifies that the gas dilution instrument used to calibrate the reference analyzers must produce a diluted gas standard that is within two percent of predicted values and that the mass flow controllers in the dilution instrument must be calibrated yearly against a National Institute of Standards and Technology (NIST) traceable standard. In addition, Method 205 requires that the gas dilution instrument must be field-checked against a precalibrated analyzer to prove that the dilution instrument meets the two percent accuracy level.