Multi-point calibrations consist of a zero and 4 upscale points, the highest being a concentration between 80 percent and 90 percent of the full scale range of the analyzer under calibration. Multi-point calibrations are used to establish or verify the linearity of analyzers upon initial installation, after major repairs and at specified frequencies.
Most modern analyzers have a linear or very nearly linear response with concentration. If a non-linear analyzer is being calibrated, additional calibration points should be included to adequately define the calibration relationship, which should be a smooth curve.
Calibration points should be plotted or evaluated statistically as they are obtained so that any deviant points can be investigated or repeated immediately.
Most analyzers have zero and span adjustment controls, which should be adjusted based on the zero and highest test concentrations, respectively, to provide the desired scale range within the analyzer’s specifications.
For analyzers in routine operation, unadjusted (’'as is") analyzer zero and span response readings should be obtained prior to making any zero or span adjustments. NO/NO2/NOx analyzers may not have individual zero and span controls for each channel; the analyzer’s operation/instruction manual should be consulted for the proper zero and span adjustment procedure.
Zero and span controls often interact with each other, so the adjustments may have to be repeated several times to obtain the desired final adjustments. After the zero and span adjustments have been completed and the analyzer has been allowed to stabilize on the new zero and span settings, all calibration test concentrations should be introduced into the analyzer for the final calibration.
The final, post-adjusted analyzer response readings should be obtained from the same device (chart recorder, data acquisition system, etc.) that will be used for subsequent ambient measurements. The analyzer readings are plotted against the respective test concentrations, and the best linear (or nonlinear if appropriate) curve to fit the points is determined.
Ideally, least squares regression analysis (with an appropriate transformation of the data for non-linear analyzers) should be used to determine the slope and intercept for the best fit calibration line of the form, y = mx + a, where y represents the analyzer response, x represents the pollutant concentration, m is the slope, and a is the xaxis intercept of the best fit calibration line.
When this calibration relationship is subsequently used to compute concentration measurements (x) from analyzer response readings (y), the formula is transposed to the form, x = (y - a)/m. For the gaseous pollutants, the verification/calibration is considered acceptable if all calibration points fall within 2% of the full scale, best fit straight line.
For manual samplers, devices (flow rate, temperature, pressure) are checked at different settings. Acceptance criteria for these devices can be found in the MQO Tables in Appendix D. As a quality control check on calibrations, the standard error or correlation coefficient can be calculated along with the regression calculations.
A control chart of the standard error or correlation coefficient could then be maintained to monitor the degree of scatter in the calibration points and, if desired, limits of acceptability can be established.