# 40 CFR 92.120 - NDIR analyzer calibration and checks.

(a) NDIR water rejection ratio check.

(1) Zero and span the analyzer on the lowest range that will be used.

(2) Introduce a saturated mixture of water and zero gas at room temperature directly to the analyzer.

(3) Determine and record the analyzer operating pressure (GP) in absolute units in Pascal. Gauges G3 and G4 may be used if the values are converted to the correct units.

(4) Determine and record the temperature of the zero-gas mixture.

(5) Record the analyzers' response (AR) in ppm to the saturated zero-gas mixture.

(6) For the temperature recorded in paragraph (a)(4) of this section, determine the saturation vapor pressure.

(7) Calculate the water concentration (Z) in the mixture from:

(8) Calculate the water rejection ratio (WRR) from:

(b) NDIR CO 2 rejection ratio check.

(1) Zero and span the analyzer on the lowest range that will be used.

(2) Introduce a CO2 calibration gas of at least 10 percent CO2 or greater to the analyzer.

(3) Record the CO2 calibration gas concentration in ppm.

(4) Record the analyzers' response (AR) in ppm to the CO2 calibration gas.

(5) Calculate the CO2 rejection ratio (CO2RR) from:

(c) NDIR analyzer calibration.

(1) Detector optimization. If necessary, follow the manufacturer's instructions for initial start-up and basic operating adjustments.

(2) Calibration curve. Develop a calibration curve for each range used as follows:

(i) Zero the analyzer.

(ii) Span the analyzer to give a response of approximately 90 percent of full-scale chart deflection.

(iii) Recheck the zero response. If it has changed more than 0.5 percent of full scale, repeat steps in paragraphs (c)(2)(i) and (c)(2)(ii) of this section.

(iv) Record the response of calibration gases having nominal concentrations of 15, 30, 45, 60, 75, and 90 percent of full-scale concentration.

(v) Generate a calibration curve. The calibration curve shall be of fourth order or less, have five or fewer coefficients, and be of the form of equation (1) or (2). Include zero as a data point. Compensation for known impurities in the zero gas can be made to the zero-data point. The calibration curve must fit the data points within 2 percent of point or 1 percent of full scale, whichever is less. Equations (1) and (2) follow:

(vi) Option. A new calibration curve need not be generated if:

(A) A calibration curve conforming to paragraph (c)(2)(v) of this section exists;

(B) The responses generated in paragraph (c)(2)(iv) of this section are within 1 percent of full scale or 2 percent of point, whichever is less, of the responses predicted by the calibration curve for the gases used in paragraph (c)(2)(iv) of this section.

(vii) If multiple range analyzers are used, only the lowest range must meet the curve fit requirements below 15 percent of full scale.

(3) If any range is within 2 percent of being linear a linear calibration may be used. To determine if this criterion is met:

(i) Perform a linear least-square regression on the data generated. Use an equation of the form y = mx, where x is the actual chart deflection and y is the concentration.

(ii) Use the equation z = y/m to find the linear chart deflection (z) for each calibration gas concentration (y).

(iii) Determine the linearity (%L) for each calibration gas by:

(iv) The linearity criterion is met if the %L is less than ±2 percent for each data point generated. For each emission test, a calibration curve of the form y = mx is to be used. The slope (m) is defined for each range by the spanning process.