91.318—Oxides of nitrogen analyzer calibration.
(b)
Initial and periodic interference. Prior to its introduction into service, and monthly thereafter, check the chemiluminescent oxides of nitrogen analyzer for NO2 to NO converter efficiency. Figure 2 in appendix B of this subpart is a reference for the following paragraphs:
(1)
Follow good engineering practices for instrument start-up and operation. Adjust the analyzer to optimize performance.
(3)
Connect the outlet of the NOX generator to the sample inlet of the oxides of nitrogen analyzer which has been set to the most common operating range.
(4)
Introduce into the NOX generator analyzer-system an NO-in-nitrogen (N2) mixture with an NO concentration equal to approximately 80 percent of the most common operating range. The NO2 content of the gas mixture must be less than 5 percent of the NO concentration.
(5)
With the oxides of nitrogen analyzer in the NO mode, record the concentration of NO indicated by the analyzer.
(6)
Turn on the NOX generator O2 (or air) supply and adjust the O2 (or air) flow rate so that the NO indicated by the analyzer is about 10 percent less than indicated in paragraph (b)(5) of this section. Record the concentration of NO in this NO O2 mixture as value “c.”
(7)
Switch the NOX generator to the generation mode and adjust the generation rate so that the NO measured on the analyzer is 20 percent of that measured in paragraph (b)(5) of this section. There must be at least 10 percent unreacted NO at this point. Record the concentration of residual NO as value “d.”
(8)
Switch the oxides of nitrogen analyzer to the NOX mode and measure total NOX. Record this value as “a.”
(9)
Switch off the NOX generator but maintain gas flow through the system. The oxides of nitrogen analyzer will indicate the NOX in the NO O2 mixture. Record this value as “b.”
(10)
Turn off the NOX generator O2 (or air) supply. The analyzer will now indicate the NOX in the original NO-in-N2 mixture. This value should be no more than 5 percent above the value indicated in paragraph (b)(4) of this section.
(11)
Calculate the efficiency of the NOX converter by substituting the concentrations obtained into the following equation:
percent efficiency = (1 (a − b)/(c − d)) × 100
Where:
a=concentration obtained in paragraph (b)(8) of this section,
b=concentration obtained in paragraph (b)(9) of this section,
c=concentration obtained in paragraph (b)(6) of this section,
d=concentration obtained in paragraph (b)(7) of this section.
If converter efficiency is not greater than 90 percent, corrective action is required.
(c)
Initial and periodic calibration. Prior to its introduction into service, and monthly thereafter, calibrate the chemiluminescent oxides of nitrogen analyzer on all normally used instrument ranges. Use the same flow rate as when analyzing samples. Proceed as follows:
(3)
Calibrate on each normally used operating range with NO-in-N2 calibration gases with nominal concentrations between 10 and 90 percent of that range. A minimum of six evenly spaced points covering at least 80 percent of the 10 to 90 percent range (64 percent) is required (see following table).
| Example calibration points (percent) | Acceptable for calibration? |
|---|---|
| 20, 30, 40, 50, 60, 70 | No, range covered is 50 percent, not 64 percent. |
| 20, 30, 40, 50, 60, 70, 80, 90 | Yes. |
| 10, 25, 40, 55, 70, 85 | Yes. |
| 10, 30, 50, 70, 90 | No, though equally spaced and entire range covered, a minimum of six points is needed. |
(4)
Additional calibration points may be generated. For each range calibrated, if the deviation from a least-squares best-fit straight line is two percent or less of the value at each data point, concentration values may be calculated by use of a single calibration factor for that range. If the deviation exceeds two percent at any point, use the best-fit non-linear equation which represents the data to within two percent of each test point to determine concentration.
(d)
The initial and periodic interference, system check, and calibration test procedures specified in 40 CFR part 1065, subparts C and D, may be used in lieu of the procedures specified in this section.