91.325 Analyzer interference checks.§ 91.325 Analyzer interference checks.
(a) Gases present in the exhaust other than the one being analyzed can interfere with the reading in several ways. Positive interference occurs in NDIR and PMD instruments when the interfering gas gives the same effect as the gas being measured, but to a lesser degree. Negative interference occurs in NDIR instruments by the interfering gas broadening the absorption band of the measured gas, and in CLD instruments by the interfering gas quenching the radiation. The interference checks described in this section are to be made initially and after any major repairs that could affect analyzer performance.
(b) CO analyzer water and CO2 interference checks. Bubble through water at room temperature a CO2 span gas having a concentration of between 80 percent and 100 percent inclusive of full scale of the maximum operating range used during testing and record the analyzer response. For dry measurements, this mixture may be introduced into the sample system prior to the water trap. The analyzer response must not be more than one percent of full scale for ranges equal to or above 300 ppm or more than three ppm for ranges below 300 ppm.
(c) NOX analyzer quench check. The two gases of concern for CLD (and HCLD) analyzers are CO2 and water vapor. Quench responses to these two gases are proportional to their concentrations and, therefore, require test techniques to determine quench at the highest expected concentrations experienced during testing.
(1) NOX analyzer CO2 quench check. (i) Pass a CO2 span gas having a concentration of 80 percent to 100 percent of full scale of the maximum operating range used during testing through the CO2 NDIR analyzer and record the value as “a.”
(ii) Dilute the CO2 span gas approximately 50 percent with NO span gas and pass through the CO2 NDIR and CLD (or HCLD). Record the CO2 and NO values as “b” and “c”, respectively.
(iii) Shut off the CO2 and pass only the NO span gas through the CLD (or HCLD). Record the NO value recorded as “d.”
(iv) Calculate the percent CO2 quench as follows, which may not exceed three percent:percent CO2 quench = 100 − 100 × [c × a/(d × a − d × b)] × a/b Where: a = Undiluted CO2 concentration (percent) b = Diluted CO2 concentration (percent) c = Diluted NO concentration (ppm) d = Undiluted NO concentration (ppm)
(2) NOX analyzer water quench check. (i) This check applies to wet measurements only. Pass an NO span gas having a concentration of 80 percent to 100 percent of full scale of a normal operating range through the CLD (or HCLD). Record the response as “D.” Bubble through water at room temperature the NO span gas and pass it through the CLD (or HCLD). Record the analyzers response as “AR.” Determine and record the analyzers absolute operating pressure and the bubbler water temperature. (It is important that the NO span gas contains minimal NO2 concentration for this check. No allowance for absorption of NO2 in water has been made in the following quench calculations.)
(ii) Calculations for water quench must consider dilution of the NO span gas with water vapor and scaling of the water vapor concentration of the mixture to that expected during testing. Determine the mixture's saturated vapor pressure (designated as “Pwb”) that corresponds to the bubbler water temperature. Calculate the water concentration (“Z1”, percent) in the mixture by the following equation:Z1 = 100 × (Pwb / GP) Where: GP = the analyzer's standard operating pressure (pascals)
(iii) Calculate the expected dilute NO span gas and water vapor mixture concentration (designated as “D1”) by the following equation:D1 = D × (1 − Z1/100)
(iv)(A) The maximum raw or dilute exhaust water vapor concentration expected during testing (designated as Wm) can be estimated from the CO2 span gas (or as defined in the equation in this paragraph and designated as A) criteria in paragraph (c)(1) of this section and the assumption of a fuel atom H/C ratio of 1.8:1 as:Wm(%) = 0.9 × A(%) Where: A = maximum CO2 concentration expected in the sample system during testing.
(B) Percent water quench shall not exceed 3 percent and shall be calculated by:% Water Quench = 100 × (D1 − AR)/D1 × Wm/Z1 [61 FR 52102, Oct. 4, 1996, as amended at 70 FR 40451, July 13, 2005]