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concentration at the instrument inlet to a reading of 90 percent of the ultimate recorded concentration.

Rise Time (90 percent)-The interval between initial response time and time to 90 percent response after a step decrease in the inlet concentration.

Zero Drift-The change in instrument output over a stated time period, usually 24 hours, of unadjusted continuous operation, when the input concentration is zero; usually expressed as percent full scale.

Span Drift-The change in instrument output over a stated time period, usually 24 hours, of unadjusted continuous operation, when the input concentration is a stated upscale value; usually expressed as percent full scale.

Precision-The degree of agreement between repeated measurements of the same concentration. It is expressed as the average deviation of the single results from the mean. Operational Period-The period of time over which the instrument can be expected to operate unattended within specifications.

Noise-Spontaneous deviations from a mean output not caused by input concentration changes.

Interference-An undesired positive or negative output caused by a substance other than the one being measured.

Interference Equivalent-The portion of indicated input concentration due to the presence of an interferent.

Operating Temperature Range-The range of ambient temperatures over which the instrument will meet all performance specifications.

Operating Humidity Range-The range of ambient relative humidity over which the instrument will meet all performance specifications.

Linearity-The maximum deviation between an actual instrument reading and the reading predicted by a straight line drawn between upper and lower calibration points.

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[36 FR 22394, Nov. 25, 1971]

APPENDIX F-MEASUREMENT PRINCIPLE AND CALIBRATION PROCEDURE FOR THE MEASUREMENT OF NITROGEN DIOXIDE IN THE ATMOSPHERE (GAS PHASE CHEMILUMINESCENCE)

Principle and Applicability

1. Atmospheric concentrations of nitrogen dioxide (NO) are measured indirectly by photometrically measuring the light intensity, at wavelengths greater than 600 nanometers, resulting from the chemiluminescent reaction of nitric oxide (NO) with ozone (O,). (1,2,3) NO, is first quantitatively reduced to NO(4,5,6) by means of a converter. NO, which commonly exists in ambient air together with NO2, passes through the converter unchanged causing a resultant total NO, concentration equal to NO+NO2. A sample of the input air is also measured without having passed through the converted. This latter NO measurement is subtracted from the former measurement (NO+NO) to yield the final NO, measurement. The NO and NO+NO, measurements may be made concurrently with dual systems, or cyclically with the same system

provided the cycle time does not exceed 1 minute.

2. Sampling considerations. 2.1 Chemiluminescence NO/NO/NO, analyzers will respond to other nitrogen containing compounds, such as peroxyacetyl nitrate (PAN), which might be reduced to NO in the thermal converter. (7) Atmospheric concentrations of these potential interferences are generally low relative to NO, and valid NO, measurements may be obtained. In certain geographical areas, where the concentration of these potential interferences is known or suspected to be high relative to NO2, the use of an equivalent method for the measurement of NO, is recommended.

2.2 The use of integrating flasks on the sample inlet line of chemiluminescence NO/ NO,/NO, analyzers is optional and left to couraged. The sample residence time between the sampling point and the analyzer should be kept to a minimum to avoid erroneous NO, measurements resulting from the reaction of ambient levels of NO and O, in the sampling system.

2.3 The use of particulate filters on the sample inlet line of chemiluminescence NO/ NO/NO, analyzers is optional and left to the discretion of the user or the manufac

turer. Use of the filter should depend on the analyzer's susceptibility to interference, malfunction, or damage due to particulates. Users are cautioned that particulate matter concentrated on a filter may cause erroneous NO, measurements and therefore filters should be changed frequently.

3. An analyzer based on this principle will be considered a reference method only if it has been designated as a reference method in accordance with Part 53 of this chapter. Calibration

1. Alternative A-Gas phase titration (GPT) of an NO standard with O,.

Major equipment required: Stable O, generator. Chemiluminescence NO/NO,/NO, analyzer with strip chart recorder(s). NO concentration standard.

1.1 Principle. This calibration technique is based upon the rapid gas phase reaction between NO and O, to produce stoichiometric quantities of NO, in accordance with the following equation: (8)

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The quantitative nature of this reaction is such that when the NO concentration is known, the concentration of NO, can be determined. Ozone is added to excess NO in a dynamic calibration system, and the NO channel of the chemiluminescence NO/ NO,/NO, analyzer is used as an indicator of changes in NO concentration. Upon the addition of O., the decrease in NO concentration observed on the calibrated NO channel is equivalent to the concentration of NO, produced. The amount of NO, generated may be varied by adding variable amounts of O, from a stable uncalibrated O, generator. (9)

1.2 Apparatus. Figure 1, a schematic of a typical GPT apparatus, shows the suggested configuration of the components listed below. All connections between components in the calibration system downstream from the O, generator should be of glass, Teflon®, or other non-reactive material.

1.2.1 Air flow controllers. Devices capable of maintaining constant air flows within 12% of the required flowrate.

1.2.2 NO flow controller. A device capable of maintaining constant NO flows within ±2% of the required flowrate. Component parts in contact with the NO should be of a non-reactive material.

1.2.3 Air flowmeters. Calibrated flowmeters capable of measuring and monitoring air flowrates with an accuracy of ±2% of the measured flowrate.

1.2.4 NO flowmeter. A calibrated flowmeter capable of measuring and monitoring NO flowrates with an accuracy of ±2% of the measured flowrate. (Rotameters have been reported to operate unreliably when measuring low NO flows and are not recommended.)

1.2.5 Pressure regulator for standard NO cylinder. This regulator must have a nonreactive diaphragm and internal parts and a suitable delivery pressure.

1.2.6 Ozone generator. The generator must be capable of generating sufficient and stable levels of O, for reaction with NO to generate NO, concentrations in the range required. Ozone generators of the electric discharge type may produce NO and NO, and are not recommended.

1.2.7 Valve. A valve may be used as shown in Figure 1 to divert the NO flow when zero air is required at the manifold. The valve should be constructed of glass, Teflon®, or other nonreactive material.

1.2.8 Reaction chamber. A chamber, constructed of glass, Teflon®, or other nonreactive material, for the quantitative reaction of O, with excess NO. The chamber should be of sufficient volume (VRC) such that the residence time (tr) meets the requirements specified in 1.4. For practical reasons, ta should be less than 2 minutes.

1.2.9 Mixing chamber. A chamber constructed of glass, Teflon®, or other nonreactive material and designed to provide thorough mixing of reaction products and diluent air. The residence time is not critical when the dynamic parameter specification given in 1.4 is met.

1.2.10 Output manifold. The output manifold should be constructed of glass, Teflon®, or other non-reactive material and should be of sufficient diameter to insure an insignificant pressure drop at the analyzer connection. The system must have a vent designed to insure atmospheric pressure at the manifold and to prevent ambient air from entering the manifold.

1.3 Reagents.

1.3.1 NO concentration standard. Gas cylinder standard containing 50 to 100 ppm NO in N, with less than 1 ppm NO,. This standard must be traceable to a National Bureau of Standards (NBS) NO in N, Standard Reference Material (SRM 1683 or SRM 1684), an NBS NO, Standard Reference Material (SRM 1629), or an NBS/EPA-approved commercially available Certified Reference Material (CRM). CRM's are described in Reference 14, and a list of CRM sources is available from the address shown for Reference 14. A recommended protocol for certifying NO gas cylinders against either an NO SRM or CRM is given in section 2.0.7 of Reference 15. Reference 13 gives procedures for certifying an NO gas cylinder against an NBS NO, SRM and for determining the amount of NO, impurity in an NO cylinder.

1.3.2 Zero air. Air, free of contaminants which will cause a detectable response on the NO/NO,/NO, analyzer or which might react with either NO, O,, or NO, in the gas

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PR=dynamic parameter specification, determined empirically, to insure complete reaction of the available O,, ppm-minute

[NO]RC=NO concentration in the reaction chamber, ppm

R=residence time of the reactant gases in the reaction chamber, minute [NO]STD=concentration of the undiluted NO standard, ppm

FNO NO flowrate, scm3/min

Fo=O, generator air flowrate, scm3/min VRC=Volume of the reaction chamber, scm3

1.4.2 The flow conditions to be used in the GPT system are determined by the following procedure:

(a) Determine Fr, the total flow required at the output manifold (Fr=analyzer

demand plus 10 to 50% excess).

(b) Establish [NO]our as the highest NO concentration (ppm) which will be required at the output manifold. [NO]our should be approximately equivalent to 90% of the upper range limit (URL) of the NO, concentration range to be covered.

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where:

FD=diluent air flowrate, scm3/min

(h) If Fo turns out to be impractical for the desired system, select a reaction chamber having a different VRC and recompute Fo and FD.

NOTE: A dynamic parameter lower than 2.75 ppm-minutes may be used if it can be determined empirically that quantitative reaction of O, with NO occurs. A procedure for making this determination as well as a more detailed discussion of the above requirements and other related considerations is given in reference 13.

1.5 Procedure.

1.5.1 Assemble a dynamic calibration system such as the one shown in Figure 1.

1.5.2 Insure that all flowmeters are calibrated under the conditions of use against a reliable standard such as a soap-bubble meter or wet-test meter. All volumetric flowrates should be corrected to 25° C and 760 mm Hg. A discussion on the calibration of flowmeters is given in reference 13.

1.5.3 Precautions must be taken to remove O, and other contaminants from the NO pressure regulator and delivery system prior to the start of calibration to avoid any conversion of the standard NO to NO,. Failure to do so can cause significant errors in calibration. This problem may be minimized by (1) carefully evacuating the regulator, when possible, after the regulator has been connected to the cylinder and before opening the cylinder valve; (2) thoroughly flushing the regulator and delivery system with NO after opening the cylinder valve; (3) not removing the regulator from the cylinder between calibrations unless absolutely necessary. Further discussion of these procedures is given in reference 13.

1.5.4 Select the operating range of the NO/NO/NO, analyzer to be calibrated. In order to obtain maximum precision and accuracy for NO, calibration, all three channels of the analyzer should be set to the same range. If operation of the NO and NO, channels on higher ranges is desired, subsequent recalibration of the NO and NO, channels on the higher ranges is recommended.

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