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B. Suggested Definitions of Performance Specifications:

Range-The minimum and maximum measurement limits.

Output-Electrical signal which is proportional to the measurement; intended for connection to readout or data processing devices. Usually expressed as millivolts or milliamps full scale at a given impedence. Full Scale-The maximum measuring limit for a given range.

Minimum Detectable Sensitivity-The smallest amount of input concentration that can be detected as the concentration approaches zero.

Accuracy-The degree of agreement between a measured value and the true value; usually expressed at percent of

full scale. Lag Time-The time interval from a step change in input concentration at the instrument inlet to the first corresponding change in the instrument output.

Time to 90 Percent Response-The time interval from a step change in the input

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

MILUMINESCENCE)

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 (O1). (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+NO2) 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.

NO/NO/NO,

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

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 be tween 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 errone

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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 NO2 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 +2% 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 NO2 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 (t) meets the requirements specified in 1.4. For practical reasons, tr 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. Cylinder containing 50 to 100 ppm NO in N2 with less than 1 ppm NO2. The cylinder must be traceable to a National Bureau of Standards NO in N2 Standard Reference Material (SRM 1683 or SRM 1684) or NO, Standard Reference Material (SRM 1629). Procedures for certifying the NO cylinder (working standard) against an NBS traceable NO or NO, standard and for determining the amount of NO, impurity are given in reference 13. The cylinder should be recertified on a regular basis as determined by the local quality control program.

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, O1, or NO2 in the gas phase titration. A procedure for generating zero air is given in reference 13.

1.4 Dynamic parameter specification.

1.4.1 The O, generator air flowrate (F.) and NO flowrate (FNO) (see Figure 1) must be adjusted such that the following relationship holds:

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