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APPENDIX C-MEASUREMENT PRINCIPLE AND CALIBRATION PROCEDURE FOR THE MEASUREMENT OF CARBON MONOXIDE IN THE ATMOSPHERE (NON-DISPERSIVE INFRARED PHOTOMETRY)
Measurement Principle 1. Measurements are based on the absorption of infrared radiation by carbon monoxide (CO) in a non-dispersive photometer. Infrared energy from a source is passed through a cell containing the gas sample to be analyzed, and the quantitative absorption of energy by CO in the sample cell is measured by a suitable detector. The photometer is sensitized to co by employing CO gas in either the detector or in a filter cell in the optical path, thereby limiting the measured absorption to one or more of the characteristic wavelengths at which CO strongly absorbs. Optical filters or other means may also be used to limit sensitivity of the photometer to a narrow band of interest. Various schemes may be used to provide a suitable zero reference for the photometer. The measured absorption is converted to an electrical output signal, which is related to the concentration of CO in the measurement cell.
2. 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.
3. Sampling considerations.
The use of a particle filter on the sample inlet line of an NDIR CO analyzer is option al and left to the discretion of the user or the manufacturer. Use of filter should depend on the analyzer's susceptibility to interference, malfunction, or damage due to particles.
rates. Flow rates for the dilution method (Figure 1) must be measured with an accuracy of + 2% of the measured value.
2.3 Pressure regulator(s) for standard CO cylinder(s). Regulator must have nonreactive diaphragm and internal parts and a suitable delivery pressure.
2.4 Mixing chamber. A chamber designed to provide thorough mixing of CO and diluent air for the dilution method.
2.5 Output manifold. The output mani. fold 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.
3.1 CO concentration standard(s). Cylinder(s) of co in air containing appropriate concentrations(s) of CO suitable for the selected operating range of the analyzer under calibration; CO standards for the dilution method may be contained in a nitrogen matrix if the zero air dilution ratio is not less than 100:1. The assay of the cylinder(s) must be traceable either to a National Bureau of Standards (NBS) CO in air Standard Reference Material (SRM) or to an NBS/EPA-approved commercially available Certified Reference Material (CRM). CRM's are described in Reference 2, and a list of CRM sources is available from the address shown for Reference 2. A recommended protocol for certifying CO gas cylinders against either a CO SRM or a CRM is given in Reference 1. CO gas cylinders should be recertified on a regular basis as determined by the local quality control program.
3.2 Dilution gas (zero air). Air, free of contaminants which will cause a detectable response on the CO analyzer. The zero air should contain <0.1 ppm CO. A procedure for generating zero air is given in Reference
Calibration Procedure 1. Principle. Either of two methods may be used for dynamic multipoint calibration of CO analyzers: (1) One method uses a single certified standard cylinder of CO, diluted as necessary with zero air, to obtain the various calibration concentrations needed. (2) The other method uses individual certified standard cylinders of CO for each concentration needed. Additional information on calibration may be found in Section 2.0.9 of Reference 1.
2. Apparatus. The major components and typical configurations of the calibration systems for the two calibration methods are shown in Figures 1 and 2.
2.1 Flow controller(s). Device capable of adjusting and regulating flow rates. Flow rates for the dilution method (Figure 1) must be regulated to + 1%.
2.2 Flow meter(s). Calibrated flow meter capable of measuring and monitoring flow
4. Procedure Using Dynamic Dilution Method.
4.1 Assemble a dynamic calibration system such as the one shown in Figure 1. All calibration gases including zero air must be introduced into the sample inlet of the analyzer system. For specific operating instructions refer to the manufacturer's manual
4.2 Insure that all flowmeters are properly calibrated, under the conditions of use, if appropriate, against an authoritative standard such as a soap-bubble meter or wet-test meter. All volumetric flowrates should be corrected to 25° C and 760 mm Hg (101 kPa). A discussion on calibration of flowmeters is given in Reference 1.
4.3 Select the operating range of the CO analyzer to be calibrated.
4.4 Connect the signal output of the CO analyzer to the input of the strip chart recorder or other data collection device. All adjustments to the analyzer should be based on the appropriate strip chart or data device readings. References to analyzer responses in the procedure given below refer to recorder or data device responses.
4.5 Adjust the calibration system to de liver zero air to the output manifold. The total air flow must exceed the total demand of the analyzer(s) connected to the output manifold to insure that no ambient air is pulled into the manifold vent. Allow the analyzer to sample zero air until a stable respose is obtained. After the response has stabilized, adjust the analyzer zero control. Offsetting the analyzer zero adjustments to + 5 percent of scale is recommended to facilitate observing negative zero drift. Record the stable zero air response as Zco.
4.6 Adjust the zero air flow and the CO flow from the standard CO cylinder to provide a diluted CO concentration of approximately 80 percent of the upper range limit (URL) of the operating range of the analyzer. The total air flow must exceed the total demand of the analyzer(s) connected to the output manifold to insure that no ambient air is pulled into the manifold vent. The exact CO concentration is calculated from:
lyzer's operating range, and Zco=analyzer response to zero air, % scale.
If substantial adjustment of the analyzer span control is required, it may be necessary to recheck the zero and span adjustments by repeating Steps 4.5 and 4.6. Record the CO concentration and the analyzer's response. 4.7 Generate several additional concentrations (at least three evenly spaced points across the remaining scale are s gested to verify linearity) by decreasing Fco or increasing F). Be sure the total flow exceeds the analyzer's total flow demand. For each concentration generated, calculate the exact CO concentration using Equation (1). Record the concentration and the analyzer's response for each concentration. Plot the analyzer responses versus the corresponding CO concentrations and draw or calculate the calibration curve.
5. Procedure Using Multiple Cylinder Method. Use the procedure for the dynamic dilution method with the following changes:
5.1 Use a multi-cylinder system such as the typical one shown in Figure 2.
5.2 The flowmeter need not be accurately calibrated, provided the flow in the output manifold exceeds the analyzer's flow demand.
5.3 The various co calibration concentrations required in Steps 4.6 and 4.7 are obtained without dilution by selecting the ap. propriate certified standard cylinder.
Where: (COlout=diluted CO concentration at the
output manifold, ppm; (CO)stp=concentration of the undiluted CO
standard, ppm; Fco=flow rate of the CO standard corrected
to 25° C and 760 mm Hg, (101 kPa), L/
min; and Fo=flow rate of the dilution air corrected to
25° C and 760 mm Hg, (101 kPa), L/min. Sample this co concentration until a stable response is obtained. Adjust the analyzer span control to obtain a recorder response as indicated below: Recorder response (percent scale)=
REFERENCES 1. Quality Assurance Handbook for Air Pollution Measurement Systems, Volume II-Ambient Air Specific Methods, EPA600/4-77-027a, U.S. Environmental Protection AgencyEnvironmental Monitoring Systems Laboratory, Research Triangle Park, North Carolina 27711, 1977.
2. A procedure for Establishing Traceability of Gas Mixtures to Certain National Bureau of Standards Standard Reference Materials. EPA-600/7-81-010, U.S. Environmental Protection Agency, Environmental Monitoring Systems Laboratory (MD-77), Research Triangle Park, North Carolina 27711, January 1981.
EXTWA OUTLETS CATTED
WIIEW NOT IN USE
TO INI ET OF ANALYZEN UNDEN CALIBRATION
Figure 2. Multiple cylinder method for calibration of CO analyzers. (47 FR 54922, Dec. 6, 1982; 48 FR 17355. Apr. 22, 1983)
APPENDIX D-MEASUREMENT PRINCIPLE with a photometer of path length 1 and cal-
ON PROCEDURE POR Tur culated from the equation.
c(atm)= - -- (in I/I0) MEASUREMENT PRINCIPLE 1. Ambient air and ethylene are delivered
(2a) simultaneously to a mixing zone where the ozone in the air reacts with the ethylene to emit light, which is detected by a photomultiplier tube. The resulting photocurrent is amplified and is either read directly or dis
106 played on a recorder.
c(ppm)= - -- (in I/10) 2. 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
(2b) and calibrated as follows:
The calculated 0, concentrations must be
corrected for O, losses which may occur in CALIBRATION PROCEDURE
the photometer and for the temperature
and pressure of the sample. 1. Principle. The calibration procedure is
2. Applicability. This procedure is applicabased on the photometric assay of ozone
ble to the calibration of ambient air O, ana(0,) concentrations in a dynamic flow
lyzers, either directly or by means of a system. The concentration of O, in an ab
transfer standard certified by this procesorption cell is determined from a measure
dure. Transfer standards must meet the rement of the amount of 254 nm light ab
quirements and specifications set forth in sorbed by the sample. This determination
Reference 8. requires knowledge of (1) the absorption co
3. Apparatus. A complete UV calibration efficient (a) of O, at 254 nm, (2) the optical
system consists of an ozone generator, an path length (l) through the sample, (3) the
output port or manifold, a photometer, an transmittance of the sample at a wave.
appropriate source of zero air, and other length of 254 nm, and (4) the temperature
components as necessary. The configuration (T) and pressure (P) of the sample. The
must provide a stable ozone concentration transmittance is defined as the ratio I/I..
at the system output and allow the photomwhere I is the intensity of light which
eter to accurately assay the output concenpasses through the cell and is sensed by the
tration to the precision specified for the detector when the cell contains an O.
photometer (3.1). Figure 1 shows a commonsample, and I, is the intensity of light which
ly used configuration and serves to illuspasses through the cell and is sensed by the
trate the calibration procedure which foldetector when the cell contains zero air. It is
lows. Other configurations may require apassumed that all conditions of the system,
propriate variations in the procedural steps. except for the contents of the absorption
All connections between components in the cell, are identical during measurement of I
calibration system downstream of the O. and I.. The quantities defined above are re
generator should be of glass, Teflon, or lated by the Beer-Lambert absorption law,
other relatively inert materials. Additional information regarding the assembly of a UV
photometric calibration apparatus is given Transmittance =
in Reference 9. For certification of transfer standards which provide their own source of On, the transfer standard may replace the O, generator and possibly other components
shown in Figure 1; see Reference 8 for guidwhere:
ance. a=absorption coefficient of O, at 254
3.1 UV photometer. The photometer connm= 308 4 atm-'cm' at 0°C and 760
sists of a low-pressure mercury discharge
lamp, (optional) collimation optics, an abtorr. (1 2 1 4 47) C=0, concentration in atmospheres
sorption cell, a detector, and signal-processI=optical path length in cm
ing electronics, as illustrated in Figure 1. It
must be capable of measuring the transmitIn practice, a stable O, generator is used tance, I/I., at a wavelength of 254 nm with to produce O, concentrations over the re sufficient precision such that the standard quired range. Each O, concentration is de deviation of the concentration meastermined from the measurement of the urements does not exceed the greater of transmittance (I/1.) of the sample at 254 nm 0.005 ppm or 3% of the concentration. Be