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series of loadplates, one at a time, between the calibration unit and the primary standard. Placing the orifice before the inlet reduces the pressure at the inlet of the primary standard below atmospheric; therefore, a correction must be made for the increase in volume caused by this decreased inlet pressure. Attach one end of a second differential manameter to an inlet pressure tap of the primary standard and leave the other open to the atmosphere. During each of the constant airflow measurements made above, measure the true inlet pressure of the primary standard with this second differential manometer. Measure atmospheric pressure and temperature. Correct the measured air volume to true air volume as directed in 9.1.1, then obtain true airflow rate, Q, as directed in 9.1.3. Plot the differential manometer readings of the orifice unit versus Q.

8.1.2 High-Volume Sampler. Assemble a high-volume sampler with a clean filter in place and run for at least 5 minutes. Attach a rotameter, read the ball, adjust so that the ball reads 65, and seal the adjusting mechanism so that it cannot be changed easily. Shut off motor, remove the filter, and attach the orifice calibration unit in its place. Operate the high-volume sampler at a series of different, but constant, airflows (usually six). Record the reading of the differential manometer on the orifice calibration unit, and record the readings of the rotameter at each flow. Measure atmospheric pressure and temperature. Convert the differential manometer reading to m.3/min., Q, then plot rotameter reading versus Q.

8.1.3 Correction for Differences in Pressure or Temperature. See Addendum B.

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(1) Robson, C. D., and Foster, K. E., "Evaluation of Air Particulate Sampling Equipment", Am. Ind. Hyg. Assoc. J. 24, 404 (1962).

(2) Tierney, G. P., and Conner, W. D., "Hygroscopic Effects on Weight Determinations of Particulates Collected on Glass-Fiber Filters", Am. Ind. Hyg. Assoc. J. 28, 363 (1967).

(3) Unpublished data based on a collaborative test involving 12 participants, conducted under the direction of the Methods Standardization Services Section of the National Air Pollution Control Administration, October, 1970. (4) Harrison, W. K., Nader, J. S., and Fugman, F. S., "Constant Flow Regulators for High-Volume Air Sampler", Am. Ind. Hyg. Assoc. J. 21, 114-120 (1960). (5) Pate, J. B., and Tabor, E. C., "Analytical Aspects of the Use of Glass-Fiber Filters for the Collection and Analysis of Atmospheric Particulate Matter", Am. Ind. Hyg. Assoc. J. 23, 144–150 (1962). ADDENDA

A. Alternative Equipment.

A modification of the high-volume sampler incorporating a method for recording the actual airflow over the entire sampling period has been described, and is acceptable for measuring the concentration of suspended particulates (Henderson, J. S., Eighth Conference on Methods in Air Pollution and Industrial Hygiene Studies, 1967, Oakland, Calif.). This modification consists of an exhaust orifice meter assembly connected through a transducer to a system for continuously recording airflow on a circular chart. The volume of air sampled is calculated by the following equation:

V=QXT.

Q=Average sampling rate, m.3/min.
T=Sampling time, minutes.

The average sampling rate, Q, is determined from the recorder chart by estimation if the flow rate does not vary more than 0.11 m.3/ min. (4 ft./min.) during the sampling period. If the flow rate does vary more than 0.11 m. (4 ft./min.) during the sampling period, read the flow rate from the chart at 2-hour intervals and take the average.

B. Pressure and Temperature Corrections. If the pressure or temperature during high-volume sampler calibration is substantially different from the pressure or temperature during orifice calibration, a correction of the flow rate, Q, may be required. If the pressures differ by no more than 15 percent and the temperatures differ by no more than 100 percent (°C), the error in the uncorrected flow rate will be no more than 15 percent. If necessary, obtain the corrected flow rate as directed below. This correction

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1.1 This method is based on the absorption of infrared radiation by carbon monoxide. Energy from a source emitting radiation in the infrared region is directed through reference and sample cells. Both beams pass into matched cells, each containing a selective detector and CO. The CO in the cells absorb infrared radiation only at its characteristic frequencies and the detector is sensitive to those frequencies. With a nonabsorbing gas in the reference cell, and with no CO in the sample cell, the signals from both detectors are balanced electronically. Any CO introduced into the sample cell will absorb radiation, which reduces the temperature and pressure in the detector cell and displaces a diaphram. This displacement is detected electronically and amplified to provide an output signal.

1.2 This method is applicable to the determination of carbon monoxide in ambient air, and to the analysis of gases under pressure.

2. Range and Sensitivity.

2.1 Instruments are available that measure in the range of 0 to 58 mg./m. (0-50 p.p.m.), which is the range most commonly used for urban atmospheric sampling. Most instruments measure in additional ranges.

2.2 Sensitivity is 1 percent of full-scale response per 0.6 mg. CO/m.* (0.5 p.p.m.).

3. Interferences.

3.1 Interferences vary between individual instruments. The effect of carbon dioxide interference at normal concentrations is minimal. The primary interference is water vapor, and with no correction may give an interference equivalent to as high as 12 mg. CO/m3 Water vapor interference can be minimized by (a) passing the air sample through silica gel or similar drying agents, (b) maintaining constant humidity in the sample and calibration gases by refrigeration, (c) saturating the air sample and calibration gases to maintain constant humidity or (d) using narrowband optical filters in combination with some of these measures. 3.2 Hydrocarbons at ambient levels do not ordinarily interfere.

4. Precision, Accuracy, and Stability. 4.1 Precision determined with calibration gases is 0.5 percent full scale in the 0-58 mg./m.3 range.

4.2 Accuracy depends on instrument linearity and the absolute concentrations of the calibration gases. An accuracy of ±1 percent of full scale in the 0-58 mg./m.3 range can be obtained.

4.3 Variations in amblent room temperature can cause changes equivalent to as much as 0.5 mg. CO/m. per °C. This effect can be minimized by operating the analyzer in a temperature-controlled room. Pressure changes between span checks will cause

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changes in instrument response. Zero drift is usually less than 1 percent of full scale per 24 hours, if cell temperature and pressure are maintained constant.

5. Apparatus.

5.1 Carbon Monoxide Analyzer. Commercially available instruments should be installed on location and demonstrated, preferably by the manufacturer, to meet or exceed manufacturers specifications and those described in this method.

5.2 Sample Introduction System. Pump, flow control valve, and flowmeter.

5.3 Filter (In-line). A filter with a porosity of 2 to 10 microns should be used to keep large particles from the sample cell.

5.4 Moisture Control. Refrigeration units are available with some commercial instruments for maintaining constant humidity. Drying tubes (with sufficient capacity to operate for 72 hours) containing indicating silica gel can be used. Other techniques that prevent the interference of moisture are satisfactory.

6. Reagents.

6.1 Zero Gas. Nitrogen or helium containing less than 0.1 mg. CO/m.3

6.2 Calibration Gases. Calibration gases corresponding to 10, 20, 40, and 80 percent of full scale are used. Gases must be provided with certification or guaranteed analysis of carbon monoxide content.

6.3 Span Gas. The calibration gas corresponding to 80 percent of full scale is used to span the instrument.

7. Procedure.

7.1 Calibrate the instrument as described in 8.1. All gases (sample, zero, calibration, and span) must be introduced into the entire analyzer system. Figure C1 shows a typical flow diagram. For specific operating instructions, refer to the manufacturer's manual.

8. Calibration.

8.1 Calibration Curve. Determine the linearity of the detector response at the operating flow rate and temperature. Prepare a calibration curve and check the curve furnished with the instrument. Introduce zero gas and set the zero control to indicate a recorder reading of zero. Introduce span gas and adjust the span control to indicate the proper value on the recorder scale (e.g. on 0-58 mg./m. scale, set the 46 mg./m.' standard at 80 percent of the recorder chart). Recheck zero and span until adjustments are no longer necessary. Introduce intermediate calibration gases and plot the values obtained. If a smooth curve is not obtained, calibration gases may need replacement.

9. Calculations.

9.1 Determine the concentrations directly from the calibration curve. No calculations are necessary.

9.2 Carbon monoxide concentrations in mg./m. are converted to p.p.m, as follows: p.p.m. CO=mg. CO/m.3X0.873

10. Bibliography.

The Intech NDIR-CO Analyzer by Frank McElroy. Presented at the 11th Methods Conference in Air Pollution, University of California, Berkeley, Calif., April 1, 1970.

Jacobs, M. B. et al., J.A.P.C.A. 9, No. 2, 110-114, August 1959.

MSA LIRA Infrared Gas and Liquid Analyzer Instruction Book, Mine Safety Appliances Co., Pittsburgh, Pa.

Beckman Instruction 1635B, Models 215A, 315A and 415A Infrared Analyzers, Beckman Instrument Company, Fullerton, Calif.

Continuous CO Monitoring System, Model A 5611, Intertech Corp., Princeton, N.J. Bendix-UNOR Infrared Gas Analyzers. Ronceverte, W. Va.

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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 as 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 increase in the inlet concentration.

Fall 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, 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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