« PreviousContinue »
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./min., Q, then plot rotameter reading versus Q.
8.1.3 Correction for Differences in Pressure or Temperature. See Addendum B.
9.1.1 True Air Volume. Calculate the air volume measured by the positive displacement primary standard.
sure, m.8 Pa=Barometric pressure, mm. Hg. Pm=Pressure drop at inlet of primary
standard, mm. Hg. Vm=Volume measured by primary stand
9.2.1 Volume Conversion. Convert the inttial and final rotameter readings to true airflow rate, Q, using calibration curve of 8.1.2. 9.2.2 Calculate volume of air sampled
T=Sampling time, min. 9.3 Calculate mass concentration of suspended particulates
(W1-W1) X 108 S.P.
V S.P.=Mass concentration of suspended
particulates, ug/m.3 Wi=Initial weight of filter, g. Wi-Final weight of filter, g.
V=Air volume sampled, m.3 108=Conversion of g. to ug. 10. References. (1) Robson, C. D., and Foster, K. E.,
“Evaluation of Air Particulate Sam. pling 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 collabora
tive test involving 12 participants, conducted under the direction of the Methods Standardization Services Section of the National Air Pollution Con
trol Administration, October, 1970. (4) Harrison, W. K., Nader, J. S., and Fug
man, 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 Fil-
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:
T=Sampling time, minutes.
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 un. corrected flow rate will be no more than 15 percent. If necessary, obtain the corrected flow rate as directed below. This correction
applies only to orifice meters having a constant orifice coeficient. The coeficient for the calibrating orifice described in 5.1.4 has been shown experimentally to be constant over the normal operating range of the highvolume sampler (0.6 to 2.2 m.3/min.; 20 to 78 ft.3/min.). Calculate corrected flow rate:
. Qa=Corrected flow rate, m.3/min. Qı=Flow rate during high-volume sampler
calibration (Section 8.1.2), m.3/min. Ti=Absolute temperature during orifice
unit calibration (Section 8.1.1), *K
or R. Pi=Barometric pressure during orifice unit
calibration (Section 8.1.1), mm. Hg. Ty=Absolute temperature during high
volume sampler calibration (Section
8.1.2), °K or °R. Po=Barometric pressure during high-vol
ume sampler calibration (Section 8.1.2), mm. Hg.
APPENDIX C-REFERENCE METHOD FOR THE
CONTINUOUS MEASUREMENT OF CARBON
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.3 (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.8 (0.5 p.p.m.).
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/m.3 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 callbration 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.8 range can be obtained.
4.3 Variations in ambient room temperature can cause changes equivalent to as much as 0.5 mg. CO/m.3 per °C. This effect can be minimized by operating the analyzer in a temperature-controlled room. Pressure changes between span checks will cause
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.
5.1 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.
6.1 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.
7.1 Calibrate the instrument as described in 8.1. All gases (sample, zero, calibration, and span) must be introduced into the entire analyzer system. Figure ci shows a typical flow diagram. For specific operating instructions, refer to the manufacturer's manual.
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.8 scale, set the 46 mg./m.3 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
9.1 Determine the concentrations directly from the calibration curve. No calculations are necessary.
9.2 Carbon monoxide concentrations in mg./m.3 are converted to p.p.m, as follows:
p.p.m. CO=mg. CO/m.3 x 0.873
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 Infrareü 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.
A. Suggested Performance Specifications for NDIR Carbon Monoxide Analyzers: Range (minimum)
(0-50 p.p.m.). Output (minimum) - 0-10, 100, 1,000,
5,000 mv, full
scale. Minimum detectable sen- 0.6 mg./m3 (0.5 sitivity
p.p.m.). Lag time (maximum)--- 15 seconds. Time to 90 percent re
30 seconds. sponse (maximum). Rise time, 90 percent 15 seconds.
(maximum). Fall time, 90 percent 15 seconds.
(maximum). Zero drift (maximum)--- 3 percent/ week,
not to exceed 1 percent/ 24
hours. Span drift (maximum)-- 3 percent / week,
not to exceed 1 percent/24
hours. Precision (minimum)--- +0.5 percent. Operational period (min- 3 days.
imum). Noise (maximum)
+0.5 percent. Interference equivalent 1 percent of full (maximum).
scale. Operating temperature 5-40° C.
range (minimum). Operating humidity range 10–100 percent.
(minimum). Linearity (maximum de- 1 percent of full viation).
scale. B. Suggested Definitions of Performance Specifications: Range-The minimum and maximum meas
urement limits. Output-Electrical signal which is propor
tional to the measurement; intended for connection to readout or data processing devices. Usually expressed as millivolts or milliamps full scale at a given impedance.
Full Scale—The maximum measuring limit
for a given range. Minimum Detectable Sensitivity—The small
est amount of input concentration that can be detected as the concentration ap
proaches zero. Accuracy-The degree of agreement between
a measured value and the true value; usu
ally 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 in
terval 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 be
tween initial response time and time to 90 percent response after a step increase in
the inlet concentration. Fall Time (90 percent)-The interval be
tween initial response time and time to 90 percent response after a step decrease
in the inlet concentration. Zero Drift—The change in instrument out
put 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 out
put 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 devia
tion 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 nega
tive 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
lower calibration points.