Active sampling—that is, sampling that uses an air pump—of workplace air requires determination of the total volume of air sampled. Two procedures are available for this purpose: one uses a representative sample medium to set the pump flow, and the other uses the actual sampler for setting and measuring the pump flow. Both procedures begin the same way.
Step 1:
Gather the components for the sampling train. The sampling train includes the sampling pump, tubing, and the sampler. (For this discussion, the sampler and the sampling medium contained therein are not defined. The sampler could be a cassette with filter medium, a cassette with filter medium attached to a cyclone, a sorbent tube, a liquid-filled impinger, or other possibilities.) Also obtain a calibrated flow meter (for example, a bubble flow meter or rotameter calibrated by a provider accredited to ISO 17025 for such calibrations). Retain the calibration certificate and documentation for the flow meter. Keep in mind that changes in temperature, pressure, and relative humidity between the point of flow meter calibration and the time and place that it is used may require compensation to the calibration.Sampling train. Photo courtesy Zefon International, Inc.
PROCEDURE B: USING THE ACTUAL SAMPLER
The following description is an outline of a method previously taught in some industrial hygiene training sessions. At one time, it was considered an industry-accepted standard practice.
Step 2, Part 1:
On a bench or table near the workplace to be sampled, start the pump and let it run for several minutes to stabilize the flow. During this pump warm-up period, assemble the remainder of the sampling train (tubing and sampler) and ready a calibrated flow meter (typically a calibrated rotameter). At the end of the warm-up period, stop the pump and attach it to the preassembled sampling train. Attach the flow meter and start the pump. Adjust the pump flow to achieve the airflow rate specified in the sampling method you’re using, as determined by the flow meter. Remove the flow meter. Record the flow rate and sampling start time. Mount the now fully-assembled and running sampling train on the worker or to a sampling stand for area monitoring.
Note that the delay in worker sampling caused by initial adjustment of the sampler flow and mounting on the worker may affect sampling results for short-term exposure limits (15 minutes or less). Adjustment of the flow can be performed with the sampler already mounted on the worker, but this is not possible with some samplers (for example, IOM inhalable sampler and Dorr-Oliver nylon cyclone), and it requires the worker to remain reasonably motionless during the procedure.
Step 2, Part 2:
At least periodically during the sampling process, observe the sampler and the sampling medium to ensure that the medium does not become overloaded.
Step 2, Part 3:
At the end of the work shift or sampling period, attach the calibrated flow meter to the sampler intake. This may require removal of the sampling train from the worker. Record the flow rate, stop the pump, and record the time. Disconnect the sampler, cap the ports, and attach an identification and security label to the sample. To determine the volume of air sampled, average the initial and final flow rates (liters per minute) and multiply by the sampling time (minutes). Submit the sample to an accredited laboratory for analysis. As with initial adjustments to the sampler flow, the delay in worker sampling caused by final determination of the sampler flow may affect STEL sampling results.
COMPARING PROCEDURES
Both procedures have advantages and disadvantages.
Procedure A—Using a Representative Calibration Sampler
One major advantage of Procedure A is that the method is delineated in internationally recognized consensus standards published by ASTM International; in the NIOSH Manual of Analytical Methods, certainly the preferred reference source for the industrial hygiene sampling community; and in the OSHA Technical Manual from the federal regulator having jurisdiction over occupational exposure control in the United States. Another advantage is that setting the flow rate of the sampling train can be performed either in the work area to be sampled or prior to entering the work area, and on all samplers at one time.
A disadvantage of Procedure A is that the representative sampling medium used to set and verify pump flow is contaminated and must not be used for exposure monitoring. Most calibration samples may be re-used for calibration purposes, perhaps multiple times, but some may be single-use only and must be disposed of properly. They are a sunk cost of the sampling.
THE OSHA METHOD FOR AIR SAMPLING
RESOURCES
ASTM International: ASTM D3686-13, Standard Practice for Sampling Atmospheres to Collect Organic Compound Vapors (Activated Charcoal Tube Adsorption Method) (2013).
ASTM International: ASTM D4532-10, Standard Test Method for Respirable Dust in Workplace Atmospheres Using Cyclone Sampler (2015).
ASTM International: ASTM D5337-11, Standard Practice for Flow Rate Adjustment of Personal Sampling Pumps (2011).
NIOSH: Manual of Analytical Methods, 5th Edition, Chapter SA (
PDF
, April 2016).
OSHA: OSHA Technical Manual, Section II: Chapter 1, Personal Sampling for Air Contaminants; II. Pre- Inspection Activities; C. Prepare Personal Air Sampling Equipment
(February 2014).ADVERTISEMENT
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The biggest drawback of Procedure A is its assumption that the pump collects air through the sampler at a constant rate throughout the duration of the sampling. For modern, well-maintained, constant-flow pumps, this is certainly true; but for old and/or hard-used pumps, this may not be true. If the airflow diminishes due to media resistance and this change in flow is not discovered, the total air volume actually passing through the sampler is less than expected and results in an underestimation of the exposure.
Procedure B—Using the Actual Sampler
One possibly major advantage of Procedure B is that the flow rate actually drawn through the sampler is determined. The average of the initial and final measured flow rates is the best available determinate for the actual air volume sampled. (But note that the time delays in worker sampling caused by initial adjustment and final determination of the sampler flow may affect STEL sampling results.)
Offsetting this benefit of Procedure B is a possibly major disadvantage: when the final flow rate is not within 5 percent of the initial flow rate and the timing of the flow rate change is unknown, the average of the initial and final flow rates may result in an overestimation or an underestimation of the exposure, depending on when the flow rate changed. If the flow rate decays substantially before the halfway point of the sampling time, the exposure determined using the average flow rate is overestimated. If the flow rate decays substantially after the halfway point of the sampling time, the exposure determined using the average flow rate is underestimated. Use of the average only “works” when the decay from the initial flow rate to the final flow rate occurs evenly (linearly) through the sampling time.
Another major disadvantage of Procedure B is that initial setting and final monitoring of the flow rate needs to be performed on the worker being sampled. For the worker, this is inconvenient at best and possibly disruptive.
In addition, contamination might be present in the flow meter used to determine the air sampling flow rate. Use of a clean and well-maintained flow meter addresses this concern.
WHICH PROCEDURE IS BETTER?
Now consider that two side-by-side sampling trains have had the flow rates set using the different approaches. What would happen if there was a change in flow rate during sampling? Modern pumps have flow control or other methodologies to measure flow, feedback circuitry to ensure flow is maintained to within 5 percent of the set-point, and a flow-fault indicator that should mean a change of more than 5 percent in the flow rate is not possible—the pump should shut down completely on flow-fault. Thus, when using a modern pump, the flow rate cannot deviate by more than 5 percent if the sample becomes clogged. Pumps from the 1980s and before would allow a flow deviation of more than 5 percent. These elderly pumps are unlikely to pass the test for flow pulsation (periodic variations produced by high-flow pumps; these variations should not exceed 10 percent) and should in most cases be retired in favor of modern equipment.
Modern standard methods, like ASTM and NIOSH methods, assume the use of modern equipment. If you’re using modern equipment, you’ve alleviated the major disadvantage of Procedure A—the assumption that the pump collects air through the sampler at a constant rate. With modern equipment, then, Procedure A is preferred.
Editor’s note: A sentence in the section “Procedure A: Using a Representative Calibration Sampler” was open to misinterpretation and was revised on Oct. 26, 2018. The original sentence read, “If the final flow rate is less than 5 percent of the initial flow rate, both can be used, giving minimum and maximum bounds to the exposure concentration.” On the advice of the authors, this sentence now reads, “If the final flow rate deviates more than 5 percent from the initial flow rate, both can be used, giving minimum and maximum bounds to the exposure concentration.”
KENNETH T. (KENN) WHITE, MS, MM, CIH, CSP, FAIHA, is the principal of Consultive Services in Virginia Beach, Va. He is a three-time recipient of the AIHA Edward J. Baier Technical Achievement Award and is a member of the AIHA Sampling and Laboratory Analysis Committee, for which he serves as chair of the Environmental Lead Subcommittee. He can be reached via
email
. MARTIN HARPER, PhD, CChem, FRSC, CIH, FAIHA, is the director of Scientific Research at Zefon International, Inc., in Ocala, Fla. He is chair of the AIHA Real-Time Detection Systems Committee and a member of the AIHA Sampling and Laboratory Analysis Committee. He can be reached via
email
. CORNELIUS K. (NEIL) KNUTSEN, CIH, is the senior consultant at Regional Reporting, Inc., in Elgin, Ill. He is a member of the AIHA Sampling and Laboratory Analysis Committee. He can be reached via
email
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Following this initial step, the procedures diverge. This article summarizes both procedures and discusses their advantages and disadvantages.
Procedure a: Using a representative calibration sampler
The following description is an outline of the method similar to what can be found in the sources listed at the end of this article.
Step 2, Part 1:
Start the pump and let it run for 5 minutes to stabilize the flow. After this warm-up period, stop the pump. Attach the calibrated flow meter to the air inlet port of a representative sampler with sampling medium (for example, filter or sorbent) from the same sampling medium lot as in the sampler to be used, and attach the sampler to the pump. Start the pump and, using the flow meter, adjust the pump to achieve the airflow rate specified in the sampling method you’re using (for example, ASTM, ISO, NIOSH, or OSHA). Record this flow rate as the initial sampling flow rate. Stop the pump, disconnect the representative sampler and flow meter, and connect the sampler to be used for the sampling. Mount the pump and sampling train on the worker or to the sampling stand for area monitoring (preferably, mounting will occur in the work area to be monitored). Start the pump to begin sampling. Record the sampling start time.
Step 2, Part 2:
At least periodically during the sampling process, observe the sampler and the sampling medium to ensure that the medium does not become overloaded.
Step 2, Part 3:
At the end of the work shift or sampling period, stop the pump and record the time. Disconnect the sampler, cap the ports, and attach an identification and security label to the sample. Reattach the representative sampler and the calibrated flow meter as in Step 2, Part 1, and restart the pump. Record the final flow rate. If the final flow rate is plus-or-minus 5 percent of the initial flow rate, the sample is acceptable.
Some methods, such as ASTM D4532-10, advise that if the final flow rate is outside plus-or-minus 5 percent of the initial flow rate, the sample is unacceptable and resampling is needed. Other methods, such as from OSHA, require only that the initial and final flow rates be submitted to the laboratory. (Note that industrial hygiene accreditation does not require the laboratory to calculate exposure using client-supplied air volume data. The laboratory may perform such calculations as a service to the client.)
If the sample is deemed to be acceptable, submit it to an accredited laboratory for analysis. To determine the volume of air sampled in liters, multiply the average of the initial and final flow rate in liters per minute by the sampling time in minutes. If the final flow rate deviates more than 5 percent from the initial flow rate, both can be used, giving minimum and maximum bounds to the exposure concentration. How to Determine the Sampled Air Volume
Advice for Active Sampling
