Decoding Calibration: The “Working Standards”
Occupational and environmental health and safety experts play a pivotal role in ensuring the air employees breathe in workplaces remains free from harmful contaminants. One of their key tasks is to accurately sample the concentration of airborne gases, vapors, bioaerosols, and particulates. To do this, the OEHS professional must use a precisely calibrated air sampling pump. Therefore, the act of sampling begins with, and its accuracy rests on, the flow meter used to verify the pump flow rate and the standard to which that flow meter can trace its calibration. Other important factors include the flow meter’s intrinsic accuracy and its ability to compensate for environmental variables. Following is a discussion of some exciting changes in industrial hygiene flow meter classification and developments in flow meter technology.
Accurate calibration is not just a routine task; it is a safeguard. It not only ensures we gather accurate air samples but also diminishes inconsistencies in data. For those delving into the specifics, personal sampling pumps must meet ISO standard 13137:2022, Workplace Atmospheres - Pumps for Personal Sampling of Chemical and Biological Agents - Requirements and Test Methods, to ensure accurate flow performance. This respected standard provides the performance requirements for battery-powered air sampling pumps used to determine chemical and biological threats in our workplace atmosphere. In addition, for those seeking the gold standard in calibration, the ISO guideline offers testing methods to validate pump performance under lab-defined scenarios.
From soap bubbles to spirometers, calibrators play a vital role in fine-tuning air sampling pumps. In OEHS circles, the prevailing language used to classify and describe calibrators has been either “primary standard,” the designation for a device that is directly traceable to a measurement standard maintained by the National Institute of Standards and Technology (NIST); or “secondary standard,” which is traceable to a primary standard. Let us demystify these calibration categories and consider whether they accurately capture flow meter efficacy within the IH air sampling spectrum.
NIOSH delineates primary calibration as a method wherein the flow rate is derived from direct volume measurements tied to the confines of a specific space. In the 5th edition of the NIOSH Manual of Analytical Methods (NMAM), you will find electronic soap-bubble and dry-cell calibrators defined as examples of commercially available “primary calibrators.” Dive deeper, and you will discover additional tools such as wet tests, mass-flow meters, and dry-gas meters mentioned in NMAM as usable instruments for field calibration. In the “secondary standard” spectrum, devices such as rotameters take center stage—these are calibrated based on a “primary standard.”
Looking at calibration through the OSHA lens offers an intriguing twist. While OSHA does not define calibration in the conventional “primary” or “secondary” terms, Appendix F from Section II, chapter 1 of the OSHA Technical Manual details the procedures for calibrating sampling pumps and discusses the advantages and disadvantages of various calibrators. Notably, the manual highlights that inverted burets are no longer considered a primary calibration standard.
What’s New in Today’s Standards? In the world of calibration, June 2023 marked a pivotal moment. ASTM International announced the release of D5337-23, Standard Practice for Setting and Verifying the Flow Rate of Personal Sampling Pumps, heralding a new era for personal sampling pumps and the flow meters IHs use to calibrate them.
It’s important to recognize that instrument calibration and flow-rate verification have different meanings. Calibration ensures that a device is accurately measuring what it’s intended to measure. Setting and verifying a flow rate is a process for ensuring that a pump’s performance meets the requirements of an analytical method.
ASTM D5337-23 emphasizes that a flow meter is employed to set and confirm the flow rate of personal sampling pumps. It mandates that flow-rate verification should be conducted with equipment that has demonstrable traceability to national or international standards. Furthermore, according to the standard, it’s essential to routinely (typically annually) establish the traceability of all measurements used in determining the flow rate.
The ASTM standard underscores a key distinction. The labels “primary” and “secondary” standards are not just jargon; they come packed with specific meanings. ASTM cautions against their casual use, particularly when discussing the traceability of flow meters.
While the industry frequently tags flow meters with traceable calibrations as “primary” or “secondary” standards, it is crucial to recognize the weight these terms carry within the domain of metrology, which the International Bureau of Weights and Measures (BIPM) defines as the intricate “science of measurement,” spanning both experimental and theoretical assessments across every scientific and technological domain.
The guardians of this precise science are NIST and Brazil’s National Institute of Metrology, Quality, and Technology (Inmetro). Their meticulous work is harmonized under the watchful eyes of BIPM. In parallel, the International Organization for Standardization (ISO) collaborates with groups like the Versailles Project on Advanced Materials and Standards (VAMAS) to glean insights on emerging technological trajectories.
What’s in a Name for IH Sampling Calibrators? After thorough research and review, ASTM established a standard practice for the usage and characterization of flow meters in IH applications, using clear and descriptive language. The traditional terms “primary” and “secondary” have been replaced with new terminology. Flow meters (or calibrators) are now termed “working standards.” Notably, this designation applies even if they haven’t undergone traceability calibration.
Aligning with the Latest ASTM Standards Flow adjustment procedures have evolved. With advancements in technology, air sampling pumps can now be adjusted with ease. There are flow meter methods that rely on differential pressure, using sensors that gauge the pressure difference across a specified orifice. Consider the advantage of additional sensors that automatically compensate for changes in air temperature and atmospheric pressure. This leads to precise volumetric flow rate measurements, eliminating the complexities associated with traditional piston or bubble calibrators.
In the continuously advancing landscape of calibration, differential-pressure flow meters have established a distinct place. But what makes this category of flow meters unique compared to others?
Differential-pressure flow meters have several distinguishing attributes. Renowned for their precision, these devices can deliver certified accuracy in the 1 percent of reading range in commonly used flow ranges. Their design, devoid of moving parts, ensures no interference with the airflow being measured. Furthermore, unlike dry piston or soap film flow meters, they operate seamlessly in any orientation.
Per ISO 13137, the uncertainty of the test instruments (flow meters) should be plus or minus 2 percent.
Note that accuracy and uncertainty are not the same thing. Accuracy is how close the measurement is to the true value. In flow meters, that means how close the output of the meter is to the calibration curve. Uncertainty is the quantification of the doubt about the measurement result. In other words, it is the margin of error within the flow meter measurements. Differential-pressure flow meters adhere to the uncertainty specifications of ISO 13137.
These flow meters undergo rigorous calibration, benchmarked against the ISO, UK, and NIST standards. The calibration is conducted in labs that hold the ISO 17025 accreditation, guaranteeing the highest level of accuracy, dependability, and confidence in their measurements. This type of accreditation is not just a stamp of approval but a testament to the rigorous quality controls and proficiency standards the laboratory meets.
When utilizing these precise tools, OEHS professionals are leveraging a device that has been vetted by the pinnacle of calibration standards, guaranteeing unmatched accuracy in the field.
Navigating the Calibration Frontier In our journey through the labyrinth of calibration, a few salient points emerged. ASTM has clarified the nomenclature and processes, emphasizing the shift from the familiar “primary” and “secondary” standard terms to the more encompassing “working standards.” This new ASTM definition brings easier interpretation to calibration standards, particularly those used in the field, and added confidence in the accuracy of field calibration. Further, as technology continually evolves, so does the method of calibration. The differential-pressure type flow meter emerges as a beacon of this advancement, marrying precision with simplicity to provide convenient, accurate technology that is an excellent example of a “working standard.” Calibrated against global stalwarts like ISO and NIST and validated in ISO 17025-accredited laboratories, these devices are not just tools but trusted companions in achieving accuracy.
LUCINETTE ALVARADO, CIH, is the corporate industrial hygienist and technical services manager-media at SKC Inc. Send feedback to The Synergist.
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ASTM International: ASTM D5337-23, Standard Practice for Setting and Verifying the Flow Rate of Personal Sampling Pumps (2023).
International Bureau of Weights and Measures: “National Metrology Systems: Developing the Institutional and Legislative Framework” (PDF, February 2021).
International Organization for Standardization: ISO 13137:2022, Workplace Atmospheres - Pumps for Personal Sampling of Chemical and Biological Agents - Requirements and Test Methods (2022).
NIOSH: “Instruments and Techniques Used in Calibrating Sampling Equipment,” Chapter 11 in The Industrial Environment - Its Evaluation and Control (1973).
NIOSH: NIOSH Manual of Analytical Methods, 5th ed. (PDF, December 2017).
OSHA: “Calibration,” Section II, Chapter 1, Appendix F in the OSHA Technical Manual.