By Justin Stewart
Who can remember a time before the personal air sampler, or PAS? My guess is that not many do. Commercially, the PAS dates back to the early 1960s in Europe and the U.S.—a time when AIHA only had a few hundred members.
The History of Personal Air Sampling Instrumentation
The personal sampling pump was developed under contract for the U.S. Bureau of Mines in 1957. At almost the exact same time, researchers in the U.K. nuclear industry had also developed a prototype device, housing it in an old bicycle lamp. The prototype was later commercialized by Casella and featured a rechargeable nickel-cadmium (NiCad) battery. NiCad batteries were subject to “memory effect” and self-discharge issues, which could cause the batteries to hold less charge over time or lose charge when stored. But with the recent deployment of lithium-ion batteries in a PAS, gone are the days of those NiCad battery troubles, which were the cause of so many aborted samples.
In 2003, Professor John Cherrie, a former president of the British Occupational Hygiene Society, wrote that “the development of the personal sampling pump … heralded the beginning of modern occupational [industrial] hygiene and provided the foundation for a proper scientific underpinning of professional practice.” It is hard to imagine that something that was so pivotal is now somewhat taken for granted within the industry.
However, the same potential design compromises that existed 60 years ago still largely exist today, and design engineers often feel that “something has to give” in one or more performance features. This can be particularly true when trying to meet intrinsic safety (IS) requirements as evidenced by the delay between the initial launch of non-IS versions and the eventual release of IS versions. A 2008 French report provided some insight into the various performance characteristics of a number of medium-flow PAS (that is, PAS with a flow rate < 5L/min) and proffered a calculation method whereby each performance element was scored (1-5) and multiplied by a weighting (0-3) depending on whether the characteristic had no importance (0) up to critical importance (3). The resulting overall total then influenced the optimal choice of pump for any given application.
An update to the performance study against the latest pump standard, ISO 13137:2013, is due to be published later this year, and it will be interesting to see which makes the cut (no pun intended). When an occupational health and safety professional purchases a pump, he or she tends to focus on ensuring that the pump has efficient back pressure and accurate flow control. However, one little-known area of pump performance is that of pulsation, which a series of NIOSH reports highlighted in The Annals of Occupational Hygiene in 2014. The ISO standard states that “the pulsation shall not exceed 10% of the flow rate.” But what is pulsation and why is it so important?
Pulsation Explained
With every cycle of the pump, air is drawn in and expelled simultaneously, and this process of reciprocation causes an uneven flow through the sampling train. Pulsation is the measure of the difference in airflow between cycles, as shown in figure 1.
The Evolution of Personal Air Sampling
No Time Like the Present?
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A large pulsation value means that the size cut performance of the cyclones used can be affected because their performance is dependent on flow rate. In addition, less sample is collected when using pumps that generate significant pulsation. As a result, many manufacturers have included pulsation dampeners into their designs to regulate the flow. But despite this design feature, the 2014 NIOSH paper showed that most manufacturers were not able to meet the ±10% requirement. Some had pulsation values over 40% (one notable exception being the original Casella Apex). The NIOSH paper argued for relaxing the standard to ±25%, stating that “a 10% criterion as currently specified in the European standards for testing may be overly restrictive and not able to be met by many pumps on the market.” The European standard in question was EN 1232:1997 (“Workplace atmospheres. Pumps for personal sampling of chemical agents. Requirements and test methods”), which was withdrawn and replaced by ISO 13137:2013. By then, the U.S. had already “signed up” to the former. Now manufacturers simply have to work harder to meet the standard’s requirements for pulsation control with the use of effective dampening.
The good news is that what was once a laboratory test for pulsation can now be performed in the field at the same time as a normal flow rate calibration using a newly introduced proprietary airflow calibrator. Bluetooth connectivity is another recent development that has allowed the whole calibration process to be automated using a dedicated smartphone app, which can help save time and increase confidence in the calibration results. Likewise, when deployed, a Bluetooth-capable pump can be interrogated remotely from a discrete distance—meaning that the worker does not have to be disturbed and the industrial hygienist can have confidence that they are getting a valid sample.
However, as Professor Cherrie noted, the heyday of measuring personal exposure to hazardous substances may have already passed. So rather like the combination of a sound-level meter and a noise dosimeter, a hand-held instrument measuring in real time can be a perfect partner for the trusty PAS.
Hand-held Real-time Instruments
There are a limited number of hand-held, real-time instruments on the market that measure concentration by detecting the amount of light scattered when dust particles are present in the instrument’s sample chamber. As one design engineer explains, it is rather like trying to weigh somebody with a torch. But despite this shortcoming, the instruments are ideally suited for walkthrough surveys of ambient and indoor workplace environments prior to the deployment of pumps. Care should be taken in interpreting results because they typically measure total dust rather than a respirable or inhalable fraction.
Like all photometer type dust meters, the optical measurement of dust concentration is an indirect method (that is, there is no direct relationship of light scatter to mass). Several properties of dust particles affect the intensity and angles of the scattered light, including particle size and shape; the refractive index of the particle; and the color of the particle.
Calibration is another important factor to consider. Factory calibration is normally carried out in a wind tunnel using ISO 12103-1 reference dust, but in some proprietary instruments, each probe is additionally supplied with its own unique calibration insert. This creates a known optical scattering effect in the probe’s sampling chamber. This fixed reference can be used to confirm the original factory calibration point and check the instrument’s linearity. Ideally, the instrument should be calibrated against the actual dust type and local conditions. This can be achieved using a gravimetric adaptor and then simply entering a calibration factor.
Comparative measurements such as those testing the effectiveness of filters in local exhaust ventilation systems are another application for which real-time instruments are an effective tool for the industrial hygienist in checking the effectiveness of controls.
Looking Ahead
So, what does the future hold? We like to think of PAS and noise dosimeters as the “original wearable technology,” which we hear so much about today. Pumps may have reached maturity, but connectivity combined with real-time sensor development surely points the way forward.
Justin Stewart is area sales manager for Casella.
Resources
American Industrial Hygiene Association Journal: “Performance Characteristics of the Multicyclone Aerosol Sampler” (1974).
Bureau of Mines, U.S. Department of the Interior: “The Effect of Pulsation Damping on Respirable Dust Collected by Coal Mine Dust Personal Samplers” (1972).
French National Research and Safety Institute for the Prevention of Occupational Accidents and Diseases (INRS): “Performances des pompes de prélèvement individual” (2008).
International Organization for Standardization: ISO 12103-1:2016, Road vehicles -- Test contaminants for filter evaluation -- Part 1: Arizona test dust (March 2016).
International Organization for Standardization: ISO 13137:2013, Workplace atmospheres -- Pumps for personal sampling of chemical and biological agents -- Requirements and test methods (October 2013).
NIOSH: “Final Report – Evaluation of Coal Mine Dust Personal Sampler Performance” (1973).
NIOSH: “Test Procedure for Coal-Mine-Dust-Personal-Sampler Unit Pulsation Dampener” (1975).
NIOSH: “The Effect of Pulsation Dampening on the Collection Efficiency of Personal Sampling Pumps” (1971).
The Annals of Occupational Hygiene: “Evaluation of Pump Pulsation in Respirable Size-Selective Sampling: Part II. Changes in Sampling Efficiency” (January 2014).
The Annals of Occupational Hygiene: “The Beginning of the Science Underpinning Occupational Hygiene” (April 2003).
Figure 1. Calculation of Pulsation Percentage
