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?