No time like the present?
Hands up all those who can remember a time before the personal air sampler (PAS)? Not many I guess, but then they do only commercially date back to the early 1960s in Europe and the US, at a time when the American Industrial Hygiene Association (AIHA) only had a few hundred members.
The history of personal air sampling instrumentation
The personal sampling pump was developed under contract for the US Bureau of Mines in 1957, at almost the exact same time researchers in the UK nuclear industry had also developed a prototype device, housing it in an old bicycle lamp. The prototype was later commercialised by Casella and featured a rechargeable NiCad battery but with the recent deployment of Lithium Ion batteries in a PAS, gone are the days of memory effect and self-discharge which were the cause of so many aborted samples!
Commenting in 2003, Professor John Cherrie (1), said 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 exists 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 launch of non-IS and eventually IS versions. A 2008 French report (2) gave some insight into the various performance characteristics of a number of medium flow PAS i.e. 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) through to critical (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 (3) is due to be published later this year and it will be interesting to see which makes the cut, no pun intended. When you purchase a pump, you tend to focus on ensuring that the pump has efficient back pressure and accurate flow control. However, one little known area of performance is that of pulsation, which a series of NIOSH reports (4) highlighted 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?
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 shown by this calculation.