Pumps Vs Real Time Measurements; no time like the present
Hands up to all those who can remember a time before the personal air sampler (PAS)? Not many I guess, but then they only do commerically date back to the early 1960's 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 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 rechargable 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, said that 'the development of the personal sampling head 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 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 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.
f(t) is the volume flow rate over time (t) in L/min
f (with dash overhead) is the mean volume flow over time (T)
t is the time in seconds (s)
T is the time period of the pulsation (s) Figure 1: Calculation of pulsation
A large pulsation value means that the size cut performance of the cyclones used can be affected because their performance is flow rate dependent. In addition, less sample is collected using pumps that generate significant pulsation (5). As a result, many manufacturers have included pulsation dampeners into their designs to regulate the flow. But, despite this design feature, the NIOSH paper showed that the majority of manufacturers were not able to meet the ±10% requirement with some having pulsation values of over 70% (one notable exception being the original Casella Apex). The paper argued a case for relaxation of 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 EN1232:1997 , which has been withdrawn and replaced by ISO 13137: 2013. By this time the US had already ‘signed up’ to the latter, so manufacturers simply have to work harder to meet the standard with regards to 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, through a newly introduced proprietary airflow calibrator. Bluetooth connectivity is another recent development, which means that the whole calibration process can be automated using a dedicated phone App saving time and increasing 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 highlighted, 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 is a perfect partner for the trusty PAS.
There is a limited number of hand-held, real-time instruments on the market which measure concentration by detecting the amount of light scattered when dust particles are present in the instrument’s sample chamber. As one design engineer put it, 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 i.e. there is no direct relationship of light scatter to mass. There are a number of properties of dust particles, which affect the intensity and angles of the scattered light namely:
- Particle size & shape
- The refractive index of the particle
- The color of the particle
Calibration is an important factor to consider. Factory calibration is normally carried out in a wind tunnel using ISO 12103-1 (6) 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 and 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 a local exhaust ventilation (LEV) systems is another application where real-time instruments are an effective tool for the Industrial Hygienist in checking the effectiveness of controls. 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.