Casella UKWolseley Rd, Kempston,
Bedford MK42 7JY
Against the welcome addition of a basic right to a safe and healthy working environment in the recently amended ILO declaration on Fundamental Principles and Rights at Work, an article on air sampling pumps  explored the background to standards and performance criteria. It looked at why standards and performance are important to achieving an accurate exposure risk assessment for potentially toxic airborne dusts and vapours but also made the point that many pumps that claim to be ‘designed to meet’ the standards sometimes fall short as revealed by independent tests.
The article concluded that “all pumps are not created equal” and even if you disagree, there are indeed pumps on the market that differ significantly in flow rate and size/weight. They fall under the general headings, low, medium and high flow but they do possess some common performance requirements. Low flow pumps tend to be the physically smallest, in the flow range up to 0.5L/min and used for gasses & vapours; medium flow in the range up to 5L/min used with sampling heads for inhalable and respirable particulates and high flow, greater than 5 L/min typically used for asbestos clearance sampling and other applications that require a large volume to be sampled, quickly.
So, what are the critical areas of the performance standards and why are they important? And what about features that are outside the standard that might improve the quality of the sample or ultimately save time, effort and money?
The standard in question is ISO13137:2013  which is primarily intended for flow controlled, battery powered pumps categorised as Type P, which covers both medium and high flow pumps and Type G for low flow applications. Back pressure capability, constant flow/pressure control, and pulsation (in the case of Type P only) are the main considerations as presented by Casella at the AIOH Conference in 2015 .
Back pressure is the resistance to air flow caused by factors like air density, friction of the motor and resistance by the tubing that connects the sampling head to the pump. The filter used in the sample train is the biggest factor, however. The smaller the diameter and the pore size of the filter and the greater the flow rate, the greater the back pressure and the harder the motor needs to work. As the media becomes loaded during sampling, a greater back pressure is exerted. The pump needs to be powerful enough to overcome the resistance. Back pressure is generally measured in inches or centimetres of water and the table in Figure 1 shows the approximate back pressures exerted by different unloaded MCE filter at different flow rates.
If the majority of sampling involves a high amount of particulate or if the media has a small pore size (e.g., 25mm 0.45µm MCE) then the pump should be capable of overcoming a large back pressure. The pump needs to be efficient to be able to cope with high back pressure levels as this makes it work harder and drains the battery. Efficiency is achieved by improving the drive control circuitry meaning, that energy losses are minimised and an estimated 40% power saving through improved circuitry is achieved by the latest generation pumps and this extra power is harnessed to save battery life. That means a battery with a 2600mAh capacity would have the same performance as a more traditional 4400mAh, much heavier battery, important since size and weight are the primary drivers of wearer acceptance.
Constant Flow Control
Pumps have a control system in their circuitry to maintain the flow rate which must be within ±5% of the initial set flow rate. So, if there is a change in back pressure, the motor works harder to maintain the flow rate. A constant flow ensures confidence in the total volume of air drawn through the pump and in the subsequent exposure calculations. The flow rate affects the cut point and the sampling efficiency of a cyclone for respirable sampling.
Figure 2 shows a comparison of flow control with increasing back pressure at a typical 2L/min flow rate of a latest generation medium flow pump (in red) compared with its predecessor (in blue). Innovation in the pump stack, pressure sensor and control algorithm have led to this improved flow control.
Environmental factors also have an effect on the flow control. They need to work effectively for a wide range of applications and environments, e.g., down a deep mine in South Africa or on a construction site in the Middle East where ambient pressures, temperatures and humidity are at their extreme. The design of the pump needs to take into consideration these factors.
Constant Pressure Control is another method of flow control, primarily used for low flow applications where multiple sampling is taking place. Up to 4 separate samplers (usually sorbent tubes) can be attached via a manifold. This method controls the flow rate by holding a constant pressure level in the tubing between the samplers and the pump. For many older generation (medium flow) pumps, this ‘constant pressure controller’ is a separate piece of equipment which can be purchased as part of a ‘low flow adaptor kit’. However, if lots of low flow measurements are being undertaken, it is worth investing in a pump which has a constant pressure mode built-in and/or ideally a bespoke low flow pump.
The standard states that “the pulsation shall not exceed 10% of the flow rate” but what is pulsation and why is it so important? For a pump with a traditional motor driven reciprocating diaphragm mechanism which comprise a chamber sealed on one side by a flexible diaphragm, with every cycle of the pump, air is drawn in and expelled simultaneously through valves and this process of reciprocation causes an uneven flow through the sampling train. Pulsation is the measure of the difference in air flow between cycles, shown by the calculation in Figure 3.
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. As a result, many manufacturers have included pulsation dampeners into their designs to regulate the flow. These are diaphragms that stretch to provide an extra reservoir of air to draw upon to smooth the flow but because they can wear, it is highly recommended that pumps are maintained in accordance with the manufacturer’s schedule typically based on hours of use, which modern pumps display.
Useful features that have been enabled through technology advancements include:
It’s quite possible that an Occupational/Industrial Hygienist practitioner might possess all three types of pumps and having a common ‘family DNA’ in terms of the user interface and connectivity is a distinct advantage. Familiar controls and a shared app (that may also work with a noise dosimeter) all help to save time and improve the chance of a successful campaign particularly when multiple instruments are being deployed, for example during plant turnaround when there may be just one opportunity to take the measurement. At the time of writing this article, the ISO standard is in the final process of review, a revision being expected before the end of 2022. It will be interesting to see what changes are proposed; tighter standards foster innovation and product improvement to the ultimate benefit of the worker whose health outcomes depend on accurate exposure assessments.