Selecting the correct instrument for the job
Using the wrong equipment when carrying out in-situ testing of filter installations can provide misleading results, as Lars Brunberg explains
Choosing the right tool for any job is imperative, not least when it comes to selecting equipment for the inspection of cleanrooms.
So which type of instrument is it preferable to use for in-situ testing of filter installations – an aerosol-photometer or a particle-counter? Before coming to any conclusions, it is necessary to consider the task in hand. A good particle-counter must have a capacity of 1cfm and be able to count up to 400.000 or 500.000 particles per minute, or in terms of cubic meters, a total concentration on the upstream side of 14 to 17 million particles/m3. It is necessary to count as many particles as possible, as the standards accept a leakage of just 0.01% of the particle-concentration on the upstream side, which means the particles in the upstream air must be counted. If only one particle is counted on the downstream side there must be at least 350.000 particles in up-stream volume (IEST RP-6 states between 1 and 10 million particles per ft3) as that is the lowest number of particles that can be accepted in the upstream air. As examples to illustrate the point, Fig.1 shows a filter with a fixed volume of 1m3 on the upstream side without any particles present, Fig. 2 illustrates the lowest acceptable number of particles in the upstream air volume and Fig. 3 the highest.
Lowest acceptable number The lowest theoretical number of particles a particle-counter can accept in the upstream area is 350.000/m3. This is useful to know, but more pertinent is the largest number a particle counter can accurately count without a large coincidence error. As previously stated, a good particle-counter should be able to count 400.000 to 500.000 particles/min at 1cfm, that is 14-17 million/m3. But why stop at 500.000? The answer is in the technology used in the counter Firstly, there is a seven-digit display that can provide 9.999.999 counts per minute, but, depending on the electronics, it cannot be guaranteed that all particles are being counted. Secondly there is the effect of hysteresis, which means it takes some time for an electronic counter to be ready to count the next pulse. So, if the pulses are close together, there is a risk that not every pulse is counted. In the manuals it is referred to as "coincidence loss". If the concentration on the upstream side reaches 500.000 particles per cfm (17 million particles/m3) as illustrated in Fig. 3, up to 48 particles could be counted on the downstream side at 1 cfm and the leakage would be acceptable. When discussing in-situ testing of filter-installations the terminology is not "number of particles per minute", but concentrations of particles in the air. In ISO-standard 14644-3 page 29, point B.6.2.2 recommends a concentration of 20 to 100 milligrams particles in 1m3 of air – the higher the better – but which type and what size of particles would provide the optimum result? Normally an oil is employed to create an aerosol using compressed air in an aerosol generator. This provides particles of 0.1 to 0.5 micron. The oil commonly used today is poly-alfa olefin (PAO 4 centistoke – recommended in the ISO standard above) sometimes distributed under the name of Emery 3004. So how much oil would be required if there are 500.000 particles with a mean diameter of 0.5 micron? As the volume of one particle is 6.6e-17L the totally quantity of oil will be 33e-12L. One litre of oil has a weight of 809.7 grams which means the concentration will be 0.03 micrograms in 1ft3. Therefore, the largest oil quantity (or concentration) a good particle counter is able to count during one minute is 0.03 microgram at 1cfm, that is 1.06 microgram/m3, which is a long way from the 20 to 100 milligrams/m3 recommended in the standard. Even using a dilution of the particle stream from the upstream side to the counter provides little improvement. If a dilution of 1000 times were made, the result would be a concentration of 400.000.000 to 500.000.000 particles in 1cfm, but how big is the concentration? Naturally it is 1000 times higher, or 1.06 milligrams/m3. Even that is a long way from the standard. Fig 4 shows how a concentration of 100 milligrams oil/m3 would look like compared to the other figures. Incidentally, there is very little risk of damaging the filter using such a high concentration. If all the particles were put them together into one large drop of oil, it should only occupy 0.0000124% of the space accessed.
Counters not best suited For high concentrations, particle counters are not, therefore, best suited to the task in hand, only a photometer will meet the requirements of the standard. Photometers work in a similar way to a particle counter (light scattering), but do not sort and count every single particle. Instead they normalise the pulses and integrate them in a short time. In other words they view the aerosol "en masse". The instrument is then able to present the particles as a concentration. Furthermore, the photometer has an integrated signal for internal calibration that is normally similar to a concentration of 100 milligram/m3 of air. This means the concentration on the upstream side can be measured and the results produced directly in milligrams/m3 of air, which enables the user to verify if there are enough particles in the upstream air. Too low a concentration tends to be very unstable which is why it is recommended concentrations between 20 to 100 milligrams/m3 of air be used.
Choosing a reference level The concentration level to be used as a reference level can also be chosen, i.e.,100%. If there is a stable level on the upstream side, that level can be set as the reference. In that way, all leakage values are a direct percentage of the upstream level. Usually there is a concentration of between 20 to 40 milligrams/m3 of air when testing filters in laminar flow benches or other open filter units. Figures 5 and 6 illustrate such concentrations. It is therefore very easy to be sure which control method has the best possibility of discovering leakage in filtration systems, as a higher number of particles in the upstream air provides a better chance of discovering a pinhole-leak in the down-stream. It can therefore be concluded that while particle counters are the ideal tool for carrying out a test or classification of a cleanroom, as you have to know the number of particles per m3, when working with high particle concentrations, an aerosol photometer must be the equipment of choice. Which is why it is so important to always choose the right tool for the job.