Fundamentals of particle counting
Selecting an airborne-particle counter for ISO 14644-1 certification
By Bill Belew, Aerosols Product Line Manager at Particle Measuring Systems
When considering the purchase of a particle counter, the user typically evaluates factors such as price, quality, laser lifetime, warranty, and service. However, performance-related factors also must be considered if the particle counter will be used for certifying a cleanroom to ISO 14644-1. Of these, sensitivity and flow rate are the most important parameters, since both impact the collection efficiency, which, in turn, determines the required sampling time. In the less-clean zones, particle counter saturation also must be considered. (This article, however, does not discuss the critical importance of the certification software. Software from various manufacturers often differs significantly in interpretation of how ISO and EC GMP regulations should be implemented.)
The sensitivity of an airborne-particle counter is determined by the size of the smallest particle the unit can detect. Modern particle counters used for cleanroom certification typically have sensitivities of 0.1, 0.3, or 0.5 micron (μm). Particle counters with greater sensitivity can count smaller particles and, therefore, many more particles. For example, under ISO conditions, a particle sensor with a sensitivity of 0.1 μm can count 28 times more particles greater than or equal in size to 0.1 μm than a 0.5 μm instrument can count particles greater than or equal in size to 0.5 μm. (There are many more small, rather than large, particles.)
The flow rate of a particle counter is simply the rate at which its pump draws the sample air through the sample chamber. The higher the flow rate, the more data the counter collects per time period-or the faster it can collect a specified volume. Historically, the flow rates of the particle counters used in cleanroom certification were 0.1 CFM and 1.0 CFM. However, recent particle counting and regulatory trends have increased the utility of 1.78 CFM (50 LPM) and 1.0 CFM counters, while reducing the value of 0.1 CFM particle counters.
For instance, an instrument that samples 1.78 CFM (50 LPM) can sample 1.0 m3 (1000 liters) in only 20 minutes, whereas a 1.0 CFM (28.3 LPM) particle counter takes 35.3 minutes to complete the same 1.0 m3 sample size A handheld monitor with a flow rate of 0.1 CFM takes 353 minutes to sample 1.0 m3.
The collection efficiency of a particle counter describes the combined effects of sensitivity and flow rate on the counter’s ability to collect data. It is defined as the product of the expected particle concentration multiplied by the flow rate for the selected particle counter, as compared to a 0.5 μm, 0.1 CFM particle counter.
No matter the ISO classification, the higher the collection efficiency, the lower the sampling time. Figure 1 displays the collection efficiencies of several PMS air-particle counters and sensors as a function of sensitivity and flow rate.
ISO 14644-1 has become the dominant, worldwide standard for classifying the cleanliness of the air in clean areas. Figure 2 lists the maximum number of particles allowed (per cubic meter) if the zone is to meet a specified ISO class of cleanliness. It is important to note that each successively higher classification allows approximately 10 times as many particles as the previous class. Also, the ratio of particles of Size A to Size B remains nearly constant for all classes. For example:
- Class 4 allows 10,200 particles of ≥0.3 μm, or 3520 of ≥0.5 μm.
- Class 5 allows 102,000 particles of ≥0.3 μm, or 35,200 of ≥0.5 μm.
Figure 2: ISO classification versus maximum particle concentration allowed.
To determine the sensitivity and flow rate your particle counter needs in order to certify a set of cleanrooms, you also must consider the sample volume required and the resulting sampling time. Figure 3 presents the minimum required sample volume per sample location necessary to meet ISO 14644-1. Values for each ISO class have been derived from the equation:
Min. volume (m3) ≥ 20 particles/
Max. particle concentration allowed
(from Figure 2)
In other words, to certify that a cleanroom meets the standard, ISO requires you to collect a large enough volume at each location that you should expect to sample at least 20 particles (that is, a statistically adequate sample) when the room holds the maximum acceptable particle concentration.
Figure 3: Minimum sample volume required per ISO class (at a sample flow rate of 1.0 CFM)
Upon determining the minimum required sample volume to collect at each location, you can calculate the minimum time required for a specific counter to collect the sample data. Figures 4A through 4C give these calculations for a 1.0 CFM, 1.78 CFM, or 0.1 CFM sampling rate. Note that in the figures, many conditions require a sample of 1 minute; this is driven by the ISO requirement of an absolute minimum sampling time of 1 minute per location.
Figure 4A: ISO minimum sampling time. Minutes at a sample flow rate of 1.0 CFM to collect min. sample volume
Figure 4B: ISO minimum sampling time.
Figure 4C: ISO minimum sampling time.
A statistically adequate sample can be collected relatively quickly in a dirty room; it takes much longer to certify an extremely clean zone. Moreover, a statistically adequate sample can be collected much quicker with a high-sensitivity particle counter than with a low-sensitivity counter. For instance:
- It only takes 1 minute per location for a 0.1 μm, 1.0 CFM counter to sample the required volume of 0.1 μm particles to certify a Class 3 area. It takes the same unit 7.1 minutes for Class 2, and a lengthy 70.7 minutes for Class 1 (see Figure 4A).
- A 0.5 μm, 1.0 CFM unit takes 20.2 minutes per location for Class 3, and 176.7 for Class 2; it is unable to certify a room for Class 1. Considering the impact of different flow rates, a 0.5 μm, 0.1 CFM handheld counter requires 201.9 minutes to certify Class 3 (see Figure 4C).
- In pharmaceutical cleanrooms requiring a sample volume of 1.0 m3, a 1.78 CFM unit can sample one location in 20 minutes, while a 1.0 CFM unit requires 35.3 minutes.
Comparing particle counters
Reducing sampling time provides significant cost savings when certification (or daily monitoring) is done frequently or at many locations. Figure 5 provides the time required by various particle counters to complete a single ISO sample for one location in a cleanroom of designated cleanliness. Note that only a 1.00 CFM, 01 μm counter can certify effectively for ISO 1 through 3, but almost any counter can certify for ISO 6.
Figure 5: Minutes required to sample one location.
ISO Class 1 through 3 rooms are extraordinarily clean, allowing only 10 to 1000 particles/m3. At this level of cleanliness, it is important to verify that the particle counter itself is not generating particles.
Most particle counters include a HEPA filter on the output of the pump; in ultra-clean rooms, this is not enough. The rotary action of pumps and fans can generate a significant number of particles that are inside the particle counter chassis; these particles are not drawn down the sample path, so they are missed by the HEPA filter. Thus, the particle counter may require a second, high-efficiency filter that scrubs the output of particles emitted from the chassis.
If an airborne-particle counter is used in a dirty environment, eventually enough dirt particles will collect on the optics so that the unit will no longer function correctly; this is referred to as the particle counter reaching saturation. At this point, the unit will begin displaying coincidence errors until the counts eventually become unreliable.
Coincidence errors occur when the unit mistakes multiple small particles for one large particle. Thus, the counts for large particles become too high, while the counts for smaller particles become too low. When this occurs, sometimes the user is able to clean out the unit by running it for 24 hours with a zero filter. Often, however, the user will need to have the unit cleaned by a certified service representative.
If you perform ISO certification in relatively dirty rooms (for example, ISO Class 7 or 8), you can minimize this problem by selecting a particle counter with a high maximum concentration. This means the unit can sample from a higher particle concentration without incurring greater than 5 percent coincidence errors. ISO Class 9 is so dirty that most particle counters will require the addition of an aerosol diluter so it can measure without saturation.
Figure 6: Appropriate airborne-particle counters for each ISO class.
Figure 6 specifies the maximum concentration limit for a number of particle counters, plus the dirtiest ISO classification for a unit before requiring the addition of an aerosol diluter. For example, the high sensitivity of a 1.0 CFM, 01 μm particle counter makes it the best possible option for sampling ISO 1 through 3 rooms. However, this same sensitivity means it counts so many particles that it cannot be used in ISO 8 and 9 rooms without an aerosol diluter.
Figure 6 also lists the types of particle counters appropriate for monitoring each ISO classification, plus, as an example, provides data, including the sensitivity, flow rate, and collection efficiency for several PMS particle counters.
Bill Belew is aerosols product manager for Particle Measuring Systems. He has over 20 years of experience in the development and marketing of monitoring systems for medical and industrial applications. At PMS, Belew has been a leader in the development of monitors for use in pharmaceutical applications. He has written over 40 publications on topics ranging from monitoring in ultra-clean manufacturing, to the carcinogenicity of industrial chemicals, to the applications of solar energy. He can be reached at email@example.com.
Frequently asked questions concerning airborne-particle counters
Q: Which particle counter should you choose if you will need to monitor primarily ISO Class 3 and 4 cleanrooms?
A: Answering this question first requires answering a series of preliminary questions:
Which particle counters can complete sampling in the least time?
Many 1.0 CFM counters can sample an ISO Class 4 location in only 1 minute. Class 3, however, is much more demanding, clearly demonstrating the importance of counting efficiency. Thus, a 1.00 CFM, 01 μm particle counter can sample a Class 3 location in 1 minute, while a 1.78 CFM, 0.3 μm particle counter takes 4 minutes per location (see Figure 5). Other particle counters just take too long for sampling Class 3.
How much will you use this particle counter to certify other rooms?
If you will use this same unit to certify ISO 1 and 2 rooms, the timesavings from using a 1.00 CFM, 01 μm are so substantial that the extra cost generally will be justified. But if you use it for Class 5 or greater certification, a 0.1 μm particle counter may be overkill.
How much will you use this particle counter for trouble-shooting, real-time monitoring, and so forth?
You need to define particle counter requirements for these applications, and then calculate the costs and number of samples per year that will be required in these efforts. For example, how portable must the unit be? If it will be placed on a mobile cart, then the size and weight are not nearly as important, but the cost of the cart should be included. If the counter is used for real-time monitoring, what are the data downloading requirements and costs?
Do you need to worry about saturation?
Because ISO 3 and 4 are extremely clean areas, you do not need to consider the potential saturation of the particle counter. However, if you also will be using this unit to sample in ISO 8 and 9 cleanrooms, it will be important to consider the maximum concentration the particle counter could allow. It might even prove necessary to use the particle counter in conjunction with an aerosol diluter. (See Figure 6 for information on individual particle counters.)
All things considered, which particle counter should you choose?
Since a 1.00 CFM, 01 μm particle counters costs two to three times as much as a 1.78 CFM, 0.3 μm particle counter, it is important to evaluate the cost trade-offs between various particle counters and requirements. While a 1.78 CFM, 0.3 μm particle counter’s sampling time may be four times longer, this could be outweighed if the Class 3 measurements are only a small portion of the total particle counting activity. Thus, you need to estimate the following:
- What are the sampling costs per location if the sample lasts 1.0 minutes? 4.0 minutes?
- How many samples per year will be taken in ISO Class 3 cleanrooms or zones? In ISO Class 4?
- What are the frequencies and sampling costs of other uses of this equipment?
The choice between units then can be expressed as a financial comparison. For each particle counter under consideration, calculate:
Estimated total costs = Initial purchase costs + sampling costs + maintenance costs
Sampling costs = Sum (# Samples of Class N x Cost/Sample for Class N), (summed over all classes and all other uses)
Cost/Sample for Class N is proportionate to sample time per location, etc.
Time period (over which costs are projected) is defined by appropriate payback criteria
Q: Which counter should you consider if you are certifying only ISO Class 6 cleanrooms-and only certify every six months-plus perform a minimal amount of troubleshooting?
A: Almost any particle counter can perform ISO 6 through 8 certification in 1.0 minute per location (see Figure 5). With infrequent usage, you should place a relatively high weight on low equipment costs. For example, a handheld particle counter offers a combination of high quality and low cost, making it a very compelling choice if you are only certifying occasionally. However, no handheld units are adequate long term for frequent usage; the handheld’s lower pump lifetime, minimal data management, and much longer sampling times all combine to make a full-sized particle counter more cost-effective than any handheld.
Q: If you will be certifying that ISO Class 5 through 8 pharmaceutical cleanrooms also meet the CE GMP Annex #1 standards, which particle counters should you consider?
A: If the pharmaceutical drugs are to be shipped only to the US, then the ISO formulas all apply, and a 1.0 CFM, 0.3, or 0.5 μm counter would be appropriate. However, if the drugs are to be shipped to the European Union, additional regulatory requirements call for samples from Class A and B rooms to have a minimum volume of 1.0 m3 (35.3 CFM). For this case, many users are now selecting a 1.78 CFM particle counter instead of a 1.0 CFM unit; this increase in sample flow rate reduces the sample time from 35.3 to 20 minutes per location.