Basics

When considering the purchase of a particle counter, the user typically considers factors such as price, quality, laser lifetime, warranty, and service. This paper discusses the performance-related factors which also must be considered if the particle counter will be used for certifying a cleanroom to ISO 14644. Of these, sensitivity and flowrate are the most important parameters, both of which impact the collection efficiency, which in turn determines the required sampling time. In the less-clean areas, particle counter saturation also must be considered.

The sensitivity of an aerosol particle counter is determined by the size of the smallest particle the unit can detect. Modern particle counters used for ISO 14644 cleanroom certification typically have sensitivities of 0.1, 0.3, or 0.5 microns (μm). Particle counters with greater sensitivity can count smaller particles, thus many more particles.

For example, under ISO 14644 conditions a particle sensor with a sensitivity of 0.1 μm can count 28 times more particles of size ≥0.1 μm than a 0.5 μm instrument can count of particles of size ≥0.5 μm. (There are many more small than large particles.)

The flowrate of a particle counter is simply the rate at which its pump draws the sample air through the sample chamber. The higher the flowrate, the more data the counter collects per time period

or the faster it can collect a specified volume. Historically, the flowrates of the particle counters used in ISO 14644 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.

An instrument that samples 1.78 CFM (50 LPM) can sample 1.0 m3 (1,000 L) in only 20 minutes, where a 1.0 CFM (28.3 LPM) particle counter takes 35.3 minutes to sample the same 1.0 m3. A handheld monitor with a flowrate 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 flowrate on the counter's ability to collect data. It is defined as the product of the expected particle concentration times the flowrate for the selected particle counter, divided by the same product for a 0.5 μm, 0.1 CFM particle counter.

In classifying a cleanroom to ISO Class 4, the LASAIR® II - 110 (with a sensitivity of 0.1 μm and a flowrate of 1.0 CFM) can be expected in one minute to sample 283 particles of ≥0.1 μm (i.e., = 10,000 per m3 / 35.3 CF/m3). In contrast, a basic Handilaz® Mini handheld particle counter (with a sensitivity of 0.5 μm and a flowrate of 0.1 CFM) can be expected in one minute to sample only one particle of ≥0.5 μm (i.e., = 352 per m3 / 35.3 CF/m3 x 0.1 CFM).

In ISO certification, however, the goal is generally to sample >20 particles, not 10,000. In a class 4 room the LASAIR II-110 will detect and count 20 particles in much less than a minute, whereas the handheld will take about 20 minutes. Calculated precisely, the collection efficiency of the LASAIR II-110 is 284 times that of the basic 0.5 μm, 0.1 CFM counter.

No matter which 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 flowrate.

Figure 1: Collection Efficiency of Selected Particle Counters (download pdf for all tables and figures) (388.0 KB)

Cleanliness Levels

ISO 14644-1 has become the dominant, worldwide standard for classifying the cleanliness of the air in clean areas. Table 1 gives the maximum number of particles allowed (per cubic meter) if the zone is to meet a specified ISO class of cleanliness. Please note:

• Each successively higher classification allows approximately 10 times as many particles as the previous class.
  
• The ratio of particles of size A to size B remains approximately constant for all classes. For example:
  
  • Class 4 allows 1,200 particles of ≥0.3 μm, or 352 of ≥0.5 μm.
    
  • Class 5 allows 10,200 particles of ≥0.3 μm, or 3,520 of ≥0.5 μm.
    

Sample Volume

To determine the sensitivity and flowrate needed for certifying a set of ISO 14644 cleanrooms, you also must consider the sample volume required and the resulting sampling time. Table 2 presents the minimum required sample volume per sample location 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 Table 1)

In other words, to certify that a cleanroom meets the 14644 standard, ISO requires you to collect a large enough volume at each location that you expect to sample at least 20 particles (i.e., a statistically-adequate sample) when the room holds the maximum acceptable particle concentration.

Table 2: Minimum Sample Volume Required per ISO Class

Sampling Time

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. Table 3 gives these calculations for a 1.0 CFM, 1.78 CFM, or 0.1 CFM sampling rate. Note in the Table that many conditions require a sample of one minute; this is driven by the ISO requirement of an absolute minimum sampling time of one minute per location

Table 3A: ISO Min. Sampling Time

Table 3B: ISO Min. Sampling Time

Table 3C: ISO Min. Sampling Time

Table 3: Minimum Sampling Time Required

A statistically-adequate sample can be collected relatively quickly in a dirty room; it takes much longer to certify an extremely clean zone. A statistically-adequate sample can be collected much quicker with a high sensitivity particle counter than with a low sensitivity counter.

It only takes one 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 Table 3A).

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 flowrates a 0.5 μm, 0.1 CFM handheld counter requires 201.9 minutes to certify Class (see Table 3C).

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.

Comparison of Particle Counters

Reducing sampling time can provide significant cost savings when certification (or daily monitoring) is done frequently and at many locations. Table 4 provides the time required by various Particle Measuring Systems particle counters to complete a single ISO sample for one location in a cleanroom of designated cleanliness. Note that only a LASAIR-II-110 can certify effectively at ISO 1-3, but almost any counter can certify for ISO 6.

Table 4: Minutes Required to Sample One Location

Ultra-Clean Rooms: High-Efficiency Filtering of Chassis Particles

ISO Class 1-3 rooms are extraordinarily clean, allowing only 10 to 1,000 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 or 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 are missed by the HEPA filter. Thus, the particle counter may well require a second, high- efficiency filter that scrubs the output of particles coming from the chassis.

Dirty Rooms: Particle Counter Saturation

If an airborne-particle counter is used in a dirty environment, eventually enough dirt particles will collect on the optics 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 PMS service representative.

If you perform ISO certification in relatively dirty rooms (e.g., ISO Class 7 or 8), you can avoid 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 >5% coincidence errors. ISO Class 9 is so dirty that most particle counters will require the addition of an aerosol diluter to be able to measure without saturation.

Table 5 specifies the maximum concentration limit for a number of particle counters, plus the dirtiest ISO classification in which a unit can be without requiring the addition of an aerosol diluter. For example, the high sensitivity of the LASAIR II-110 makes it the best possible option for sampling ISO 1-3 rooms. However, this same sensitivity means it counts so many particles that it cannot be used in ISO 8-9 rooms without an aerosol diluter. Table 5 also provides data including the sensitivity, flowrate, and collection efficiency for each PMS particle counter. Taken together, the range of PMS instrumentation covers every combination of sensitivity and flowrate required for common airborne particle counting applications.

Table 5: Characteristics of PMS Airborne-Particle Counters

FINAL EXAM: Open Book

A) Which particle counter should you choose if you will need to monitor primarily ISO Class 3 & 4 cleanrooms? Answering this question requires first answering a series of preliminary questions:

A-1) Which particle counters can complete sampling in the least time?

Many 1.0 CFM counters can sample an ISO Class 4 location in only 1.0 minutes. Class 3, however, is much more demanding, clearly demonstrating the importance of counting efficiency. Thus, the LASAIR II - 110 can sample a Class 3 location in one minute, while the LASAIR II 350L takes 4 minutes per location (see Table 4). Other particle counters just take too long for sampling Class 3.

A-2) How much will you use this particle counter to certify other rooms?

If you will use this same unit to certify ISO 1 & 2 rooms, the time savings from certifying with a LASAIR II-110 are so substantial that the extra cost generally will be justified. If you use it for a substantial amount of Class ≥5 certification, a 0.1 μm particle counter may be over-kill.

A-3) How much will you use this particle counter for trouble-shooting, real-time monitoring, etc.?

You need to define particle counter requirements for these applications, 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?

A-4) Do you need to worry about saturation?

Because ISO 3 & 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 Table 5 for information on individual particle counters.

A-5) All considered, which particle counter should you choose?

Since a LASAIR-II-110 costs 2-3 times as much as a LASAIR II-350L, it is important to evaluate the cost trade-offs. While the LASAIR II-350L's sampling time may be 4 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:

• 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 Clas 4?
• What are the frequencies and sampling costs of other uses of this equipment?

The choice between units then can be expressed as a clear financial comparison. For each particle counter under consideration, calculate:

Estimated Total Costs = Initial Purchase Costs + Sampling Costs + Maintenance Costs,

where: Sampling Costs = Sum (# Samples of Class N x Cost/Sample for Class N), (Summed over all Classes & all other uses)

Cost/Sample for Class N ~ Sample time per location, etc.,

and where : Time period (over which costs are projected) is defined by appropriate payback criteria.

B) Which counter should you consider if you are certifying only ISO Class 6 cleanrooms

and only certify every 6 months -- plus perform a minimal amount of trouble-shooting?

Almost any particle counter can perform ISO 6-8 certification in 1.0 minute per location (see Table 4). With infrequent usage, you should place a relatively high weight on low equipment cost. For example the Handilaz Mini handheld particle counter offers a combination of high quality and low cost that makes 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.

C) If you will be certifying that ISO Class 5-8 pharmaceutical cleanrooms also meet the CE GMP Annex #1 standards, which particle counters should you consider?

If the pharmaceutical drugs are to be shipped only to the US, then the ISO formulas all apply and a counter like the LASAIR II-510 or the LASAIR II-310 would be appropriate. However, it the drugs are to be shipped to the European Union, there is an extra regulatory requirement that the sample in Class A & B rooms have a min. 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; this increase in sample flowrate reduces the sample time from 35.3 to 20 minutes per location. The LASAIR-II-350L and LASAIR-II-550L both offer 1.78 CFM (50 LPM) sample flows, hence reduced sample times.

D) Are there additional considerations when selecting a particle counter?

Consideration should be given to how the particle counter will be used. Physical size, weight, and materials of construction should be all taken into account when selecting a particle counter. If sterilization chemicals are to be used on the particle counter make sure to check the materials of compatibility prior to purchase.

FOOTNOTE

 1 This paper does not discuss the critical importance of the certification software. Software from different manufacturers often differs significantly in interpretation of how ISO and EC GMP regulations should be implemented.

Contact us if you need more information or have questions.

Authors: Paul Hartigan and Bill Belew, Particle Measuring Systems

Published: 02/2006 Cleanrooms Magazine; Updated Nov 2007

Lasair and HandiLaz are registered trademarks of Particle Measuring Systems, Inc.