Particle counter: Wikis

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Diagram Particle Counter

A particle counter is an instrument that detects and counts particles. Applications of particle counters are separated into three primary categories:

  • Aerosol particle counters
  • Liquid particle counters
  • High-energy particle counters (see Particle detector)

Contents

Aerosol particle counters

Aerosol Particle Counters are used to determine the air quality by counting and sizing the number of particles in the air. This information is useful in determining the amount of pollutants inside a building or in the ambient air. It also is useful in understanding the cleanliness level in a controlled environment. A common controlled environment aerosol particle counters are used is a cleanroom. Cleanrooms are used extensively in semiconductor manufacturing, biotechnology, pharmaceutical, disk drive, aerospace and other fields that are very sensitive to environmental contamination. Cleanrooms have defined particle count limits. Aerosol particle counters are used to test and classify a cleanroom to ensure its performance is up to a specific cleanroom classification standard. Several standards exist for cleanroom classification. The most frequently referred to classification is from the United States. Though originating in the United States, the standard Federal Standard 209E was the first and most commonly referred to. This standard was replaced in 1999 by an international standard, however Federal Standard 209E remains today the most widely referenced standard in the world.

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US FED STD 209E cleanroom standards

  Particles/ft³
Class 0.1 µm 0.2 µm 0.3 µm 0.5 µm 1 µm 5 µm
1 35 7 3 1    
10 350 75 30 10 1  
100 3500 750 300 100 10 1
1,000       1,000 100 10
10,000       10,000 1,000 100
100,000       100,000 10,000 1,000


The Replacement standard is ISO 14644-1 and is meant to completely replace Federal Standard 209E. This ISO Standard can be found through the non-profit organization, IEST. Each of these standards represents the maximum allowable number of particles in a unit of air. The typical unit is either cubic feet or cubic meters. The particle counts are always listed as cumulative.

ISO 14644-1 cleanroom standards

  particle/m³
Class 0.1 µm 0.2 µm 0.3 µm 0.5 µm 1.0 µm 5.0 µm
ISO 1 10 2        
ISO 2 100 24 10 4    
ISO 3 1,000 237 102 35 8  
ISO 4 10,000 2,370 1,020 352 83  
ISO 5 100,000 23,700 10,200 3,520 832 29
ISO 6 1,000,000 237,000 102,000 35,200 8,320 293
ISO 7       352,000 83,200 2,930
ISO 8       3,520,000 832,000 29,300
ISO 9       35,200,000 8,320,000 293,000

Cleanroom class comparison

ISO 14644-1 FED STD 209E
ISO 3 1
ISO 4 10
ISO 5 100
ISO 6 1,000
ISO 7 10,000
ISO 8 100,000

Liquid particle counters

Liquid Particle Counters are used to determine the quality of the liquid passing through them. The size and number of particles can determine if the liquid is clean enough to be used for the designed application. Liquid particle counters can be used to test the quality of drinking water or cleaning solutions, or the cleanliness of power generation equipment, manufacturing parts, or injectable drugs.

Liquid particle counters are also used to determine the cleanliness level of hydraulic fluids, the reason being that 75-80% of hydraulic breakdowns can be attributed to contamination. There are various types, installed on the equipment, or portable units that can be transported to site ( e.g. a construction site) and then used on the machine ( e.g. a bulldozer) to determine fluid cleanliness. By determining and monitoring these levels, and following a pro-active or predictive maintenance program, the user can reduce hydraulic failures, increase uptime and machine availability, and to reduce oil consumption. They can also be used to assure that hydraulic fluids have been cleaned using filtration, to acceptable or target cleanliness levels. There are various standards in use in the hydraulic industry, of which ISO 4406:1999, NAS1638 and SAE AS 4059 are probably the most common.

Detection methods

There are several methods used for detecting and measuring particle size or size distribution (though many exist); Light Blocking, Light Scattering, coulter principle and direct imaging.

The Light Blocking optical particle counter method is typical useful for detecting and sizing particles greater than 1 micrometre in size and is based upon the amount of light a particle blocks when passing through the detection area of the particle counter. This type of technique allows high resolution and reliable measurement.

The Light Scattering Method is capable of detecting smaller sizing particles. This technique is based upon the amount of light that is redirected by a particle passing through the detection area of the particle counter. This redirection is referred to as light scattering. Typical detection sensitivity of the Light Scattering method is 0.05 micrometre or larger. However, employment of the condensation nuclei counter (CNC) technique would allow a higher detection sensitivity in particle sizes down to nanometre range. A typical application is monitoring of ultrapure water in semiconductor fabs.

The light blocking method is specified for particle counters that are used for counting in hydraulic and lubricating fluids. Particle counters are used here to measure contamination of hydraulic oil, and therefore allow the user to maintain their hydraulic system, reduce breakdowns, schedule maintenance during no or slow work periods, monitor filter performance etc. Particle counters used for this purpose typically use ISO Standard 4406:1999 as their reporting standard, and ISO Standard 11171 as the calibration standard. Others also in use are NAS 1638 and its successor SAE AS4059D.

Direct imaging is a technique that uses the light emitted by a laser as a source to illuminate a cell where particle are passing through. The technique does not measures the light blocked by the particles but rather measures the area of the particles functioning like an automated microscope. A pulsed laser diode freezes the particle motion. The light transmitted through the fluid is imaged onto an electronic camera with macro focusing optics. The particles in the sample will block the light and the resulting silhouettes will be imaged onto the digital camera chip.

Other types of particle counters

Remote Particle Counters

Small particle counters that are used to monitor a fixed location typically inside a cleanroom or Minienvironment to continuously monitor particle levels 24 hours / day seven days a week. These smaller counters typically do not have a local display and are connected to a network of other particle counters and other types of sensors to monitoring the overall cleanroom performance. This network of sensors is typically connected to a facility monitoring system (FMS), data acquisition system or programmable logic controller.

This computer based system can integrate into a database, alarming and may have e-mail capability to notify facility or process personnel when conditions inside the cleanroom have exceeded predetermined environmental limits. Remote particle counters are available in several different configurations, from single channel to models that detect up to 8 channels simultaneously. Remote particle counters can have a particle size detection range from 0.1 to 100 micrometres and may feature one of a variety of output options including 4-20 mA, RS-485 Modbus, ethernet and pulse output.

Manifold particle counters

Modified aerosol portable particle counter that has been attached to a sequencing sampling system. The sequencing sampling system allows for one particle counter to sample multiple locations, via a series of tubes drawing air from up to 32 locations inside a cleanroom. Typically less expensive then utilizing remote particle counters, however each tube is monitored in sequence.

Handheld particle counters

Small self contained particle counter that is easily transported and used. Though lower flow rates of 0.1 ft³/min (one 10th of a cubic foot) (0.2 m³/h) than larger portables with 1 ft³/m (2 m³/h), handhelds are useful for most of the same applications. However longer sample times may be required when doing cleanroom certification and testing. Most handheld particle counters have direct mount isokinetic sampling probes. You may use a barbed probe on a short piece of sample tubing, but it is recommended that the length of the tubing not exceed 6 ft, due to loss of larger particles in the sample tubing.

External links


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