Particle counter

Optical counting

The nature of particle counting is based upon either light scattering, light obscuration, or direct imaging. A high intensity light source is used to illuminate the particle as it passes through the detection chamber. The particle passes through the light source (typically a laser or halogen light) and if light scattering is used, then the redirected light is detected by a photo detector. If direct imaging is used, a halogen light illuminates particles from the back within a cell while a high definition, high magnification camera records passing particles. Recorded video is then analyzed by computer software to measure particle attributes. If light blocking (obscuration) is used the loss of light is detected. The amplitude of the light scattered or light blocked is measured and the particle is counted and tabulated into standardized counting bins. The image to the right shows a light scattering particle counter diagram. More information about types of particle counters and types of particle detection follow in this article.

Direct imaging particle counting employs the use of a high resolution camera and a light to detect particles. Vision based particle sizing units obtain two dimensional images that are analyzed by computer software to obtain particle size measurement in both the laboratory and online. Along with particle size, color and shape analysis can also be determined.

Applications of particle counters are separated into three primary categories:

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 particles 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 in is a cleanroom. Cleanrooms are used extensively in semiconductor device fabrication, biotechnology, pharmaceutics, disk drives, 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, but Federal Standard 209E remains today the most widely referenced standard in the world.

U.S. FED STD 209E cleanroom standards

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, Institute of Environmental Sciences and Technology (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.

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 proactive 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.

Solid particle counters

Solid particle counters are used to measure dry particles for various industrial applications. One such application could be for the detection of particle size coming from a rock crusher within a mining quarry. Sieves are the standard instruments used to measure dry particle size. Vision based systems are also used to measure dry particle size. With a vision based system quick and efficient particle sizing can be done with ease and tremendous accuracy.

Detection methods

There are several methods used for detecting and measuring particle size or size distribution — light blocking (obscuration), 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 micrometer 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 called 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 fabrication facilities.

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 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.

Remote particle counters

Small particle counters that are used to monitor a fixed location typically inside a cleanroom or mini-environment to continuously monitor particle levels. 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, each tube is monitored in sequence.

Hand-held particle counter

A hand-held particle counter is a small, self-contained device that is easily transported and used, and designed for use with Indoor Air Quality (IAQ) investigations. Though lower flow rates of 0.1 ft³/min (0.2 m³/h) than larger portables with 1 ft³/m (2 m³/h), hand-helds are useful for most of the same applications. However longer sample times may be required when performing cleanroom certification and testing. (Hand-held counters are not recommended for cleanrooms). Most hand-held particle counters have direct mount isokinetic sampling probes. One 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 (1.8 m), due to loss of larger particles in the sample tubing.