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Particle counter calibration


What are Particle Counters?

Aerosol particle counters, such as optical particle counters (OPCs) and condensation particle counters (CPCs) are widely deployed in industry, air quality and academia. They measure the concentration and often the size of particles by light scattering (in the case of the CPC particles too small to otherwise interact with light are first grown in size by condensing a fluid upon them).

They find applications in determining the concentration (and often size) of airborne particles in scenarios such as cleanrooms, occupational health, atmospheric monitoring and indoor air quality. The response of OPCs varies to an extent based on the material composition of the particles, and is dependent upon the particles' size according to Mie scattering theory. For those reasons, alongside manufacturing variability, it is common practice to traceably calibrate optical particle counters against a trusted standard counter as a function of particle size.

How are calibration particles selected by size?

For smaller (<< 1 micron) particles, a differential mobility analyser (DMA) has historically been used to size select particles, whether from a broad aerosol source, or to remove "residue" particles from Polystyrene Latex (PSL) Spheres. This however has a number of disadvantages due to the fact that the technique relies upon electrically charging the particles:

  • The size of particle is dependent upon the number of charges gained by it. Electrostatic charging is a statistical process, and whilst most small particles will only gain 1 charge (if they gain any at all), as they increase in size there becomes an increasing likelihood of 2 or more charges being gained, which causes the DMA to pass particles substantially larger than intended, thus producing inaccurate and ambiguous results:

Calibration of an OPC with a DMA shows multiple ambiguous peaks

Calibration of an OPC with a DMA can lead to multiple ambiguous peaks

  • This means that the source aerosol has to be carefully controlled to avoid larger than needed particles being present and changed as the size is increased — it's not possible to use "one jar" of source aerosol material across the full size range of the counter.

  • Multiple charging can only be overcome to a certain extent, which means the DMA is not suitable for particles any greater than a few hundred nanometres, which only covers the very smallest particles detectable by an ordinary OPC.

  • Only a maximum of 20% of particles will gain the optimal 1 charge — many of the smaller particles will not be charged at all and will not pass through the DMA. Therefore up to 80% of the particles are wasted, which means that more material is needed and/or the concentration will be low.

  • It requires a radioactive or X-ray charger which can be subject to strict regulatory requirements in many territories

By contrast, an Aerodynamic Aerosol Classifier classifies by size without needing the particles to be charged — it just uses the motion of the particles in a stream of air under the influence of centrifugal force to classify. This means that:

  • Just one peak is produced; there is no ambiguity:

Just one peak is seen when calibrating an OPC with an AAC

Only one peak is ever produced by the AAC

  • The choice of underlying aerosol distribution can be made freely — you can use just one "jar" of material provided it can make particles covering the whole range of interest.

  • The AAC works up to 5 μm (microns!). Whilst the AAC classifies by "aerodynamic diameter" and the DMA by "electrical mobility diameter", for an aerosol of known density the conversion is simple, and indeed automatic in the AAC's software.

  • Around 80% of particles will pass through the AAC above 100 nm (there are increased losses for smaller particles, but these are still fewer than for a DMA system)

  • No radioactive or X-ray source is required.

The AAC has been adopted by several national metrology institutes, as well as other laboratories for OPC calibration (see reference list below). As of 2023 it is included in ISO 21501-4:2018+A1:2023 Determination of particle size distribution. Single particle light interaction methods. Light scattering airborne particle counter for clean spaces.

What laboratory setup is required?

At its simplest all that is required is an aerosol source (such as a nebuliser for liquid particles or a burner for soot), an AAC, filtered makeup air to balance the flows, a flow splitter, a reference particle counter and the counter to be calibrated (plus electrically conductive tubing to connect everything together):

Calibration of an OPC using an AAC

Calibration of an OPC against a reference OPC using an AAC to size select

As ever, one has to be careful about particle losses, especially for very small particles (by diffusion) or very large particles (by impaction or gravitational settling), and mixing; for a more detailed experimental setup, see Horender et al. (2019).

What about CPC calibration against an electrometer?

ISO 27891 describes the calibration of condensation particle counters using a reference electrometer. For an electrometer to be able to count particles, they must be charged — hence a DMA must be used to size select particles in this case. However, it is critical that each particle exiting the DMA has just one charge — or the electrometer count will be inaccurate. The AAC can be used as a pre-classifier for the DMA to ensure the larger particles which would gain additional charges do not enter the DMA:

Using an AAC as a pre-classifier for CPC calibration against an Electrometer ISO 27891

Using an AAC as a pre-classifier for CPC calibration against an electrometer

For an example see Owen and Grisham (2023).

In many ways, the function of the DMA here is just to ensure the particles have a charge for the electrometer. The AAC is itself capable of accurately doing the size classification to a high resolution (whilst making the density based conversion from mobility to aerodynamic diameter for the known test material) . Therefore it is possible to run the DMA with a very broad transfer function (a low sheath flow) which increases the possible maximum size classifiable by the DMA. The low resolution of the DMA does not matter as the AAC can provide the size classification function. For more details, see Symonds (2018).

References

A number of national metrology institutes, universities and laboratories now use the AAC for particle counter calibration. Here are some references:

Federal Institute of Metrology (METAS), Switzerland (2019): Facility for calibration of optical and condensation particle counters based on a turbulent aerosol mixing tube and a reference optical particle counter

University of Alberta (2019): Calibration of optical particle counters with an aerodynamic aerosol classifier

National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST) (2020): Determining the cutoff diameter and counting efficiency of optical particle counters with an aerodynamic aerosol classifier and an inkjet aerosol generator

U.S. Army Primary Standards Laboratory (2023): Tandem aerodynamic aerosol classifier – differential mobility analyzer as a single charged aerosol source for diameters up to one micrometer

ISO 21501-4:2018+A1:2023 Determination of particle size distribution. Single particle light interaction methods. Light scattering airborne particle counter for clean spaces.

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