Calibrating Light-Scattering Air Particle Counters

ISO 21501-4 is a new type of international calibration standard that deals with periodic and performance calibration of light-scattering air particle counters (LSAPC or APC) throughout the service life of the instrument. This international standard is based on regional standards and recommended practices characterized by a wide range of tests, and defines the performance parameter limits applied in routine periodic calibration.

The purpose of this standard is to reduce any inaccuracies in the results of measurements carried out using the particle counters, and variations in the measurement results of different instruments. This article discusses the different parameters of the methods, required limits and standards in addition to a discussion on the combined uncertainty of calibration.

Particle Counting Technology

Particle detection technology is based on the light scattering principle. When a particle is illuminated with a light wave, the radiation is scattered at the frequency similar to that of the incident light. Figure 1 shows the design of a typical optical particle sensor consisting of the primary elements.

The scattered light is focused on a photo-diode and collected using a set of mirrors or lenses. An electric pulse is created from the scattered light of each particle on the photo-diode. Figure 2 illustrates the process of amplification of these particles using an analog to digital converter.

Figure 1. Typical light scattering air particle sensor. Image credit: Beckman Coulter

Figure 2. Conversion of electrical pulses to particle counts. Image credit: Beckman Coulter

Calibration of APC Instruments

Most of the APCs are calibrated using NIST traceable polymer microspheres or calibration spheres. Some of the parameters of the spheres are certified by the Manufacturer.

In order to calibrate an APC, a wide range of standard calibration spheres are used in the particle sensors at a controlled rate through an aerosol nebulizer, such that the standard particles are atomized, dried and maintained in a controlled concentration. Figure 3 shows the statistical parameters relevant to NIST standard parameters.

Figure 3. Statistical parameters relevant to NIST standard particles. Image credit: Beckman Coulter

The distribution of pulse heights generated by the microspheres at the time of each calibration sphere challenge are collected and displayed on a Pulse Height Analyzer (PHA). Figure 4 shows the example of a PHA output of a mono-sized calibration sphere challenge.

Figure 4. Typical output form a PHA connected to an APC being challenged with calibration spheres. Image credit: Beckman Coulter

Using the relationship between pulse height voltages for each particle in the dynamic range of the sensor, a calibration curve can be obtained. An example of APC calibration curve in shown in the Figure 5.

Figure 5. Example APC calibration curve. Image credit: Beckman Coulter

Performance Metrics of Particle Counting Instruments

ISO 21501-4 defines certain performance parameters such as flow rate, false count rate, size resolution and counting efficiency, in addition to the actual size calibration.

Counting Efficiency

When the APC is introduced with calibration spheres with size similar to that of a particular APC size channel threshold, only 50% of these spheres will be recorded to be greater than that of the size channel.

However, in real time, the probability of a particle transmitted via the sensor producing pulse exactly equal to the channel size threshold will be 50%, which is illustrated in Figure 6.

Figure 6. Explanation of Counting Efficiency. Image credit: Beckman Coulter

Moreover, ISO 21501-4 specifies the point at which the first channel must reach 100% +/-10% counting efficiency. Figure 7 shows the ISO 21501-4 specifications for channel 1 counting efficiency.

Figure 7. ISO 21501-4 specifications for channel 1 counting efficiency. Image credit: Beckman Coulter

Following the calibration of APC size channels to the 50% counting efficiency point, it is necessary to verify the count with the comparison against a 100% counting efficiency reference APC. Figure 8 shows an example illustrating the verification of APC counting efficiency.

Figure 8. Verification of APC counting efficiency using a 100% counting standard. Image credit: Beckman Coulter

Sizing Resolution

Sizing resolution is defined as the difference in the size of the particles measured by the optical particle sensor. Variance around the mean size of the particle is associated with the variance in pulse heights produced by the sensor with respect to identically sized particles. The size resolution of a particle counter is not more than 15% for the ISO 21501-4 standard.

Size resolution of the particle is calculated using the Equation 1:

where

R = Sensor particle size resolution in %
χ_ρ = Particle size of the calibration particles
σ_P = Standard deviation
σ = Observed standard deviation measure on the PHA

False Count Rate

False count rate or zero-count rate is defined as the number of false counts recorded by the APC with respect to the time. The false particles are associated with the number of intrinsic and extrinsic sources to the system such as cosmic radiation, optoelectronic noise or contamination.

Sampling Flow Rate and Sample Time

Particle counts are measured by APC instruments as a function of concentration per unit volume, for instance, particles per cubic meter or cubic foot. Sample flow rate accuracy is, therefore, critical to mitigate the occurrence of flow rate errors caused while sampling actual volume for a fixed sample time. Sample time accuracy is also critical for measuring the sample volume at a given sample rate.

Conclusion

ISO 21501-4 standard ensures more sophisticated calibration systems which provide accurate characterization of particles. Calibrating APC to this standard requires a thorough insight of the calibration process and associated parameters in a statistical manner.

This information has been sourced, reviewed and adapted from materials provided by Beckman Coulter, Inc. - Particle Characterization.

For more information on this source, please visit Beckman Coulter, Inc. - Particle Size Characterization.