ISO 21501-4 is the recognized standard for optical particle counter calibration. In this blog, we look at the basics of the ISO 21501-4 standard for those new to particle counter calibration.
Why do I need to care about ISO 21501-4?
Your aerosol particle counter is going to need calibration maintenance over its lifetime. In the broad sense, this calibration means more accurate counts that make trending and comparisons possible.
There are four parts to ISO 21501. ISO 21501-4 is titled “Determination of particle size distribution—Single particle light interaction methods” and is specifically for the Light Scattering Aerosol Particle Counter (LSAPC). LSAPCs include the Lasair® III, IsoAir® Pro-E, Airnet® II, etc.
ISO 21501-4 Calibration Requirements
The ISO 21501-4 standard has requirements around the following:
- Sample flow rate
- Counting efficiency
- False count rate
- Particle size setting
What is the sample flow rate?
The standard uncertainty of volumetric flow rate shall be equal to or less than 5%.
If the LSAPC does not have a flow rate control system this ISO 21501-4 subclause does not apply, however the manufacturer needs to specify the allowable limit of its flow rate of the LSAPC. The flow rate calibration is performed to ensure a known volume is presented for counting particles. The standard provides two different requirements based upon the type of instrument being tested:
(1) Instrument with a Flow Control System (i.e. pump or blower)
- Compare the Unit Under Test (UUT) flow to certified reference flow meter
- Passing range is ± 5% of nominal
(2) Instrument without a Flow Control System
- UUT samples from a flow set by certified flow system
- Manufacturer specifies the allowable limits of its flow rate
What is particle counter counting efficiency?
The particle counter counting efficiency is a ratio of the measured particle data between a UUT and a reference instrument. The test is performed using calibration particle standards, known as polystyrene latex spheres (PSLs), with two sizes: one that is close to the minimum detectable reported size range and another that is 1.5 to 2 times larger than the minimum detectable size. An image of PSLs is shown at right. PMS employ Universal Reference Instrument (URI) particle counters that are calibrated against a Scanning Mobility Particle Sizer and Condensation Particle Counter.
The counting efficiency shall be 50% ± 20% for calibration particles with a size close to the minimum.
The counting efficiency shall be 100% ± 10% for calibration particles with a size of 1.5 to 2 times larger than the minimum detectable particle size.
What is the particle counter resolution?
The resolution test verifies the instrument’s ability to resolve small differences in particle size.
The size resolution shall be equal to or less than 15% for calibration particles of a size specified by the manufacturer.
Why isn’t particle counter resolution perfect?
The laser beam’s intensity is highest in the center and degrades towards the edges, as shown in the diagram at left. As a result, particles size larger when crossing through the beam’s center than similar particles crossing the beam’s edge. The Gaussian-shaped curve (shown below the beam pattern) illustrates the resultant light scattering peak as a particle passes through the laser beam. Resolution is improved if the particles are illuminated by the most uniform and highest intensity light. During instrument design and development, PMS optimizes resolution by integrating beam-shaping lenses and masks that can produce more consistent particle sizing, with accurate and repeatable results.
What is the Pulse Height Analyzer (PHA)?
Particle counters employ solid-state photodetectors that convert scattered light energy into electrical signals. The signals, known as pulses, are proportional amplitudes of energy that represent the particles or photons witnessed by the detectors. The pulses are sent to the PHA, and the PHA correlates the relative pulse amplitude to the instrument’s particle size thresholds or bins. Each particle channel is represented by a counting bin and totaled with other pulses that are of the same amplitude.
What is a Particle Counter False Count Rate?
The false count rate is determined by measuring the particle number concentration in the unit of counts-per-cubic-meter at the minimum reported size range when sampling clean air.
No Range Limits
The zero count test is a measurement of “electrical or signal noise”.
ISO 21501-4 requires these specifications to be met when performing the zero count test:
“…the measured particle number concentration per m³ when the LSAPC is set to the minimum detectable size and particle free air flows to the LSAPC.”
In addition, the ISO 21501-4 standard requires that only the count rate be reported. Some standards, including the Japanese Industrial Standard (JIS), do not have this requirement. Going beyond this requirement, PMS particle counter calibrations include a final zero count test, which challenge the instrument to achieve restrictive limits on false counts. The use of restrictive limits, even when not required by the standard, guarantee higher accuracy in the instrument performance and avoids false counts that appear as particle counts.
What is the Particle Size Setting?
The particle sizing error is calculated for the minimum detectable size and additional sizes. The test demonstrates that if an instrument accurately sizes particles, it will also accurately count particles. ISO 21501-4 requires the use of a PHA to determine the particle distribution of three different calibration particles that span the range of particle sizing.
During instrument design, PMS develops a particle sizing curve. The curve defines calibration points and their associated threshold voltages or values. The curve also shows the associated values for other particles, across the entire sampling range of the instrument.
The graph at left shows a threshold/voltage values between the “as found” condition and the calibration’s final result. The sizing error is the difference between the instrument’s reported particle size range and the calculated particle size.
The error in the particle sizes shall be less or equal than 10%.