Metrolab Blog

Particle Counter Technology and the importance of ISO 21501-4 Calibration vol2

Airborne Particle Counter Technology and critical parameters
Particle Counters measure what we cannot see. To get some perspective a 0.5μm particle is about 200 times smaller than the diameter of a strand of your hair. Refer to fig.1 to get a perspective of particle sizes which particle counters count and you will begin to understand the importance of particle counter technology. In order to count particles laser light is used to reflect light off of the particles as they pass through the laser beam. Airborne particle counters using laser diode technology count particles by
collecting scattered light inside the sensor of the particle counter. This scattered light occurs when a particle goes through the “view volume” of the sensor. The view volume is the target location in the sensor where the laser beam and the flow path converge. The laser light generated from the laser diode crosses the view volume and particles are directed through the view volume by an internal air pump.
The flowrate of the internal pump is controlled by a feedback loop and a mass flow controller which keeps the flowrate constant. There are a few options when choosing a Portable Particle Counter when it comes to flowrate. The options are typically 1CFM, 50L/min and 100L/min. A 1CFM flowrate will pull a 1m³ volume of air in under 36 minutes. A 50L/min flowrate will take 20 minutes and a 100L/min flowrate will take 10 minutes to sample a 1m³ volume of air.


The importance of an accurate flowrate – The particle counter sensor is tuned into the
flowrate and so sample flow plays a critical role in the accuracy of the particle sizing. If the flow is too slow the particle dwells longer in the view volume and scatters more light and is perceived as a large particle than it actually is. If the flowrate is higher than nominal flow then the particle dwell time in the view volume is less and the light energy picked up by the photodetector is less than normal and the particle is perceived as a smaller particle than it actually is. Streetlights that magically switch on when it
gets dark at night and switch off in the morning when it gets light are triggered by photodetectors. The photodetectors change in resistivity based on light intensity or lack of is used to trigger the on/off switch for the street light. The photodetector inside a particle counter sensor is more advanced and basically converts the scattered laser light into electrical energy in the millivolt (mV) range. The amount of light scattered is proportional to the size of the particle. Big particles scatter more light than small particles.
This is the fundamental conversion of a physical particle into electrical energy that can be measured. Once measured the mV signal is captured then threshold comparators size the particle and the sized particles fall into different size ranges which are displayed on the particle counter as count data at different sizes. If the flow rate is not within specified tolerance then sizing errors can occur.

Laser integrity and alignment – If the laser light is not running at its optimum intensity then there is an accuracy error factor that comes into play. Laser health should be monitored consistently and error signals communicated if laser levels drop below optimum ranges. If the laser is not illuminated at its nominal working range then there is the possibility of some small particles passing through the view volume undetected. Laser Alignment is also a critical factor. If the laser is not aligned correctly then the
particle count accuracy will be wrong as miss-alignment will also have the same effect of missing particles and the overall result will be inaccurate particle counts. Therefore laser health and alignment is critical and your particle counter should alarm on laser intensity issues and regular service should pick up miss alignment problems which mostly occur under heavy handling of the particle counter.