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Air Particle Counter Sensors use Lasers to Detect PM2.5

particulate matter in air

 

Particulate matter like dust, pollen, mold, smoke, and even metals are in the air we breathe. Too much particulate matter in the air lessens the ability of the lungs to take in oxygen. With repeated exposure, this leads to difficulty breathing, lung disease and even death.

Particles in the Air

For centuries scientists have known about the adverse health effects of breathing contaminated air.

  • Georgius Agricola, a German mineralogist, first described lung disease in coal miners in the 16th century. He found that the disease was caused by coal dust. “Black lung disease” is now widely recognized worldwide as an occupational illness.
  • The widespread use of Asbestos beginning in World War II led to many asbestos-related lung diseases 20 years later.
  • Smog, a term derived from combining smoke and fog is primarily found in big cities and other industrial areas. It is composed of nitrogen and sulphur oxides, ozone, smoke and other particulates. Smog is considered so dangerous that countries have “smog alerts” where city residents are instructed to stay indoors or to only travel wearing a mask.

The understanding of the dangers of particles in the air led scientists to develop laser particle counters to quickly and accurately measure the number of particles in the air.

Particle Size

When discussing particles, it’s important to comprehend how small airborn particles are. For example, a human hair is approximately 50 to 70 um (micrometers or microns) in diameter. Dust, pollen and mold are 10 um or less in diameter, while soot, airborne chemical compounds and metals are in the range of 2.5 um in diameter.

When we think of particles we may think of dust or viruses like Covid-19. In fact, everything you smell is the result of micron-sized particles entering your nose. While our nasal hair and mucus is perfectly designed to trap large particles, smaller ones can evade this filter and be drawn into the lungs. These particles can lodge in the alveoli in the lungs and cause long-term damage.

Laser Particle Counters

particle matter sensor

While there are many different ways to count particles, the most common method is light scattering airborne particle counters. Known as optical particle counters (OPCs), they use a laser directed at a stream of air.

Particles in the air deflect the light. You’ve seen this if you’ve ever looked across the still air in a dusty room on a sunny day. While the sun shows larger particles, a laser particle counter with precision optics is able to precisely measure the size and quantity of the particles.

Inside an optical particle counter is a fan, a laser and a light detector. As the fan pushes the air sample across the laser beam, the particles in the air deflect the laser light.

However, instead of measuring the particles, the photon (light) detector measures the deflections. Like shadows, the diameter of the deflections are measured by photon detectors that convert the size and frequency of the deflections into a signal that tells not only how many particles are in the stream of air, but their size.

Advances in Laser Particle Counter Technology

The need for “clean air” both indoors and outdoors encouraged scientists and engineers to develop new, lower cost optical particle counters using lasers. An early example of low-cost lasers are the common “laser pointers” used in classrooms and “laser scopes” used on hunting rifles. With further modification, lasers have decreased in size and cost to the point that they are now available to anyone.

PM2.5 sensor

An example is the Cubic PM (particulate matter) Sensor. This small sensor contains a laser, fan and photon detector capable of measuring particles in the air between 0.3μ-10μm and real-time output of PM1.0, PM2.5, PM10 in μg/m³.

Overall, the sensor sends 11 different measurements via UART or I2C that can be used to monitor the particle count in real-time including:

  • PM1.0, PM2.5 and PM10 GRIMM mass concentration
  • PM1.0, PM2.5 and PM10 TSI mass concentration
  • Particles count at 0.3um, 0.5um, 1.0um, 2.5um, 5.0um and 10um

GRIMM in Germany and TSI in the US are companies that make laboratory-quality, high-accuracy particle counters. Because of their precision, they are considered as standards for optical particle matter counters worldwide.

 

Photo by Anastasia Shuraeva from Pexels


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