PM2.5 Monitor

A brief on PM2.5

  • Make a portable PM2.5 monitor for less than $40.
  • Parts are low-cost and with resonable quality. It should be fine for personal uses.
  • Functionality can be extended from base model to transfer data to MQTT server, integrate other sensors

If you want to move on with part and wiring, go the page 2.

1. What is PM2.5?

In the world of abbreviation, we are hardly to keep track of what people are talking about by three-letters sounds. But this is an important one. PM2.5 is a collection of suspended particulate matter (PM) in the air with diameter no more than 2.5 μm. The smaller end of size is 0.1 μm. PM2.5 is often called fine particles to differentiate coarse particles and recently, ultra-fine particles.

scale Courtesy of U.S. EPA

With small size and relatively long half-life in the air, lasted from hours to days, PM2.5 is imposing health threats to almost everyone. If you live in city, traffic fumes from engine combustion, residental and industrial burnings make an obvious case for air pollution. People live in developing countries are in a direr situation, in part because of trade-offs of using cheap fuels such as coals, biomass and almost no end-pipe treatment in motobikes' exhaust, not to mention old buses and trucks to reduce cost of production in a short term.

Air pollution from fine particles is not limited to city dwellers. In rural area, there is episodic biomass burning after harvest seasons. Daily uses of biomass residuals for cooking causes eye irritation at the least. Indoor environment is not be guaranteed to safe either. Smoking is one. Cooking method such as frying also releases fine particles in smoke form.

Looking back, I often used solder without fume extraction, and some days I can smell it after taking shower. Soldering actually releases smoke and if you have a PM2.5 monitor, you could see the reading jumps up quickly. Calling PM2.5 particles leads to some misconception as well. It is really size of smokes and fumes. In research, PM2.5 is often called "aerosol" because of a mix-composition and well-suspended in the air. So, if you are in DIY, and make electronics yourself, you may need to know the concentration of PM2.5 in your room.

Adverse effects of PM2.5 is clearer with elderly, those with respiratory deficiency, and chronic lung diseases. In Air Quality Index (AQI), an representation of level of air pollution from six pollutants in the air, vulnerable people are placed in warning with a lower concentration. With that, I hope to make a case that making a PM2.5 can be useful and put your hobby in electronics into practice.

2. How PM2.5 is measured?

Standard methods using weight change of a filter with a constant flow rate called gravimetric method (Method 201A). Approved monitors by U.S EPA is used for continuous monitoring mostly use the attentuation of beta rays absorbed into a filter by suspended matters. I wrote another brief on different methods is here.

Low-cost sensor used a method called Light scattering, in which a laser beam is diffused when shining to a flow contains small particles. The diffusing light is detected and with a conversion equation converted to a number of particles in the flow. The number of particle is subsequently converted to the concentration in micro-gram in cubic meter. To keep the accuracy of the sensor is a long the way, both by electronic components and by appropriate adjust factor between the counting number vs. observed concentration from a known PM2.5 monitor. In a base case, PM2.5 monitor can be used to track the trend of concentration.

Below is the inside Nova Fitness SDS011, a popular choice if you need more fan power for sampling. Although I called this a low-cost option, but not the lowest ones. Sensors with a lower cost even uses a 5mmLED with a lens as the light source instead of a laser diode. Those sensors also have no fan. I attempted to build with a cheaper dust sensor, but so far the reading is not stable.

sds011

Sensor with light scattering method can produce readings in every seconds. For PMS7003, each reading takes 700ms. In practice, you would not need such resolution. The particle concentration changes slowly over a few seconds to minutes. If data is collected for further analysis, collecting high-resolution data could be cumbersome for clean up.