PurpleAir Sensors Functional Overview

PurpleAir makes sensors that a community of citizen scientists use to collect hyper-local, real-time air-quality data and share it on a map that is accessible to everyone. This category will establish a necessary overview for understanding your PurpleAir sensor.

WiFi Range

The WiFi range is typically about 100 feet. This range can vary depending on anything that impedes a direct path between the sensor and WiFi router. Learn more about what may affect WiFi range from the article, “How Far can a WiFi Signal Travel?”, by Actiontec. Prior to mounting your sensor, use a WiFi-enabled device to test your WiFi’s signal strength at the potential installation location. If you’re having trouble obtaining a signal, slightly adjust your router’s antennae or alter your sensor’s position.

A Note on Temperature and Humidity

The temperature and humidity data are for the sensor itself, and not meant to reflect environmental readings. Heat generated by the WiFi module causes an increase in temperature, and a decrease in humidity within the housing. We’ve found that a correction of 8°F correlates closely to the ambient temperature, while a 4% correction correlates closely to the ambient humidity. We have adjusted these values on the PurpleAir map to reflect this.

Power Consumption

All PurpleAir sensors run on 5 volts and the current draw is 180mA. This means the power consumption of a sensor is approximately 1 watt. The daily consumption would be 0.0216 kWh.

A PurpleAir sensor’s power supply will work with an AC input of 100-240V, and uses a Type A (USA style) plug.

Data Usage

The sensor sends roughly 8 kB every two minutes, and may use up to 526 MB per month if it was registered to send data to two 3rd party services (Weather Underground, Microsoft Power-Bi, Microsoft Azure, or custom data processor).

What PurpleAir Sensors Measure

At this time, standard PurpleAir sensors measure particulate matter, temperature, humidity, and pressure. However, we do have a limited number of Bosch BME680 sensors that also measure volatile organic compounds (VOC) emitted as gases. We are still testing the gas sensor to determine if the data it produces are valid. Further details about the BME680 are available from the data sheet. If you’d like to purchase one preinstalled in sensor, or to add the BME680 to your current sensor, please contact us here.

LED Brightness Control

The LEDs in your PurpleAir sensors can be turned off or have their brightness changed. To learn how to do this, view the article Change the LED Settings on your Sensors.

PA_LEDBrightness

How We Measure Air Quality

PurpleAir sensors use PMS*003 series laser counters to measure particulate matter in real time. Each laser counter within a pair alternates five-second readings averaged over two minutes. Each laser counter uses a fan to draw a sample of air past a laser beam. The laser beam is reflected off any present particles onto a detection plate. The reflection is measured as a pulse by the detection plate. The length of this pulse determines the size of the particle. The number of pulses determines the particle count. These particle measurements are used to infer the mass concentrations of \text{PM}_{1.0}, \text{PM}_{2.5}, and \text{PM}_{10} for standard indoor and outdoor atmospheric particles. View the product data manual from Plantower.

PurpleAir sensors report real-time data that is updated every two minutes. Since air quality can fluctuate greatly throughout the day, the real-time PurpleAir AQI reading may appear “high” when compared to 12-hour averaged AQI data.

Life Expectancy

The life expectancy of a PurpleAir sensor is roughly two years, although some run much longer than that. Regarding the laser counters themselves, much like the cartridge in a printer, these have a lifespan and may need to be replaced over time.

Security

To enhance firmware security, there is no way to load firmware onto PurpleAir sensors over a local network; any updates must come from PurpleAir’s own server as a response to a sensor initiated request. Additionally we’ve made it impossible to change any settings on the sensor with the exception of WiFi settings (SSID and password for the WiFi network).

For those concerned about network security, we recommend connecting security-sensitive devices to their own SSID (WiFi network) or fire-walling them from the rest of your local network.

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Can you share if you are also using the Bosch Software Environmental Cluster (BSEC) with the BME680 output? Or are you using/developing some other sensor output and conversion algorithm? If you are using the BSEC then I suppose what we see as VOC readings from our PurpleAir sensors corresponds directly to the IAQ values in Table 4 of the datasheet.

I think you are not using the BSEC algorithm as I have seen values well over 500 on the map, and the Bosch IAQ scale is 0-500. [I would also imagine that the BSEC is a pretty pricey software solution to license].

Thanks!

We do use BSEC, but the problem is that it is an early adoption that does not store the calibration data. This was due to the sensor not having enough free FLASH. We do have a solution but it requires updating to an interim version, then the more memory version. We will do this soon and also implement a proper BSEC driver.

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That is a very helpful response. Thank you.

As a new owner of one of these devices, this was a pretty disappointingly short number to see. Can you give us a bit more resolution on when sensors and cartridges fail and what indications we should be looking for, please?

Agreed, I was surprised by the number as well. My original PA-II has been running for several years, and makes me wonder about the fidelity of the data now.

Regards,
Robert