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 are 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. A sensor can peak up to 600mA. 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. Power supplies for different countries are available in our online store.

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

Data Usage

PurpleAir devices send roughly 8 Kilobytes of data to our servers every two minutes. Thus, if a device were connected to WiFi for 30 days without interruption, it would send approximately 173 Megabytes of data.

PurpleAir sensors will also send roughly 8 Kilobytes every two minutes to each registered 3rd party data provider.

For example, a PurpleAir sensor registered with Weather Underground as a 3rd party data provider would send roughly 16 Kilobytes every two minutes. This would be approximately 346 Megabytes every 30 days if WiFi is uninterrupted.

Weather Underground is enabled by default on the PurpleAir registration page. This can be disabled or changed to another 3rd party data provider by updating your device’s registration. A second data provider can also be enabled during registration.

What PurpleAir Sensors Measure

This article has moved here: What Do Purpleair Sensors Measure, and How Do They Work?.

How We Measure Air Quality

This article has moved here: What Do Purpleair Sensors Measure, and How Do They Work?.

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.

4 Likes

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.

2 Likes

That is a very helpful response. Thank you.

1 Like

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

It would be interesting to know what the other potential points of failure are, other than the laser sensors which are meant to be easily replaceable.

Most of our sensors last for much longer than two years. However, environmental conditions are different for our various sensors throughout the world. Exposed to certain conditions, the laser counters can become inaccurate due to the presence of debris or even insects/spiders that enter the sensor housing. This is the most common reason for a sensor to begin reporting inaccurately and can often be resolved by performing a few simple cleaning steps.

This is one of the reasons that we include two laser counters in our outdoor sensors. We compare the readings of the two channels to create a measure of sensor health called the Confidence Score, which you can see by selecting any outdoor sensor on the map (additionally, certain users will use outdoor sensors in an indoor environment, in which case, those “indoor” sensors will also include the Confidence Score as they include two laser counters). This Confidence Score is used to determine if the laser counters are reporting accurately.

3 Likes