What PurpleAir Sensors Measure
Measurements taken by PurpleAir sensors include:
- Particulate matter
- Temperature
- Relative humidity
- Pressure
Particulate matter measurements are taken from Plantower laser counters, which provide measurements for particles between 0.3μm and 10μm in diameter. You can view the PMS5003 laser counter data manual for further information. PMS5003 laser counters are specifically used by PurpleAir Classic sensors, with PMS1003 and PMS6003 laser counters used by other sensors.
PurpleAir Flex, Zen, and Touch Indoor sensors use Bosch BME680/688 sensor boards that also measure volatile organic compounds (VOC) emitted as gases. At this time, VOC readings are in an experimental phase. Further details about the BME680 are available in its data sheet. VOC data reported by PurpleAir sensors corresponds to “Static IAQ” found in the datasheet.
VOC readings are experimental, and we are still evaluating what they mean. The first report is available on the SCAQMD’s PurpleAir PA-II Flex VOC Evaluation page. It shows the data may be useful for short spikes or transients but longer events are less detectable.
PurpleAir sensors do not generally measure Ozone, but there are an extremely small number that do as the result of a project with Citizens of Clean Air. The small number is due to the cost of Ozone sensors, which can exceed 500USD for an accurate detector.
Particulate Matter
Can my Sensor Detect Something Specific in the Air?
PurpleAir sensors only differentiate particulates by size. They cannot tell if something specific is in the air.
Why is my Sensor Not Detecting Something?
PurpleAir sensors primarily measure particulate matter (PM) between 0.3μm and 10μm in diameter. This information can then be used when estimating PM levels.
PM estimates provided by PurpleAir sensors include:
- PM1 - this includes particulates up to 1µm in diameter
- PM2.5 - this includes particulates up to 2.5µm in diameter
- PM10 - this includes particulates up to 10µm in diameter
PM2.5 and PM1 estimates from PurpleAir sensors have been found to be highly precise. However, it has been demonstrated that they drastically underestimate PM10 pollution levels.
If something is outside the size range detected by the sensor’s laser counters (0.3-10µm), it may not be detected. If something is above 2.5µm, it may also be underrepresented or not detected.
How PurpleAir Sensors Measure Air Quality
PurpleAir sensors use PMS*003 series laser counters to measure particulate matter in real time (the asterisk ‘*’ represents a number determining the version of the laser counter). 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 PM1.0, PM2.5, and PM10 for standard indoor and outdoor atmospheric particles. The product data manual from Plantower can provide more information.
PurpleAir sensors report real-time data that is updated every two minutes. Since air quality can fluctuate greatly throughout the day, real-time readings from PurpleAir devices may appear “high” when compared to 12-hour averaged data.
It should be noted that while PurpleAir sensors have been found to be highly precise in their PM2.5 and PM1 estimates, it has been demonstrated that they drastically underestimate PM10 pollution levels. As such, we do not generally recommend using them.
Temperature and Humidity
Measurements Taken by the Sensor
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.
This data can be viewed on the map using the “Operating Temperature” and “Operating Relative Humidity” data layers. This is also the temperature data provided from the PurpleAir API, recorded to SD Cards, and locally queried on your network .
Estimating Ambient Conditions
Conversions can be applied to temperature and humidity readings to more closely match ambient conditions. The equations for these are found in descriptions for the following PurpleAir Map data layers:
- Conversions for temperature and humidity developed by Lance Wallace are used in the “Estimated Temperature” and “Estimated Relative Humidity” data layers.
- Simple conversions, which were previously recommended, are used in the “Temperature (Simple Correction)” and “Relative Humidity (Simple correction).”
Humidity readings are less precise than temperature. Thus, although the correction will bring humidity closer to ambient readings, it still may not match closely.
Learn More
Air Quality Index
PurpleAir Sensors Functional Overview
What’s the Difference Between CF1, ATM, and ALT?