What Do PurpleAir Sensors Measure and How Do They Work?

Sensors
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What PurpleAir Sensors Measure

Measurements taken by PurpleAir sensors include:

  • Particulate matter
  • * VOCs
  • Temperature
  • Pressure
  • Relative humidity

These and other derived data can be viewed on the PurpleAir Map by opening map settings and selecting the appropriate data layer.

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.

* Not all PurpleAir sensors report VOC data. Read the VOC section below for more.

Particulate Matter

Particulate matter measurements are taken from Plantower laser counters, which provide measurements for particles between 0.3ÎŒm and 10ÎŒm 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.

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 0.3ÎŒm and 10ÎŒm 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 1”m in diameter
  • PM2.5 - this includes particulates up to 2.5”m in diameter
  • PM10 - this includes particulates up to 10”m 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 (0.3-10”m), it may not be detected. If something is above 2.5”m, 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.

VOCs

Certain PurpleAir sensors use the Bosch BME68X (680 or 688) to measure volatile organic compounds (VOC) emitted as gases.

The BME688 is used on all new sensors. Older sensors may instead use a BME680. There is currently no difference in how the sensor uses the BME680 vs the BME688.

The Bosch BME680 measures TVOCs, which is a measure of all VOCs that may be present. In PurpleAir’s current implementation, the BME68X can’t tell you which VOCs are present.

  • The PurpleAir Flex, Zen, and Touch sensors include VOC readings by default.
  • Classic sensors can be upgraded by purchasing the necessary BME board. Send an email to contact@purpleair.com for more information.

VOC data reported by PurpleAir sensors corresponds to “Static IAQ” found in the BME688 datasheet.

What do the VOC Readings Mean?

PurpleAir VOC data is reported using our firmware-specific implementation of the BME680/688 sensor. It is by no means the only way to interpret or configure the BME680/688 sensors. PurpleAir uses a basic interface with the BME680/BME688 sensors and does not implement “AI gas training.”

VOC readings from PurpleAir sensors remain experimental as we continue to assess their accuracy and applicability. The first evaluation report from SCAQMD is available on the PurpleAir PA-II Flex VOC Evaluation page. The report suggests that while the data may effectively capture short-term fluctuations, it is less reliable for detecting prolonged events.

The BME’s data sheet provides a scale for IAQ readings. However, based on the findings from the South Coast Air Quality Management District’s evaluation, we do not currently apply this scale to PurpleAir sensor readings.

Given these results, we will continue labeling Bosch Static IAQ as “Experimental.” We plan to review our firmware implementation using the BME data in which we will explore improvements to enhance its accuracy and reliability.

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?

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