Please note: This article only refers to Purple Classic SD sensors (PA-II-SD).
PurpleAir sensors sample particulate matter every second. These samples are averaged in 2-minute intervals, which are sent in data reports to our database. Those 2-minute reports are the smallest frequency (“real-time”) averages you can see on the map. Sensors that have an SD logger and an SD card will store these 2-minute reports on the SD card as well.
Occasionally, a user will have an interest in storing data on the SD card at more frequent intervals. This can be done with some slight modifications to the sensor’s hardware and one of the files on the SD card.
For example, one of our users was performing research on a wildfire by attaching one of our sensors to a drone and flying the drone over the fire. In this case, the second-by-second data was important to the researcher.
Step 1: Hardware Alteration
You will need to disconnect the sensor from power and remove the components from the sensor housing. Once this is done, you will cut the black wire that connects the SD logger to the mainboard.
Step 2: SD Card Alteration
Next, you will remove the SD card from the logger and connect it to a computer. On the SD card, you will find a file called “config.txt”. The contents of this file need to be edited.
Original:
115200,26,3,0,1,1,0
baud,escape,esc#,mode,verb,echo,ignoreRX
After alteration:
115200,26,0,0,1,1,0
baud,escape,esc#,mode,verb,echo,ignoreRX
Note that the third value of the first line—corresponding with “esc#”—needs to be changed from “3” to “0”.
How Will the Data Appear
After these two steps have been completed, you can insert the SD card back into the sensor, return the components into the housing, and power on the sensor. The SD card will no longer store data in CSV format. Rather, all data will be stored in text files called “LOGXXXXX.TXT”. For more information about how to understand what this data output means, you can refer to this article: Understanding PurpleAir Serial Output .
Additional Information
Be aware that making the above alterations will change how the sensor stores data on the SD card; it will not change how frequently the sensor sends its 2-minute averages to our database. These will still be sent at 2 minute intervals.
The above alterations can be undone by soldering the black cord back to its original location on the logger and reverting the change in the config.txt file.
A full reference for the config.txt file can be found here: