PurpleAir sensors equipped with an SD logger can log data offline to a microSD card. These are our Classic-SD, Flex, and Zen models; the normal Classic and Indoor Touch do not have SD capability. When an SD card is inserted correctly, a blue LED will flicker roughly every two minutes. This indicates data is being logged correctly.
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Whenever you insert a microSD card or reboot the sensor, you’ll want to watch the SD LEDs to ensure the SD card is properly detected. Upon starting, the SD LEDs should flicker a few times and then remain quiet. This indicates that the SD card was detected correctly.
microSD cards must be formatted to FAT32 to work with PurpleAir sensors.
LED Behavior
If your PurpleAir sensor displays any SD LED behavior other than what is described above, it could indicate an issue. Below are some other LED behaviors you might experience.
Blue Light Flickers More Frequently Than Every Two Minutes
The blue light flashing means data is being written to the SD card at that moment. If the blue light flashes more often than what is described above, the sensor could be outputting supplemental status information such as WiFi connection and disconnection, uploading of data, or other information that is not necessary to record. The logging of this additional information can cause the SD card to reach capacity much sooner than you might anticipate. If you experience this issue, you’ll want to check the SD card to ensure it’s not at capacity and no data is being lost. If it is at capacity, you can delete files from the SD card to make room.
Blue Light Flickers Three Times in Quick Succession
If the blue light is flickering three times in quick succession, it means that an SD card isn’t being detected correctly. This will happen when no SD card is inserted.
However, if it occurs when an SD card is inserted, you’ll want to try the following:
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Ensure the microSD card is inserted correctly. It is possible that an SD card can be inserted halfway. You should feel a click when it is fully inserted.
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Power cycle the sensor with the SD card inserted. If that is unsuccessful, remove the SD card, power cycle again, and then reinsert it.
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Check that the inserted SD card is formatted to FAT32.
A Green LED is Present
You might see a green LED, which indicates that the logger is being initialized or that the sensor is starting up/rebooting. The green LED should only flicker once on startup. If it is flashing more often, it is most likely crashing. Connecting your sensor to the PurpleAir Utility and viewing its logs can help you troubleshoot this further.
If you have a device with full-color AQI LEDs (the Flex, Zen, or Touch), these are separate from this green light.
Connecting to WiFi
If you are unsure about anything above or think your sensor could be experiencing a different issue, you can connect it to WiFi to see if the Map reports any errors. Below are examples of two different sensors’ pop-up windows on the PurpleAir map. On the left is a window for a sensor whose SD card is detected and logging data properly. The window on the right is for a sensor whose SD card is not detected.
Additionally, connecting to WiFi will automatically update the sensor if a firmware update is available. Keep in mind that it is recommended to perform a power cycle after inserting a microSD card.
Reviewing SD Data
When looking at the data recorded to your SD card, a few different issues can occur. These issues usually involve data appearing as incorrect or missing.
Why Does the SD Data Have an Incorrect Date or Time?
The timestamps in your SD data might appear incorrect for a couple of reasons.
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Dates are recorded in UTC and will not reflect your timezone. Check for the differences between UTC and your timezone to see if the timestamps are accurate.
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SD-capable sensors include a real-time clock to keep track of time even when powered off. If the dates recorded are in the years 1970 or 2000, then your sensor’s real-time clock will need to be resynced. This can be done by connecting your device to WiFi. When connected, the sensor will attempt to synchronize its time over the internet. You can check whether this is successful by checking the WiFi Status Indicator for NTP.
If the dates are still off after accounting for UTC, try the following troubleshooting steps:
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Move the SD data onto a computer and reinsert the SD card to verify that incorrect dates are still being recorded. Dates previously written to the SD card are not updated when time is synchronized.
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Power cycle the sensor by unplugging it for 30 seconds and plugging it back in.
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If the above steps don’t work, please email contact@purpleair.com. It’s possible that your sensor may need a new SD logger or that there is a board issue with the device.
Why Are There Gaps in My SD Card Data?
If no data is being recorded to the SD card, you’ll want to follow the other troubleshooting steps in this article. However, if there is data on the SD card, but there are gaps where nothing seems to be reported, sensor data may have been improperly recorded within .TXT or .log files. You can install the PurpleAir Utility and use its SD Data Tool to search for any recoverable data in these other files.
If the SD data tool doesn’t recover the missing data, it is likely that the sensor was without power during those data gaps. View our Data Collection and Reliability article for tips on preventing data loss.
Frequently Asked Questions
Do I Need to Get My SD Cards from PurpleAir?
We offer 32GB SD cards in our store that are pre-formatted to FAT32. You do not need to purchase your SD cards from us, but you must ensure that the cards you are going to use are formatted to FAT32. There are online tutorials available on how to do this for both Mac and Windows.
If you are not going to purchase from us, we recommend using a card with a minimum writing speed of 10 megabytes per second or a Speed Class of C10/U1/V1. The SD Association explains more about Speed Classes in this article. The maximum supported microSD card capacity is 32GB.
A 32GB card is usually able to hold about 4 years worth of data.
Can I Push SD Card Data to the Cloud?
At this time, when the sensor is out of network range and recording to the microSD card, the data is solely stored on the card. When you come back into the network range, the data that was logged offline is not synced up with the cloud. The microSD card would have to be pulled to extract that data.
Why Does the SD Card Fall Out or Not Insert Properly?
If you cannot hear a click when inserting the SD card, the logger’s latch may be broken.
Another possible issue, though rare, is that the SD logger is inserted into a bad plastic bracket. This was corrected in May 2023, so only sensors before that date can be possibly affected.
In either of these cases, the problem is that the SD card is not being held in the logger. You can try securing the SD card into the logger for a temporary solution, though getting a replacement logger is what would permanently fix the problem.
If you would like a replacement SD logger, send an email to contact@purpleair.com with a description of the issue and your sensor’s device ID.
What are the SD Card File Headers?
You can learn about SD card file headers here: SD Card File Headers
Learn More
How to Obtain Sensor Data
The PurpleAir Utility
SD Card File Headers