SD Card Overview

Sensors Troubleshooting
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PurpleAir sensors with SD logging capability (Classic and Classic Plus with SD capability, which both come with a microSD card, and Flex and Zen sensors) can record air quality data directly to a microSD card, providing offline data collection that doesn’t require WiFi connectivity. This overview will cover the installation process, as well as general information you may want to know about using SD cards with your PurpleAir sensor.

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What Are SD Cards and Why Should You Use Them?

SD (Secure Digital) cards are small storage devices that PurpleAir sensors can use to record data locally, without requiring an internet connection. This makes them ideal for:

  • Remote deployments where WiFi may be unavailable or unreliable

  • Long-term data collection without depending on network connectivity

  • Backup data storage in addition to cloud uploads

  • Offline research where you need complete control over your data

How SD Logging Works

When a microSD card is used with one of our sensors:

  • Data is recorded every 2 minutes to the microSD card

  • Data is written in CSV (comma-separated values) format

  • Each day’s data is typically stored in a separate file named by date (e.g., 20240415.csv)

For a complete reference of all SD data columns, see Understanding SD Card Data Columns.

Important: SD card data is not uploaded to the map; it is separate. If you want data to be uploaded to the map/API, your sensor will need a WiFi connection as well.

SD Card Requirements

Before purchasing or using a microSD card with your PurpleAir sensor, ensure it meets these specifications:

Card Type:

You will need a microSD card, not a full-size SD card.

Format: FAT32

Your microSD card must be formatted as FAT32. Cards formatted as exFAT, NTFS, or other file systems will not work with PurpleAir sensors.

You can purchase pre-formatted cards from PurpleAir, or format your own card using your computer. There are online tutorials available for confirming the current format of a card and formatting it to FAT32 on both Mac and Windows.

Important: Reformatting a card will erase all data currently on it

Capacity: Up to 32GB Maximum

PurpleAir sensors support microSD cards up to 32GB in capacity. Larger cards (64GB, 128GB, etc.) will not work.

  • A 32GB card can hold approximately 4 years of data

  • Once the card is full, the sensor will stop recording (it will not overwrite old data)

Speed Class: Minimum C10/U1/V1

Your SD card must have a minimum write speed of 10 MB/s (indicated by C10, U1, or V1 speed class markings). Slower cards may cause data loss or recording failures.

For more details on speed classes, visit the SD Association’s Speed Class page.

Setting Up Your SD Card

Step 1: Format the Card

Note: Formatting will erase all data on the card. Make sure you’ve backed up anything important before proceeding

If you’re using a new card or one that’s been used elsewhere:

  1. Insert the SD card into your computer (using an SD card reader if needed)

  2. Back up any existing data on the card if needed

  3. Format the card as FAT32

  4. Ensure the card is completely empty after formatting

Step 2: Insert the Card into Your Sensor

  1. Locate the SD card slot on your sensor (varies by model)

  2. Insert the microSD card fully with the label facing out, until you hear and feel a click

Cards can be inserted halfway—make sure it’s fully seated

Step 3: Verify SD Card Detection

After inserting the card, observe the sensor’s LED behavior underneath the unit. When your sensor has power and a microSD card inserted, it should flicker a blue color only once every 2 minutes as it writes data to the card.

If you don’t see a blue flicker every 2 minutes after inserting a microSD card or you see any other unexpected LED behavior, proceed to the next section on Understanding SD Logging Behavior with Sensor LED Indicators.

Understanding SD Logging Behavior with Sensor LED Indicators

LED Indicators

The blue LED on your sensor provides important feedback about its SD card status, and there are two primary LED indicators to observe:

  1. Flickers every ~2 minutes: Normal operation, data is being written as expected.
  2. Flickers three times in quick succession repeatedly: SD card is not detected. If you see this indicator, go to and proceed from this section on your SD card not being detected in our SD troubleshooting article.

If you’re seeing any other LED indicators that seem unusual, refer to the following list of indicators.

  • Flickers more frequently than every 2 minutes erratically: Excessive amounts of data being logged. If the blue LED continues flickering erratically without a distinctive pattern for a couple of minutes after inserting the microSD card, review our troubleshooting steps for this here: Blue LED Flickers More Frequently Than Every Two Minutes

  • Green LED constantly flashing: Sensor may be crashing

  • If the green LED continues flickering several times, review our troubleshooting steps for this here: Green LED Flickers Frequently

Best Practices for SD Card Data Collection

During Deployment

  1. Verify SD Data is Being Written** by visually identifying that the blue LED flickers every 2 minutes

  2. Ensure consistent power, as data cannot be recorded without power

  3. Periodically check your sensors’ SD status via the map if WiFi is available

  4. Sync the clock by connecting to WiFi occasionally (prevents timestamp drift)

  5. Monitor card capacity, and plan to retrieve data before the card fills up

After Retrieval

  1. Use our SD Data Tool to recover all available data and ensure it appears as expected
    a. You can download our Utility software needed to use our SD Data Tool here
    b. Instructions for using the SD data tool in that article are here
  2. Back up your data once you’ve retrieved it

You may also find value in reviewing our best practices for data collection with our article on Data Collection and Reliability

Learn More

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