***Please be aware that any data collected from affected devices is safe and completely valid. This is a minor issue that will not alter data that has been collected in any way.***
During manufacturing, each ActiGraph activity monitor is calibrated to compensate for known variations in the sensitivity of the solid state accelerometers from unit to unit. An issue was discovered recently, wherein some incorrect calibration values were applied to an isolated batch of GT9X Link devices after an initial power up and calibration. The issue has since been resolved with a coding revision.
When a GT9X Link device powers up, it reads calibration values from memory to make sure they are not default values, and then uses those values to compute a scale and bias value for each axis. As data comes in from each accelerometer axis, the scale and bias values for that axis are applied to the raw values from the accelerometer, resulting in a more accurate measurement.
Affected devices were properly calibrated, and had valid computed scale and bias values (because when they are powered up originally, calibration values are not default) that inadvertently had the backing calibration values in memory set to default. These calibration values were changed after the device powers up, and are not a factor until the device has been powered down and then powered back up again (at which point the default calibration values would be used, resulting in invalid scale/bias values as well as failing the default calibration check). Once the battery dies the device cannot be reinitialized. Any attempt to do so will result in the screen displaying "Device Error 08", indicating that default calibration values were present at startup.
Are any of my devices affected?
Please refer to the attached spreadsheet at the bottom of this article to see a list of affected device serial numbers. To best look through this list, type Ctrl+F (see picture) and search for any serial numbers you'd like. If we haven't reached out to you already, you most likely will not have any devices on this list. If you find any of your devices to be on this list, please reach out to firstname.lastname@example.org with a list of any affected devices.
What has been done to resolve the issue?
We are sending out replacement units for any that have been found to be affected, and getting all affected units back to ActiGraph. Our engineers have already worked swiftly to remove the bug that produced this issue, so that it will not affect any devices manufactured in the future.
How am I impacted?
Your data is not affected by this issue. If you were able to collect data with an affected device, the data is valid because the correct calibration values are applied originally. The incorrect calibration values do not become a factor until the device has been restarted (due to firmware update, loss of battery power, etc.). In the event that a restart has occurred, safeguards are in place that prevent devices with incorrect calibration from collecting data. When initialization is attempted after a restart, "Device Error 08" will be displayed on the device's screen and any attempt to initialize it will fail. Data cannot be collected on an affected device that has been restarted.
What do I need to do?
If you haven't been contacted by us regarding this issue, then chances are you are not affected; as this only affected an isolated batch of GT9X Link devices. If you notice that any of your devices are displaying "Device Error 08" on screen, please get in touch with us and list the serial number for any devices showing this (serial numbers are on the back of the device, starting with "TAS").
If you have any further questions, you can contact us via email (email@example.com), or give us a call at (877) 497-6996, Monday through Friday, 8 am to 5 pm CST.