I purchased a pair of self contained data loggers, which have a 3 axis accelerometer, 3 axis magnetometer, 2GB of internal storage, and a 250 mAh 3.7v Li-po battery. They measure about 50mm X 26mm X 12mm, and are surprisingly light at just around 10g (not including the housing).
The device is recharged through a micro-USB port (its sole interface), and when connected to a PC it appears as a flash drive. Inside, there is a folder containing recorded data files, a primitive Java application which can help with data analysis and scheduling, and a few text files which can be edited to adjust configuration parameters (like sampling frequency). After reading the device manual (GCDC_X8M-3_User_Manual), I configured the device to sample continuously at 25Hz using a modified configuration file:
With the settings downloaded, it was time to take the XM8-3 for a spin. While recording, I moved the device so that each one of its axes were parallel to the force of gravity for about 10 seconds, and generated about 900 points of raw data (DataPost1). I then scaled the accelerometer data to g units by dividing each data point by 1024 ( no need to call Curtis Jackson) , and used Matlab to generate a plot:
The data was generally as expected. In each position, one axis should have experienced 1g while the other two axes experienced no force. Minor variations can be explained by the surface the device was sitting on during the test not being completely level, which is compounded by the fact that faces of the case with seams bulge out in the middle. This might have impacted the readings in which the X and Y axes were parallel to gravity. In any case, the stated zero-g offset accuracy for the device was +/- .15g for the X and Y axes and +/- .25g for the Z axis, and the data are roughly within those bounds. More important than device accuracy, however, is its precision, since we will be most interested in relative motion in future experiments. Form time t = 2 seconds to t = 12, the standard deviation of the measurements in each axes was calculated: X = 0.012g Y = 0.011g Z = 0.012g. This tight clustering shows high precision in the device.