Recently I followed with a certain interest a project trying to calculate the 3D printers filament consumption during jobs. The ReFILL agent project first published on Hackster.io bases the calculation on the number of rotations of the filament roll detected by an RFID sensor. Then when the filament is near to finish an automatic order can be sent to amazon for replacement.
To be honest the idea has not convinced me too much because it is too complex to manage (every roll should be modified by the user adding a printed RFID tag and the measure is subject to a too big systematic error.
As I have explained in my comments on the Element14 blog post ReFILL-ament for3D printers the base principle is not correct and the systematic error is amplified and distributed up to the finished project.
To manage the math of the filament we should start from a different approach, not the too effimere roll rotation approach. This is the reason I started the 3D printer filament monitor project a couple of weeks ago.