Which Sensor can I use to measure the material depth in a Ball Mill ?

Master one or at the maximum two in each domain. Experiment as much as possible to master them.   In  2D: AutoCAD (Must) In3D: Solidworks, Solidedge, Inventor, Catia, Pro E etc ( Master any one) In FEM: Ansys/ Nastran/ Comsol etc (Try to learn one) All tools are similar. And if shift is required you can very easily make the transition in a week or so. Also always keep updated. With every latest releases, a lot of addition is happening. Software wont attack us. So freely exploit them to the core. Discussion forums are also a best place to get updated.