Optical Flow Meters use light to determine flow rate. Small particles which accompany natural and industrial gases pass through two laser beams focused a short distance apart in the flow path in a pipe by illuminating optics. Laser light is scattered when a particle crosses the first beam.
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You can study Thermodynamics at the University of Michigan via Coursera for free! The course description: Introduction to Thermodynamics: Transferring Energy from Here to There.