Two other non-contact methods I've seen are direct mass measurement (probably ebay the scales, it'll be like $500 new) or the most innovative thing I've seen is flow rate meters watching the CO2 source. See you can measure flow rate of the beer but that can get weird with bubble formation and sterilization but CO2 is clean and biologically boring and single phase. You can buy a flow rate meter with digital output for immense expense but its easier to use an analog one with a webcam. Big fun!