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Tampering with Mass Flow Meters: "The magnets caused the meters to record a higher amount of fuel than what was actually delivered to the buyer vessel"
https://www.channelnewsasia.com/singapore/southernpec-bunker-marine-fuel-oil-cheat-mpa-spf-2211806
Mass Flow Meter looks fascinating inside ... Tampering with a Magnet looks plausible
https://en.m.wikipedia.org/wiki/Mass_flow_meter