Flowmeters have been in use for a long time, and up until recently were mostly mechanical devices, but recent advances in electronic technology have given us the Laser flow meter
, (sometimes referred to as an Ultrasonic flowmeter) which uses two laser beams to determine the velocity of small particles present in liquids and industrial gases as they pass through the flowmeter. Two laser beams (focused across the flow a short distance apart) are scattered by particles crossing the the beams with the amount of scattering indicative of the speed of the flow Their accuracy is derived from the fact that their speed is not dependent on thermal conductivity of gases, variations in gas flow or composition of gases and thus these factors have little effect on these flowmeters.
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Risks associated with the microbiological quality of bodies of fresh and marine water used for recreational purposes: summary estimates based on published epidemiological studies