Flow measurement
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The flow rate of a fluid can be measured in a variety of ways. In fluid mechanics, flow rate is usually given the symbol Q.
The frequency of rotation of a turbine, anemometer or other rotary mechanism immersed in the passing fluid is linearly related to the flow rate over a range of flow rates, allowing simple measurement of the flow rate.
Another method of measurement, known as a venturi meter, is to constrict the flow in some fashion, and measure the differential pressure that results across the constriction. This method is widely used to measure flow rate in the transmission of gas through pipelines, and has been used since Roman Empire times.
Another simple method of measurement uses an orifice plate, which is basically a plate with a hole through it. It is placed in the flow and constricts the flow. It uses the same principle as the venturi meter in that the differential pressure relates to the velocity of the fluid flow (Bernoulli's equation).
A variation on this method is to measure the pressure changes that are created by the vortexes that are caused by the flow passing an object. It is usually measured with a quartz crystal. This kind of flow-meter is called a vortex-meter.
Measurement of the pressure within a pitot tube in the flowing fluid, or the cooling of a heated element by the passing fluid are two other methods that are used. These types of sensors are advantageous in that they are rugged, so not easily damaged in an extreme environment.
A pitot tube is an L shaped tube which is also able to measure fluid flow. An advantage is that it does not disturb the flow as much as a venturi meter or an orifice plate would. It works by measuring the difference between the static pressure and the dynamic pressure.
Modern innovations in the measurement of flow rate incorporate electronic devices that can correct for varying pressure and temperature (i.e. density) conditions, non-linearities, and for the characteristics of the fluid.
Measurement of the doppler shift resulting in reflecting an ultrasonic beam off the flowing fluid is another recent, accurate innovation made possible by electronics.
Companies selling flow measurement devices
Sierra instruments inc. (http://www.sierrainstruments.com/) mcmillan flow (http://www.mcmflow.com/) hedland (http://www.hedland.com/) Rosemount flow technoligies (http://www.rosemount.com/products/flow/) water control flowmeters (http://www.environmental-expert.com/resulttechquery.asp?cod=Water%20/%20Wastewater%20Control:%20Flowmeters) www.flowmeters.com/