Flow Sensors


  There are many different technologies used in the measurement of flow. The choice of the most suitable type depends on a wide range of factors including the nature and viscosity of the fluid, the flow rate range and the accuracy of measurement required. Below are a brief description of the main types...



    Differential pressure flowmeters insert an obstruction in the flowstream to reduce the flow rate and thus the pressure. Flow rate is calculated by taking the difference between upstream and downstream pressures.
    Positive displacement flowmeters capture a liquid sample in a small container and calculate flow rate by counting the number of captures and fills.
    The rotor in turbine flowmeters spins at a rate proportional to liquid flow rate.
    Variable-area flowmeters typically contain a float that is forced upward by the fluid passing through. The point of equlibrium between this force and that of gravity indicates the volumetric flow rate.
     As a liquid or gas passes through the (usually) bent vibrating tubes of a Coriolis flowmeter, the tubes twist to a degree proportional to the fluidís mass flow. The deflection is measured by a position sensor.
    Magnetic flowmeters operate on Faradayís law of electromagnetic induction and therefore work only with conductive liquids. A current applied to coils mounted on or outside the flow pipe generates a magnetic field inside the pipe. The liquid passing through generates a voltage proportional to flow rate, which is detected by electrodes on either side of the pipe.
   Vortex flowmeters measure flow by placing a bluff body across the stream. Flow rate is calculated by multiplying the pipe area by the flow velocity, which is proportional to the frequency of the vortices generated by the bluff body. These devices are used with liquid, gas, and steam.
    Multivariable differential pressure transmitters, when attached to or integrated with a primary element, measure temperature and/or pressure and use these values to calculate mass flow.
    Thermal flowmeters measure mass flow directly. Some of these devices put heat into the flow stream and measure how long it takes to dissipate; others measure the amount of energy required to maintain a constant temperature in the stream. They are used for low flows, primarily gas.


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