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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 fluid’s mass flow. The deflection is measured by a
position sensor.
Magnetic flowmeters operate on Faraday’s 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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