Flow measurement of fluids, including air and liquids, are measured
in a variety of ways. Each type of measurement technique has
advantages and disadvantages and therefor implementation varies
across the broad field of flow measurement. The applications of
different technology vary due to some of the inherent properties of
the fluid being measured, resolution of the results,
life-span/capability of materials, cost, pipe size, pressure and
velocity. Other factors such as cost vary and are sometimes a
function of the primary variables, such as reading resolution and
operating pressures.
Measurement of
flow parameters are often performed by positive displacement methods.
An analogy to this method is a bucket and a stopwatch. The bucket is
filled with the fluid and the time is recorded that it takes to reach
a filled state. The volumetric capacity of the bucket is known and
the time duration to fill is known, which constitutes a flow rate.
The sensors that can be implemented for this type of monitoring would
be a mechanical switch paired with a floating device, which trigger
the timer when the cavity is empty and full. The most popular types
of positive displacement meters use pistons that operate in a cavity
of known volume. Every time the cavity is filled to capacity, the
piston is forced to move and subsequently rotate an axle that it is
connected to. The signal from the rotating axle can be transmitted to
the user by a magnetic drive, needle dial and a counter such as an
odometer. A turbine also uses mechanics to produce flow measurements,
but instead of positive displacement, the fluid is exerting force on
the components and creating work. A turbine is place in the path of
the fluid being measured, so that the fluid produces a force on the
area of the turbine. The force causes the turbine to rotate, which
once established at a steady speed, is proportional to the velocity
of the fluid.
A vortex meter
uses the phenomenon of Van Karman forces that are created using an
object that is located in the flow path of the pipe or channel. The
object that is placed in the path is known as a bluff body and
results in vortices created in the wake of the body. The Van Karman
forces vary between the two sides of the bar at a rate proportional
to the fluid velocity. For measurement, a piezoelectric sensor
records the number of times the vortices are created by transmitting
a voltage pulse.
Similar to the Van Karmen forces, where an object is placed in the path of the pipe, Jalpesh describes in his post the affect that a restriction can cause. The post did an excellent job explaining how the flow was measured using the differential pressures through the restrictions. Each type of restriction was presented and explained correctly. This type of meter was one of the few I saw in the flow sensors postings that relied mostly on heavy theory from fluid dynamics. The other sensors definitely included theory, some of which might have been more complicated than Bernoulli Theorem, but they didn't seem as classic. One thing that wasn't mentioned in the posts I looked was the types of sensors that relied heavily on more calibration and empirical data, such as those that are transcribing through magnetic needles and other sensitive components.
Aside from
mechanical flow meters, fluid velocity and flow can be measured using
optic sensors. The optic sensors take advantage of lasers of light
passing through a tube or pipe containing fluid. Two lasers are
contained in a small area of the pipe which track particles suspended
in the flow path. The first laser sends a beam of light through the
medium and the particles scatter the laser. On the oppisite side of
the pipe, a photo detector records the amount of light and sends an
electric pulse through a circuit. The same suspended particle then
passes another laser beam that completes the same process the first
has done with a photo detector sending another pulse. The time
between pulses is known as well as the distance between the two
lasers, therefor it is possible to calculate the flow rate.
Source:
http://en.wikipedia.org/wiki/Flow_measurement
It was really interesting reading your post specially the part about positive displacement because when researching the topic of flow sensors not once did I come across this type of method of measuring flow of fluids. But based on the way you described the process of measuring the flow with this method is definitely adequate for none continuous flows. Using optic sensors to measure flows was a new concept overall and seems like more and more people are moving towards using this innovated sensor technology.
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