Flow Transmitters PDF
Flow Transmitters PDF
Flow Transmitters PDF
Introduction
Flow meters are an integral tool for measuring the flow
of liquid, gas, or a mixture of both in applications used in
the food and beverage industry, oil and gas plants, and
chemical/pharmaceutical factories. There are many different types of flow meters available on the market. Fluid
characteristics (single or double phase, viscosity, turbidity,
etc.), flow profile (laminar, transitional, or turbulent, etc.),
flow range, and the need for accurate measurements are
key factors for determining the right flow meter for a particular application. Additional considerations such as
mechanical restrictions and output-connectivity options
also impact this choice. The overall accuracy of a flow
meter depends to some extent on the circumstances of
the application. The effects of pressure, temperature,
fluid, and dynamic influences can potentially alter the
measurement being taken.
Industrial flow meters are used in environments where
noise and sources of high-voltage surges proliferate. This
means that the analog front end (AFE) needs to operate
at high common-mode voltages and have extremely good
noise performance, in addition to processing small electrical signals with high precision and repeatability. The 4- to
20-mA loop is the most common interface between flow
transmitters and flow-control equipment such as programmable logic controllers. Flow transmitters can either be
powered by this loop or have a dedicated power line. Flow
transmitters designed to use the loop have extremely
stringent power constraints, as all of the electronics for
signal acquisition/processing and transmission may need
to operate solely off the 4- to 20-mA loop. Ultra-low-power
processors such as the Texas Instruments MSP430 and
TMS320C5000 DSP families, in conjunction with highprecision, low-power AFE solutions, are commonly used in
loop-powered transmitters. Transmitters with digitalconnectivity features such as a process field bus
(PROFIBUS), I/O links, and/or wireless connectivity are
increasingly popular, as they reduce start-up times and
provide continuous monitoring and fault diagnostics. All
these factors greatly improve productivity and efficiency
of the automation loop.
This article provides an overview of the working operation of the four most common flow meters: differentialpressure, electromagnetic (magmeter), Coriolis, and
P2
Orifice
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2Q 2012
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General Interest
eq (voltage) = dFB/dt
B = Magnetic field
w = Speed of rotation
vr = Radial velocity
30
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General Interest
Pickoff
(Inlet Side)
Pickoff
(Outlet Side)
(microseconds)
Phase Shift
f2
Flow
f1
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General Interest
Transit-time meter
On the contrary, the transit-time ultrasonic meter can be
used for measuring only extremely clean liquids or gases.
It consists of a pair of ultrasound transducers mounted
along an axis aligned at an angle with respect to the fluidflow axis (Figure 6). These transducers, each consisting of
a transmitter/receiver pair, alternately transmit to each
other. Fluid flowing through the pipe causes a difference
between the transit times of beams traveling upstream and
downstream. Measuring this difference in transit time gives
flow velocity.
The difference in transit time is typically on the order of
nanoseconds. Hence, precise electronics are needed to
make this measurement, whether the time is measured
directly or a conversion corresponding to frequency difference is made. The latter is more popular and involves an
Conclusion
t2
Flow
t1
DIFFERENTIAL-PRESSURE
ELECTROMAGNETIC
CORIOLIS
ULTRASONIC
Volume
Mass
Volume
Fluid/flow rate
Particulate flow/slurries
Conditionally suitable
Suitable
Conditionally suitable
Conditionally suitable
Liquid/gas mixture
Not suitable
Conditionally suitable
Conditionally suitable
Conditionally suitable
Liquid conductivity
Not suitable
Suitable
Suitable
Installation/maintenance
Typical accuracy
0.2 to 1% of reading
Doppler-shift meter:
1% of reading to 2%
of full scale
Transit-time meter:
0.35% of reading to 2%
of full scale
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