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Texas Instruments Incorporated
General Interest
Industrial flow meters/flow transmitters
By Deepa Kalyanaraman
Business Development Manager, End-Equipment Solutions
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 differ-
ent 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 par-
ticular 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 electri-
cal 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 program-
mable 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 high-
precision, low-power AFE solutions, are commonly used in
loop-powered transmitters. Transmitters with digital-
connectivity 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 opera-
tion of the four most common flow meters: differential-
pressure, electromagnetic (magmeter), Coriolis, and
ultrasonic, the last of which includes Doppler-shift and
transit-time flow meters. The key uses of these meters are
presented along with their advantages/disadvantages and
system considerations.
Differential-pressure flow meter
This meter operates based on Bernoulli’s principle. It mea-
sures the differential-pressure drop across a constriction
in the flow’s path to infer the flow velocity. Common types
of differential-pressure flow meters are the orifice, the pitot
tube, and the venturi tube. An orifice flow meter (Figure 1)
is used to create a constriction in the flow path. As the
fluid flows through the hole in the orifice plate, in accord-
ance with the law of conservation of mass, the velocity of
the fluid that leaves the orifice is more than the velocity of
the fluid as it approaches the orifice. By Bernoulli’s princi-
ple, this means that the pressure on the inlet side is higher
than the pressure on the outlet side. Measuring this differ-
ential pressure gives a direct measure of the flow velocity
from which the volumetric flow can easily be calculated.
System considerations for differential-pressure flow meters
• Robust and mature technology with easy maintenance
(no moving parts)
• Suitable for turbulent flow
• Poor accuracy for low-flow measurements
• Uses extractive flow-measurement technique, so there
is always a permanent pressure loss that must be over-
come with extra pumping energy
• Requires strict placement of pipe fittings, elbows, and
bends for downstream and upstream constriction taps
Figure 1. Differential-pressure orifice
flow meter
P1
P2
Orifice
29
Analog Applications Journal
2Q 2012
www.ti.com/aaj
High-Performance Analog Products
General Interest
Texas Instruments Incorporated
Electromagnetic flow meter (magmeter)
The electromagnetic flow meter, also known as a magmeter,
is based on Faraday’s law of electromagnetism and can be
used to measure the flow only of conductive fluids. Two
field coil magnets are used to create a strong magnetic field
across a pipe (Figure 2). Per Faraday’s law, as the liquid
flows through the pipe, a small electric voltage is induced.
This voltage is picked up by two sensor electrodes located
across the pipe. The rate of fluid flow is directly propor-
tional to the amplitude of the electric voltage induced.
System considerations for electromagnetic
flow meters (magmeters)
• Can measure only fluids with conductivity greater than
10 µS/cm, eliminating their use in the petroleum, oil,
and gas industries, since hydrocarbons have poor
conductivity
• Sensor-electrode choices change depending on fluid
conductivity, pipe construction, and type of installation
• No losses in system pressure, which may be critical in
applications that cannot tolerate pressure drops, such as
applications with low-velocity flow
• Ideal for corrosive and dirty fluids, slurries, etc., pro-
vided the liquid phase has sufficient conductivity, since
the flow meter has no internal parts
• High accuracy to within ±1% of indicated flow
• Higher cost
Coriolis flow meter
This popular flow meter directly measures mass flow rate.
The installation can include a single straight tube or, as
shown in Figure 3, a dual curved tube. The architecture
with a single straight tube is easier to construct and main-
tain because it is subject to fewer stress forces, but it is
susceptible to interference and noise. The architecture
with dual curved tubes cancels out any noise picked up
because the two tubes oscillate in counterphase.
In Coriolis meters, the tubes through which the fluid
flows are made to oscillate at a particular resonant
frequency by forcing a strong magnetic field on the
Figure 2. Electromagnetic flow meter
B
Flow
e
e
q
(voltage) = –d
F
B/dt
B = Magnetic field
The coils used to create the magnetic field can be
excited with AC or DC power sources. In AC excitation,
the coils are excited with a 50-Hz AC signal. This has the
advantage of drawing a smaller current from the system
than the DC excitation technique. However, the AC excita-
tion method is susceptible to interference from nearby
power cables and line transformers. Thus, it can introduce
errors into the signals measured. Furthermore, null drift-
ing is a common problem for AC-powered systems and
cannot be calibrated out. Pulsed DC excitation, where the
polarity of the current applied to the field coils is periodi-
cally reversed, is commonly employed as a method to
reduce the current demand and mitigate the problems
seen with AC-powered systems.
Figure 3. Coriolis flow meter
F
C
(Coriolis force) = 2m
w
v
r
m = Moving mass
w
= Speed of rotation
v
r
= Radial velocity
30
High-Performance Analog Products
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Analog Applications Journal
Texas Instruments Incorporated
General Interest
tubes. When the fluid starts flowing through the tubes, it
is subject to Coriolis force. The oscillatory motion of the
tubes superimposes on the linear motion of the fluid, exert-
ing twisting forces on the tubes. This twisting is due to
Coriolis acceleration acting in opposite directions on either
side of the tubes and the fluid’s resistance to the vertical
motion. Sensor electrodes placed on both the inlet and
outlet sides pick up the time difference caused by this
motion. This phase shift due to the twisting forces is a
direct measurement of mass flow rate. Figure 4 shows
typical detection results.
System considerations for Coriolis flow meters
• Direct measurement of mass flow rate eliminates
effects of temperature, pressure, and flow profile on
the measurement
• High accuracy
• Sensor can make simultaneous measurements of flow
rate and density because the basic oscillating frequency
of the tube(s) depends on the density of the fluid flow-
ing inside
• Cannot measure flow rate of fluids with entrained
particles (liquids with gas or solid particles; gas with
liquid bubbles; etc.) because such particles dampen the
tube’s oscillations, making it difficult to take accurate
measurements
Ultrasonic flow meter
Doppler-shift meter
The Doppler-shift ultrasonic meter, as the name suggests,
is based on the Doppler effect. This meter (Figure 5) con-
sists of transmit- and receive-node sensors. The transmit
node propagates an ultrasound wave of 0.5 to 10 MHz into
the fluid, which is moving at a velocity v. It is assumed that
the particles or bubbles in the fluid are moving at the same
velocity. These particles reflect the propagated wave to
the receiver with a frequency shift. The difference in fre-
quency between the transmitted and received ultrasound
wave is a measure of the flow velocity. Because this type
of ultrasound flow meter requires sufficient reflecting
particles in the fluid, it does not work for extremely pure
single-phase fluids.
Figure 4. Signals detected by sensor in
Coriolis flow meter
t
Pickoff
(
Inlet Side
)
Pickoff
(Outlet Side)
(a) Time difference between inlet and outlet
t
(microseconds)
Figure 5. Doppler-shift ultrasonic flow meter
Transmitter
and Receiver
Phase Shift
f
2
f
1
Flow
Q (volumetric flow rate) = K
D
(f
1
, f
2
)
f
1
and f
2
= Incident and reflected frequencies, respectively
K = A constant that is a function of angle of incidence/reflection,
reflective-particle position, cross section
(b) Phase shift translates to flow rate
31
Analog Applications Journal
2Q 2012
www.ti.com/aaj
High-Performance Analog Products
General Interest
Texas Instruments Incorporated
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 fluid-
flow 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 differ-
ence is made. The latter is more popular and involves an
FFT analysis of the difference in frequency between waves
received in and against the flow direction.
System considerations for ultrasonic flow meters
• The Doppler-shift flow meter is relatively inexpensive
• The transit-time flow meter provides one of the few
techniques for measuring nonconductive slurries and
corrosive fluids
• The ultrasonic flow meter is externally clamped onto
existing pipes, allowing installation without cutting or
breaking pipes, which minimizes personal exposure
to hazardous liquids and reduces possible system
contamination
• The ultrasonic flow meter’s most significant disadvantage
is its dependence on the fluid’s flow profile; for the same
average flow velocity, the meter could give different out-
put readings for different flow profiles
Conclusion
This article has discussed the working operation of the four
most common flow meters. Their key uses and design con-
siderations, summarized in Table 1, were also discussed.
There is a wide range of solutions available for flow
meters, including interfaces for industrial field-bus trans-
ceivers, a variety of AFEs, and low-power processing solu-
tions. Selecting the right flow meter for an application from
the various different technologies and designs available on
the market can be rather challenging. By understanding
the properties of the fluid being used, knowing the appli-
cation’s flow rates and required measurement accuracy,
and being aware of physical constraints and operating con-
ditions, the designer can narrow down the choices faster.
Related Web site
www.ti.com/solution/flow_meter
Figure 6. Transit-time ultrasonic flow meter
t
2
Flow
t
1
Q (volumetric flow rate) = K × (t
1
– t
2
)/(t
1
× t
2
)
K = A constant that is a function of acoustic-path length, ratio
between the radial and axial distances from the sensors, velocity
distribution (flow-velocity profile), cross section
t
1
= Transit time for downstream path
t
2
= Transit time for upstream path
Table 1. Characteristics of the four most common flow meters
FEATURE
DIFFERENTIAL-PRESSURE
ELECTROMAGNETIC
CORIOLIS
ULTRASONIC
Volume/mass measurement Volume
Volume
Mass
Volume
Fluid/flow rate
Not suitable for gases with
low flow rate
Not suitable for gas flow
Not suitable for very high
flow rates (>20,000 l/min)
Not suitable for gas flow
Particulate flow/slurries
Conditionally suitable
Suitable
Conditionally suitable
Conditionally suitable
Liquid/gas mixture
Not suitable
Conditionally suitable
Conditionally suitable
Conditionally suitable
Liquid conductivity
Suitable for all
Only conductive liquids
Suitable for all
Suitable for all
Food and beverage
(consumable liquids)
Not suitable
Suitable
Suitable
Most suitable for non-
intrusive measurement
Installation/maintenance
Easy installation; periodic
cleaning required
Moderate installation effort;
minimal maintenance
Installation outlay can be
considerable; relatively
maintenance-free
Easy installation and
maintenance
Typical accuracy
0.6 to 2% of full scale
0.2 to 1% of reading
0.1 to 0.5% 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
32
High-Performance Analog Products
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2Q 2012
Analog Applications Journal
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SLYT471
© 2012 Texas Instruments Incorporated
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