A flow meter (or flowmeter) is an instrument used to measure linear, nonlinear, volumetric or mass flow rate of a liquid or a gas.
When choosing flow meters, one should consider such intangible factors as familiarity of plant personnel, their experience with calibration and
maintenance, spare parts availability, and mean time between failure history, etc., at the particular plant site. It is also recommended that
the cost of the installation be computed only after taking these steps.
One of the most common flow measurement mistakes is the reversal of this sequence: instead of selecting a sensor which will perform properly,
an attempt is made to justify the use of a device because it is less expensive. Those "inexpensive" purchases can be the most costly installations.
This page will help you better understand flow rate meters, but you can also speak to our application engineers at anytime if you have any
special flow measurement challenges.
Spring and Piston Flow Meters for Gases and Liquids
Mass Gas Flow meters
Ultrasonic Flow meters
Turbine Flow meters
...and More Flow Meter Types
volumetric or mass Flow Rate?
Popular Flow Meter Models
Learn more about flow meters
Flow Measurement Orientation
The basis of good flow meter selection is a clear understanding of the requirements of the particular application. Therefore, time should
be invested in fully evaluating the nature of the process fluid and of the overall installation.
First Steps to Choose the Right Flow rate Meter
The first step in flow sensor selection is to determine if the flowrate information should be continuous or totalized, and whether this information
is needed locally or remotely. If remotely, should the transmission be analog, digital, or shared? And, if shared, what is the required (minimum)
data-update frequency? Once these questions are answered, an evaluation of the properties and flow characteristics of the process fluid, and of the
piping that will accommodate the flow sensor, should take place. In order to approach this task in a systematic manner, forms have been developed,
requiring that the following types of data be filled in for each application: Download the Flow meter Evaluation Form.
Fluid and flow characteristics
The fluid and its given and its pressure, temperature, allowable pressure drop, density (or specific gravity), conductivity, viscosity (Newtonian or not?)
and vapor pressure at maximum operating temperature are listed, together with an indication of how these properties might vary or interact. In addition,
all safety or toxicity information should be provided, together with detailed data on the fluid's composition, presence of bubbles, solids
(abrasive or soft, size of particles, fibers), tendency to coat, and light transmission qualities (opaque, translucent or transparent?).
Pressure & Temperature Ranges
Expected minimum and maximum pressure and temperature values should be given in addition to the normal operating values when selecting flow meters.
Whether flow can reverse, whether it does not always fill the pipe, whether slug flow can develop (air-solids-liquid), whether aeration or pulsation
is likely, whether sudden temperature changes can occur, or whether special precautions are needed during cleaning and maintenance, these facts, too,
should be stated.
Piping and Installation Area
Concerning the piping and the area where the flow meters are to be located, consider: For the piping, its direction (avoid downward flow in liquid
applications), size, material, schedule, flange-pressure rating, accessibility, up or downstream turns, valves, regulators, and available
straight-pipe run lengths. The specifying engineer must know if vibration or magnetic fields are present or possible in the area, if electric or
pneumatic power is available, if the area is classified for explosion hazards, or if there are other special requirements such as compliance
with sanitary or clean-in-place (CIP) regulations.
Key Questions to Ask when choosing a Flow Meter
What is the fluid being measured?
Do you require rate measurement and/or totalization?
If the liquid is not water, what viscosity is the liquid?
Do you require a local display on the flow meter or do you need an electronic signal output?
What is the minimum and maximum flowrate?
What is the minimum and maximum process pressure?
What is the minimum and maximum process temperature?
Is the fluid chemically compatible with the flow meter wetted parts?
If this is a process application, what is the size of the pipe??
Flow rates and Accuracy
The next step is to determine the required meter range by identifying minimum and maximum flows (volumetric or mass) that will be measured.
After that, the required flow measurement accuracy is determined. Typically accuracy is specified in percentage of actual reading (AR),
in percentage of calibrated span (CS), or in percentage of full scale (FS) units. The accuracy requirements should be separately stated at
minimum, normal, and maximum flowrates. Unless you know these requirements, your flow meter's performance may not be acceptable over its full range.
In applications where products are sold or purchased on the basis of a meter reading, absolute accuracy is critical. In other applications,
repeatability may be more important than absolute accuracy. Therefore, it is advisable to establish separately the accuracy and repeatability
requirements of each application and to state both in the specifications.
When a flow meter's accuracy is stated in % CS or % FS units, its absolute error will rise as the measured flow rate drops. If meter error is
stated in % AR, the error in absolute terms stays the same at high or low flows. Because full scale (FS) is always a larger quantity than the
calibrated span (CS), a sensor with a % FS performance will always have a larger error than one with the same % CS specification. Therefore,
in order to compare all bids fairly, it is advisable to convert all quoted error statements into the same % AR units.
In well-prepared flow meter specifications, all accuracy statements are converted into uniform % AR units and these % AR requirements are
specified separately for minimum, normal, and maximum flows. All flow meters specifications and bids should clearly state both the accuracy
and the repeatability of the meter at minimum, normal, and maximum flows.
Accuracy vs. Repeatability
If acceptable metering performance can be obtained from two different flow meter categories and one has no moving parts, select the
one without moving parts. Moving parts are a potential source of problems, not only for the obvious reasons of wear, lubrication, and
sensitivity to coating, but also because moving parts require clearance spaces that sometimes introduce "slippage" into the flow being
measured. Even with well maintained and calibrated meters, this unmeasured flow varies with changes in fluid viscosity and temperature.
Changes in temperature also change the internal dimensions of the meter and require compensation.
Furthermore, if one can obtain the same performance from both a full flow meter and a point sensor, it is generally advisable to use
the flow meter. Because point sensors do not look at the full flow, they read accurately only if they are inserted to a depth where
the flow velocity is the average of the velocity profile across the pipe. Even if this point is carefully determined at the time of
calibration, it is not likely to remain unaltered, since velocity profiles change with flowrate, viscosity, temperature, and other factors.
Flow Measurement in History
Our interest in the measurement of air and water flow is timeless. Knowledge of the direction and velocity of air flow was essential
information for all ancient navigators, and the ability to measure water flow was necessary for the fair distribution of water through
the aqueducts of such early communities as the Sumerian cities of Ur, Kish, and Mari near the Tigris and Euphrates Rivers around 5,000 B.C.
Volumetric or mass Units
Before specifying a flow meter, it is also advisable to determine whether the flow information will be more useful if presented in mass
or volumetric units. When measuring the flow of compressible materials, volumetric flow is not very meaningful unless density (and sometimes
also viscosity) is constant. When the velocity (volumetric flow) of incompressible liquids is measured, the presence of suspended bubbles will
cause error; therefore, air and gas must be removed before the fluid reaches the meter. In other velocity sensors, pipe liners can cause
problems (ultrasonic), or the meter may stop functioning if the Reynolds number is too low (in vortex shedding meters, RD > 20,000 is required).
In view of these considerations, mass gas flow meters, which are insensitive to density, pressure and viscosity variations and are not affected by
changes in the Reynolds number, should be kept in mind. Also underutilized in the chemical industry are the various flumes that can measure
flow in partially full pipes and can pass large floating or settleable solids.
Omega Engineering as your flow meters supplier.
Originally founded in 1962, many big corporations and prestigious institutions acquire from OMEGA accrediting us as a quality manufacturer. OMEGA is your single-source provider with exceptional customer service at a global level where you can buy more than 100,000 innovative products for measurement and control.
OMEGA receives thousands of inquiries from around the world. Our customers buy from a basic thermometer to a unique request for their purchase. OMEGA's degreed engineers are always ready to discuss special requirements to support your applications. Support is available via email, telephone or live online through our website.
Our commitment to maintaining the leading edge through research development and state-of-the art manufacturing keeps OMEGA firmly at the forefront of technology and a well-known supplier. OMEGA's extensive world-wide inventory ensures off-the-shelf delivery in your purchases. We have an unmatched record of getting you the products you buy, when and where you need them; we'll even inventory specific products just for your acquisition.
Rotameters or Variable Area Flow sensor
A variable area flowmeter consists
of a tapered tube and a float. It is
most widely used for gas and liquid
flow measurement because of its
low cost, simplicity, low pressure
drop, relatively wide rangeability,
and linear output.
Spring and Piston Flow Meters Also used in gas and liquid flow
measurement, these flowmeters
can be mounted in any orientation.
Scales are based on specific
gravities of 0.84 for oil meters,
and 1.0 for water meters. Their
simplicity of design and the ease
with which they can be equipped to transmit electrical
signals has made them an economical alternative to
variable area flowmeters for flowrate indication and
Mass Gas Flow meters
Thermal-type mass flow meters are
used for the measurement of mass
flow rate of a fluid, primarily gases.
Popular applications include leak
testing and low flow measurements
in the milliliters per minute range.
They operate with minor dependence on density,
pressure, and fluid viscosity. This style of flowmeter
utilises either a differential pressure transducer and
temperature sensor, or heated sensing elements and
thermodynamic heat conduction to determine the true
mass flow rate.
Ultrasonic Flow meters
Ultrasonic flowmeters are noninvasive
and commonly used
in clean or dirty applications
that ordinarily cause damage
to conventional sensors. The
basic principle of operation
employs the frequency shift (doppler effect) of an
ultrasonic signal when it is reflected by suspended
particles or gas bubbles in motion. The transit time
method depends on the slight difference in time
taken for an ultrasonic wave to travel.
Turbine Flow transmitter
Turbine meters give very accurate
readings and can be used for the
measurement of clean liquids.
They require a minimum of 10 inch
pipe diameters of straight pipe on
the inlet and 5 inch on the outlet. The unit consists of a
multi-bladed rotor mounted within a pipe, perpendicular
to the liquid flow. The rotor spins as the liquid passes
through the blades. The rotational speed is a direct
function of flow rate and can be sensed by a magnetic
pick-up, photoelectric cell, or gears. Turbine meters are
particularly good with low-viscosity liquids.
One of the most popular
cost-effective fluid and water
flowmeters. Many are offered with
flow fittings or insertion styles.
These meters require a minimum of
10 inch pipe diameters of straight
pipe on the inlet and 5 inch on the outlet. Chemical
compatibility should be verified when not using water.
Outputs come in Sine wave, Square wave, and also
transmitters for panel mounting and built-in systems.
Positive Displacement Flow meters
These meters are used for low to
high viscous applications when no
straight pipe is available. Operation
of these units consists of separating
liquids into accurately measured
increments and moving them on.
These meters are good for liquids
where a simple mechanical meter system is needed.
The positive displacement are also used for viscous liquids.
Vortex meters are able to measure
high temperatures in steam, gas
and liquids. The main advantages
of vortex meters are their low
sensitivity to variations in process
conditions and low wear. Vortex
meters make use of a natural phenomenon called vortex
shedding that occurs when a liquid flows around an
object. The frequency of the vortex shedding is directly
proportional to the velocity of the liquid flowing through
Pitot Tubes or Differential Pressure Sensor for Liquids and Gases
The pitot tubes offer the following advantages easy, low-cost installation, much lower permanent pressure loss,
low maintenance and good resistance to wear. The pitot tubes do require sizing, contact our flow engineering.
Magnetic Flow transmitters for Conductive Liquids
Electromagnetic meters can
handle most liquids and
slurries that are electrically
conductive. Pressure drop
across the meter is the same
as it is through an equivalent
length of pipe because there are no moving
parts or obstructions to the flow. Electromagnetic
flowmeters operate in accordance to Faraday’s
law of electromagnetic induction, which states
that a voltage will be induced when a conductor
moves through a magnetic field. The liquid serves
as the conductor; the magnetic field is created by
energised coils outside the flow tube.
Renowned for their
outstanding accuracy and
versatility in measuring
challenging flow applications,
Coriolis meters can detect
the flow of all liquids, as
well as that of moderately dense gases. These
meters are excellent on applications where multiple
measurements such as mass flow, volume flow,
temperature, and density are needed. A Coriolis
flow meter works on the principle that the inertia
created by fluid flowing through an oscillating tube
causes the tube to twist in proportion to mass
So you want to measure flow? The answer would seem to be to purchase a flow meter. With fluid flow defined as the amount of fluid
that travels past a given location, this would seem to be straightforward — any flow meter would suffice. However, consider the
following equation describing the flow of a fluid in a pipe.
Q = A x v
Q is flow rate, A is the crosssectional area of the pipe, and v is the average fluid velocity in the pipe. Putting this equation
into action, the flow of a fluid traveling at an average velocity of a 1 meter per second through a pipe with a 1 square meter
cross-sectional area is 1 cubic meter per second. Note that Q is a volume per unit time, so Q is commonly denoted as the
“volumetric” flow rate. Now consider the following equation:
W = rho x Q
Where W is flow rate (again - read on), and rho is the fluid density. Putting this equation into action, the flow rate will be
1 kilogram per second when 1 cubic meter per second of a fluid with a density of 1 kilogram per cubic meter is flowing.
(The same can be done for the commonly-used “pounds”. Without getting into details — a pound is assumed to be a mass unit.)
Note that W is a mass per unit time, so W is commonly denoted as the “mass” flow rate. Now — which flow do you want to measure?
Not sure? In some applications, measuring the volumetric flow is the thing to do.
Consider filling a tank. Volumetric flow may be of interest to avoid overflowing a tank where liquids of differing densities can
be added. (Then again, a level transmitter and high level switch/shutoff may obviate the need for a flow meter.) Consider controlling
fluid flow into a process that can only accept a limited volume per unit time. Volumetric flow measurement would seem applicable.
In other processes, mass flow is important. Consider chemical reactions where it is desirable to react substances A, B and C. Of interest
is the number of molecules present (its mass), not its volume. Similarly, when buying and selling products (custody transfer) the mass
is important, not its volume.
How much maintenance does a flow meter require?
A number of factors influence maintenance requirements and the life expectancy of flow meters. The major factor, of course, is matching the
right instrument to the particular application. Poorly selected devices invariably will cause problems at an early date. Flow meters with no
moving parts usually will require less attention than units with moving parts. But all flow meters eventually require some kind of maintenance.