Flow Meter

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

1. What is the fluid being measured?
2. Do you require rate measurement and/or totalization?
3. If the liquid is not water, what viscosity is the liquid?
4. Do you require a local display on the flow meter or do you need an electronic signal output?
5. Do you require a local display on the flow meter or do you need an electronic signal output?
6. What is the minimum and maximum flowrate?
7. What is the minimum and maximum process pressure?
8. What is the minimum and maximum process temperature?
9. Is the fluid chemically compatible with the flow meter wetted parts?
10. 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.

You can view our range of flow meters

Variable Area Flow Meters 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 control.

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.

Paddlewheel Sensors
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
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 the meter

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.

Coriolis
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 flowrate.

volumetric or mass Flow Rate?

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.