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As the name suggests, a flow meter is a device used to measure the volume or mass of a gas or liquid that flows in a pipe of other vessel in a given time.
Although the name flow meter is probably the most widely used term, these devices are also referred to by a number of other names as well: flow gauge, flow indicator, liquid meter, flow rate sensor, etc. The actual name will depend on the particular industry and application.
Whatever the name, all these devices measure the flow of a liquid or gas and they are widely used with data acquisition systems which collect data and monitor the performance of a process.
Flow meters are used in many industrial processes and as such they very frequently used with data acquisition systems, monitoring a process and also enabling other control circuitry to control the flow in various parts of a system.
Flow measurement units
Flow meters actually measure what is termed volumetric flow rate. There are a number of units that can be used and flow meters may be calibrated in a variety of them. Alternatively any data acquisition system is likely to be able to convert between the different units very easily.
The way a flow is measured is in a given volume passing a point in a particular time. Thus using SI Units the flow rate measurement would be made in cubic metres per second. However cubic centimetres a second, litres per second, gallons per second and others may also be used as flow rate units.
In addition to the basic flow rate measurement, the temperature and pressure must be given (particularly for gasses) as the density will vary. Often the term "standard" may prefix the units, e.g. NN standard cubic metres a second. This indicates that the measurement has been made at standard temperature and pressure.
Types of flow meter
There are many different ways in which the flow of a fluid can be measured. Accordingly there are a number of different types of flow meter that are in use.
Each different type of flow meter uses a different approach, and accordingly different types of flow meter are used in different applications.
Some of the many types of flow meter are listed below:
Positive displacement: Positive displacement, PD, or volumetric flow meters measure flow of the fluid by passing by allowing a precise volume of fluid to flow with each revolution. Mechanical in nature, these flow meters have internal moving components that are hydraulically linked in tandem with the volume of fluid moving through the flow meter. These meters can measure fluid flow for fluids with any viscosity and they also can measure intermittent flows and flow of variable flow rates. However, be aware that these meters are not economical for large pipe diameters such as a 30 cm diameter line
Mechanical flow meters: There is a variety of different types of mechanical flow meter. These types all require the transfer of some of the energy from the flow to make a mechanical movement, and as a result they may not be suitable for all applications.
Magnetic flow meters: After mechanical flow meters, the magnetic flow meters are the type that is in most widespread use. This type of flow meter uses Faraday's law of magnetic induction as the basis of its operation. A magnetic field is applied to a tube through which the fluid is flowing and as a result this results in an electric potential being generated proportional to the velocity of the fluid flowing through the pipe. Knowing the dimensions of the pipe, it is then possible to relate this to the flow of the fluid.
In order that this type of flow meter is able to work, it is necessary that the pipe be an insulating type, so that the potential that is generated is not short circuited, and it is also necessary that the magnetic field is not disturbed, i.e. steel pipes are not normally used. A further requirement for magnetic flow meters is that the fluid that is flowing needs to be able to conduct electricity, at least to some degree so that Faraday's laws of induction apply. Water can be used, for example, as this has a sufficiently high level of conduction for the flow meter to operate.
Ultrasonic flow meters: Ultrasonic flow meters are used in a number of applications where they can be convenient and relatively easy to use. Ultrasonic flow meters use detect the difference in propagation time or transit time of an ultrasonic signal directed into a flowing fluid.
Normally the pulses that are used are fired into the fluid by the ultrasonic flow meter at an angle of between 30 and 45 degrees. The ultrasonic pulses will take a certain time to travel between the transmitter and receiver if the fluid is stationary. If the fluid is flowing in the same direction as the sound pulses, then they will travel more quickly, and similarly if the fluid flow is against that of the sound, then it will travel more slowly. By measuring the time taken for the ultrasonic pulses to travel, then it is possible for the ultrasonic flow meter to calculate the flow of the fluid.
Coriolis flow meters: The Coriolis flow meter uses an effect known as the Coriolis effect, from which it derives its name. The technique involves laterally vibrating the tube or pipe in which the fluid is flowing and monitoring the way in which it vibrates as this changes according to the fluid flow in the pipe.
The fluid flowing through the vibrating pipe has kinetic energy resulting from its movement. This causes the vibration in the inlet section of the pipe to be dampened while the vibration towards the outlet is enhanced. By measuring the vibration in the pipe, it is possible to determine the flow in the pipe.
The advantage of Coriolis flow meters is that they can be very accurate and insensitive to the medium that is being measured. As a result they can provide an ideal flow meter for many applications. However they do not respond sufficiently fast for use with injection flow systems.
Mass, velocity or volume flow
Different types of flow meter measure fluid flow in different ways and they can also measure different aspects of the fluid flow.
As different types of flow meter measure different attributes of the fluid flow, it is necessary to ascertain exactly what needs to be measured and then select the right type of flow meter to take the measurement required either directly, or indirectly with some form of conversion in the processing.
|Measurement Type||Details||Types of Flow Meter|
|Volumetric||These meters directly measure the volume of fluid that flows||Positive displacement, PD|
|Mass||Output signal is proportional to the mass of fluid passing through the meter||Thermal, Coriolis|
|Velocity||The output signal is proportional to the velocity of the fluid passing through the flow meter||Magnetic, ultrasonic, propeller, paddle wheeel|
Understanding flow meter accuracy
One important specification for any flow meter is its accuracy. It is necessary to understand how accurate the readings will be, especially for some situations. It is also important to understand the different ways that flow meter accuracy can be specified.
The accuracy can be specified in a number of ways: a percentage of actual reading, AR, also defined as %F, i.e. percentage of the flow; as a percentage of calibrated span, CS; or as a percentage of full scale, FS.
Another factor that needs to be considered is the repeatability. This describes the capability of the meter to display or indicate the same reading for identical flow rates. Meters will not always indicate the same reading for identical flow rates at different times, so this can be an important factor.
Selecting a flow meter for a DAQ system
In view of the wide variety of flow meter types and actual flow meters there is a huge selection from which to choose the right one.
Each type of flow meter type has its own attributes and within each type of flow meter, there are very many different models, each having its own level of performance.
Fortunately it is possible to narrow the field. There are usually trade-offs to be made, so the starting point is to ascertain the critical aspects of the operation for the meter, along with a target specifications.
It is also important to select the right type of flow meter for a given application. Selecting the right type will ensure the meter operates correctly and gives the measurements required. Selecting the wrong type of meter can mean that the readings are not suitable.
It helps to have the basic requirements for the meter to hand when looking at the options available:
|Requirements check for Flow Meter|
|Accuracy requirements||Different meters and different meter technologies will have different accuracy levels|
|Min / max flow rates||Different flow meters and technologies will be able to accommodate different levels of flow.|
|Continuous / intermittent flow||Some meter technologies are able to accommodate intermittent flow and others now. Check what is required.|
|Temperature range||With some processes requiring high or low temperatures, it is necessary to ensure the meter technology and meter itself can tolerate the temperatures.|
|Fluid type||The type of fluid is important because some fluids may react wit the flow meter material or they may be abrasive, etc.|
|Fluid viscosity||The viscosity can have an effect on the performance of the flow meter. Some do not work with highly viscous fluids, etc.|
|Pipe diameter||The diameter of the flow meter is important as some will be designed for narrow bore tubes, others will be used with much wider pipes. It is necessary to match the meter to the pipe / tube.|
|Maximum pressure||Some lines carry high pressures and it is necessary to ensure the flow meter can accommodate the level of pressure.|
|Pressure drop||The act of measuring the flow may result in a small pressure difference across the flow meter. This must be acceptable for the system.|
|Electrical interface||The format for the electrical interface must be compatible with the data acquisition system. Factors such as the actual interface as well as reading frequency, resolution etc must be as required.|
Once the basic requirements have been assembled, it is possible to eliminate the types of meter that will be unsuitable. This will leave the flow meter types that can be used.
Often it is best to determine the flow meter type or technology that best suits the application and then look to see the best fit for the requirements vs the meter specification.
As with many items, saving on cost can result in poor quality performance, especially in terms of the long term reliability. In situations where a large process is involved, a few pence or cents saved on the meter could result in many thousands of pounds, dollars, euros, etc lost in production.
It is also best to understand the expected life of the meter and then it can be incorporated in any preventive maintenance schedules.
Each type of flow meter has its own advantages and disadvantages. This makes the different types of flow meter more suitable for one application or another. For any given application, it is therefore necessary to choose the most applicable type of flow meter.
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