Flow meters for data acquisition

- an overview or tutorial about the different types of flow meters for fluids used in many applications including manufacturing processes and data acquisition systems.

There are many applications where the flow of a gas or liquid needs to be monitored. Many chemical and other processes require flow measurement and flow meters to enable their processes to be accurately controlled and monitored. Accordingly flow meters are widely used in conjunction with data acquisition and control systems to provide measurement.


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:

  • Mechanical flow meters
  • Magnetic flow meters
  • Ultrasonic flow meters
  • Coriolis flow meters

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.


Summary

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.

By Ian Poole


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