An electromagnetic flow meter is a volumetric flow meter that does not have any moving parts. It is ideal for wastewater applications or any dirty liquid which is conductive or water based. Magnetic flow meters are also ideal for applications where low-pressure drop and low maintenance are required.

Magnetic flow meters will generally not work with hydrocarbons, distilled water, and many non-aqueous solutions.

How does a magnetic flow meter work?

Magnetic flow meters use a magnetic field to generate and channel liquid flow through a pipe. A voltage signal is created when a conductive liquid flows through the flowmeter’s magnetic field. The faster the flow of the fluid, the greater the voltage signal generated. Electrode sensors located on the flow tube walls pick up the voltage signal and send it to the electronic transmitter, which processes the signal to determine liquid flow.

What is the operating principle of a magnetic flow meter?

The operation of a magnetic flow meter or mag meter is based upon Faraday's Law, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field is proportional to the velocity of that conductor.

As applied to the design of magnetic flow meters, Faraday’s Law indicates that signal voltage (E) is dependent on the average liquid velocity (V) the magnetic field strength (B) and the length of the conductor (D) (which in this instance is the distance between the electrodes).

Vortex Flow Meter

A vortex flow meter is a flow measurement device best suited for flow measurements where the introduction of moving parts presents problems. They are available in industrial grade, brass or all plastic construction. Sensitivity to variations in the process conditions are low and with no moving parts have relatively low wear compared to other types of flow meters.

Vortex flow meters operate under the vortex shedding principle, where an oscillating vortexes occur when a fluid such as water flow past a bluff (as opposed to streamlined) body.

The frequency that the vortexes are shed depend on the size and shape of the body. It is ideal for applications where low maintenance costs are important. Industrial size vortex meters are custom built and require appropriate sizing for specific applications.

The Working Principles of Turbine Flow Meters

Turbine flow meters are employed for both lower viscosity gases and liquids with turn down ratios from 7 to 30:1. The use of an appropriate zero drag electronic pick off in place of the standard magnetic type is the reason behind achieving this extended range. The turbine experiences too much extra drag at low fluid velocities due to the standard magnetic type. As a result, the rotational speed is slowed down.

The typical accuracy and repeatability of turbine flow meters are ±0.5% and ±0.1%, respectively. However, it is possible to achieve accuracy of ±0.25% and repeatability of ±0.05%.

Turbine flow meters (Figure 2) are inherently sensitive to Reynolds number. They will not be linear at Reynolds numbers nearing or in the laminar region. Therefore, they should be carefully used in the case of fluids with a broad temperature and viscosity range such as some oils. It is not a problem at constant conditions, but is often hard to achieve practically.

Meter Design

Designing an axial turbine in principal is simple, i.e., putting a propeller in a tube. With years of experience, the basic design has been developed into a relatively complex assembly. The key aspect of any design is keeping the turbine spinning freely. Hence, bearing design and sources of drag within the assembly have been given special focus.

Due to this, it is difficult to make miniature axial turbines as the relative turbine energy is largely negated by the bearing and sensor drags. For rotational efficiency, plain bushes or roller/ball bearings are employed. A ball is generally employed to handle the end thrust. The presence of hydrodynamically designed turbine and bearing supports in some designs creates a low pressure zone in front of the turbine. As a result, the turbine is pulled forward, thus mitigating or removing the end thrust.

The bearing supports lower pressure drop but increases the fluid velocity over the turbine blades. The sensor is typically magnetic and provides a low voltage sine wave output. An electronic pick-up is generally employed for extended flow range, extra accuracy or in the case of operating the flow meter in an electrically noisy environment.

What is an Ultrasonic Flow Meter?

Definition: An ultrasonic flow meter can be defined as, a meter that is used to measure liquid velocity with ultrasound to analyze the volume of liquid flow. This is a volumetric flow meter that needs bubble or minute particles within the liquid flow. These meters are suitable in the applications of wastewater but they will not work with drinking/distill water. So this type of flow meter is ideal for the applications wherever chemical compatibility, low maintenance, and low-pressure drop are required.

These meters will affect the audio properties of the liquid and also impact through viscosity, density, temperature, etc. Like mechanical flow meters, these meters do not include moving parts. The price of these meters will change greatly so frequently it can be used and maintained at a low cost.

Ultrasonic Flow Meter Working Principle

An ultrasonic flow meter construction can be done by using upstream and downstream transducers, sensor pipe and reflector. The working principle of ultrasonic flow meter is, it uses sound waves to resolve the velocity of a liquid within a pipe. There are two conditions in the pipe like no flow and flowing. In the first condition, the frequencies of ultrasonic waves are transmitted into a pipe & its indications from the fluid are similar. In the second condition, the reflected wave’s frequency is dissimilar because of the Doppler Effect.

Whenever the liquid flows in the pipe quickly, then the frequency shift can be increased linearly. The transmitter processes the signals from the wave & its reflections determine the flow rate. Transit time meters transmit & receive ultrasonic waves in both the directions within the pipe. At no-flow condition, the time taken to flow in between upstream & downstream in between the transducers is the same.

Under these two flowing conditions, the wave at upstream will flow with less speed than the downstream wave. As the liquid flows faster, the distinction between the up & downstream times raises. The times of the upstream & downstream processed by the transmitter to decide the flow rate.

Types of Ultrasonic Flow Meter

Ultrasonic flow meters available in the market are radar, Doppler velocity, ultrasonic clamp-on, and ultrasonic level.

Doppler velocity type meters use reproduced ultrasonic noise to calculate the liquid’s velocity.

Radar type meter employs microwave technology for transmitting small pulses to reflect off a flowing surface back to the sensor for deciding velocity.

Ultrasonic clamp-on type meter is ideal for applications wherever accessing the pipe is difficult otherwise not possible.

Ultrasonic level type meter is ideal for determining the fluid level in both open & closed channels.

Advantages of Ultrasonic Flow Meter

The advantages are

It does not block the path of liquid flow.

The o/p of this meter is different for density, viscosity & temperature of the liquid.

The flow of liquid is bidirectional

The dynamic response of this meter is good.

The output of this meter is in analog form

Conservation of energy

It is appropriate for huge quality flow measurement

It is handy to fit and maintain

Versatility is good

There is no contact to liquid

There is no leakage risk

There are no moving parts, pressure loss

High accuracy

Disadvantages of Ultrasonic Flow Meter

The disadvantages are

It is expensive as compared with other mechanical flow meters.

Design of this meter is complex

Auditory parts of this meter are expensive.

These meters are complicated as compared with other meters, thus it requires specialists for maintaining and repairing these meters

It cannot measure cement or concrete pipes one they rusted.

It doesn’t work once the pipe contains holes or bubbles in it

Can’t measure cement/concrete pipe or pipe with such material lining

Metal tube rotameter and other material flowmeter are used to test the flow parameters of gas or liquid. We have mentioned a lot about the difference between metal tube rotameter and its flowmeter, so we should have a lot of understanding for such a test instrument. However, when using this kind of flowmeter with a float, we will encounter a problem, that is, the pointer of the flowmeter vibrates, so that it is unable to accurately detect the flow.

In the use process of metal tube rotameter, if the site environment is corrosive, acid or alkaline gas will also have a certain corrosion effect on the flowmeter in the air. Rust / rust is a big defect of metal products. Once the article begins to rust, its corrosion will continue, and then it will affect the equipment Very bad. As a metal measuring instrument, metal tube rotameter is easy to rust.

Corrosion will not only affect our use effect, but also affect our service life, so we also need to deal with the corrosion of the surface? So how should we deal with it? This seems to be a problem that users need to understand.

Step or need to start from the usual use of work, that is to say, we should do a good job in the maintenance of the instrument, so as not to make the instrument quickly problems, our use will become more smooth.

And if the beginning of corrosion, then we need to spend a lot of energy to complete the rust treatment work, we can remove its rust surface by cleaning agent. Or solve it in other ways.