**Flow meter** is a equipment used to accomplish the task of flow measurement by measuring the flow rate of solids/liquids/gases linearly/non-linearly/volumetrically/on weight-basis. Flow gauges, flow indicators and liquid meters are the other names by which they are known.

## Types of Flow Meters

Besically we can found three **types of flow meters** which are-

- Mechanical Flow Meters
- Optical Flow Meters
- Open Channel Flow Meters

### Mechanical Flow Meters

#### Positive Displacement Flow Meters

These meters measure the flow rate by measuring the volume of the fluid passing through them. The actual process includes entrapment of the fluid into the certain container-like thing so as know its flow rate. This is very much similar to the case wherein we allow water to fill a bucket up to a certain predetermined level, after which it is allowed to flow out. These **flow meters** can measure intermittent flows or meager flow rates and are suitable for any liquid irrespective of their viscosity or density. Positive displacement flow meters can be considered to be robust as they remain unaffected by the turbulence in the pipe.

Nutating disc meter, Reciprocating piston meter, Oscillatory or Rotary piston meter, Bi-rotor type meters like those of Gear meter, Oval gear meter (Figure 1) and Helical gear meter fall under this category.

#### Mass Flow Meters

These meters provide the user with an estimate of the flow rate by measuring the mass of the substance flowing through them. This kind of weight-oriented flow meters is usually employed in chemical industries wherein the weight-based measurement is essential in comparison to volumetric analysis.

Thermal meters (Figure 2a) and Coriolis flow meters (Figure 2b) fall under this category. In the case of thermal meters, the fluid flow cools-off the probe which is pre-heated to a certain degree. The heat loss can be sensed and will be used to determine the rate at which the fluid will be flowing. On the other hand, Coriolis meters work on the principle of Coriolis principle in which the fluid flow through the vibrating tube causes a change in frequency or phase shift or amplitude, which gives a measure of its flow rate.

#### Differential Pressure Flow Meters

In this kind of flow meters, the flow is measured by measuring the drop in pressure as the fluid passes through the obstructions which are inserted into the path along which it flows. This is because, as the flow of the fluid through the pipe increases, there will be more drop in the pressure across the constriction (Figure 3) which can be recorded by the meters. From this, one can compute the flow rate as it will be proportional to the square root of the pressure drop (Bernoulli’s equation).

Orifice plate meter, Flow nozzle meter, Flow tube meter, Pilot tube meter, Elbow tap meter, Target meter, Dall tube meter, Cone meter, Venturi tube meter, Laminar flow meter and Variable Area meter (Rotameter) are a few examples of differential pressure flow meters.

#### Velocity Flow Meters

These flow meters estimate the flow rate of the fluids by measuring the velocity of the fluid which flows through them. Here the velocity of the fluid gives a direct measure of its flow rate as they are directly proportional to each other. In these meters, one can measure the velocity in a variety of ways in which the use of the turbine is also one (Figure 4). Depending on the way used to find out the velocity, we have various kinds of velocity flow meters like Turbine flow meter, Vortex Shedding flow meter, Pitot tube flow meter, Propeller flow meter, Paddle or Pelton wheel flow meter, Single jet flow meter and Multiple jet flow meter.

Flow rate measurement of fluids in hazardous environments including those in the case of mining demands for the non-intrusive flow meters. SONAR flow meters which are a type of velocity flow meters serve this kind of requirements. Besides, ultrasonic flow meters, as well as electromagnetic flow meters, also form a part of velocity-type flow meters.

### Optical Flow Meters

These kind of **flow meters** work on the principle of optics i.e. they measure the flow rate using light. Usually, they employ a set-up consisting of a laser beam and photodetectors. Here, the particles of the gas flowing through the pipe scatter the laser beam to produce pulses which are picked-up by the receiver (Figure 5). Then, the time between these signals is determined as one would know the distance by which the photodetectors would be separated, which in turn leads to the measurement of the speed of gas.

As these meters measure the actual speed of the particles constituting the gases, they remain unaffected by the thermal conditions and the variations in gas flow. Hence, they are capable of providing highly accurate flow data even when the environment is most unfavorable, say, for example, when we have high temperature and pressure, high humidity, etc.

### Open Channel Flow Meters

Open channel flow meters are used to measure the flow rate of a fluid whose flow path involves a free surface. Weir meters and Flume meters (Figure 6) are the open channel flow meters which use secondary devices like bubblers or float to measure the depth of the fluid at a particular point. From this depth, the flow rate of the fluid can be obtained.

On the other hand, in dye-testing based open channel flow measurement case, a certain predetermined amount of dye or salt is used to change the concentration of the flowing stream of the fluid. The resulting dilution gives a measure of flow rate of the fluid. Next, it is to be noted that the precision with which the flow meters need to operate is decided by the application for which they are used. For example, when we want to monitor the flow of water through a pipe in our garden, it would be enough if we use a flow meter which has a lower precision than that which will be used when we need to monitor the flow of an alkali meant for a chemical process. Besides, one more factor which requires to be noted is that the **flow meters**, when used in conjunction with the flow valves, can perform the controlling actions successfully.