Flow Measurement

What is a flow meter and What are the types of flow meter

What is a flow meter and what does it do

Flow meters are devices that can be used to measure the amount of liquid, or gas which passes through it. It is an instrument which is used to conduct the flow measurement. The flow rate of liquid and gases must be measured accurately for the better quality of the industrial process. Accurate measurement of gases is needed for the industrial process and it also does the control of flow rate. Flow meters measure the flow by two methods, some flow meters measure the flow as the amount of the liquid passes through the flowmeter during a period of time. While other flow meters measure the flow by measuring the total amount of fluid that has passed through the flow meter.

How does a flow meter work

Flowmeter consists of devices such as transducer and transmitter, the transducer will sense the fluid that passes through the primary device. The transmitter will receive a signal from the transducer, so the transducer changes this signal into a usable flow signal. So a flow meter can be considered as the combination of these physical devices.

What are the factors that affect a flow meter selection

  • The phase of the fluid-like gas, liquid, steam
  • Flow conditions and flow range, flow conditions like clean, dirty, abrasive, viscous will affect
  • Process conditions like pressure and temperature
  • Preferred material mostly in case of corrosive fluid
  • Pipe size and accuracy
  • Repeatability and cost

What are the types of flow meter

Flow meters can be classified into differential pressure meters, variable area meter, positive displacement meters, magnetic, turbine, ultrasonic, vortex, and Coriolis. In differential pressure meters a restriction is used to create a pressure drop. So the flow rate can be determined by measuring the pressure drop across the restriction. In positive displacement meters, the measuring process takes place by precision-fitted rotors as flow measuring elements.

Orifice meter

Orifice meters are commonly used flow meter, an orifice plate is a plate with a hole in it placed in the flow it constricts the flow and measures the pressure differential across the construction gives the flow rate. It is used extensively because of its simple shape, and easy manufacture. Orifice type can be used for clean liquid, gases, and steam. It can’t be used for multi-phase fluids. This meter can be used to determine the rate of flow through a pipe. orifice plate installed in the pipeline creates a pressure differential as the fluid flows through it and the differential pressure is proportional to the rate of flow.

Venturi meter

Venturi tubes are capable to handle large flow volumes at low-pressure drops, it is used to determine the measurement of flow in a pipe line. In this meter, the fluid is accelerated through the converging cone and the pressure difference between the upstream side of the cone and the throat is measured and it provides the signal for the flow rate. Pressure recovery of the venturi meter better than the orifice plate. Pressure loss is low for venture meter, venture tubes are suitable for clean, dirty, and viscous liquid. High pressure and energy recovery make the venture meter suitable where only small pressure heads are available.

Magnetic flow meter

Magnetic flow meter depended on faraday’s law of electromagnetic induction, so it states that voltage will be induced when a conductor moves through a magnetic field. The liquid acts as the conductor and the magnetic field are created by energized coils outside the flow tube. The voltage produced is directly proportional to the flow rate, two electrodes are attached to the pipe wall to detect the voltage and it is measured by a secondary element. These flow meters can measure corrosive liquids and slurries

Elbow tap meters

This meter works on the principle that, if liquids travel in a circular path centrifugal force is exerted along the outer edges. The force on elbows interior when liquid flows through it is proportional to the density of the liquid times the square of its velocity. The force is inversely proportional to the elbows radius. Elbow taps are easy to implement because most piping configurations already contain elbow in which taps can be located and there will be no added pressure loss.

Ultrasonic flowmeters

Ultrasonic waves can propagate through any substances like solid, liquid, gases and this property of the wave is utilized to measure the fluid flow in a pipe the ultrasonic signal is sent to the pipe at an angle, and the time taken for the signal to reach from one side to another is measured. The difference between the transit time across the pipe and that of the signal traveling in the reverse direction is proportional to the flow rate.

Vortex flowmeter

This flow meter is also known as oscillatory flow meters, it utilizes the natural phenomena when a liquid is allowed to flow in a bluff object. The frequency of the vortex shedding is directly proportional to the velocity of the liquid flowing through the r, this meter is widely used to measure the steam flow.

Coriolis meter

In this meter, the measurement is done by using newton’s second law which is force= mass × acceleration. Mass measurement is done in this it is not sensitive to changes in temperature, pressure, viscosity, and density. It can be used to measure liquids, slurries, and gases. The fluid which runs through the tube will cause it to vibrate in an angular harmonic oscillation. Due to the Coriolis force the tubes will deform and an additional the tubes will deform and an additional vibration component is added to the oscillation and this component will create phase shift on some places of tubes which can be measured with sensors.

Turbine flow meter

In this meter the fluid which flows through the pipe comes in contact with the vanes of the turbine then the turbine will start to spin and rotate and to calculate the flow the spin rate is measured. Multiple blade rotor is mounted with a pipe perpendicular to the liquid flow, the rotational speed is the direct function of the flow rate and they are sensed by magnetic pick-up or photoelectric cell.

Thermal flow meter

Thermal flow meters can be used to measure the gas and liquid flow, this meter operates independently of density, pressure, and viscosity. This meter uses a heated sensing element that is isolated from the fluid flow path. The flow stream conducts heat from the sensing element and the conducted heat is directly proportional to the mass flow rate.


post-graduate in Electronics & communication.

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