Introduction to Flowmeter Selection - Advanced ( 1 )

Introduction to Flowmeter Selection Advanced )

Type of fluid flow meter

Introduced different types of liquid flowmeters - orifice plate, Venturies , nozzle, rotor flowmeter, pitot tube, Calorimetrics , turbine, vortex, electromagnetic, Doppler, ultrasonic, thermal mass, mass, Coriolis

The most common calibration fluid flow measurements are:

  • Differential pressure flowmeter
  • Speed ​​flow meter
  • Positive displacement flowmeter
  • Mass flowmeter
  • Open Channel Flowmeter

Differential pressure flowmeter

The method of calculating the differential pressure flow of the drop device is to measure the pressure drop over an obstacle insertion flow.

The differential pressure flowmeter is based on the Bernoullis equation, where the pressure drops and the measured signal is a function of the squared flow velocity.

The most common types of differential pressure flow meters are:

  • Orifice plate
  • Flow nozzle
  • Venturi tube
  • Variable zone-rotor flowmeter

Orifice plate

With the orifice plate, fluid flow is measured, blocking the pipe from one part of the side from upstream to downstream by different pressures.

Accurate measurement of the baffle plate provides a way for pipes to clog, shrink, and force the liquid to flow.

Orifice plates are simple and inexpensive, and can be supplied in almost any material for any application.

The accuracy conversion efficiency of the orifice plate is less than 5: 1 . Its accuracy is poor at low flow rates.

High accuracy depends on the orifice plate in good condition, with sharp edges upstream.

Reduced wear accuracy.

  • Orifice, Nozzle and Venturi Meter

Venturi tube

Due to simplicity and reliability, venturi flowmeters are often used in applications where high range rates are required, or to reduce pressure drops, than orifice plates can provide.

In the Venturi tube, the interception area where the fluid flow is reduced is measured, and a pressure difference is generated in the flow path.

After the contraction zone, the fluid passes through a pressure recovery outlet section. As much as 80% of the pressure difference is generated in the contraction zone and is recovered.

With proper instrumentation and flow calibration, venturi flow can be reduced by approximately 10% of full scale with appropriate accuracy .


This provides a precision conversion efficiency of 10: 1.

  • Orifice, Nozzle and Venturi Meter

Flow nozzle

Flow nozzles are often used as industrial applications for measuring air and gas flow of components .

Nozzle flow is relatively simple and inexpensive and can be used in many applications on many materials.

The conversion efficiency and accuracy are compatible with orifice plates.

  • Orifice, Nozzle and Venturi Meter

Nozzle at Sonic Serious (呛) Flow Nozzle

As the gas accelerates through the nozzle , the speed increases, and the pressure and gas density decrease.

The highest speed reaches the smallest area in the throat, where Mach 1 or Sonic rests.

It is not possible at this point to increase flow and reduce downstream pressure.

Flow is choked.

This situation is used in many control systems to maintain a fixed, accurate, and repeatable gas flow independent of downstream pressure.

Recovery pressure drop tips, nozzles and venturi gauges

In the differential pressure flowmeter that has been generated after the pressure difference, the fluid passes through the pressure recovery outlet section, and the generated pressure difference is partially recovered in the contraction zone.

As we can see, the orifice pressure drop is significantly higher than that of the venturi .

Variable area flowmeter or rotor flowmeter

Rotary flowmeter consists of a vertical glass (or plastic) tube with a larger end on it, which is a freely moving tube within a measuring floating range.

The fluid flow causes the rising pressure difference of the floating riser and the buoyant fluid overcomes the effects of gravity.

The float rises to the annular tube between the zones. The float rises enough to allow a dynamic equilibrium state between the pressure difference and the buoyancy factor, the downward factor.

The height of the float is the displayed flow.

The tube is available in appropriate calibration and graduated flow units.

Typical float flow meters have range ratios up to 12: 1 .

Accuracy may not be as good as a 1% full scale rating.

Magnetic buoy for alarm and signal transmission functions.

Speed ​​flow meter

The flow rate calculation method in a speed meter is to measure the flow rate at one or more points, the flow rate and the integrated flow area .

Pitot tube

Pitot tube is one of the most commonly used (cheapest) method to measure the flow of fluids, especially in air applications, ventilation and air conditioning systems, even at speeds used in aircraft

Pitot tube measures the flow velocity of a fluid by converting kinetic energy into potential energy.

Measurement using a pitot tube's limit point.

With "Aniu", or multiple mouthpieces of the Pito probe, dynamic pressure can be measured over the entire velocity profile, and Aniu obtains an average effect.


The flow measurement principle of calorimetric fluid is based on the close relationship between the two temperature sensors and the thermal insulation of the fluid, but on the other side.

Two of these sensors are constantly heating and the effect of coolant flow is used to monitor the flow.

There is a constant temperature difference between the two temperature sensors in the fixed (no-flow) oil state.

As the fluid flow increases, the difference between the thermal energy from the heated temperature sensor and the sensor decreases.

This reduction is proportional to the velocity of the fluid.


Response time will vary due to the heat transfer fluid.

Generally lower thermal conductivity requires higher speeds for proper measurements.

Calorimeters can achieve high accuracy and low flow rates.

Turbine flowmeter

There are many different designs and manufactures of turbine flow meters , but in general they are based on the same simple principle:

If the turbine blades on the pipeline fluid movement and behavior, the turbine begins to spin and rotate.

Rotational speed measurement to calculate flow.

The adjustment ratio may exceed 100: 1 if the turbine flowmeter is calibrated for single flow and uses fixed conditions.

Accuracy may be better than +/- -0.1%.

Vortex flowmeter

A vortex downstream flow is generated in the obstacle fluid flow.

Every time a critical fluid blocks the flow velocity, a vortex occurs.

An example of vortex shedding, in which the alternating low pressure zone is downstream.

These AC low-voltage areas cause blocking towards low-voltage areas.

The intensity of the eddy current with the sensor can be measured.

  • Vortex flowmeter principle-it brings the principle of vortex flowmeter.

Electromagnetic Flowmeter

The electromagnetic flowmeter operates the Faraday's law of electromagnetic induction, pointing out that it will cause a voltage conductor to move through a magnetic field.

The liquid acts as a conductor and the magnetic field coil is energized to create a flow tube outside .

The generated voltage is proportional to the flow rate.

Two electrodes installed on the inner wall of the pipeline to detect the voltage is a secondary factor to measure.

Electromagnetic flow meters can measure difficult and corrosive liquids and slurries, they can measure flow in both directions with equal accuracy.

Electromagnetic flowmeters have a relatively high power consumption and can only be used for conductive liquid water.

  • Principle of electromagnetic flowmeter-it brings the principle of electromagnetic flowmeter

Ultrasonic Doppler Flowmeter

Movement effect sound source, its frequency of sound, observed and described by Christian John Doppler.

The frequency of the reflected signal is modified by the speed and direction of the flowing fluid

If the liquid is moving towards a sensor, the election of the frequency signal will increase.

As the fluid moves away from the transducer, the frequency of the return signal drops.

The frequency difference is equal to the reflection frequency minus the original frequency and can be used to calculate the speed of fluid flow.

  • Ultrasound Doppler flowmeter and time of flight
  • .Ultrasonic Flowmeter Tutorial-Basic tutorial on ultrasonic flowmeter.

Positive displacement flowmeter

Positive displacement flowmeter measures the rotor element of the flow measurement device for accurate flow measurement during the process.

Known and fixed number of rotors between displacement.

The rotor is replaced with fluid proportional to the amount of rotation.

The number of rotating rotors is calculated by a complete electronic pulse transmitter and converted into volume and flow.

The construction of the positive displacement rotor can be in the following aspects:

  • Reciprocating piston flow meters are single and multiple piston types.
  • The ellipse has two rotating gear meters, and the oval gear is synchronized with the teeth.
  • A fixed amount of liquid flows through each revolution of meter.
  • Shaft rotation can be monitored for specific flow.
  • There are removable disk meters. Nutating disks are mounted in a concentric ball located in the side wall of the ball.
  • The passage of pressurized liquid through the measuring chamber causes the disk rock circulation path to have no axis of its own.
  • This is the only moving part in the measurement room.
  • Made of rotating blades with equal meters, rotating impellers, two or more compartments, and instrument housings inside.
  • The continuous impeller is in contact with the housing.
  • A fixed volume of liquid is swept to the meter's outlet to rotate from the impeller of each carriage.
  • The revolutionary impeller is counted and registered in volume units.

The positive displacement flowmeter can be used for all relatively non-abrasive fluids such as heating oils, lubricants, polymer additives, animal and vegetable oils, inks, freons, and more.

Accuracy can be as high as 0.1 % of full scale and above 70: 1.

Mass flowmeter

Measure mass flow directly.

Thermal flow meter

Thermal mass flowmeters operate independently for density, pressure, and viscosity.

The hot water heat sensing element is used to isolate the fluid flow path from the sensing element in the hot air flow.

The amount of heat carried is proportional to the mass flow and temperature difference, and he calculates the mass flow.

The reliability of an accurate heat flow calibration device depends on the actual process and changing temperature, pressure, flow, heat capacity and viscosity of the fluid.

Coriolis flowmeter

Direct mass measurement Coriolis flowmeters integrate among other technologies.

Changes in mass measurement are not sensitive to pressure , temperature, viscosity and density .

.Because it can measure liquid, mud and gas ,

Coriolis flow meters are popular meters.

The Coriolis mass flowmeter uses the Coriolis effect to measure the amount of mass moving through the element.

The fluid to be measured flows through a U-shaped tube, which is an angular harmonic oscillation causing vibration.

Due to the Coriolis force, a deformed tube and an additional vibration component will be added to the oscillation.

This additional component will cause the phase shift to some places where the tube can measure the sensor.

Coriolis flowmeters are generally very accurate, with a bed rate of better than +/- -0.1% over 100: 1.

Coriolis meters can also be used to measure liquid density .

Open Channel Flowmeter

A common method to measure the flow through an open channel is to measure the height of the liquid, because it passes a multi-obstructed sink or weir channel.

Commonly used are Sharp top weir, V-notch weir Cipolletti weir, rectangular tip weir, Parshall flume or Venturi flume.

  • Weir flow measurement standards -important and commonly used standards in weir flow measurement
  • Flow measurement and weir -weirs are commonly used to measure flow velocity in open channels and to connect river water supply and sewage treatment plants