Liquid flowmeter is an instrument used to measure linear, nonlinear, mass or volumetric flow rate of liquids. Measuring the flow of liquids is a critical need in many industrial plants. In some operations, the ability to conduct accurate flow measurements is so important that it can make the difference between making a profit or taking a loss. In other cases, inaccurate flow measurements or failure to take measurements can cause serious (or even disastrous) results.

With most liquid flow measurement instruments, the flow rate is determined inferentially by measuring the liquid’s velocity or the change in kinetic energy. Velocity depends on the pressure differential that is forcing the liquid through a pipe or conduit. Because the pipe’s cross-sectional area is known and remains constant, the average velocity is an indication of the flow rate. The basic relationship for determining the liquid’s flow rate in such cases is: Q = V x A where Q = liquid flow through the pipe, V = average velocity of the flow, and A = cross-sectional area of the pipe. Other factors that affect liquid flow rate include the liquid’s viscosity and density, and the friction of the liquid in contact with the pipe.

Direct measurements of liquid flows can be made with positive-displacement flowmeters. These units divide the liquid into specific increments and move it on. The total flow is an accumulation of the measured increments, which can be counted by mechanical or electronic techniques.

Liquid Flow Meter Types

  • Differential Pressure Flowmeters – In a differential pressure drop device the flow is calculated by measuring the pressure drop over an obstructions inserted in the flow. The differential pressure flowmeter is based on the Bernoullis Equation, where the pressure drop and the further measured signal is a function of the square flow speed.
    • Orifice Plate Liquid Flowmeters
    • Flow Nozzle Liquid Flowmeters
    • Venturi Tube Liquid Flowmeters
    • Pitot Tube Liquid Flowmeters
    • Variable Area – Rotameter Liquid Flowmeters
  • Velocity Flowmeters – In a velocity flowmeter the flow is calculated by measuring the speed in one or more points in the flow, and integrating the flow speed over the flow area.
    • Calorimetric Flowmeter
    • Turbine Flowmeter
    • Vortex Flow Meter
    • Electromagnetic Flowmeter
    • Ultrasonic Doppler Flowmeter
  • Positive Displacement Flowmeters – positive displacement flowmeter measures process fluid flow by precision-fitted rotors as flow measuring elements. Known and fixed volumes are displaced between the rotors. The rotation of the rotors are proportional to the volume of the fluid being displaced. The number of rotations of the rotor is counted by an integral electronic pulse transmitter and converted to volume and flow rate.
    • Reciprocating piston Liquid Flowmeter
    • Oval-gear Liquid Flowmeter
    • Nutating disk Liquid Flowmeter
    • Rotary vane Liquid Flowmeter
  • Thermal Mass Flowmeters – Thermal mass flowmeters have traditionally been used for gas measurements, but designs for liquid flow measurements are available. These mass meters also operate independent of density, pressure, and viscosity. Thermal meters use a heated sensing element isolated from the fluid flow path. The flow stream conducts heat from the sensing element. The conducted heat is directly proportional to the mass flow rate.
  • Open Channel Flowmeters – A common method of measuring flow through an open channel is to measure the height of the liquid as it passes over an obstruction as a flume or weir in the channel.