The flow meter is one of the most important instruments in industrial measurement. With the development of industrial production, demands for measurement accuracy and range have grown increasingly stringent, driving rapid advancements in flow measurement technology. To accommodate diverse applications, various types of flow meters have been developed. Currently, over 100 types of flow meters are in use. Flow meters can be classified according to various criteria.

Three common classification methods exist:
First, classification by measured medium;
Second, by the measurement principle;
Third, by structural principle.

I. Classification of Flow Meters

A. By Medium:
Liquid flow meters, gas flow meters, steam flow meters, and solid flow meters.

B. By Measurement Principle:
(1) Mechanical Principles:
Instruments based on this principle include:
Differential pressure type and rotor type, utilizing Bernoulli’s principle; impulse flow meters and movable tube flow meters based on momentum theory; direct mass flow meters based on Newton’s second law; target flow meters based on fluid momentum principles; turbine flow meters based on angular momentum theory; vortex flow meters and vortex street flow meters based on fluid oscillation principles; Pitot tube flow meters based on total static pressure difference; volumetric flow meters; weir and flume flow meters, etc.
(2) Electrical Principles:
Instruments based on these principles include electromagnetic, differential capacitance, inductive, and strain gauge types.
(3) Acoustic Principles:
Flow measurement utilizing acoustic principles includes ultrasonic and acoustic (impulse wave) types.
(4) Thermal Principles:
Flow measurement based on thermal principles includes heat-based, direct calorimetric, and indirect calorimetric types.
(5) Optical Principle:
Laser-type and photoelectric-type instruments belong to this category.
(6) Physical Principle:
Nuclear magnetic resonance-type and nuclear radiation-type instruments belong to this category.
(7) Other Principles:
Marker principle (tracer principle, nuclear magnetic resonance principle), correlation principle, etc.

C. Classification by Flowmeter Structural Principle:
Based on current flowmeter products and their structural principles, they can be broadly categorized into the following types:
1. Positive Displacement Flowmeters
A positive displacement flowmeter functions as a container with a fixed volume, continuously measuring the flowing medium. Higher flow rates result in more frequent measurements and increased output frequency. Their relatively simple principle makes them suitable for measuring high-viscosity fluids with low Reynolds numbers.
Based on the shape of the rotating element, currently produced models include:
– For liquid flow measurement: Elliptical gear flow meters, rotary piston flow meters, and rotary paddle flow meters;
– For gas flow measurement: Servo-type positive displacement flow meters, diaphragm flow meters, and rotary cylinder flow meters.

2. Impeller Flow Meters

Impeller flow meters operate by placing an impeller within the measured fluid. The impeller rotates under fluid impact, with rotation speed reflecting flow rate. Typical examples include water meters and turbine flow meters, available in mechanical transmission output or electrical pulse output configurations. Generally, mechanically transmitted water meters have lower accuracy with errors around ±2%, but they feature simple structures and low costs. They are mass-produced domestically and standardized, universalized, and series-produced. Electrically pulsed turbine flow meters offer higher accuracy, typically with errors of ±0.2% to ±0.5%.

3. Differential Pressure Flow Meters (Variable Differential Pressure Flow Meters)

Differential pressure flowmeters consist of primary and secondary assemblies. The primary assembly, known as the flow measurement element, is installed within the measured fluid pipeline. It generates a pressure differential proportional to flow rate (velocity), which is transmitted to the secondary assembly for flow display. The secondary assembly is the display instrument. It receives the differential pressure signal from the measuring element and converts it into a corresponding flow rate for display. The primary assembly of a differential pressure flowmeter is often an orifice plate or a dynamic pressure measuring device (Pitot tube, average velocity tube, etc.). The secondary assembly consists of various mechanical, electronic, or combined differential pressure gauges paired with flow display instruments. The differential pressure-sensitive elements of differential pressure gauges are typically elastic components. Since differential pressure and flow rate exhibit a square-root relationship, flow display instruments incorporate square-root extraction mechanisms to linearize flow rate scales. Most instruments also feature flow totalization devices to display cumulative flow for economic accounting purposes. This method of flow measurement via differential pressure has a long history and is relatively mature. It is widely employed in critical applications worldwide, accounting for approximately 70% of all flow measurement methods. Power plants employ this type of meter for measuring main steam, feedwater, and condensate flow.

4. Variable Area Flowmeter (Constant Differential Pressure Flowmeter)

A float placed within a conical flow channel—wider at the top and narrower at the bottom—moves under the force exerted by the upward-flowing fluid. When this force balances the float’s “display weight” (its own weight minus the buoyancy force exerted by the fluid), the float remains stationary. The height at which the float remains stationary indicates the flow rate. Since the flow cross-sectional area varies with the float’s height, and the pressure difference between the upper and lower sections is equal when the float is stable, this type is termed a variable area flowmeter or equal pressure drop flowmeter. A typical example is the rotor (float) flowmeter.

5. Momentum Flow Meters

Flow meters that measure flow rate by detecting the momentum of the fluid are called momentum flow meters.
Most meters of this type use sensing elements to convert momentum into pressure, displacement, or force, which is then measured. Typical instruments include target flow meters and rotary vane flow meters.

6. Impulse Flow Meters

Flow meters that measure flow using the impulse theorem are called impulse flow meters. They are primarily used to measure the flow of granular solid media and are also employed for measuring slurries, crystalline liquids, and abrasive materials.
Flow measurement ranges from several kilograms per hour to nearly ten thousand tons per hour. A typical example is the horizontal component impulse flowmeter. Its measurement principle is based on the impulse generated when the measured medium freely falls from a height h onto a detection plate at an inclination angle θ. The horizontal component of this impulse is proportional to the mass flow rate, so measuring this horizontal component reflects the magnitude of the mass flow rate. Based on signal detection methods (9), this type of flowmeter is categorized into displacement detection and direct force measurement variants.

7. Electromagnetic Flowmeter

The electromagnetic flowmeter operates on the principle that conductive materials moving within a magnetic field generate an induced electromotive force (EMF). This EMF is directly proportional to flow rate, enabling pipeline flow measurement through EMF detection. It offers high measurement accuracy and sensitivity.
Industrially, they are widely used to measure the flow of media such as water and mineral slurry. They can measure pipes up to 2m in diameter with minimal pressure loss. However, they cannot be applied to media with low conductivity, such as gases and steam.
Electromagnetic flowmeters are relatively expensive, and their signals are susceptible to interference from external magnetic fields, limiting their widespread use in industrial pipeline flow measurement. Consequently, products are continuously being improved and updated, evolving toward microcomputer-based designs.

8. Ultrasonic Flowmeter

The ultrasonic flowmeter operates on the principle that the propagation speed of ultrasonic waves through a flowing medium equals the geometric sum of the medium’s average flow velocity and the wave’s own velocity. It also measures flow rate by detecting flow velocity. Although ultrasonic flowmeters only emerged in the 1970s, they have gained popularity due to their ability to be manufactured in non-contact configurations. They can be integrated with ultrasonic water level meters for open-channel flow measurement without causing disturbance or resistance to the fluid, making them a promising flowmeter type. Ultrasonic Doppler flowmeters, which utilize the Doppler effect, have received widespread attention in recent years and are considered ideal instruments for non-contact measurement of two-phase flows.

9. Fluid Oscillation Flow Meters

Fluid oscillation flow meters operate on the principle that fluids oscillate under specific flow conditions, with oscillation frequency proportional to flow velocity. When the flow cross-section is constant, velocity is directly proportional to volumetric flow rate. Thus, measuring oscillation frequency directly determines flow rate. Developed in the 1970s, these meters combine the advantages of no moving parts with pulse digital output, making them highly promising.
Current representative products include vortex flow meters and vortex-induced flow meters.

10. Mass Flow Meters

Since fluid volume is affected by parameters such as temperature and pressure, specifying medium parameters is required when expressing flow rate in volumetric flow. Under conditions of constantly changing medium parameters, meeting this requirement often proves difficult, leading to distorted instrument readings. Consequently, mass flow meters have gained widespread application and prominence.
Mass flow meters are categorized into direct and indirect types. Direct mass flow meters utilize principles directly related to mass flow for measurement. Commonly used types include calorimetric, angular momentum, vibrating gyro, Magnus effect, and Coriolis force mass flow meters. Indirect mass flow meters calculate mass flow by directly multiplying the volumetric flow rate obtained from a density meter.
In modern industrial production, operating parameters such as temperature and pressure of flow media continue to increase. Under high-temperature and high-pressure conditions, direct mass flow meters face application challenges due to material and structural limitations. Indirect mass flow meters, constrained by the humidity and pressure ranges of density meters, also prove difficult to implement practically.
Therefore, temperature-pressure compensated mass flow meters are widely adopted in industrial production. These can be regarded as a type of indirect mass flow meter. Instead of using a density meter, they leverage the relationship between temperature, pressure, and density. Temperature and pressure signals undergo functional calculations to generate a density signal, which is then multiplied by the volumetric flow rate to obtain the mass flow rate.