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Classification and significance of flow meters
Dec 12, 2018

Classification and significance of flow meters


Flow measurement is a science to study the quantitative change of matter, and the law of mass mutual change is the basic law of the development of things related to each other. Therefore, the measurement object is not limited to the traditional pipe liquid in the sense, and there are flow measurement problems wherever the quantitative change needs to be mastered.The flow rate, pressure and temperature are listed as three detection parameters.For a certain fluid, as long as these three parameters are known, its energy can be calculated, and these three parameters must be detected in the measurement of energy conversion.Energy conversion is the basis of all production processes and scientific experiments, so the flow rate and pressure, temperature meter has been the most widely used.

Classification and significance of flow meters

I. classification according to measurement principle

(1) mechanical principle: instruments belonging to this principle have differential pressure type and rotor type using Bernoulli's theorem;Using momentum theorem of impulse type, movable tube type;Using the direct mass formula of Newton's second law;Target formula using the principle of fluid momentum;Turbine form using the angular momentum theorem;Vortex type and vortex street type using the principle of fluid oscillation;Pitot tube type, volumetric type, weir type, groove type and so on are used to make use of total static pressure difference.

(2) electrical principle: the instruments used for this principle are electromagnetic type, differential capacitor type, inductive type, strain resistance type, etc.

(3) acoustic principle: ultrasonic wave is used for flow measurement based on acoustic principle.Acoustic (shock wave), etc. 

(4) thermal principle: heat, direct calorimetry and indirect calorimetry are used to measure the flow rate.

(5) optical principle: laser type, photoelectric type and so on are instruments belonging to this principle.

(6) original principle of physics: nuclear magnetic resonance type, nuclear radiation type and so on are instruments belonging to this principle.

(7) other principles: labeling principle (tracer principle, nuclear magnetic resonance principle), correlation principle, etc.

Ii. Classification according to the principle of flow meter structure

According to the actual situation of current flowmeter products, according to the structure principle of flowmeter, it can be roughly summarized as the following types:

1.Volumetric flowmeter

Volumetric flowmeter is equivalent to a container of standard volume, which continuously measures the flow medium.The larger the flow, the more measurements, and the higher the output frequency.Volumetric flowmeters are simple in principle and suitable for measuring fluids with high viscosity and low Reynolds number.According to the different shape of the rotating body, the current products are divided into: suitable for measuring the flow of liquid elliptic gear flow meter, waist wheel flow meter (luo zi flow meter), rotary piston and scraper type flow meter;Servo volumetric flowmeter, membrane flowmeter and reducer flowmeter are suitable for measuring gas flow.

2.Impeller type flowmeter

The working principle of the impeller flowmeter is to put the impeller in the flow body to be measured and rotate under the impact of the fluid flow. The speed of the impeller rotation reflects the flow rate.The typical impeller flowmeter is a water meter and a turbine flowmeter. The structure of the impeller flowmeter can be mechanical transmission output type or electrical pulse output type.General mechanical transmission output of the water meter accuracy is low, the error of about plus or minus 2%, but simple structure, low cost, domestic mass production, and standardization, generalization and serialization.The accuracy of the turbine flowmeter output by electrical pulse signal is higher, and the general error is plus or minus 0.Two percent one zero.5%.

3.Differential pressure flowmeter (variable-pressure drop flowmeter)

Differential pressure flowmeter is composed of primary device and secondary device.The primary device is called the flow measurement element, which is installed in the pipeline of the flow body being measured to generate pressure difference proportional to the flow rate (flow rate) for the secondary device to display the flow rate.The secondary device is called the display instrument.It receives the differential pressure signal generated by the measuring element and converts it into the corresponding flow rate for display.The primary device of differential pressure flowmeter is usually throttling device or dynamic pressure measuring device (pitot tube, mean velocity tube, etc.).Secondary device for a variety of mechanical, electronic, combined differential pressure meter with flow meter.Differential pressure sensitive elements of differential pressure meter are mostly elastic elements.Due to the square root relationship between differential pressure and flow, flow display instruments are equipped with a square root device to linearize the flow scale.Most instruments also have flow integrators to display accumulated flow for economic accounting.This method of measuring flow by differential pressure has a long history and is relatively mature. It is generally used in important occasions all over the world, accounting for about 70% of all flow measurement methods.This kind of meter is used to measure the flow rate of steam, feed water and condensate water in the power plant.At present, our products include orifice flowmeter, wedge flowmeter, venturi flowmeter and average pitot tube

4.Variable area flowmeter (isobaric flowmeter)

The float, placed in a conical flow path with a large top and a small bottom, is moved by a fluid flowing from the bottom up.When this force is in equilibrium with the float's "indicated weight" (the float's own weight minus the buoyancy of the fluid it is subjected to), the captive is at rest.The static height of the float can be used as a measure of the flow rate.Since the cross-sectional area of the flowmeter varies with the height of the float, and the pressure difference between the upper and lower parts is equal when the float is stationary, this type of flowmeter is called variable area flowmeter or isobaric flowmeter.The typical meter of this type of flowmeter is rotor (float) flowmeter.

Classification and significance of flow meters

5.Momentum type flowmeter

A flowmeter that measures the momentum of a fluid to reflect the flow is called a momentum flowmeter.Since momentum P of the flowing fluid is directly proportional to the density of the fluid and the square of velocity v, namely, P v2, when the flow cross section is determined, v is directly proportional to volume flow Q, so P Q2.If the proportionality coefficient is set as A, then Q = A. Therefore, if P is measured, it can reflect the flow Q.This type of flowmeter, most use detection elements to convert momentum into pressure, displacement or force, and then measure the flow.Typical instruments of this kind of flowmeter are target-type flowmeter and rotating-vane flowmeter.

6.Impulse type flowmeter

The flow meter which USES the impulse theorem to measure the flow is called the impulse flowmeter, which is mainly used to measure the flow of granular solid medium, but also used to measure the flow of mud, crystalline liquid and abrasive.Flow measurements range from a few kilograms per hour to nearly ten thousand tons.A typical instrument is a horizontal component type impulse flowmeter. Its measuring principle is that when the medium under test falls freely from a certain height h to a detection plate with an inclination Angle, an impulse is generated. The horizontal component of the impulse is proportional to the mass flow.According to the detection method of signal (9), the flow score displacement detection type and the direct force measurement type.

7.Electromagnetic flowmeter

Electromagnetic flowmeter is the application of conductive body movement in the magnetic field to produce induced electromotive force, and the induced electromotive force and flow size is proportional to the measurement of electromotive force to reflect the principle of pipe flow and made.Its measurement accuracy and sensitivity are high.Industrial used to measure water, pulp and other media flow.The maximum pipe diameter can be measured up to 2m, and the pressure loss is minimal.However, medium with low conductivity, such as gas and steam, cannot be used.

Electromagnetic flowmeter cost is high, and the signal is easy to be interfered by external magnetic field, which affects the wide application of industrial pipe flow measurement.To this end, the product is constantly improving and updating, to the development of microcomputer.

8.Ultrasonic flowmeter

Ultrasonic flowmeter is designed based on the principle that the velocity of ultrasonic wave propagating in the flow medium is equal to the geometric sum of the average velocity of the medium under test and the velocity of sound wave itself.It is also measured by the flow rate to reflect the size of the flow.Although ultrasonic flowmeter only appeared in the 1970s, but because it can be made into non-contact type, and can be linked with ultrasonic water level meter for open flow measurement, there is no disturbance and resistance to the fluid, so it is very popular, is a very promising flowmeter.

Ultrasonic flowmeter classification: ① multispectral ultrasonic flowmeter: transducer 1 emits ultrasonic signal with frequency f1, which is offset after passing through suspended particles or bubbles in the liquid in the pipeline and reflected to transducer 2 with frequency f2. This is the multispectral doppler equation. The difference between f2 and f1 is multispectral doppler frequency difference fd.Let the fluid velocity be v, the ultrasonic sound velocity be c, and the multispectral shift fd is proportional to the fluid velocity v.15. As soon as conduit conditions, transducer installation location, emission frequency, and sound velocity are determined, c, f1, and theta are constants, and fluid velocity is directly proportional to the frequency shift of the multispectral ler. By measuring frequency shift, fluid velocity can be obtained, and then fluid flow rate can be obtained.② Moveout ultrasonic flowmeter: moveout ultrasonic flowmeter measures the flow rate of the fluid by using the principle that the time difference between the forward and backward propagation of sound waves in the fluid is proportional to the flow rate of the fluid.

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9.Fluid oscillating flowmeter

Fluid oscillating flowmeter is designed based on the principle that the fluid will oscillate when it flows in a specific flow channel and the frequency of oscillation is proportional to the flow rate.When the flow cross section is constant, the flow rate is proportional to the volume flow.Therefore, the flow rate can be measured by measuring the oscillation frequency.This kind of flowmeter was developed and developed in the 1970s.Because it has both the advantages of non-rotating parts and pulse digital output, it has a bright future.At present, typical products include vortex flowmeter and precession vortex flowmeter.

10.Mass flowmeter

Since the volume of the fluid is affected by temperature, pressure and other parameters, the volume flow is used to indicate the medium parameters when the flow is large.In the case of constant change of medium parameters, it is often difficult to achieve this requirement, which results in distortion of instrument display value.Therefore, mass flowmeter has been widely used and paid attention to.There are two kinds of direct and indirect mass flow scoring.The direct mass flowmeter USES the principle which is directly related with the mass flow rate to carry on the measurement, at present commonly used has the calorimetry type, the angular momentum type, the vibration gyro type, the magnus effect type and the coriolis force type and so on mass flowmeter.Indirect mass flow meter is obtained by directly multiplying density meter and volume flow rate.


There are also weir type flowmeters and groove type flowmeters suitable for open channel flow measurement.Plug - in flowmeter suitable for large diameter flow measurement;Laminar flow meter for measuring laminar flow;A correlative flowmeter suitable for two-phase flow measurement;And laser method, nuclear magnetic resonance method flow meter and a variety of tracer method, dilution method flow measurement.