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Types and Principles of Flowmeters

Ⅰ. The flowmeter is classified according to the measurement principle


1. Principles of mechanics: Instruments belonging to such principles include differential pressure type and rotor type using Bernoulli's theorem; impulse type and movable tube type using the momentum theorem; direct mass type using Newton's second law; using fluid momentum The target type of the principle; the turbine type using the angular momentum theorem; the vortex type and vortex street type using the principle of fluid oscillation;


2. Electrical principle: The instruments used for this principle include electromagnetic type, differential capacitive type, inductive type, strain resistance type, etc.


3. Acoustic principle: There is an ultrasonic type for flow measurement using the acoustic principle. Acoustic type (shock wave type), etc.


4. Thermal principle: There are thermal type, direct thermal type, indirect thermal type, etc. that use thermal principle to measure flow.


5. Optical principle: laser type, photoelectric type, etc. are instruments that belong to this type of principle.


6. Physical principles: nuclear magnetic resonance, nuclear radiation, etc. are instruments that belong to such principles.


7. Other principles: there are marking principles (tracer principle, nuclear magnetic resonance principle), related principles, etc.


Ⅱ. Classification according to the structure principle of flowmeter


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


1. Volumetric flowmeter


A positive displacement flowmeter is equivalent to a container of standard volume, which continuously measures the flowing medium. The larger the flow, the more times the measurement is performed, and the higher the frequency of the output.


The principle of positive displacement flowmeter is relatively simple, and it is suitable for measuring fluids with high viscosity and low Reynolds number. According to the shape of the rotating body, the products currently produced are divided into: oval gear flowmeters, waist wheel flowmeters (Roots flowmeters), rotary piston and scraper flowmeters suitable for measuring liquid flow; servo-type flowmeters suitable for measuring gas flow Volumetric flowmeter, membrane type and rotary flowmeter, etc.


2. Impeller flowmeter


The working principle of the impeller flowmeter is that the impeller is placed in the fluid to be measured, and is rotated by the impact of the fluid flow, and the flow rate is reflected by the speed of the impeller rotation. Typical impeller flowmeters are water meters and turbine flowmeters, and their structures can be mechanical transmission output type or electrical pulse output type. Generally, the accuracy of the water meter output by mechanical transmission is low, and the error is about ±2%, but the structure is simple and the cost is low. It has been mass-produced in China, and is standardized, generalized and serialized. The accuracy of the turbine flowmeter output by the electrical pulse signal is relatively high, and the general error is ±0.2%~0.5%.


3. Differential pressure flowmeter


The differential pressure flowmeter consists of a primary device and a secondary device. The primary device is called a flow measuring element, which is installed in the pipeline of the measured fluid to generate a pressure difference proportional to the flow rate (velocity) for the secondary device to display the flow. The secondary device is called display instrument. It receives the differential pressure signal generated by the measuring element and converts it into the corresponding flow for display.


The primary device of the differential pressure flowmeter is often a throttling device or a dynamic pressure measuring device (pitot tube, uniform velocity tube, etc.). The secondary device is a variety of mechanical, electronic and combined differential pressure gauges with flow display instruments. Differential pressure sensitive elements of differential pressure gauges are mostly elastic elements.


Since the differential pressure and flow are in a square root relationship, the flow display instruments are equipped with a square root device to linearize the flow scale. Most instruments are also equipped with a flow accumulating device to display the accumulated flow for economical accounting. This method of measuring flow using differential pressure has a long history and is relatively mature. It is generally used in more important occasions around the world, accounting for about 70% of various flow measurement methods. This kind of meter is used in the flow measurement of main steam, feed water, condensate, etc. in power plants. The products currently produced are: orifice flowmeter, wedge flowmeter, venturi tube flowmeter, and average pitot tube. LEFOO Industrial CO LTD can offer you more information about flowmeters.