What are the principles of flow measurement

Release time：

2023-01-12 17:12

Source：

Flow measurement is the measurement of common fluid physical indicators in production and life. There are many principles of flow measurement, including ultrasonic, electromagnetic, pressure, temperature and other measurement principles. Generally, the signal of the flowmeter comes from flow, pressure, and temperature detection. Components, reasonable selection of the installation position of different detection components can reduce the impact of fluid disturbance and noise, and improve the detection effect.

(1) Pressure detection

This type of flowmeter is commonly used by throttling devices such as orifice flowmeters, Annubar flowmeters, and Venturi tubes.

Fluid pressure detection mostly uses diffused silicon pressure sensors or ceramic pressure sensors. According to the principle of vortex precession, the position of the pressure measuring hole should be selected at the throat of the measuring tube, because the throat pressure can represent the pressure in the area where the vortex core precesses.

(2) Vortex signal detection

The most common products are vortex flowmeters, turbine flowmeters and other products. Thermal, force-sensitive, capacitance, ultrasonic, optical fiber, electromagnetic and other detection elements can be used to detect local flow velocity changes and fluid pressure fluctuations caused by vortices, and realize vortex signals. detection.

Reasonable selection of the installation position of the detection element has a good effect on obtaining a vortex signal with high intensity and low noise. According to the test, it is better to install the vortex signal detection element near the junction of the throat of the measuring tube and the expansion section, where the vortex precession is strong and the noise is relatively small.

The detection method with double detection elements has better measurement effect. A pair of eddy signal detection elements are arranged symmetrically on the two sides of the measuring tube. Using the 180-degree phase difference between the two vortex signals, the differential detection method can obtain doubled signals, and can effectively eliminate common-mode interference, such as pipeline vibration, flow field disturbance, etc., thereby greatly improving signal-to-noise Ratio, expanding the flow measurement range.

The size of the detection element should be as small as possible to reduce the disturbance of the detection element to the flow field. If an insertion type detection element is used, the influence of the insertion depth of the detection element into the fluid should be considered. The large size and deep insertion of the detection element will increase the throttling effect and increase the disturbance to the flow field, which will also affect the stability and frequency of vortex core precession. The test shows that the instrument coefficient K of the flowmeter increases with the increase of the insertion depth, but the linearity of the flowmeter becomes worse.

(3) Fluid temperature detection

The temperature detection of fluid mostly adopts platinum resistance detection element, which has good stability and reliability, low price, small size and convenient installation. The thin-film platinum resistance detection element that has appeared in recent years is small in size and fast in response, and is a better temperature measuring element.

The position of the temperature measuring point should be selected in the outlet section of the measuring tube, where the temperature of the fluid is basically the same as that of the throat, and the temperature measuring element will not interfere with the precession of the vortex core.

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Precautions for Flow Meters

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Precautions for Flow Meters

2021-09-15

Precautions for Flow Meters

2021-09-15

Flow meters are mainly used in chemical production. The general selection is mainly based on the following principles:

Electromagnetic flowmeter technology development history

2021-09-15

From the AC sine wave power frequency excitation developed in the 1950s to the low-frequency rectangular wave excitation in the 1970s and 1980s, and then to the high-frequency excitation, dual-frequency excitation, and programmable control excitation after the 1990s, the excitation methods are constantly changing.

Maintenance and repair skills of industrial instruments

2021-09-15

As long as proper maintenance steps are taken, scientific maintenance methods are followed, the technical performance of the instrument is understood, the working principle of the circuit is well studied, the basic measurement methods are familiar, and the fault inspection skills are mastered, it is not difficult to find out its regularity.

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