What are the Principles of Flow Measurement?
Time:2022-11-04 Read:741
Reading guide: Generally, the signals of the flowmeter come from flow, pressure, and temperature detection elements. Reasonable selection of the installation location of different detection elements can reduce the impact of fluid disturbance and noise, and improve the detection effect.

(1) Pressure detection

The most common flowmeters of this type are orifice plate flowmeters, Annubar flowmeters, Venturi tubes, etc

Diffusion silicon pressure sensors or ceramic pressure sensors are usually used for fluid pressure detection. According to the vortex precession principle, the position of the pressure tap should be selected at the throat of the measuring pipe, because the throat pressure can best represent the pressure in the precession area of the vortex core.

Flow Measurement Meters principle

(2) Vortex signal detection

The most common products are rotary vortex flowmeters, turbine flowmeters, and other products. Thermal sensing, force sensing, capacitance, ultrasonic, optical fiber, electromagnetic, and other detection elements can be selected to detect local velocity changes and fluid pressure pulsations caused by the vortex, so as to realize vortex signal detection.

The reasonable selection of the installation position of the detection element has a good effect on obtaining the vortex signal with high intensity and low noise. The test shows that the vortex signal detection element is better installed near the junction of the throat and expansion section of the measuring pipe, where the vortex precession is strong and the noise is low.

The detection method of double-detection elements has a better measurement effect. A pair of vortex signal detection elements are symmetrically arranged in the two measurements of the measuring tube. Using the 180-degree phase difference between the two vortex signals, the differential detection method can obtain double signals and effectively eliminate common mode interference, such as pipe vibration and flow field disturbance, thus greatly improving the signal-to-noise ratio and expanding the flow measurement range.

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

Flow Measurement Meters detection

(3) Fluid temperature detection

Platinum resistance detection element is mostly used for fluid temperature detection, which has good stability and reliability, low price, small size, and convenient installation. In recent years, the thin film platinum resistance detection element, which is small in size and fast in response, is a better temperature measurement element.

The temperature measuring point shall be located in the outlet section of the measuring pipe, 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.