The flowmeters are widely used in industrial productions; it includes measurement flow of water, steam, purified water, compressed air, natural gas, etc. electromagnetic flowmeters, vortex flowmeters, turbine flowmeters, etc. are mostly used, we often talk about low flow cut off, this article is mainly discussing the purpose for low flow cut off operation.
Electromagnetic flowmeter low flow cut-off
The main reason why the magmeter has zero drift and need low flow cut off operation is as below:
The conductivity of the fluid changes significantly;
Magnetic flow meter is interfaced by the outside nose;
The millivolt signal is sent by the electrodes, during amplification and conversion, zero drift occurs;
When a certain section of the upstream and downstream tubes of the electromagnetic flow tube is not filled with liquid and the pipeline pressure fluctuates, the average flow rate is zero, and the axial flow velocity flowing through the electrode has a certain swing. Although the value is small, since the accuracy of the electromagnetic flowmeter has now reached a very high level, it can still be observed the interference.
Vortex flowmeter low flow cut-off
Some people say that there is no zero drift in the vortex flowmeter, but when the flow signal is 4-20 mA, the analog circuit always has a certain zero drift, it is necessary to set a small signal cut-off.
When the vortex flow meter signal is pulse, small flow cut-off is still needed. Why ?
From the principle of the vortex flowmeter, there is no zero drift in the vortex flowmeter. The reason is that when the flow velocity is zero, there will be no vortex behind the vortex generator, so there is no pulse output. In fact, the flow rate is not zero, as long as the flow velocity of the fluid is low to a certain value, and the corresponding Reynolds number is smaller than the turbulent flow interval, and the vortex is not generated. Then why set up a small signal cut? The reason is very simple: when the Reynolds number is very low, the interference will come in, such as the interference caused by the vibration of the pipeline, the interference generated by the radio frequency, etc.,
Why Ultrasonic flowmeter need low flow cut-off
Ultrasonic flowmeter is a kind of flowmeter which is widely used nowadays. It adopts the time difference method. It is based on the measurement of the ultrasonic downstream flow time and the countercurrent propagation time difference. It can measure not only forward flow but also reverse flow. When the flow velocity is zero, the downstream propagation time and the countercurrent propagation time should be equal, but due to the incomplete symmetry of the transducer and the time drift and temperature drift, the zero drift of the flow indication is also caused. Therefore, in the flow transducer Small signal cut-0ff are also set, but the cutoff value is set to 1% FS.,
The power generated by the fluid on the turbine blades causes the turbine to rotate. When the fluids flow rate is too weak to push the rotor to rotate, the turbine cannot work. so in principle, zero drift should not occur. However, if the flow signal is output in the form of 4-20 m, zero drift occurs due to the characteristics of the analog circuit, so small signal cut-off is still needed.