Our industrial production is more widely used in flow measurement, and it is mainly used for internal cost accounting and trade transactions. Flow meters include primary water, steam, pure water, compressed air, natural gas, and other flow measurement. When we are in the flow measurement, we use the current application of more flow meters: differential pressure flowmeter, electromagnetic flowmeter, vortex flowmeter, ultrasonic flowmeter, turbine flowmeter, and so on, but the flow meter small-signal excision problem has been a hot topic, today I will discuss with you the purpose and significance of small-signal excision.
1. Small signal cut-out for differential pressure flow meters
Differential pressure flowmeter is put into industrial applications of various flowmeter applications in the most general, the most extended history of a flowmeter. It is also the earliest people apply small-signal excision technology to a flowmeter.
In the early differential pressure flowmeter, people use U-shaped tube differential pressure meter display differential pressure device, it will then send the differential pressure signal, then it will be converted into instantaneous flow; we use the float type differential pressure meter, ring scale type differential pressure meter display instantaneous flow and conversion. However, it is impossible to apply the small-signal excision technique due to technical constraints, so we must use differential zeros to find the zero drift. Then, we eliminate the effect of zero sense using correction.
Since the introduction of electronics into differential pressure flowmeters, analog and logic circuits have been rapidly developed to provide a technical basis for small signal excision.
When the flow rate is close to zero, the slope of the curve dq/d A∆p – near infinity determines the instability of the flow rate zero indication value. The early differential pressure meter is 1.5 level of accuracy. Differential pressure zero point drift ± 1.5% is also considered normal in a period. Such a drift corresponding to the flow value is considerable—instrument manufacturers on the flow of the following 30% of the indicated value without counting the error. The former instrument engineers and early flow meter books and magazines said the differential pressure flow meter range ratio of 3:1 from.
When the electric unit combination instrument was developed for the DDZ-11 series, the differential pressure transmitter accuracy class was increased to 0.5. The instrument manufacturer-recommended adjusting the small-signal cut-off point to 1% of the upper differential pressure limit, corresponding to a low value of 10% FS.
When the differential pressure transmitter accuracy is further improved to 0.2 level, the differential pressure transmitter zero drift performance is further enhanced. However, the differential pressure transmitter zero drift is only one aspect of the differential pressure flow meter zero drift, differential pressure method flow measurement is a system, in addition to differential pressure transmitter zero drift, it also has a differential pressure signal transmission failure; however, its flow algorithm, etc. will also introduce zero drift; comprehensive multifaceted However, its flow rate calculators and so on also introduce zero drift; combining various factors, many instrument manufacturers around the world have reduced the differential pressure cut-off point to 0.75%, which corresponds to a flow rate value of 8.7%.
Instrument manufacturing technology is still improving, and now the differential pressure transmitter has been raised to 0.065 level, 0.04 level, and 0.02 level. Differential pressure signal transmission has also been improved. Due to the use of integrated structure, differential pressure signal transmission distortion can be negligible, so for the low differential pressure transmitter composition of integrated throttle flowmeter, small-signal cut value down to 2% max, our system can still Excellent work, will not appear “out of nothing” phenomenon.
2. Meaning and purpose of flow small-signal removal
2.1 The electromagnetic flowmeters
The main reasons for the zero point drift of electromagnetic flowmeters are the following.
2.1.1 An insulating layer has adhered to the electromagnetic surface.
2.1.2 There is a significant change in the conductivity of the fluid.
2.1.3 The electromagnetic flowmeter is subject to external interference.
2.1.4 The electrode sends out a millivolt signal. Therefore, it will have a zero-drift when amplifying and converting.
2.1.5 Electromagnetic flow tube upstream and downstream a section of the line is not filled with liquid and pipe pressure fluctuations, so that the average flow is zero. In contrast, the flow through the electrode axial flow rate, but there is a particular swing. This swing amplitude is small. However, we can still observe because the electromagnetic flowmeter accuracy has reached a very high level.
2.2 The vortex flowmeters
2.2.1 Why is there no zero-point drift in vortex flowmeters?
Some people say vortex flowmeter does not exist zero drift. Still, its flow signal with 4-20mA output, due to the analog circuit always has a certain amplitude of zero drift, so need to set a small signal cut, and when the flow signal with pulse form output must also select a small signal cut.
From the principle of vortex flowmeter, vortex flowmeter does not exist zero drift, and the reason is that when the flow rate is zero, the vortex generator behind the vortex will not be generated, so there is no pulse output. Therefore, it does not need to flow to zero. Therefore, as long as the fluid flow rate is low to a specific value, the corresponding Reynolds number is less than the turbulent flow interval, and the vortex will not already be generated.
2.2.2 why do we need to set up small-signal cut-outs?
The reason for this is simple: when the Reynolds number is deficient, it will take advantage of the situation, or interference from pipeline vibrations, interference from RF, etc.
2.3 The ultrasonic flowmeters
The ultrasonic flowmeter is a very widely used flowmeter. It uses the time difference method, which is based on determining the difference between ultrasonic downstream propagation time and countercurrent propagation time. It can measure the forward flow and reverse flow when the flow rate is zero; downstream propagation time and countercurrent propagation time should be equal. However, due to the incomplete symmetry of the transducer and performance time drift, temperature drift, which will also cause the zero drift of the flow indication, therefore, the flow transducer is also set small-signal excision—still, we excision point set to 1% FS completely.
2.4 The turbine flow meters
Turbine flowmeters are based on the force generated by the fluid on the turbine blades as it flows through the turbine flow sensor causing the turbine to rotate. When the flow rate is zero, and the flow rate is not zero but below the start-up sensitivity, the turbine will not rotate. The turbine will not rotate. When the flow rate is zero and when the flow rate is not zero, but it is below the start-up sensitivity, the turbine will not rotate, and therefore, it should not have a zero-drift problem. However, if the flow signal is output in the form of a 4-20m, zero drift will also occur due to the nature of the analog circuit, so a small signal cut of no more than 1% FS should also be introduced.