Small signal excision in flow meter

Through the analysis of flow measurement principle, field conditions and measurement requirements, the Flow Meter is selected correctly, so that the performance of the selected instrument can meet the requirements of the actual operation parameters, and the purpose of accurate measurement is achieved. Based on the working practice, this paper discusses the optimization of small signal excision of flow measuring instrument.
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Abstract: The small signal excision in flow meter is to overcome the adverse consequences caused by the small signal caused by various reasons. Various flowmeters with different principles have different mechanisms for producing small signal phenomena. The same principle of the flow meter, due to its accuracy, instrument quality, measured media, field environment and installation differences, so the zero point uncertainty is also very different. The debugging personnel should reasonably set the cutting point according to the specific requirements and specific conditions.
Key words: flow measurement zero drift small signal excision optimization

For steam measurement, the standard orifice flowmeter is used, and the measuring range is 0~20t/h. After running for a period of time, it was found that when the user's main production device was running, the flow meter could be measured normally, and when the production device was stopped, the meter detected the flow rate, indicating that there was a deviation in the measurement. In order to maintain the principle of justice, the small signal cutting point was reasonably determined, and the steam used in the operation results could be shown truthfully.

1 Small signal removal of Differential Pressure Flowmeter

Differential pressure flowmeter is a variety of flowmeters put into industrial applications, the most widely used, the longest history of a flowmeter, is also the earliest application of small signal cutting technology a flowmeter.
In the use of the old differential pressure flowmeter to display instantaneous flow and integration, the user is always dissatisfied with the zero drift of the flowmeter, the differential pressure meter is very small zero drift will lead to the actual flow of zero, there is a large flow indication, since the successful application of small signal cutting method in the instrument, can cover up the contradiction of zero drift.
The importance of the small signal cutting method in the differential pressure flowmeter is determined by the square root characteristic of the input-output relationship of the differential pressure flowmeter. In the differential pressure flowmeter there is the following relationship:
Pictures (1)
In the formula, q is the flow indication value, and the unit qma is consistent with; A△P is the differential pressure signal after dimensionless processing, 0~1; qmax is the upper limit of flow measurement, such as kg/h or N/s. The relation corresponding to equation (1) is shown in Figure 1.
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FIG. 1 Relationship between flow rate q and differential pressure A△P
As can be seen from Figure 1, when the flow rate is close to 0, the slope of the curve dq/dA△P is approximately infinite, which determines the instability of the flow indication. The accuracy of the early differential pressure meter is 1.5, that is, in a period of time, the differential pressure indicating the zero drift of ±1.5% is also normal, and the corresponding flow value of such a drift is 12.2%.
When the accuracy of the Differential Pressure Transmitter is further improved to 0.2, the zero drift performance of the differential Pressure Transmitter is further improved, but the zero drift of the differential Pressure transmitter is only one aspect of the zero drift of the differential pressure flowmeter. Differential pressure flowmeter is a system that in addition to differential pressure transmitter zero drift, differential pressure signal transmission distortion and flow calculus will also introduce zero drift. Combining various factors, many instrument manufacturers in the world have reduced the differential pressure cutting point to 0.75%, and the corresponding flow value is 8.7%. Instrument manufacturing technology is still improving, and now the differential pressure transmitter has been raised to 0.075, and the differential pressure signal transmission has also been improved. For the integrated throttling flowmeter composed of low and medium differential pressure transmitters, the small signal cutting value is reduced to 2%qmax, and the system can still work well.

1.1 Electromagnetic Flowmeter

The main reasons for electromagnetic flowmeter zero drift are as follows:
(1) Flow measuring tube up and down a certain section of the tube is not filled with liquid, and the axial flow rate flowing through the electrode has a certain amplitude of swing. Although this swing amplitude is small, it can still be observed because the accuracy of the electromagnetic flowmeter has reached a very high level.
(2) The electrode surface is adhered to a layer of insulation.
(3) There is a significant change in the conductivity of the fluid.
(4) The millivolt signal sent by the electrode has zero drift during amplification and conversion.
(5) External interference.
The small signal cutting point of the electromagnetic flowmeter is generally much smaller than the differential pressure flowmeter, and it can meet the requirements when set at 1%FS.

1.2 Vortex flow meter

Vortex flowmeter is said to have no zero drift, but when the flow signal is output with 4~20mA, because the analog circuit always has a certain amplitude of zero drift, it is necessary to set a small signal cutting, and when the flow signal is output in the form of a pulse, it must also set a small signal cutting. Since the pulse signal output of the probe of the vortex flowmeter is proportional to the square of the flow rate of the fluid flowing through the vortex generator, the amplitude of the signal output of the probe is larger, so the signal-to-noise ratio is also larger, and interference is not easy to invade, and when the flow rate is low and interference is easy to invade, the amplitude of the pulse signal is proportional to the square of the flow rate of the fluid flowing through the vortex generator. The small signal (characterized by frequency) excising method is used, so that the instrument can ensure normal measurement when the flow rate is high, and can achieve a stable indication of zero when the flow rate is lower than the promised value and the corresponding value of the minimum flow can be measured.

2. Realization method and selection of flow small signal cutting

2.1 Implementation method in non-intelligent instrument

Before the CPU enters the flow measuring instrument, the small signal cutting is implemented by hardware method. The schematic diagram of the typical small signal cutting circuit in the open square integrator of DDZ-III series instrument is shown in Figure 2.
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FIG. 2 Schematic diagram of small signal cutting circuit in square integrator
It consists of a comparison circuit and an electronic switch. The VL in the figure is composed of a high stability voltage source and a divider circuit containing a resistor and potentiometer to provide a small signal cutting set point. Since the flow measuring instrument adopts CPU, the small signal cutting is realized by software method, which is not only reliable, but also easy to operate. As long as you select the cutting mode and the cutting point value in the dialog box or menu, the cutting operation can be completed accurately after the instrument runs.

2.2 Selection of excision mode

The relationship curve between differential pressure and flow under two typical switching modes of intelligent differential pressure transmitter is shown in Figure 3. Among them, Figure 3a shows that the square root relationship is switched to zero, and the cutting point can be selected either by the manufacturer's recommended value or by itself. The advantage of this switching mode is that after the flow signal is lower than the set value, the transmitter can stably output a zero signal (4mA or digital signal), but when the actual flow is indeed zero (for example, the cut-off valve has been closed and confirmed that there is no leakage), what is the zero drift value, but it is completely unknown. If the switching mode in Figure 3b is selected, Then the drift can be observed from the residual output of the transmitter. The residual output, though small, is not zero. Since the output signal of the differential pressure transmitter is generally sent to the post-stage instrument such as flow calculator, integrator or DCS, PLC, it is sufficient to set the cutting point at 0.75%FS of the input signal in the post-stage instrument.
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FIG. 3 Flow differential pressure curve of two switching modes

3 Optimization of cutting point selection

Flow small signal excise has the concept of optimization, is a method to solve the flowmeter drift to cover up the contradiction, not the use of small signal excise after the instrument zero drift does not exist, but zero drift is still, but do not let the user see it, if you do not set small signal excise to meet the requirements, it is best not to excise. However, the zero drift and time drift of the simulator are often bidirectional, and the transmission distortion of the differential pressure signal and the intrusion of vibration interference are uncertain factors, so the small signal excision is necessary. For small signals to be excised or not, how to take the value between large and small excisions, this is the concept of optimization. The general principle is to choose the excision value as small as possible under the premise that the purpose can be achieved.

4 Closing remarks

Small signal excision in measuring instrument is a special need in flow measurement, and the mechanism of small signal phenomenon is different in different flowmeters. The same principle of the flow meter, due to its accuracy, instrument quality, measured media, field environment and installation differences, the zero point uncertainty is also very different. There is no fixed formula for the optimization of flow small signal excision, it is just to find an optimal point between the use requirements and the measurable minimum flow that can be achieved, and the debugger reasonably sets the excision point according to the specific requirements and specific conditions, usually determined by the field debugging.