Electromagnetic flowmeter signal accuracy and DC noise interference

Since the advent of electromagnetic flowmeters in 1950, with the development of electronic technology and computer digital technology, electromagnetic flowmeters have now become very rich in varieties and types, and the production technology is also quite mature and perfect. They are welcomed by the flowmeter instrument market. One of the varieties. The original working model of the electromagnetic flowmeter was proposed by Faraday. As the pioneer and founder of the principle of electromagnetic induction, Faraday made a huge contribution to mankind's move from the era of lead and fire to the era of light and electricity. There is an electromagnetic flowmeter in human history. The measurement case was also carried out by Faraday in the Thames River in 1832. At that time, he was conducting a measurement experiment of river water velocity, but the experiment was not successful. The reason was that the DC signal contained a drifting DC polarization voltage. The value is difficult to distinguish from the signal. Although later electromagnetic flowmeters have experienced technological progress and development such as AC excitation and low-frequency rectangular wave excitation, it is difficult to distinguish the orthogonal interference and in-phase interference caused by electromagnetic induction and the series mode interference and common mode interference caused by electrostatic induction. It also effectively solves problems such as zero point instability and measurement output swing caused by the sharp interference caused by the friction of the slurry on the measuring electrode. For measuring electrolyte fluid, the DC polarization voltage that drifts between the grounded (liquid-contacted) component and the measuring electrode still exists, which will still affect whether the reference point of the flow signal is stable or not, thereby affecting the stability and reliability of the output signal.

Therefore, it is necessary to correctly understand the benchmark of traffic signals and take effective solutions. It can be known from electrical knowledge that for the measurement of electromagnetic flow signals as electromotive force, it is important to have a stable potential difference reference point, that is, the signal must be well grounded. In the past, some people often only pursued the grounding resistance as small as possible, thinking that they could get a stable flow signal. In fact, it is not the case. The conductive fluid medium is more important as the reference point of the signal. Based on many years of experience in researching and applying electromagnetic flowmeters, this paper analyzes the actual measurement problems encountered on site, tries to understand the importance of conductive fluid as a reference point for signals, and provides methods for connecting liquid at reference points for reference. For more detailed instructions on electromagnetic flowmeter product selection, please click to enter 1. The conductive fluid is the reference potential point* of the flow signal voltage. For a voltage signal, there is always a reference "ground" point. and a changing "signal" endpoint to form a potential difference. The early electromagnetic flow sensors used one measuring electrode as the "ground" point of the signal and the other measuring electrode as the "signal" point. This type of signal transmission is called a "single-ended signal", and like other voltage signals, it can be illustrated in Figure 1a. The amplification of single-ended signals is to superimpose various DC and AC interference voltages and signals and input them to the amplifier input terminal at the same time. Usually, we call these interferences series-mode interference, normal interference or lateral interference, etc. Amplifiers have difficulty separating interference from signals, whichThese interference signals are often very large in amplitude, much larger than millivolt or microvolt level flow signals. As a result, these interferences cause distortion of the amplified signal, making the amplifier saturated and blocked, so that it cannot work. The flow signal of modern electromagnetic flowmeter is transmitted from the sensor to the conversion amplifier in differential form. Like other differential voltage measurements, the two electrodes that pick up the electromagnetic flow signal are not directly connected to the signal "ground" of the conversion amplifier. Instead, the "zero resistance" fluid medium is connected to the signal "ground" terminal of the conversion amplifier. Figure 1b shows the equivalent circuit of this differential flow signal. The signals entering the two signal terminals of the differential signal amplifier have equal amplitude and opposite polarity to the "ground" terminal. What the differential amplifier amplifies is the difference between the signal terminals of the two electrodes.

Therefore, for the flow signal, the differential amplifier is in an amplified state. However, for common-mode interference with equal amplitude and same polarity, the difference entering the differential amplifier is almost zero, and the output is almost zero. The differential amplifier attenuates common mode interference. Although due to reasons such as the ground current of the ground loop, the polarization voltage, the electrostatic coupling between the excitation power supply and the electrode, when the differential flow signal contains common mode interference, as long as the parameters of the voltage amplifier are symmetrical, unless the common mode interference can be converted into a certain Series mode interference, these interference will not affect signal amplification. In fact, with the development of integrated op amp circuit manufacturing technology, the common mode rejection ratio of the device is getting higher and higher. If measures such as power supply floating circuit are adopted, the common mode rejection ratio will be higher, and the measurement accuracy will become higher and higher. high. In the application of electromagnetic flowmeter, the signal reference and DC noise conductive fluid medium as the reference point of the signal can divide the flow signal into a differential differential state, and it has been repeatedly emphasized that the measured fluid must be reliably connected to the ground terminal of the signal conversion amplifier. This is because the change in the reference point of the differential signal will cause the common-mode interference that originally had the same voltage amplitude and polarity to become differential-mode interference voltages with different amplitudes, that is, it will be converted into series-mode interference. As mentioned earlier, the differential amplifier at this time is powerless to suppress series-mode interference. 2 Reliable signal reference and correct grounding It is emphasized again that the measured liquid conductive medium is regarded as zero resistance, and then used as the reference point of the differential flow signal. Theoretically, the smaller the reference point value, the better. The smaller the resistance value, the closer to zero, and the more equal the differential signal amplitudes will be.

This means that the conductive fluid to be measured should be in a large-area container or in a long pipeline. It has been analyzed in the literature [1]. The volume resistance Rt of the fluid can be obtained from the resistivity formula: Signal reference and DC noise in the application of electromagnetic flowmeter. Here, the conductor length is the inner diameter D of the measurement pipe, and the resistivity of the conductor material is the electrical conductivity. The reciprocal of σ, the pipe length is recorded as l. Generally speaking, liquid transportation pipelines are connected to the earth. This requirement, which assumes that the volume resistance of the fluid is zero, is relatively easy to achieve. However, in some simulation tests, using a bucket of water or a basin of water may not necessarily satisfy this requirement.One request. With the conductive liquid as the signal reference, this reference must be led to the midpoint of the differential signal terminal using the correct method. In practical applications, the following methods are used to derive the measured fluid medium as the signal reference point of the electromagnetic flowmeter: the flow sensor is installed in a pipe with metal pipes in front and behind it. At this time, the conductive fluid can pass through the metal pipes in front and behind the flow sensor and be electrically connected to it.

Connect, then electrically connect the front and rear pipes to the sensor's ground terminal with a wire. Sometimes, this situation does not necessarily ensure that the sensor is electrically connected to the front and rear pipes, because the insulating lining and insulating gasket of the sensor may still electrically isolate the sensor from the front and rear pipes. In this case, metal wires need to be used to connect the front and rear pipes to the sensor. . When the pipes before and after the sensor are non-metallic or the inner wall of the metal pipe is lined with an insulating lining, a flowmeter with a metal grounding ring connected to the front and rear flanges of the sensor should be used. The conductive fluid is connected to it by means of a metal grounding ring (more accurately called a wetted ring). Then, use a grounding ring to connect to the sensor signal ground. For situations where the conductivity of the measured fluid is relatively low, due to the relatively large volume resistance of the liquid, a conductive metal short pipe can be used instead of the grounding ring. In some cases, such as the measurement of highly corrosive liquids, in order to save expensive metal materials, a ground (liquid contact) electrode method can be used to connect the reference to the sensor ground point. Because this method often measures the conductivity of corrosive liquids is relatively high, and the volume resistance of the liquid is very small, so just use a point electrode to connect it.

Of course, in practical applications, in addition to the fluid being grounded as a signal reference, it is also important to note that the front and rear pipes are metal pipes, and the front and rear pipes should be well electrically connected to the sensor. This is because there are often ground currents, stray currents, and three-phase unbalanced currents in metal pipelines. These currents will form a large voltage drop in the pipelines at both ends that are not well electrically connected to the sensor measurement tubes, forming a large common mode. The voltage is then added to the signal electrode through the ground resistance to affect the measurement. It should also be noted that for the purpose of anti-corrosion or electrolytic wastewater measurement, cathodic protection current and large DC current may flow through the front and rear metal pipelines. In this case, large area copper plates with low resistance should be used to connect the front and rear metal pipelines. up, allowing large current to flow through the copper plate bypass, without forming a large voltage drop on the sensor measuring tube. As for the grounding resistance, as long as the sensor, front and rear metal pipes, and grounding rings are connected to the earth according to the principle of the one-point grounding method, the grounding resistance size requirements are not strict. Under normal circumstances, the grounding resistance is below 100Ω. If there are explosion-proof requirements, it should be less than 10Ω. 3 DC noise 3.1 Polarization voltage in fluid We know that when the electrode is buried in the electrolyte liquid, positive and negative ions will move directionally, and a certain electric field will be formed between the electrode and the fluid medium. This is what is commonly known as polarization. This phenomenon can be observed through an experiment. When the test pen of a millivolt voltmeter (voltage range of a digital multimeter) is inserted into a glass of water, the voltmeter can read the voltage value. This is because the materials of the voltmeter test pens are different, and the polarization potential formed on the test pens is different.A potential difference is formed. If the electrode and the grounding ring (metal pipe, grounding electrode) are made of different materials, the magnitude and direction of the polarization voltage formed will be different. Polarizing voltage is a drifting DC voltage. As shown in Figure 2, the voltages of the measuring electrode, metal pipe (or ground ring, ground electrode) to the fluid (regarded as a resistance of 0Ψ) are e1, e2 and e3 respectively. It can be seen that e3 is the common mode voltage, which is superimposed with the differential flow signals e1 and e2 respectively, and enters the differential amplifier of the converter. Excessive polarization voltage (for example, the situation we analyze below may be as high as hundreds of mV) directly enters the differential amplifier and often blocks the amplifier, and the flow signal cannot be amplified. Even if it can be amplified, since the superimposed common mode voltage drifts and changes, the output swing of the flow signal is also very large.

In this way, how to reduce the polarization voltage is very important. Signal Reference and DC Noise in Electromagnetic Flowmeter Applications When any metal is immersed in an electrolytic solution, its charged positive ions tend to dissolve while the metal itself maintains a negative charge, which forms an electrode with a certain potential. This kind of electrode forms a potential difference in the medium and generates current, which causes the electrode to continue to dissolve, that is, to continue to corrode. This is the process of electrochemistry. The potential of the formed electrode can be expressed by the Nernst equation [2]: Signal reference and DC noise in electromagnetic flowmeter applications. In the formula: n is the valence of the metal; T is the temperature; R is the molar constant of the ideal gas, 8.31 Joules/ Mol·K; F is Faraday’s constant; C is the constant of metal ion concentration; c is the activity of metal ions in solution. The potential of the ion standard solution under study is called the standard potential, represented by E0, so the electrode potential of the metal at 25 hours is Standard potential (see Table 1). The signal reference and DC noise in the application of electromagnetic flowmeters are based on metal materials science [3]. Adding another alloy material to a metal can increase the electrode potential of the matrix. For example, when 11.7% chromium is dissolved in ferrite, the electrode potential will jump from -0.56V to +0.20V. Adding a large amount of chromium or chromium-nickel alloy enables the steel to form a single-phase austenite structure to avoid the formation of micro-batteries and reduce the DC polarization voltage, thus significantly improving corrosion resistance. 3.2 Reduction of DC noise According to the polarization potential of metal materials introduced above, and combined with Figure 2, it can be seen that when two metals of different materials are contacted in the same electrolyte fluid, the direction and size of their polarization potentials are different. The voltage magnitude and polarity between two metal electrodes change with the direction and magnitude of the polarization potential. For example, the material of the measuring electrode is stainless steel containing chromium and nickel, and their potential to the measured fluid medium is + 0.2V; the front and rear pipes in contact with the liquid are carbon steel, and their potential to the measured fluid medium is - 0.58V. Then, it can be calculated from Figure 2 that the voltage of the measuring electrode butt liquid pipe is + 0.78V. If you do not use the front and rear metal pipes as the reference point connection method, but use a grounding ring, the material of the grounding ring should also be the same stainless steel containing chromium and nickel as the measuring electrode. At this time, the measurementThe voltage of the measurement electrode to the reference point will become 0V.

In other words, DC common mode interference is reduced. On the contrary, if the electrode material is more expensive, such as tantalum or platinum, and the material of the metal wetted parts is carbon steel or stainless steel, the DC noise on the measuring electrode will also be large. When measuring highly corrosive media such as hydrochloric acid and sulfuric acid, although the measuring electrode is made of tantalum or platinum and can withstand strong acid corrosion, the metal wetted parts are made of carbon steel or stainless steel and cannot withstand strong acid corrosion, and the DC noise also increases. , a large swing in the output will occur. Therefore, while paying attention to the measurement electrode not being corroded, we must also pay attention to the corrosion resistance of the material of the liquid contact ring of the signal reference. It can be seen from equation (2) that the polarization potential is affected by temperature (in the equation, T is the temperature). This shows that DC noise is related to temperature and is a drift amount. Its presence will cause the flowmeter to drift and oscillate. Therefore, in addition to reducing the polarization voltage, the converter must be able to have capacitors for DC noise isolation to prevent it from entering the amplifier and being amplified. 4 Conclusion DC noise is very important to the stability of the reference of electromagnetic flow signal. The causes of DC noise are not limited to metal polarization voltage (material corrosion) of wetted parts, but also include geomagnetic induction voltage, thermoelectric potential, contact potential, electrode pollution and many other causes. Here, we won’t discuss it much.