For flowmeter sensors that have been immersed in water for a long time, manufacturers use appropriate sealing structures and strict manufacturing processes to ensure their tightness. And in use, strict waterproof sealing measures are used to solve the sensor terminal box sealing of the cable outlet. It is customary to name the electromagnetic flowmeter used for immersion in water as an underwater type (or submerged type, buried type), and in accordance with the relevant IP68 enclosure protection in accordance with the IEC standard (IEC529-76) and national standard GB4208-84 Design and manufacture. Of course, the manufacturing cost of underwater models is higher than that of general electromagnetic flow meters. The electromagnetic flow sensor structure usually manufactured by the manufacturer is generally only suitable for the form of a splash-proof sealed structure. These enclosures are IP65 (splash-resistant) or IP67 (water-immersed).
广泛地用于城市供水、水处理过程中水的测量和计量。 Electromagnetic flow meters are widely used for water measurement and metering in urban water supply and water treatment processes. However, water pipelines in these industries are often buried underground. In this way, electromagnetic flow sensors are also often installed underground or in underground measurement wells. It is not difficult to imagine that long-term immersion in water, or due to the closure of the measuring well, the temperature inside the well is different from the outside temperature, resulting in the presence of a large amount of water vapor in the well. If an electromagnetic flow sensor of a general structure is used, water or water vapor will seep into the sensor when it is subjected to groundwater pressure and long-term immersion. The water vapor in the well may also penetrate the terminal box through the cable lead-out sleeve, thereby reducing the insulation resistance and insulation strength of the excitation coil to the ground, causing a large zero offset of the instrument, causing unreliable measurement and even measurement. Although the manufacturer can consider the sealing precautions of using the above sensors, often due to insufficient communication with the user, improper selection will occur. Or, the installation and commissioning personnel did not really understand the detailed requirements of the operation, which caused problems such as water in the sensor and moisture in the terminal. In this way, when the flow meter is put into operation, some strange phenomena may occur. Finding the cause of these failures often takes a lot of time and effort, and may also affect the normal operation of the production process. If we can have a deeper understanding of this kind of problem, that is to recognize the nature of the failure phenomenon, we must do the selection, manufacturing, installation, wiring and other debugging processes carefully and carefully according to the technical requirements of waterproof performance Work to prevent problems before they occur; and during the maintenance process, you can quickly and accurately troubleshoot based on theoretical guidance, which will play a positive role in the correct use and maintenance of electromagnetic flowmeters.
零点漂移的实例 Several examples of zero drift of electromagnetic flowmeter
1. A Sanshui plant in a certain city in Henan Province is a medium-sized modern water plant constructed by a foreign government loan. Among them, the surface water entering the plant is measured using a DN1200 electromagnetic flowmeter; the groundwater entering the plant is collected by several wells and measured using a DN900 electromagnetic flowmeter. Factory water metering connected in parallel uses two DN1000 electromagnetic flow meters. The four electromagnetic flowmeters are all advanced products imported from a famous foreign company. The flowmeter sensor installation complies with the electromagnetic flowmeter standard requirements. It is installed in measurement wells with more than 10 times upstream and 5 times downstream straight pipe sections. The measuring well also considers that a clamp-type ultrasonic flowmeter can be installed for comparison measurement with an electromagnetic flowmeter.
The system has been in normal operation for almost two years. It was found in November 1999 that the total amount of incoming water measurement was about 10% lower than the total amount of outgoing water measurement. On the surface, the flowmeter works fine. The problem is there, it is more difficult to find. Because general water plants have clear water tanks, especially for the case of unsatisfactory production load, the inlet pump may be turned on and off. Therefore, it is difficult to use the instantaneous amount of the meter to balance the amount of water in and out of the production process. By comparing with the ultrasonic flowmeter and using different combinations of opening and closing valves, we confirmed that the zero offset of the two factory water flowmeters occurred. One has an offset of up to 200 m3 / h and the other has an offset of up to 300 m3 / h. Consistent with about 10% higher display shipments. According to experience, we think that it may be caused by damp of the terminal and the insulation resistance of the excitation circuit falling to ground. Checking the sensor found that a round signal-specific cable should have been used. Two round shielded cables were used instead. In winter, the temperature of the measurement well is high, and the moisture enters the junction box along the cable from the tightly sealed terminal port. The excitation terminal is only 5 to 6 MΩ to ground. Dry with an electric hair dryer. After the insulation resistance of the excitation circuit to the ground rises to tens of MΩ, the zero point of the meter drops to zero, and the measurement returns to normal, which can correctly reflect the balance of the amount of water entering and leaving the production process.
2. A county-level water plant in Zhengzhou City, Henan Province used an LD-800 electromagnetic flowmeter.
The excitation terminal has only 5MΩ insulation resistance to ground. When the range is 1000 m3 / h, the result is a zero point display of 300 m3 / h. Zeroing with the converter cannot go down.
3. In a water plant in Shenzhen, Guangdong, after an LD-800 electromagnetic flow sensor entered the water, the insulation resistance of the excitation coil to ground dropped to 8MΩ. When the range is 1000 m3 / h, the result is a zero point display of 160 m3 / h. It cannot be lowered with zero adjustment.
4. A sewage treatment plant in a city in Zhejiang province introduced a DN1000 electromagnetic flowmeter terminal box into the water, untreated dry sealant, and the insulation resistance of the excitation terminal to the ground was only 2MΩ, resulting in a zero output of hundreds of m3 / h. The converter zero cannot be lowered.
Causes of the zero drift caused by the reduction of the insulation resistance of the excitation circuit to the ground due to the reduction of the insulation resistance of the magnetic flowmeter. The equivalent circuit of the sensor shown in Figure 1 can be used to analyze the cause of the zero drift. In the figure, Rj: insulation resistance of the excitation coil to the electrode; C0: distributed capacitance between the excitation coil and the electrode; Rs: internal resistance of the sensor induction signal. In fact, Rs is the sum of the capacitive reactances of the two measuring electrodes of the sensor to the capacitance formed between the measuring liquids. The volume resistance of the measurement liquid is regarded as approximately zero, and the measured liquid is used as the ground reference point. Therefore, the internal resistance of the signal in the figure is divided into two equal parts (except for the case where the electrode is contaminated); rd: sensor ground resistance. Es: Induction of the flow velocity signal, and is divided into differential signals + Es / 2 and-Es / 2 of equal size and opposite directions by the internal resistance of the signal. Generally, the insulation resistance Rj of the sensor designed and manufactured by the manufacturer to the ground is above 50MΩ; the value of the distributed capacitance C0 may be tens of μF. When the sensor is assembled, the method of shielding the electrode or the excitation coil can reduce the effect of the distributed capacitance to a small extent. In practical applications, the ground resistance rd of the sensor can be several Ω, with a maximum of 100 Ω. The internal resistance Rs of the sensor's induction signal can be obtained by the formula (1), where σ is the conductivity of the measured fluid, S / m, and d is the diameter of the sensor measurement electrode, m. For the measured fluid being tap water, its conductivity is about 10-2S / m. Usually, the diameter of the sensor measurement electrode is about 0.01∽0.02m. We take d = 0.016m to calculate the internal resistance Rs of the signal at about 6k25Ω. When the fluid fills the measuring pipe and does not flow, the induced velocity signal is zero. However, from the equivalent circuit diagram, we can see that, in fact, the insulation resistance and the internal resistance of the signal divided into two parts form the voltage division relationship of the formula (2). On the electrode, a part of the excitation voltage ec is divided to the ground. For two electrodes, the voltages ec they distribute to ground are equal in magnitude and in the same direction, forming interference signals. This is commonly referred to as common-mode interference voltage. Of course, here we only talk about this form of common mode interference. Other forms are not discussed here. Almost all signal flow amplifiers of electromagnetic flow converters use a preamplifier circuit with a high common-mode rejection ratio to suppress common-mode interference signals and reduce their mixing into the velocity signal. The preamplifier circuit with high common-mode rejection ratio amplifies the difference of the differential flow velocity signal; for common-mode interference signals, the difference is zero, and the differential amplifying circuit plays an attenuation role. Due to the imperfect symmetry of the actual differential circuit components, the common-mode interference after differential amplification will have a small residual output. For the characteristics of the differential amplifier circuit, we use the common mode rejection ratio CMRR to characterize it. CMRR is the logarithm of the ratio of the amplification factor kd of the differential signal to the amplification factor kc of the common-mode interference signal (far less than 1), and multiplying by 20, the unit is decibel (dB). At present, the common-mode rejection ratio of electromagnetic flow converters is around 80dB. For electromagnetic flux converters that require high requirements such as capacitive transmission types and use power supply floating ground, the common mode rejection ratio can reach more than 120dB. According to the CMRR definition, for a general CMRR = 80dB electromagnetic flowmeter, take the differential flow velocity signal amplification factor kd = 1, then the common mode amplification factor kc = 0.0001. This means that the converter preamplifier circuit can reduce the common-mode interference signal voltage rate by 10,000 times. When the excitation coil is connected to ground (calculated as above, the electrode is only 3k2Ω to ground, it can also be regarded as the excitation coil counter electrode) when the insulation resistance Rj = 50MΩ and the excitation voltage Ec = 30V, calculated by formula (2), common mode interference voltage ec = 19.2mV. Attenuated by a preamplifier 10,000 times, only 1.92μV. For a typical electromagnetic flowmeter that senses a 0.2mV voltage at 1m / s, the residual common-mode interference voltage accounts for only 1%. Zero can be lowered to zero. For the case where the insulation resistance of the terminal due to moisture, water in the sensor, etc. is below 10MΩ, for example, Rc = 5MΩ, the common-mode interference voltage ec = 192mV, and after the attenuation of the preamplifier by 10,000 times, the residual common-mode interference voltage 19.2 μV. For 0.2m signal voltage of 1m / s, it accounts for 10%. The results are basically consistent with the above examples.
上面解析，说明了励磁线圈对电极绝缘电阻降低引起零点偏移的实质是共模干扰电压增大所致。 The above analysis of the electromagnetic flowmeter shows that the reduction of the insulation resistance of the excitation coil to the electrode causes the zero offset to be caused by an increase in the common mode interference voltage. This reminds us that in applications that are susceptible to moisture and water, we must use products that meet the IP68 underwater structure. During the installation process, care must be taken to protect the terminal box from water and moisture before packaging the terminal box. Operate in strict accordance with the technical specifications of the installation and operation manual provided by the manufacturer.
Of course, in the operation of the electromagnetic flowmeter, when the zero offset or the flow increases, we can first check the insulation resistance of the terminal or the coil to ground. Because this check method is very convenient and the problem handling is simple.