ADMAG AXR Two-wire Magnetic Flow Meter

Yutaka Aoyama¹ Fujikazu Sugawara¹ Tooru Shimura¹ Yuuichi Kaneko¹ Hisashi Noda¹ Akio Yasumatsu¹

Yokogawa has developed the ADMAG AXR magnetic flow meter, the world's first two-wire system applying the dual frequency excitation method. In order to achieve steady flow measurement in a two-wire system with limited power consumption, the S/N ratio was improved by reducing power consumption and converter noise, improving excitation efficiency of the detector by using a high-density coil, and lowering noise caused by the electrodes of the detector. This report describes the technologies used in the ADMAG AXR.

1. Field Instruments Business Center, Industrial Automation Business Headquarters

INTRODUCTION

Figure 1 External Views of ADMAG AXR

More than half a century has passed since Yokogawa developed Japan's first magnetic flow meter for industrial use in 1955, and today, Yokogawa's magnetic flow meters are widely used in a variety of applications thanks to features such as low pressure loss and maintenance-free operation due to no obstacles and moving parts in the fluid flowing pipeline.

While previous magnetic flow meters require a total of four wires, a pair of wires each for power supply and output, a two-wire system that supplies power and provides current output with a pair of wires as in transmitters and other flow meters has been much awaited because of lower wiring costs. Since two-wire systems supply power and provide output with a small current of 4-20 mA, they consume less power compared to four-wire systems that require commercial power, and this helps reduce running costs. However, they had problems with performance aspects such as accuracy and output stability, which were caused by the limitation of the about 50 times lower power consumption.

Yokogawa has solved these problems and developed the ADMAG AXR Two-wire Magnetic Flow Meter with performance equivalent to the four-wire systems^{1 2}. Figure 1 shows external views. This report describes the features and key technologies used in the ADMAG AXR.

KEY TECHNOLOGIES USED IN ADMAG AXR

Figure 2 Measurement Principle of Magnetic Flow Meter

The following describes key technologies used in the converter and detector to increase the performance of the ADMAG AXR.

Key Technologies Used in Converter

Dual Frequency Excitation Method

Figure 2 shows the measurement principle of a magnetic flow meter. Current is applied to the coil in the flow meter to generate a magnetic field in the pipe (excitation). When fluid flows through the pipe, an electromotive force that is proportional to the flow rate is generated according to the Faraday's law of electromagnetic induction. This is detected by the electrodes.

The followings are fluid noises caused by fluid itself that may disturb the flow measurements except for the electrical noise from the surrounding environment.

• Slurry noise generated by the collision of solid particles with the electrode
• Flow noise generated by the friction between the fluid and pipe wall
• Noise generated by an sudden change in the conductivity

Figure 3 illustrates the slurry noise and flow noise.

Figure 3 Fluid Noises	Figure 4 Frequency Characteristic of Fluid Noise

These noises have a 1/f characteristic with which the fluid noise decreases as the frequency increases as shown in Figure 4. Therefore, increasing the excitation frequency improves the S/N ratio.

Figure 5 Comparison of Excitation Methods

Still, with high excitation frequency alone, differential noise generated by a temporal change in the magnetic field strength does not attenuate sufficiently in the sampling interval, and so it is difficult to stabilize the zero point and the accuracy is low. To overcome this disadvantage keeping the advantage of the high S/N ratio, Yokogawa adopted a unique method called a dual frequency excitation method in the four-wire systems, which performs excitation by a current with two frequency components in which the high frequency is superimposed on the low frequency, thus achieving high accuracy and high noise resistance³. Figure 5 shows a comparison of the excitation methods.

During the ADMAG AXR development, we drastically reduced the power consumption of the electronic circuits and improved the efficiency of the signal processing software in order to adopt dual frequency excitation method, and released the world's first two-wire magnetic flow meter using the dual frequency excitation method.

Excitation Currents
Switching To effectively use power in a two-wire system with a limited power supply which varies according to the output, three levels of excitation current are switched according to the output as shown in Figure 6.

Figure 6 Flow Meter Output and Excitation Current

Although calibration time is required to prevent errors caused by the small magnetic circuit nonlinearity due to the excitation current switching, flow calibration with actual fluid is performed with each excitation current.

Employing Low Noise Amplifier
Since the signal electromotive force is weak, noise in the converter also affects the flow measurements. Therefore, a low-noise and low-power consumption operational amplifier was selected as the head amplifier in the signal input part.

Removing Electrical Noise and Increasing Immunity from Electrical Noise
Since the signal electromotive force is weak, two-wire systems are susceptible to the effect of electrical noise from the surrounding environment in principle. Accordingly, the ADMAG AXR utilizes feedthrough capacitors in the wall separating the terminal box of the case to remove noise, and the filter and shield of the converter circuit are designed to improve the electrical noise resistance to almost the same level as that of four-wire systems.

Improving Detector S/N Ratio
The proven converter of the ADMAG AXF four-wire magnetic flow meter was redesigned to improve the S/N ratio.

Figure 7 Magnetic Circuit of ADMAG AXR

Increasing Signal Electromotive Force
The basic structure of the magnetic circuit is based on that of the AXF, which has a frequency characteristic capable of high frequency excitation required for dual frequency excitation and a structure where the optimization of the magnetic field distribution to increase the accuracy is easy by changing the aperture angle of the plate core⁴. Figure 7 shows the magnetic circuit of the ADMAG AXR and its periphery.

Since the excitation current of the ADMAG AXR is limited to less than or equal to one-eighth of the previous four-wire systems due to the available power, the magnetic field generated by the excitation is small. To compensate for that, the magnetic field strength was increased by using a conductor with a larger diameter than that of the four-wire systems and increasing the number of turns of the coil without increasing coil resistance. Since the coil space is limited for the ADMAG AXR, a high-density coil was developed. The base of the high-density coil is a rectangular air-core coil with regular alignment that requires advanced technology for formation, and it is transformed to a saddle coil while keeping the alignment intact. Figure 8 shows the external view of the high-density coil and its enlarged cross-section view.

Figure 8 High-Density Coil and Its Cross-Section

For the production of ADMAG AXR, new coil winding and forming machines and jigs were developed and optimized. This enabled mass production of the high density coils, in which the conductor ratio per unit cross-sectional area of the coil (conductor space factor) increased by 30%. Moreover, a change of the coil dimensions increased the number of turns of the coil, which resulted in an about 50% increase in the signal electromotive force per unit current of the ADMAG AXR compared to the previous four-wire ADMAG AXF.

Low-noise Electrodes
Since the fluid noise superimposed on the signal mainly depends on the flow velocity regardless of the magnetic field, the effect of noise is relatively large in two-wire systems in which the signal electromotive force is small. Therefore, the electrode was changed to an insert type and the fluid contact area was expanded from 3 mm to 10 mm in diameter in order to reduce noise by averaging. Moreover, polishing and oxidizing treatment for the surface of the electrode were added in order to reduce noise.

Figure 9 XPS Surface Analysis Results of Stainless Steel Electrode

 

Figure 10 Internal Lining Surface

Figure 9 shows the results of X-ray photoelectron spectroscopy (XPS) analysis of stainless steel electrode in the direction of depth. A chrome-rich layer (Cr > Fe) is found in the surface layer, which is not found in the untreated surface, thus a thin and dense passivation film is formed, reducing the noise of the electrode.

Mirror-finished Lining Surface
Flow noise produced by friction between the fluid and pipe wall is also affected by the surface roughness of the pipeline. Therefore, for sizes up to 100 mm susceptible to the internal pipeline surface, the injection molding die and molding conditions for the PFA lining were improved and mirror-finished lining surface is provided as standard. Figure 10 shows the effects on the internal pipeline surface and internal surface of the ADMAG AXR.

IMPROVED OPERABILITY

Figure 11 Typical LCD Panel Displays

Development of the ADMAG AXR placed emphasis not only on enhancing performance but also usability and features like the development of the previous models.

A Variety of Displays

As shown in Figure 11, the ADMAG AXR is equipped with a full-dot matrix LCD, which has a good reputation with the four-wire AXF. The LCD displays measured flow rates and serves as an interface for setting parameters as well.

The power consumption is minimized despite a 128 x 64 dot wide display, and the LCD panel is designed to be able to withstand high temperature and humidity considering the installation environment of the flow meter. Since the specifications required for the LCD panel for durability at a high temperature and humidity is stricter than those for automotive use, Yokogawa worked closely together with a panel manufacturer to design the panel.

As shown in the figure, the panel displays from one to three lines, the character size and items (current flow rate, current flow rate in %, and accumulated flow volume) are selectable, and the bar graph can also be displayed. The display language is selectable from six languages, including English and Japanese (katakana). The panel displays not only the names of the parameters to be set and items to be selected, but also the alarm information with countermeasure when an alarm occurs.

Operational Switch

An infrared switch was used in the four-wire AXF to make it possible to operate without opening the cover while operating in explosion hazardous areas and outdoors. Since the power consumption is limited with the ADMAG AXR, new magnet switches utilizing a hall effect device are added to the push button switches instead of infrared switches. Figure 12 shows operations with magnet and push button switches.

PERFORMANCE VERIFICATION

Figure 12 Setting Switch Operations	Figure 13 Accuracy Test Results

 

Figure 14 Output Stability Comparison at Low Conductivity (Measuring Object: Water, Conductivity: 10μS/cm)

The following shows some test results of the newly developed ADMAG AXR. Accuracy Figure 13 shows the results of flow measurement accuracy testing with a typical size. The red vertical bar in the figure shows 3 s variation at the flow velocity points. The variation is somewhat larger in the low flow velocity range in which the excitation current is low but well within the specified accuracy range.

Output Stability

Output fluctuations in flow measurements generally tend to be larger in two-wire systems in which the signal electromotive force is low because of limited excitation current; in particular, significantly larger at a high flow velocity of low-conductivity fluids.

Figure 14 shows the output of the newly developed ADMAG AXR, four-wire AXF, and two-wire flow meters of third parties. With all the flow meters, output fluctuations increase as the flow velocity increases. However, the ADMAG AXR is nearly as stable as the four-wire AXF, demonstrating the effect of the dual frequency excitation.

Conductivity Fluctuation Test

A test simulating the chemical injection process was conducted by intermittently injecting high-concentrated salt water into the flow of tap water. The output does not change significantly as a result of the injection of highly concentrated salt water as shown in Figure 15.

Slurry Fluid Test

Figure 15 Conductivity Fluctuation Test	Figure 16 Actual Flow Test of Plastic Slurry

The quantitative effect of slurry noise produced by the collision of solid particles with the electrode varies greatly depending on the type and amount of solid particles. The effect of the dual frequency excitation method is large for a relatively light slurry consisting of plastic pieces and the effect on the output is insignificant. Figure 16 shows the actual flow test results of plastic slurry.

Figure 17 Actual Fly Ash Flow Test

However, hard fly ash (generated in combustion) has a large effect on the output because it damages the surfaces of electrodes. Therefore, it is advisable to use the four-wire AXF with large signal electromotive force when measuring fluid including hard fly ash. Figure 17 shows the actual flow test results of fly ash.

Electrical Noise Test

Figure 18 shows a radiated immunity test environment and Figure 19 shows the results of the test. This test was conducted by generating radio waves at the frequencies of such as radio devices and cell phones using the antenna and confirming the output of the ADMAG AXR. The output is stable as shown in the chart, with no abrupt change in the output at any of the frequencies.

Figure 18 Radiated Immunity Test Environment	Figure 19 Radiated Immunity Test Results

FIELD TESTS

Figure 20 Field Test with Cooling Water Line

Not only internal tests but also field tests were conducted in actual plants in order to confirm reliability and long-term stability.

Cooling Water Line

Figure 20 shows an cooling water line (Size: 80 mm) with an ADMAG AXR and four-wire AXF installed in series. In this application, the flow rate changes every hour from 30 m³/h, nominal flow rate, to 45 m³/h in a short interval. The output readings of both flow meters are nearly identical to each other.

Dust Collection Water Line

Figure 21 Field Test with Dust Collection Water Line

This test was conducted jointly with a Yokogawa's customer who is a steel maker by installing an AXR in place of the existing four-wire magnetic flow meter on a dust collection water line (Size: 80 mm) for a steel converter. Figure 21 shows the environment and results of the test.

In spite of a harsh environment with contamination and conductive adhesion of solid materials, the output of the ADMAG AXR was always stable during the six-month test period. Adhesion on the lining was relatively less than that of the existing flow meter, thus the improvement effect of the smoothing of the internal lining surface against adhesion was confirmed. Figure 22 shows the internal lining surface adhesion.

CONCLUSION

Figure 22 Internal Lining Surface Adhesion on Dust Collection Line

The ADMAG AXR Two-wire Magnetic Flow Meter has achieved flow measurement accuracy and stability equivalent to those of four-wire systems by improving the S/N ratio, which was accomplished by adopting the world's first dual frequency excitation method in two-wire systems, high- density coil, and low-noise electrodes.

Yokogawa will extend specifications such as support of a large size, and expand the range of applications for the two- wire magnetic flow meters.

 

 

REFERENCES

1. Masayuki Shiomi, "Two-wire Magnetic Flow Meter Using the Dual Frequency Excitation Method," Instrumentation Control Engineering, Vol. 53, No. 5, 2010, pp. 72-76 in Japanese
2. Masayuki Shiomi, "Latest Trends and Applications of Magnetic Flow Meters," Instrumentation and Automation, Vol. 38, No. 2, 2010, pp. 62-65 in Japanese
3. Kenichi Kuromori, Shigeru Goto, et al., "Advanced Magnetic Flow Meter with Dual Frequency Excitation," Yokogawa Technical Report, Vol. 32, No. 3, 1988, pp. 13-18 in Japanese
4. Osamu Yoshikawa, Norihiro Shikuya, et al., "New ADMAG AXF Series Magnetic Flow Meters," Yokogawa Technical Report English Edition, No. 37, 2004, pp. 15-20

• ADMAG, AXR, and AXF are registered trademarks of Yokogawa Electric Corporation.