2019-10-17 16:15:24

Electromagnetic flowmeter features

Frequency programmable low frequency rectangular wave excitation improves stability of flow measurement and low power loss.

The new 16-bit ultra-low power microprocessor with FLASH memory has high integration, fast calculation speed and high calculation accuracy.

Full digital processing, strong anti-interference ability and reliable measurement.

Ultra-low EMI switching power supply, suitable for power supply voltage variation range, high efficiency, small temperature rise; good EMC performance.

Chinese and English menu operation, easy to use, easy to operate, easy to learn and understand.

High definition backlight wide temperature LCD display.

It can perform bidirectional flow measurement and bidirectional total accumulation; it has automatic range switching function, which can effectively improve the measurement accuracy of analog current and frequency output, especially suitable for occasions where the diurnal flow range changes greatly and needs to send control signals; flow measurement range Up to 1500:1.

There are three totalizers inside, which record and display the forward cumulative amount, the reverse cumulative amount and the accumulated difference integrated amount, which are convenient for fluid metering and custody transfer.

Provide isolated or non-isolated RS485/RS232C digital communication interface, and support fieldbus communication modes such as MODBUS, PROFIBUS-DP and HART.

Constant current source fluid resistance measurement can accurately measure the internal resistance of the electrode signal in the case of long-line transmission. It can be used not only to determine whether the fluid in the sensor is empty or not, but also to identify abnormal phenomena such as contamination and coverage of the electrode, and to provide cleaning for the user. Fault processing information such as electrodes.

Several problems of solutions

Recalculating the differential pressure scale

Temperature and pressure compensation can only reduce the measurement error, not only can not solve the problem fundamentally, but also the measurement signal exceeds 20mA, resulting in steam leakage measurement. The transmitter measurement signal exceeds 20 mA, indicating that the actual measured differential pressure signal ΔP exceeds the design differential pressure value.

Increase temperature and pressure compensation

When the temperature and pressure of the steam change, the density of the steam changes, and the steam flow measurement produces an error. Measurement error can be reduced by temperature and pressure compensation. Since the temperature of the saturated steam is a single-valued function of the pressure, the temperature and pressure compensation of the saturated steam can be pressure compensated or temperature compensated. Because the pressure signal detection is sensitive and the compensation accuracy is high, it is compensated by pressure and realized by DCS.

Conclusion

Steam is a special medium. As the pressure and temperature change, the density of steam changes. Therefore, it is necessary to compensate for temperature and pressure. When the pressure and temperature fluctuation of the steam are not large, that is, when the operating condition parameters deviate from the design parameters and the influence on the measurement is small, the temperature and pressure compensation measures can achieve the purpose of accurate measurement. However, when the operating parameters deviate too much from the design parameters or the operating parameters fluctuate frequently and are too large, even with the temperature and pressure compensation, it is difficult to meet the measurement accuracy requirements. At this point, only differential pressure or flow can be recalculated for a particular throttling element.

Ultrasonic flowmeter measurement principle

When the ultrasonic beam propagates in the liquid, the flow of the liquid will cause a small change in the propagation time, and the change in the propagation time is proportional to the flow velocity of the liquid, and its relationship conforms to the following expression.

among them

θ is the angle between the sound beam and the direction of flow of the liquid

M is the number of linear travels of the sound beam in the liquid

D is the inner diameter of the pipe

Tup is the propagation time of the sound beam in the positive direction

Tdown is the propagation time of the sound beam in the reverse direction

ΔT=Tup –Tdown

Let the speed of sound in the stationary fluid be c, the velocity of the fluid flow be u, and the propagation distance be L. When the sound wave is in the same direction as the fluid flow direction (ie, the downstream direction), the propagation velocity is c+u; otherwise, the propagation velocity is cu. Two sets of ultrasonic generators and receivers (T1, R1) and (T2, R2) are placed at two places separated by L. When T1 is in the forward direction and T2 transmits ultrasonic waves in the reverse direction, the time required for the ultrasonic waves to reach the receivers R1 and R2 respectively is t1 and t2, then

T1=L/(c+u); t2=L/(c-u)

Since the flow velocity of the fluid in the industrial pipeline is much smaller than the sound velocity, that is, c>>u, the time difference between the two is ▽t=t2-t1=2Lu/cc. Thus, the propagation velocity of the acoustic wave in the fluid is known. When it is known, the flow rate u can be obtained by measuring the time difference ▽t, and the flow rate Q can be obtained. The method of measuring the flow using this principle is called the time difference method. In addition, a phase difference method, a frequency difference method, or the like can be used.