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Description of the accuracy and flow rate of the electromagnetic flowmeter

1, accuracy level and function

The performance of general-purpose electromagnetic flowmeters on the market is quite different, some have high precision, many functions, some have low precision and simple functions. The basic error of the instrument with high precision is (±0.5%～±1%)R, and the instrument with low precision is (±1.5%～±2.5%) FS. The price difference between the two is 1-2 times.

Some models claim to have higher accuracy, the basic error is only (±0.2% ~ ±0.3%) R, but there are strict installation requirements and reference conditions, such as ambient temperature 20 ~ 22 ° C, front and rear straight pipe length requirements More than 10D, 3D (usually 5D, 2D), respectively, even proposed that the flow sensor should be integrated with the front and rear straight tubes for real-flow calibration on the flow standard device to reduce the impact of poor clamping. Therefore, when comparing multiple models, do not simply look at high indicators. Read the manufacturer's samples or instructions for a comprehensive analysis.

The function of the electromagnetic flowmeter on the market is also very different. The simple one is to measure the one-way flow, and only output the analog signal to drive the rear position meter; the multi-function meter has the measurement bidirectional flow, range switching, upper and lower limit flow alarm, empty tube and power supply. Cut off alarm, small signal cutoff, flow display and total calculation, automatic check and fault self-diagnosis, communication with host computer and motion configuration. The serial digital communication function of some models can be selected from a variety of communication interfaces and dedicated chips (ASIC) to connect HART protocol system, PROFTBUS, Modbus, CONFIG, FF field bus and so on.

2, flow rate, full flow, range and caliber

The selected instrument diameter is not necessarily the same as the pipe diameter and should be determined by the flow rate. The process industry transports liquids with different viscosities such as water. The flow rate of the pipeline is generally 1.5 to 3 m/s. The electromagnetic flowmeter is used on such a pipe, and the diameter of the sensor is the same as the diameter of the pipe.

When the EMF full flow rate, the liquid flow rate can be selected within the range of 1 to 10 m/s, and the range is relatively wide. The upper limit flow rate is not limited in principle, but it is generally recommended not to exceed 5 m/s unless the lining material can withstand scouring, and practical applications rarely exceed 7 m/s, and more than 10 m/s is even more rare. The minimum flow rate for full flow is typically 1 m/s, and for some models it is 0.5 m/s. In some pipelines with low flow rate or low flow rate at the beginning of new construction, from the perspective of measurement accuracy, the gauge diameter should be changed to less than the diameter of the pipe and connected by a reducer.

It is used for fluids with easy adhesion, deposition, scaling and other materials. The flow rate is not less than 2m/s, preferably increased to 3-4m/s or above, which can be used for self-cleaning and adhesion prevention. For fluids with high abrasiveness such as pulp, the flow rate should be less than 2~3m/s to reduce the wear on the lining and the electrode. When measuring a low conductance liquid close to the threshold, select a lower flow rate (less than 0.5 to 1 m/s) as much as possible, and the flow noise will increase due to the flow rate increase, and output sloshing will occur.

The range of the electromagnetic flowmeter is relatively large, usually not less than 20, and the meter with automatic range switching function can exceed 50-100. Domestically available styling products range in size from 10mm to 3000mm, although the actual application is mostly in medium and small caliber, but with most other principle flow instruments (such as volumetric, turbine, vortex or Coriolis mass, etc.) Compared to large-diameter instruments, they account for a large proportion.

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