In practice, the choice of the same precision instrument, consider the actual measured flow rate range, to compress the range meter is the nearest range of the measured values in order to a chieve the purpose of improving measurement accuracy. And the amount of 1cm with a ruler with the meter stick something of precision than that is a reason. As an example of electromagnetic flowmeter The performance of general-purpose market are very different, and some high precision, multi-function, some precision is low, functional simplicity. High precision instruments fundamental error (± 0.5% ~ ± 1%) R, the instrument was low precision (± 1.5% ~ ± 2.5%) FS, the price difference between 1 to 2 times. Therefore, measurement precision is not very high places (for example, control of non-trade accounts only for the purpose of electromagnetic flowmeter only require high reliability and good repeatability of the place) on the selection of high-precision instruments in the economy does not pay. Some models instrument claimed to have higher accuracy, the basic error is only (± 0.2% ~ ± 0.3%) R, but there are strict installation requirements and the reference conditions, such as ambient temperature of 20 ~ 22 ℃, the length of straight pipe before and after the requirements were greater than the 10D and 3D (usually 5D and 2D), even with the proposed flow sensor composed of straight pipe before and after the one in the flow standard device for real-calibration, to reduce the clamping effect. Therefore, when comparing the various models do not simply look at selected high targets, vortex flow meter to read the manufacturer to do comprehensive analysis of samples or brochures. EMF features on the market differ greatly, a simple one-way traffic on the only measure, only after driving the output analog signal level meter; multifunctional measuring instrument with two-way flow, range switching, the minimum flow alarm, air traffic control and power cut off the alarm , small signal removal, the total flow rate display and calculation, automatic checking and fault diagnosis, communication and movement with the host computer configuration and so on. Some models feature digital communication instrument optional serial communication interface and a variety of ASIC (ASIC), turbine flow meters to connect the HART protocol system, PROFTBUS, Modbus, CONFIG, FF Fieldbus and so on. Measured using the EMF on the premise that the liquid must be conductive, and can not be below the threshold (ie, lower limit). Conductivity below the threshold will produce measurement errors can not even use, even if the changes exceed the threshold value can be measured, showing little change in error, the threshold of universal EMF in 10-4 ~ (5 × 10-6) S / cm, depending on the model to another. Shihai depends on the use of sensors and signal converters length between the flow and distribution of capacitors, factory manual that corresponds to the conductivity is usually provided in the signal line length.

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