How can the small signal of vortex shedding flowmeter be excised?
When the flow signal is output in pulse mode, small signal excision must also be set. According to the principle of operation, there is no zero drift in the vortex flowmeter with pulsed output. The reason is that when the flow rate is zero, the vortex generation device will not produce vortices, so there is no pulse output. In fact, if the flow velocity is not zero, as long as the flow velocity of the fluid is low to a certain value, the corresponding Reynolds number is separated from the turbulent zone (Re<2300), and the vortex will not be produced. So, the evil street flowmeter why set up small signal excision? The reason is very simple, that is when the Reynolds number is very low, although there is no vortex also did not produce the output pulse interference, but will enter, such as pipeline vibration interference or radio frequency interference. The pulse signal amplifier of vortex flowmeter is a variable gain amplifier, the input pulse signal amplitude is lower, the greater the gain, so the maximum gain in pulse signal input, and this is the interference signal into open convenient ways.
Because the probe output pulse signal of vortex flowmeter, flow velocity of fluid flowing through the square and the magnitude of the swirl generating body is proportional to the velocity measurement, and ensure the normal range in instrument factory is committed to the Re is more than 20000, the amplitude of the signal probe output is large, so the signal-to-noise ratio is large, the interference is not easy to invade, and in the flow rate is low, the easy invasion of interference when using small signal (with frequency representation) resection method, so that the instrument can be done to ensure the normal measurement at a higher flow rate, and can be in the measurable velocity is lower than the minimum flow of value commitment, stable zero indicator.
Small signal elimination refers to the flow flow totalizer to overcome interference and zero drift transmitter, differential pressure transmitter or to ensure the normal operation and the establishment of the flowmeter system is lower than the specific flow value of instantaneous flow rate show zero, higher than that of the normal operation of the instrument function of this value. China's JJG 1003-2005 "flow totalizer" verification procedure provides specific requirements of small signal resection: 1, set the flow totalizer with standard throttling device (refer to the standard orifice, standard nozzle, classic Venturi tube and wing wind measuring device, double venturi tube in V cone flowmeter, Wedge Flowmeter and plug-in multi jets supporting the use of diameter flowmeter), small signal cut point should not be greater than the maximum flow design condition 8%. 2, flow meter and turbine flowmeter, vortex flowmeter, electromagnetic flowmeter, ultrasonic flowmeter using flow sensor or transmitter matching, small signal cut point should not be greater than the maximum flow design condition 5%.
