Thread Content
Vortex flowmeters are now being used more and more frequently, mainly because of their simple structure, as well as their significant advantages over other similar products in terms of repeatability, reliability, and maintainability. However, as the diameter of the vortex flowmeter sensor increases, its upper frequency limit decreases; especially when the medium is a liquid, this upper frequency limit is only in the range of a dozen hertz or even just a few hertz. In such cases, the volume flow rate of the medium per unit frequency is very high, resulting in large fluctuations in the current output of the transmitter, which is why such meters are generally not used. When displayed directly by a secondary instrument, the flow rate experiences significant fluctuations. Generally, the filtering time constant is increased to reduce these fluctuations in the displayed flow rate; however, this makes the instrument less responsive, and the flow rate shown on the instrument lags behind the actual changes in flow rate within the pipeline, thereby causing errors. As large-diameter flow sensors are being used more and more often, failure to resolve this issue will limit the application of vortex flow sensors in such projects. Emerson’s vortex flowmeters can address this problem effectively by using spread spectrum technology to amplify the frequency signal output by the sensor by several times or even dozens of times. Spread-spectrum methods are divided into traditional types and intelligent types with a CPU; the latter can also correct the non-linearity of sensors. The conventional circuit of vortex flowmeters is simple, but it suffers from two types of conversion errors. Especially at low frequencies, the average current is very small; as a result, the filtering time constant of the amplifier must be larger than the period of the maximum signal, which requires a high amplification factor. It is also necessary that the amplifier’s offset voltage be low and that there be minimal thermal noise in the resistors. Even so, experiments show that errors remain significant at 5 Hz. To address this, phase-locked loops are used for frequency scaling. There is still a nonlinear error between the frequency signal and the flow rate, and nonlinear correction is required when high precision is needed. Since there is a nonlinear relationship between the frequency output by the vortex street sensor and the flow rate, nonlinear compensation should be applied when high measurement accuracy is required. Therefore, intelligent spread-spectrum and linearization transformation circuits were designed.