How big is the error of the positive displacement flow meter?

Abstract: The information on the error of the positive displacement flow meter is provided by the excellent flow meter and flow meter manufacturers. For any flow measurement value, it must include two parts: one is the flow measurement value itself; the other is its allowable error range. Otherwise it is incomplete. Therefore, for any kind of flowmeter, it is necessary to understand its error characteristics. so-called. More flowmeter manufacturers choose models and price quotations. You are welcome to inquire. The following is the article details of the error of the positive displacement flowmeter. For any flow measurement value, it must include two parts: one is the flow measurement value itself; the other is its allowable error range. Otherwise it is incomplete. Therefore, for any kind of flowmeter, it is necessary to understand its error characteristics. The so-called error characteristic is the relationship between the error value of the flowmeter and the flow measurement value. To discuss the error characteristics is to discuss and study the trend of the measurement error value changing with the flow measurement value. The measurement error value E of the positive displacement flowmeter can be expressed by the ratio of the difference between the indicated value and the true value to the indicated value. Let: V be the true value of the fluid volume passing through the flowmeter; I is the indicated value of the flowmeter, then the error value E can be expressed as E = (2-5) The relationship between the fluid volume V and the indicated value I (2- 3) Substitute into it, we can get: E=1- (2-6) It can be seen from Equation 2-6 that the error characteristic of the positive displacement flowmeter is only related to the measurement space volume v inside the flowmeter and the gear ratio constant a of the instrument. That is to say, from the point of view of the measurement principle, the measurement error of the positive displacement flowmeter is only related to the geometric structure of the flowmeter, and has nothing to do with the fluid properties and flow value. We call this error characteristic the ideal of the positive displacement flowmeter. Error characteristics. Draw a curve, which is a horizontal line parallel to the horizontal axis, as shown in Curve 1 in Figure 2-6. However, when we calibrate the positive displacement flowmeter and draw the actual error characteristic curve, it is closer to the form of curve 2. When the flow rate is small, the error value slopes sharply to the negative direction; as the flow rate increases, the error value gradually moves from the negative direction to the positive direction, and stabilizes at a certain position, and the error curve is parallel to the horizontal axis. As the flow continues to increase, the error value will shift to the negative direction again. The fact that the actual error characteristic curve shows this trend is because there is an inevitable leakage phenomenon in the positive displacement flowmeter. The so-called leakage flow means that the fluid flows directly from the inlet to the outlet through the gap between the rotating part and the casing without being measured. The following will discuss the relationship of error characteristics considering the leakage phenomenon. Assume that the fluid leakage flow in unit time is expressed by g; the flow through the flowmeter is qv; the total volume of fluid passing through the flowmeter is V; the volume of the total leakage during this period is V. This strain V can be expressed as * V = g (2-7) Therefore, when there is leakage flow, when the rotor discharges N amount of measuring space fluid, the actual volume of fluid passing through the flow meter is V = Nv + V (2-8) Substitute equations 2-2 and 2-7 into equation 2-8, we can get: V = v + g (2-9) equation (2-9) can be organized into: V = (2-10) ) into the error definition formula (2-5), we can get: E=1- (2-11) Analytical formula 2—11 It can be seen that since the measurement space volume v and the gear ratio constant a are both fixed values, the relationship between the error E and the flow rate is affected by the leakage flow rate g per unit time. If the leakage flow g of the positive displacement flowmeter is assumed to be a constant value, the formula (2—11) Discuss the changing trend of its error curve. When the flow rate is very small, in extreme cases, qr = g, then the formula (2—11) The square brackets are 0, and the error value E tends to negative infinity. As the flow qv increases, the formula (2—11) The value in the brackets increases gradually, and the error value E also increases gradually in the positive direction. When the flow rate continues to increase until g is very small compared to qv, that is, when g/qv is very close to 0, the formula (2—11) Converted to formula (2-6), the flowmeter error curve tends to the ideal error curve. The above is the whole content of this article. You are welcome to inquire about the flowmeter selection and quotation of our factory. 'How big is the error of the positive displacement flow meter'

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