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Theory and practice of viscosity compensated turbine flowmeter

preface

traditional turbine flowmeter can only be used to measure the flow of low viscosity liquids, such as water, gasoline, etc. The turbine flowmeter manufacturer also generally stipulates that the viscosity of the measured liquid shall not be greater than 5mpas, otherwise serious errors will occur. How to use the traditional turbine flow sensor to accurately measure the flow of liquid whose viscosity is greater than 5mpas, such as the flow of crude oil, machine oil, etc., will attract our extensive attention

in this study, LW type traditional turbine flow sensor is used as the research object. Extensive theoretical and experimental research has been carried out on it, and the medium viscosity compensation model of this flow sensor has been obtained. At the same time, we have also developed a turbine flow meter integration display instrument with viscosity automatic compensation function. In this way, the traditional turbine flow sensor uses the corresponding viscosity compensation model and is equipped with this integration display instrument, The flow measurement of viscous liquid can be realized

two medium viscosity effect test

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hochreiter (1) and Shafer (2) have given a physical model of the viscosity effect of turbine flow sensing medium

(1)

where

f --------- the frequency sent by the sensor

q ------- instantaneous flow

v --------- kinematic viscosity of the measured liquid

equation (1) is called "general viscosity curve of turbine flowmeter". Among them, Φ Is a polynomial; Φ The form of must be determined by experiment

to this end, we first conducted a test on the influence of medium viscosity. The test device is shown in Figure 1, polycarbonate production technology with independent intellectual property rights. The viscosity of the test medium varies from 1 to 95.6mpas; The model of the test turbine flow sensor is lw-25

(2)

the test results are given in Figure 2. In the figure, K is the actual instrument constant

e is the same error, which is defined as

(3)

known from the test results. When the viscosity reaches 8.91mm2/s or more, the sensor almost loses its linear range. It can also be seen from the figure that the sensor instrument constant changes greatly with the viscosity at a small flow rate; Moreover, the higher the viscosity is, the smaller the instrument constant is, while at a larger flow rate, the viscosity effect is much smaller. From the composition of this fatigue testing machine and the basic management test results in ordinary use, it shows that the medium viscosity plays an important role in small flow, while in large flow, the role of viscosity is not important

tri orthogonal polynomial viscosity compensation model

it is known from the general viscosity curve model of turbine flowmeter that the instrument constant is only inferior to f/v, that is,

therefore, we re plot the test data on a single logarithmic coordinate paper with f/v as the abscissa, as shown in Figure 3. It was found that the original dispersed viscosity curves were combined into one curve, which is the general viscosity curve

we use the orthogonal curve fitting method with any step length, reorganize the test data in f/l mode, and then carry out orthogonal polynomial fitting

the formula in the formula is coefficient, and the calculation method is referred to Literature (5)

the calculation shows that the general viscosity curve shown in Figure 3 can be fitted in segments, and the curve fitted by the segmented orthogonal curve is shown in Figure 4 and figure 5. As can be seen from the figure, when f/l 30 (about re 5000), Ф The curve is close to the horizontal straight line, that is, at this time, the number of meters is "constant". The formula of the curves in Figure 4 and figure 5 is expressed as

the above is the viscosity compensation model of the test turbine flowmeter. Where δ It reflects the relative error of the instrument constant calculated by the model from the actual instrument constant. In the model, when f/l is 30, the maximum absolute value of the instrument constant calculated by the model deviates from the actual value is 2.03%, so the fitting accuracy of this section of curve is ± 2.5%. When f/l ≥ 30, the maximum absolute value of the calculated value deviates from the actual value is 0.96%, so if the instrument works in this range, its accuracy can reach ± 1%

four viscosity compensation

in order to enable the turbine flowmeter to realize automatic viscosity compensation measurement, we also developed a viscosity compensation turbine flowmeter flow calculation display instrument (hereinafter referred to as the instrument). Before real-time measurement, the instrument can enter the measurement state as long as the viscosity V of the fluid (unit: mm2/s) is input. In the model

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