RU2235977C2 - Gas-flow counter - Google Patents

Abstract

FIELD: measurement technology.

SUBSTANCE: proposed gas-flow counter includes rotameter with cylindrical measuring tube , circular float and electronic measuring circuit. Surface of central cone passing via through central hole in float is brought in contact with taper side inner wall of float when it is in lower position. Tube, float and cone made from conducting material are linings of capacitance converter are coated with insulating layer. Electronic measuring circuit consists of high-frequency self-excited oscillator whose driving RC-chain is provided with capacitance converter for motion of float.

EFFECT: enhanced accuracy of measurement; simplified construction.

2 cl, 2 dwg

Description

The invention relates to measuring technique and can be used to measure the flow rate of liquid and gaseous media and, in particular, to measure the flow rate of natural gas.

There are many designs of primary flow converters for liquid and gaseous media based on various physical principles. Including: on the basis of measuring the pressure drop in a special constricting device (diaphragm, Venturi pipe) inserted into the line; based on the measurement of the pressure of a moving medium in a collision with an obstacle of various shapes placed on a highway (the force of the influence of the medium flow on the obstacle is measured); based on measuring the speed of rotation of a turbine flowing around a stream of liquid or gas; based on measuring the height of the float in a conical or cylindrical tube under the pressure of a liquid or gas flow (when the float rises, the annular gap between the float and the walls of the conical tube or conical nozzle passing through the central hole of the float increases); on the basis of measuring the frequency of vortex formation when a medium flows around a poorly streamlined body placed in a medium flow (this frequency is proportional to the flow velocity); based on measuring the electrical resistance of a thermistor placed in a stream of a controlled environment and heated by an electric current passing through it, etc. [1, 2].

To construct a gas flow meter with a wide range of measured flow rates and especially low flow rates (domestic use), it is most convenient to use rotametric primary transducers based on the differential pressure method. These are the already mentioned converters, in which, under the influence of the flow of a controlled medium on the body streamlined by it (float), the latter rises to a certain height, depending on the flow velocity (instantaneous flow). In [1, p.228-233] two of the most widely used designs of float rotameters are described: with a conical measuring tube and a cylindrical measuring tube and a cone coaxially placed in it. To convert the position of the float into an electrical signal, a differential transformer converter can be used, the movable core of which is connected by a rod to the float [1. c.232, fig. 7.16 c], the magneto-induction method [3], the system of reed switches included by a permanent magnet integrated in the float [4], or the capacitive method [5].

Closest to the technical nature of the present invention is a capacitive rotameter as.with. No. 1682792 [5]. It consists of a conical tube made of dielectric material, inside which a float is placed so that in the lowermost position it completely overlaps the lower (smaller) hole of the conical tube. Under the pressure of the gas flow supplied from below, the float rises to a certain height, and an annular gap is formed between it and the inner surface of the tube, through which the gas flow passes. The area of this gap will be proportional to the height of the float, which, in turn, depends on the gas flow rate (i.e. its flow rate). This height will correspond to the moment of equilibrium between the lifting force of the gas flow acting on the float and the weight of the float. To stabilize the position of the float along the axis of the conical tube when it is raised, oblique grooves are made on the lateral surface of the float, passing through which the gas gives the float a rotational movement, and its center of gravity is below the section with a maximum area, which prevents it from tipping over. On the outer surface of the conical tube there are three plates of the differential capacitive transducer, one of which has an elongated rectangular shape and is located along the generatrix of the conical tube, and the other two are triangular, and the triangular plates are placed on both sides of the rectangular so that their high sides are on the same distance from the rectangular lining and parallel to it, and their bases are located in opposite directions (one from the bottom, the other from the top). The float is made of a dielectric material with a high dielectric constant. Therefore, when it rises, both capacitances of the differential capacitive transducer, measured between the rectangular plate and each of the triangular ones, will change (one will increase and the other will decrease).

The main disadvantages of the described rotameter are the small size of the measured capacities (according to the approximate calculations - less than one picofarad) and the insufficient sensitivity of the capacitive transducer. The small size of the measured capacitances is determined by the large distance between the rectangular and triangular plates (especially between the opposite sides of the rectangular plate of the triangular plates). A low sensitivity is determined by the weak dependence of these capacities on the position of the float. Indeed, even if you make a float made of capacitor ceramics with a relative dielectric constant of about 100 (all other dielectric solid materials, including polymeric ones, have a dielectric constant of 2 to 8), then in this case the main fraction of capacities between rectangular and triangular plates will be determined by force by lines of electrostatic scattering fields that are closed between the high sides of the triangular plates located in the immediate vicinity of the rectangular plate, moreover, these power lines will be closed not through the float, but through the pipe material and the air gap. Moreover, as the float rises, the radial clearance between the plates and the float will increase, which means that the effect of the float on these containers will weaken even more. This, by the way, will lead to nonlinearity of the conversion function of the capacitive converter. All these shortcomings sharply worsen the accuracy of gas flow measurement.

The technical problems to which the invention is directed are: a sharp increase in the sensitivity of the flow sensor, an increase in the capacitance value of the capacitive transducer for displacing the float and, as a result, an increase in the accuracy of measuring the gas flow, as well as simplification of the design and manufacturability of the float rotameter, which is a sensor for instant gas flow in the gas meter in question. These problems are solved by using a float rotameter with a cylindrical measuring tube, a central cone and a float with a central hole through which the cone passes, a capacitive transducer for displacing the float and an electronic measuring circuit, and the measuring tube, float and central cone are made of electrically conductive material and are capacitive plates the float displacement transducer, the float has a conical lateral inner wall adjacent to the central cone at the lower position of the float, and the capacity formed by the serial connection of the containers between the inner side surface of the float and the cone and between the outer side surface of the float and the cylindrical measuring tube is included in the master RC circuit of the high-frequency oscillator, the output of which is connected to the serial port of the microcomputer, and the output of the microcomputer is connected with a digital indicator.

The device float rotameter shown in figure 1 (a), the equivalent circuit of a capacitive transducer displacement of the float in figure 1 (b), and the measuring circuit of the gas flow meter in figure 2. The float rotameter consists of a cylindrical measuring tube 1, a central cone 2 and an annular float 3 with a conical lateral inner wall adjacent to the central cone 2, with the float in the lower position, and the cylindrical measuring tube 1, and the central cone 2, and the float 3 are made made of electrically conductive material, and to prevent electrical shortage between them, the surfaces of the float are covered with an insulating layer, and the central cone is attached using a dielectric sleeve with through holes tii for the passage of gas.

The measuring circuit (figure 2) consists of a high-frequency oscillator 4, a microcomputer 5 and a digital indicator 6, and a capacitive transducer for displacing the float formed by the series connection of the capacitance between the inner wall of the float and the central cone and between the outer wall of the float and a cylindrical measuring tube, and the output of the oscillator 4 is connected to the serial (i.e., single-bit) port of the microcomputer 5, and its output is digitally in indicator 6.

The gas flow meter works as follows. The float rotameter is built into the gas line strictly vertically so that the gas flow is directed from below (according to the arrows in FIG. 1). At zero gas flow, the pressure from the bottom and top of the float 3 is the same and the float under its own weight drops to its lowest position, at which the annular gap between the float 3 and the central cone 2 is zero. If there is a gas flow, the pressure on the top of the float drops and under the pressure of the gas flow from below the float rises to such a height that the pressure of the gas from below is balanced by the weight of the float and the decreased gas pressure on top of the float (see [1. p.228-231]). When the float rises, the capacitance C _in between the inner side wall of the float 3 and the central cone 2 will decrease due to an increase in the annular gap between them. The capacity C _n between the outer side wall of the float and the cylindrical measuring tube 1 will remain unchanged, because when moving the float 3, both the gap value and the overlapping area of the electrodes of this capacity do not change. Therefore, the total capacitance between the central cone 2 and the cylindrical measuring tube 1 will be a series connection of the two above-mentioned containers. Since the capacitance C _in the lower position, even when the float (when the maximum capacity) is much smaller than in series with it combined capacitance C _n (since the final area of overlap capacitance electrodes substantially greater than the area of overlap of the first electrodes are at the same values of gap) , then changes in the total capacitance C _ISM between the cylindrical measuring tube 1 and the central cone 2 will be determined by changes in the smaller of the series-connected capacities, i.e. capacity C _in . It can be shown that when the float moves from the lowest to the highest position, the change in this capacity will be very significant. Considering the field to be uniform in the air gap between the inner side wall of the float 3 and the central cone 2 and considering that the height of the float 3 is much less than the height of the central cone 2, for the approximate calculation, you can use the formula for the capacity of a cylindrical capacitor [6, p.45]:

,

,

where h is the height of the float;

ε and ε ₀ are the relative permittivity of the dielectric separating the capacitor plates, and the absolute permittivity of the vacuum, respectively;

D and d are the diameters of the outer and inner cylinders, respectively (in our case, these are the diameters of the inner side wall of the float 3 and the central cone 2 in the middle section of the float, respectively).

Since when the float moves, the values of h, D, and ε remain unchanged and only the diameter d of the cross section of the central cone 2, corresponding to the position of the middle section of the float 3, changes, to determine the relative change in this capacity at the extreme positions of the float 3, it suffices to calculate the corresponding ratios of these diameters.

When the float 3 is in the lowest position, its inner wall is adjacent to the surface of the central cone 2 and only an insulating layer separates them, the maximum thickness of which can be assumed to be 0.5 mm (if the float is made of an aluminum alloy and oxidizes its surface for insulation, then the thickness of this layer will be even less). Having taken D = 20 mm, we obtain d _n = 19 mm and D / d _n = 1.0526, and ln (D / d _n ) = 0.05126.

With the highest position of the float 3, we take d _in = 6 mm and, accordingly, D / d _in = 3.333 and ln (D / d _in ) = 1.204.

The ratio of C _in / C _n will be 23.5.

Actually, the change in capacity will be less, because the measured capacitance C _meas between the central cone 2 and the cylindrical measuring tube 1 will be determined not only by the capacities formed by the gaps between the side surfaces of the float 3 with these elements, but also by the constant distributed capacitance between the cylindrical measuring tube and the central cone due to fields closing above and below float 3 at its arbitrary location. Approximately this capacity can be estimated by the same formula, given the diameter (internal) D _{m of the} cylindrical measuring tube 1 and its height H _m . If we take D _m = 50 mm and H _m = 100 mm, then taking d, corresponding to the average section of the central cone 2, equal to d = (20 + 6) / 2 = 13 mm, we obtain

.

.

With the same dimensions and height of the float h = 10 mm, the component of the useful capacity (through the float) between the inner side wall of the float 3 and the central cone 2 at the lower position of the float C _int will be

;

;

at the upper position of the float

;

;

and the capacitance C _n between the outer lateral surface of the float 3 and the cylindrical measuring tube 1 (it does not depend on the position of the float) will be

.

.

Thus, with given dimensions of the rotameter, the measured capacity will be at the lower position of the float

,

,

and at the top

.

.

Therefore, at the extreme positions of the float, the measured capacitance will change in C _meas.n / C _meas.v = 2.6 times.

As you can see, due to the influence of the distributed capacitance C _{0, the} useful change in the measured capacitance significantly decreases, and the conversion function of the capacitive transducer of displacement of the float becomes nonlinear (at first the sensitivity is high, and then as the float rises, when the proportion of the distributed capacitance C ₀ in the total measured capacitance becomes prevailing, it falls). To reduce the nonlinearity of the conversion function, it is necessary to increase the diameter of the cylindrical measuring tube 1 and use a truncated central cone 2 so that when the float 3 is in the upper position, the magnitude of the changing component of the capacitance C _in would be comparable with the value of the distributed capacitance C ₀ .

As a measuring circuit, a circuit should be used that allows you to ground one of the plates of the measured capacitance (cylindrical measuring tube 1), because in this case, no additional shielding of the capacitive transducer is required (the cylindrical measuring tube 1 itself will serve as a screen).

For the convenience of constructing a gas flow meter and improving the accuracy of measurements, it is desirable to use a measuring circuit with a frequency output. Such a circuit can be a high-frequency oscillator, in the master RC-circuit of which is included a capacitive transducer for moving the float. The frequency of the oscillator can be measured programmatically using a microcomputer, which converts the measured frequency into the current value of the gas flow and the accumulation of flow for any time interval. Such a measuring circuit is shown in figure 2. On it, the capacitive transducer for displacing the float is designated as the capacitance C _ISM included in the RC oscillator driver circuit 4, and the output of the oscillator 4 is connected to the serial (single-bit) port of the microcomputer 5, the output of which is connected to the digital indicator 6. The range of measured frequencies should be selected in accordance with the speed of the used microcomputer. Microcomputer 5 programmatically measures the frequency of the signal coming from the output of the oscillator 4 and converts it into values of the instantaneous gas flow rate using the calibration characteristic stored in the memory of the microcomputer 5. Summing up the instantaneous gas flow rate at equal time intervals (1 s), it determines the total gas flow rate for any time intervals and displays it on the digital indicator 6. The auxiliary and service functions performed by the microcomputer are not considered here. To reduce the influence of network interference, it is advisable to choose the time of frequency measurement as a multiple of the period of the industrial network (20 ms).

Thus, while simplifying the design and increasing the manufacturability of the production of the float rotameter, the following results were achieved: a significant increase in its sensitivity, an increase in the absolute values of the capacitance of the capacitive transducer for displacing the float, and, therefore, an increase in the accuracy of measuring gas flow. Therefore, all the assigned technical tasks are solved.

Literature

1. Farzane N.G., Ilyasov L.V., Azim-zade A.Yu. Technological measurements and instruments: Textbook for universities. - M .: Higher school. 1989 .-- 456 p.

2. Spector S.A. Electrical measurements of physical quantities: Measurement methods: Textbook. manual for universities. - L .: Energoatomizdat, 1987 .-- 320 p.

3. A.S. No. 595625, USSR. Rotameter. / Brukhanov B.K., Grigorovsky B.K. Zavyalov V.G. Publ. 02/28/1978, Bull. Number 8.

4. A.S. No. 792076, USSR. Rotameter. / Kolmykov S.P., Tarkhanov O.V. Publ. 12/30/1980, Bull. Number 48.

5. A.S. No. 1682792, USSR. Capacitive flowmeter. / Dubrovsky M.G., Zelenin A.N. Publ. 10/07/1991, Bull. Number 37.

6. Turichin A.M. Electrical measurements of non-electrical quantities. Ed. 4. - M.-L.: Energy, 1966 .-- 690 p.

Claims (2)

1. A gas flow meter comprising a rotameter with a measuring tube, a float, a capacitive transducer for moving the float and an electronic measuring circuit, characterized in that it is provided with a central cone passing through the central hole in the ring-shaped float, the conical side inner wall of which is adjacent to the surface of the central the cone when the float is in the lower position, the measuring tube is made cylindrical, the float and the central cone are made of electrically conductive material and are obk adkami capacitive transducer float movement, and to prevent electrical insulation between the cylindrical measurement tube, a central cone and float them to the surface coated with an insulating layer. 2. The gas flow meter according to claim 1, characterized in that its electronic measuring circuit consists of a high-frequency oscillator, a microcomputer and a digital indicator, and a capacitive transducer of displacement of the float formed by a series connection of capacities between the inner side wall of the float and the central cone and between the outer side wall of the float and the cylindrical measuring tube, and the output of the oscillator is connected to been consistent port of the microcomputer, the output of which is connected to the digital display.

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