RU2113694C1 - Device for measuring the conducting medium level - Google Patents

Abstract

FIELD: electrical engineering. SUBSTANCE: insulated electrodes are positioned in separate communicating cylindrical vessels of different diameters equidistantly from vessel bottom. Electrodes are connected to capacitive transducers. Adjustable capacitor is connected in parallel to electrode immersed in large-diameter vessel. Vessels are made of conducting material. EFFECT: higher accuracy, extended measurement range. 2 cl, 2 dwg

Description

 The invention relates to instrumentation and measuring equipment, in particular to devices for measuring the level of conductive media.

 A device for determining the level of electrically conductive liquid in a metal tank (AS USSR N 1719912, class G 01 F 24/21), containing a level sensor made in the form of a plugged from the lower end of a metal tube with two insulated electrodes attached to the plug , power supply, indicator, bipolar switch and electrical meter. The electrodes are made in the form of a thermocouple and are connected to the input of the switch. An electrical measuring device is connected to one of the inputs of the switch, one of the indicator inputs and one of the outputs of the power source are connected to the other. The second input of the indicator and the second output of the power source are connected to the tank.

 However, the described device, designed to control a certain, predetermined level, cannot be used to measure any arbitrarily changing liquid level.

 Known capacitive compensation level meter (AS USSR N 1695139, class G 01 F 23/26), containing measuring and compensation sensors, a generator, two capacitance to voltage converters, each of which includes an operational amplifier and capacitor, two circuits subtraction, two scale controls, indicator, two voltage dividers, two voltage to frequency converters and a frequency divider. In this case, the output of the generator through the scale controllers is connected to the first inputs of the subtraction circuits and the inputs of the voltage dividers, the outputs of which are connected to the non-inverse inputs of the operational amplifiers of the capacitors to the voltage, and the outputs of the latter are connected to the second inputs of the subtraction circuits, the outputs of which are connected to the voltage-to-frequency converters to two inputs of the frequency divider. The third input of the frequency divider is connected to the output of the generator, and the output to the input of the indicator.

 The disadvantage of the described device is that it does not take into account the electric capacitance of the non-liquid part of the sensor, which leads to an increase in error at low liquid levels and thereby narrows the dynamic range of measurements. So, it can be shown that when filling the sensor by 10%, the error of the device will be 12.5%, and when filling by 20% - 6.25%.

 Closest to the proposed is a device for measuring levels of electrically conductive media (AS USSR N 1721442, CL G 01 F 23/26, 1992, BI 1992, N 11, p. 142), containing an amplifier, recorder and bridge, one arm of which includes a capacitive sensor made in the form of insulated electrodes, and the other arm contains a capacitor of variable capacitance.

 However, this device has the same drawback as described previously - it does not take into account the influence of the electric capacitance of the unfilled part of the sensor.

 The objective of the invention is to expand the measurement range by eliminating the influence of the unfilled part of the sensor that changes during the measurement of electric capacity and increasing the accuracy of measurements in the entire range.

 The solution to this problem is achieved by the fact that in the device for measuring the level of electrically conductive media containing a recorder, the electrodes made insulated, and the capacitance made with the possibility of adjustment, the electrodes are located in separate cylindrical vessels of various diameters interconnected, made of conductive material and connected with the inputs of the capacitance converter, and the capacity, made with the possibility of adjustment, connected in parallel to the electrode, immersed in a vessel of larger diameter.

 In addition, the electrodes are installed at the same distance from the bottom of the vessel.

 In FIG. 1 shows the design of the device; in FIG. 2 is a diagram of its inclusion.

 The positions in the drawings indicate: the body of the cylindrical communicating vessels 1, the measuring electrode 2, the compensation electrode 3, the additional capacity 4, the generator 5, the recorder 6, the transducer of the tank 7.

 The device comprises a housing 1 in the form of two cylindrical vessels of different diameters interconnected, made of conductive material, measuring 2 and compensation 3 electrodes, one of which is immersed in a vessel of a larger diameter, filled with liquid, the level of which must be measured, and the other in a vessel of a smaller diameter filled with the same liquid (according to the law of communicating vessels). To the electrode, immersed in a vessel of a larger diameter, in parallel is connected an additional capacity 4 made with the possibility of adjustment. Measuring 2 and compensation 3 electrodes, made isolated from the vessels and installed at the same distance from the bottom of each vessel, are connected to the inputs of the capacitor transducer. A recorder is connected to the output of the capacitance converter.

The electrical capacitance of a vessel with a compensation electrode consists of two components:
C _ac = c $\genfrac{}{}{0ex}{}{\text{X}}{\text{ac}}$ + C $\genfrac{}{}{0ex}{}{\text{Lx}}{\text{ac}}$ ,
where c $\genfrac{}{}{0ex}{}{\text{Lx}}{\text{ac}}$ - the electric capacity of the unfilled part of the vessel of larger diameter.

C $\genfrac{}{}{0ex}{}{\text{X}}{\text{ac}}$ - the electrical capacity of the filled part of the vessel of larger diameter,
L is the height of the vessel,
X is the height of the liquid column in the vessel.

где
ε_o,ε_в,ε_c - диэлектрические проницаемости вакуума, воздуха и среды (жидкости, заполняющей датчик) соответственно.Using formulas for calculating the capacity of a cylindrical capacitor, we obtain

Where
ε _o , ε _in , ε _c - dielectric constant of vacuum, air and medium (liquid filling the sensor), respectively.

 δ is the thickness of the insulation on the electrodes.

где

,

,
r₁ - радиус электродов,
r₂, r₃ - внутренние радиусы цилиндров сообщающихся сосудов.The total electric capacitance Cac of a vessel of a larger radius is thus equal to

Where

,

,
r ₁ is the radius of the electrodes,
r ₂ , r ₃ - the inner radii of the cylinders of communicating vessels.

где

Нормированные (т. е. умноженные на заранее известные постоянные коэффициенты) значения этих емкостей равны

Разность же нормированных емкостей

прямо пропорциональна высоте X столба жидкости в сосудах и не зависит от переменной емкости незаполненных объемов сосудов.By analogy for a vessel of smaller diameter we get:

Where

The normalized (i.e., multiplied by previously known constant coefficients) values of these capacities are equal

The difference is normalized capacities

It is directly proportional to the height X of the liquid column in the vessels and does not depend on the variable capacity of the unfilled volumes of the vessels.

 Thus, in the proposed device, the error associated with the influence of the capacitance of the sensor volume not filled with liquid is completely eliminated, which allows to expand the dynamic range of measurements due to the possibility of taking them at small (less than 0.1) X / L values and to increase accuracy in the entire measurement range .

The capacitance converter can be made, for example, in the form of a divider on the resistances Z1 and Z2, powered by a generator 5 connected between a common point of capacitors C _ac and C _sun (vessel body) and a common point of resistance Z1 and Z2. The resistances Z1 and Z2 together with the capacities C _ac and C _sun represent a classic bridge powered by generator 5. The vessel capacities C _ac and C _sun form two adjacent shoulders of this bridge, and the other two, the resistances Z1 and Z2, selected from the condition Z1 / Z2 = K2 / K3. In this case, the balance of the bridge should be ensured at zero level, i.e. in the absence of fluid in the vessels. However, in practice, the full balance of the bridge cannot be obtained with this choice of resistances Z1 and Z2 due to the difference in the end capacitances of the measuring and compensation electrodes relative to the bottom of each vessel. This difference can be compensated by connecting an additional trimmer tank C _add . With full compensation of the difference in the end capacitance, the change in the output voltage Δ _Uout taken from the bridge is proportional to the difference in capacitance (C _ac - C _sun ), level X, or liquid volume V _liquid. , i.e. Δ U _out = K4 • (C _as - C _Sun ) = K5 • X = K6 • V _liquid , where K4, K5 and K6 are pre-calculated constants determined by the design of the sensor.

Claims (2)

 1. A device for measuring the level of electrically conductive media, containing a recorder, electrodes made insulated, and a capacitance made with the possibility of adjustment, characterized in that the electrodes are located in separate, interconnected cylindrical vessels of different diameters made of conductive material, and connected to the inputs of the capacitance converter, and the capacitance made with the possibility of adjustment is connected in parallel to the electrode immersed in a vessel of a larger diameter.  2. The device according to p. 1, characterized in that the electrodes are installed at the same distance from the bottom of the vessels.

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