RU2231028C2 - Method of liquid level determination (variants) - Google Patents

Abstract

FIELD: operation of stationary reservoir filled with fuel, water or other liquids.

SUBSTANCE: method involves directing optical radiation through elongated substantially transparent item (band, rod or fiber) immersed into liquid, wherein optical radiation is directed at an angles to side item surface to provide maximum radiation emitting into liquid through side item surface; recording radiation emitted from side item surface near liquid level by coordinate light sensor arranged along item for determining liquid level into reservoir. Another method involves directing optical radiation through elongated substantially transparent item (band, rod or fiber) immersed into liquid; recording radiation emitted from side item surface by float indicator for determining liquid level into reservoir; creation relation between radiation intensity and liquid depth by including transforming and/or dispersing additives into item. Additives have specified bulk density dependence on item length.

EFFECT: increased accuracy and reliability.

3 cl, 6 dwg

Description

The invention relates to the field of operation of stationary tanks with fuel, water or other liquids and can be used in various works related to determining the height of a liquid level.

There are several ways to determine the height of the liquid level in the tanks. The most widely known of them consists in measuring the height of a level with a level indicator of the UDU type [2, 4], as well as with a standard tape manually (tape measure with a lot or meter stock).

The UDD level indicator includes the following elements: a float with a steel measuring tape on which it is suspended; two guiding strings passing through two rings on the sides of the float, along which (the strings) the float glides; a piping system with blocks inside at the turning sections of the tape, which (system) allows you to remove the tape from the tank; a counting mechanism, fixed, as a rule, on the side surface of the tank; system for remote transmission of liquid level readings A method for measuring the height of a level with an indicator of the type of UDD consists in exposing the buoyant force of the liquid to the float and transmitting the movement of the float (following the liquid level) through the tape to the counting mechanism of the level height. The readings of the counting mechanism are often transmitted via the remote transmission system to the warehouse console. The error of direct measurement of the level height (without the error of the remote system) is ± 3 and ± 4 mm at a level height of 5 and 10 m, respectively [4]. Approximately the same error should be expected when using roulette. Using a potentiometric data transmission system adds ± 15 mm to the specified error, and using a code system ± 1 mm.

The main disadvantages of the method for determining the liquid level by UDD are as follows: at negative temperatures in separate periods of time, ice appears in the counting mechanism, which is the result of condensation of water vapor from the liquid of the tank and which complicates the operation of UDU; jamming of the float is sometimes observed due to skew arising from movement along the strings; in some cases, the above-mentioned errors in measuring the height of the level, taking into account the errors in remote transmission, may not be acceptable for large-capacity tanks.

In another method of measuring the height of a level, a UDAR type indicator [4] is used, consisting of two measuring tubes with sources and gamma-radiation counters, an electronic-mechanical unit, a source-sensor system (moves after the level) and a control panel for registration of level height, liquid overflows, etc. A method for determining the level height by an indicator of this type consists in the effect of gamma radiation on the liquid between the pipes and in recording its absorption by the liquid and the gaseous medium, which makes it possible to determine the relative density of these media, and therefore the level height. The disadvantages of this method are the excessive bulkiness of the equipment, the potential danger from sources of radioactivity, a number of limitations in operation due to the presence of sources of radioactivity.

The height of the liquid level in stationary tanks is also determined by other methods using, for example, a capacitive sensor of the DUE type, a magnetic float indicator like MPU, simply measuring glasses, etc. They are not so widespread and have a number of advantages and disadvantages associated with the accuracy of level measurement, the absence of a telemetric part, etc. So, the recently appeared DUE sensor (based on the registration of an electric capacitance between two cables immersed in a liquid, depending on the height of the level) has a rather large error. For example, the use of a DUE of accuracy class 1.5 [9] should lead to errors of up to ± 7.5 cm at a 5-meter level height. If we take into account the additional errors listed in [9], then the general error will be approximately 2 times larger.

The task of accurate and reliable determination of the height of the liquid level can be solved by using well-known methods of optical location. So, to determine the height of the level, the method by which the pulsed light range finder works [10] could be applied: a light pulse is sent to the object, and the distance to the object is determined by the speed and time the pulse travels to and from the object. This method is reliable and extremely accurate. However, its use for liquids in tanks is associated with relatively complex equipment, difficulties in operation, etc.

The industry also uses level gauges based on a pair of light emitting diode - photodetector. A device based on this basis contains two of these pairs — one for controlling the upper level of the liquid and one for controlling the lower level [7]. When the level is lower than the required level, the beam from the LED starts to arrive at the photodetector, which in turn turns on the level control relay; two pairs of LED - photodetector allow you to maintain the level in the desired range. The method on the basis of which the device operates is thus reduced to directing the radiation to the liquid and registering it with a photodetector or not registering it depending on whether the beam path to the receiver is open or if the liquid interferes with it. This method and the device in its current form are not suitable for the current (and not discrete) determination of the liquid level.

Of all the above methods, the closest in technical essence to the invention is the method by which a pulsed light range finder operates.

The aim of the present invention is to improve the accuracy and reliability of determining the height of the liquid level with simple equipment for this, i.e. the goal is to eliminate most of the disadvantages listed above in the description of analogues.

The goal is achieved by the method described below in two independent versions.

According to the first variant, the method of determining the height of the liquid level is that they direct optical radiation along a sufficiently transparent extended body (tape, rod, fiber) immersed in the liquid, and the radiation emerging from the body is recorded, which is used to judge the height of the liquid level, characterized in that the radiation in the body is directed at angles of incidence to the side surface of the body, providing the greatest output of radiation into the liquid from the side surface, and a burst of radiation output from the side surface to the liquid is recorded near the level with a coordinate photodetector (FP) located along the body.

The fact that the body is an optically denser medium than the gaseous medium above the liquid leads, as a rule, to an increased output of radiation from the body near the level. This output can be enhanced by: selecting the material of the body so that its refractive index relative to the liquid n _{tj is} as small as possible; reducing the cross section of the body and thereby increasing the frequency of reflection and refraction of light in the body per unit length; selection of a radiation source with the desired radiation pattern.

If the calculation of the implementation of the above variant of the method does not lead to reliable registration of increased radiation by the coordinate AF, then an additional and in some cases a sharp increase in the radiation yield and thereby create a reliable light probe for the AF can be achieved as follows:

1) if n _t > 1, then the radiation in the body is directed at angles of incidence to its lateral surface θ _n greater than the angle of total internal reflection for rays in the body relative to the gaseous medium above the liquid θ _g , but smaller than the angle of total internal reflection for rays in the body relative to the fluid θ _w ;

2) if n _t <1, then the radiation in the body is directed at angles θ _g <θ _n <90 °.

As an example, confirming the possibility of implementing the method according to this embodiment, Figures 1-5 show diagrams of the method, and explanations are given below.

Figure 1 shows a fragment of a tank with jet fuel 5 and its level 8. To measure the current height h level in the jet fuel, dip tape 2 with a coordinate photodetector 3 located along the tape through gaskets 4 for fuel access to the FP. The tape and the FP are fixed on the base 9 having a light-absorbing surface. The radiation comes from the source 7 through the adapter 6, forming and directing the rays into the tape. The radiation source can be taken out to the control panel, and the signal from it can enter the tape through a fiber optic cable. The tape is made of polystyrene having an absolute refractive index at a wavelength of 589 nm at 20 ° C equal to n _t = 1.59 [11] (hereinafter it will be taken the same for waves λ = 800 ... 900 nm); the attenuation of the light signal — about 500 dB / km at a wavelength of λ≈850 ... 900 nm — was taken for polymer optical fibers [8] for 1977, taking into account the promise of nylon and polystyrene [3]; tensile strength σ _bp = 40-50 MPa; bending modulus E = 3.2 GPa. Other indicators are also quite acceptable for our case. The disadvantage is the fragility of polystyrene, which can be compensated by enclosing the tape in a thin, transparent and reliable polymer shell with an acceptable refractive index, and, if necessary, in a special wire sheath common for the tape and FP.

According to these data the maximum possible thickness permitted by the belt at a bend radius R = 50 cm b = 2σ amount _Bp R / E = 2 · 40 MPa · 50 cm / 3.2 cm = 1.25 GPa.

Assuming that the jet fuel from the optical indices close to the mark AVKP (USA), taking the refractive index, as well as at AVKP-n _w = 1.40 at 25 ° C [6]. Since in our case, n _m n _w is substantially greater in the body under the direct rays of incidence angles in θ _d <θ _f <θ _w. The angles θ _g and θ _W have the following meanings:

These angles are shown in figure 2, where it is also indicated: 2 - tape; 3 - AF; 4 - gaskets for fuel access to the FP; 8 - fuel level. The distance between the tape and the FP is chosen so that there is no capillary rise in fuel. In addition, the thickness of the tape is checked so that the main radiation from the side surface of the tape is not concentrated in the adjacent volume of liquid with a radius r (figure 2). To do this, it is sufficient to have b> (0.1-0.3) r.

Thus, we direct the rays in the tape at angles of incidence in the range 38 ° 58'-61 ° 42 '. Moreover, the best result will be at angles closer to 38 ° 58 '. In this case, the radiation will propagate in the tape to the liquid level mainly without loss; with perfect tape manufacturing technology, losses should be expected only due to Rayleigh scattering, i.e. insignificant. Below the fuel level due to the fact that θ _p <θ _w = 61 ° 42 ', the rays will not only begin to reflect from the side surface, but also, refracting, leave the tape. Assuming that it is possible to provide 38 ° 58 '<θ _n <45 °, the rays after several reflections will mainly leave the tape, forming a light reference at the fuel surface. In this regard, to reduce the error in determining h, we take the width of the tape as small as possible, but at the same time significant, so as not to complicate the work of manufacturing the tape, polymer sheath, wire hose with fixing the minimum bending radius, connection with the radiation source. To a first approximation, such a width will be b = 0.5-1.0 mm, and the minimum allowable bending radius, taking into account about 4 times the margin of safety of the tape as a brittle element, R≈20 cm.

The spectral matching of the materials for the tape (polymers, borosilicate glasses with attenuation of the order of 50 dB / km and, in the extreme case, quartz glass) with the radiation source and phase transition is best achieved by the band at 800 and 600 nm. Below, the choice of the radiation source and the phase transition is oriented at a wavelength of 800 nm.

For the radiation source, we take the GaAlAs light-emitting diode (LED), taking into account its operation according to schemes providing compensation for the temporal and temperature instability of radiation [7]. LEDs are taken with an admixture of 0.05 ... 0.1, providing λ _max = 800 nm (maximum generation) [8]. We connect it to the tape according to the scheme in figure 3, located on the left. The angle of inclination of the diode 1 to the tape 2, the radiation pattern and the focusing element 3 are calculated so as to ensure the most efficient input of radiation, taking into account the need for the propagation of rays in the tape with incidence angles in the range 38 ° 58'-45 °. If it is difficult to fulfill these conditions, we take for the radiation source a GaAlAs DGS laser with λ _max = 800 ... 900 nm. In this case, the focusing element 3 (FIG. 3) is optional. If you select the material tape would n _w> n _t, it is possible to use LEDs without tilting to the tape, ie the connection of the LED with the tape is simply an optical glue 4, which is shown on the right side of figure 3 and that is used as one of the methods of radiation input [8].

By directing the rays in the tape at angles of incidence of 38 ° 58'-61 ° 42 ', we provide, as was said, a burst of radiation output near the level. The plot of the intensity of the radiation flux from the side surface of the tape is shown in Fig.4. The areas above and below the maximum in the diagram correspond mainly to Rayleigh scattering (insignificant background).

The emitted radiation corresponding to the peak on the plot forms a photoelectric contact in the coordinate photodetector. The preferred coordinate FP for our case is a functional photoresistor (FFR) with a variable interelectrode distance and a compensating load, which can significantly eliminate interference from temperature and other influences [5]. Its circuit is depicted in figure 5. On it, a light probe 6 formed by a peak of increased radiation output from the tape passing through the dielectric substrate 2 and photo layer 1 forms a contact between the electrode 3 and parallel equipotential electrodes 4 and 5. We set the distance between the electrodes, for example, by a linear dependence on h and remove the voltage V ₂ (h).

For our case, the FSF is taken from a CdSe-based film, the spectral sensitivity of which is close to the highest at λ = 800 nm [1]. The calculation of the FFR linked to the calculation of the index for this method as a whole is not given here.

FPs of the FFR type, as practice shows, have reliable performance indicators, and the resolution remains constant in time and amounts to several microns [5]. The peculiarity of a film based on CdSe (a relatively large tape) should not cause fundamental difficulties in its production by the technological methods described in [1]. Nevertheless, to reduce the complexity of manufacturing a film with a variable interelectrode distance, it is possible to replace FFR with a variable interelectrode distance with a slotted type FFR [5]. In addition, the entire FFR can be made in the form of segments (sections). In technological evaluation, it is also necessary to take into account the possibility of using a photopotentiometer with a compensating load by analogy with the FFR [5].

With the described system, the source — transparent tape — AF — the main measurement error h will mainly depend on the width of the peak of lateral radiation from the tape. With a successful selection of the tape material according to the refractive index, tape width and thickness, directivity of the radiation source, the error in determining h can be brought to 1 mm.

A variant of the above method for determining the height of the liquid level is that an additional burst of radiation output from the side surface of the body is created by incorporating transforming and / or scattering additives, for example, phosphors, into the body. As phosphors can be used salts of rare earth elements or actinides that form glow centers. The rod for this subvariant can be made of borosilicate glass with n _t ≈ 1.5 and attenuation of about 50 dB / km [8]. It is possible to use other materials for the rod, including transparent polymers, if this is linked to the technology of incorporating phosphors into them. When choosing activators, it is best, as mentioned above, to focus on wavelengths in the region of 800 ... 850 nm, and to set their density but volume constant. The radiation source may be a GaAlAs LED. In this case, the LED and the coordinate FP should correspond to the selected activator in wavelength. A probable measurement error of h when using this subvariant can be expected up to 1 mm.

According to the 2nd variant, the method of determining the height of the liquid level consists in directing optical radiation along a sufficiently transparent extended body (tape, rod, fiber) immersed in the liquid and registering the radiation emerging from the side surface of the body with an indicator on the giveaway, according to which ( radiation) they judge the height of the liquid level, characterized in that they create a predetermined depth regularity of the intensity of radiation from the side surface, achieving this by incorporating transforming and / or scattering radiation into the body of additives, e.g., phosphors, with their bodies to specify the length change in the bulk density.

Figure 6 shows a diagram corresponding to this variant of the method. A rod 2 of circular cross section is immersed in the liquid 4 of the tank 1. The rod is mounted on a stand 6 and has a radiation source 5. The characteristics of the rod and source can be the same as those proposed in the description of the sub-variant of the first embodiment using additives, with the exception of the bulk density of the activators. In this case, it is laid along the length of the rod such that the intensity of radiation from the side surface corresponds to the selected regularity in h. From the photodiode (one or more) mounted on the float 7, the signal is transmitted via cable 8 to the registration. The magnitude of the signal depends on the established pattern of radiation from additives (for example, phosphors) along the length of the body and, therefore, determines h. The photodiode can be taken out of the tank, and in its place is an optical receiver connected to the photodiode with a fiber-optic cable. The accuracy of measuring h according to the 2nd embodiment can be expected up to 1 mm.

If it is difficult to implement the 1st or 2nd variant of determining h, the extended body and (or) the AF together or separately are taken to be made in a protective housing and, if necessary, in the form of sections with connectors. The use of one or another variant of determining h depends on many reasons: the transparency of the liquid, the type of liquid and the associated work safety, the possibility of using wetting regulators, etc.

Sources of information

1. Anisimova I.D. Semiconductor photodetectors: Ultraviolet, visible and near infrared spectral range. -M.: Radio and Communications, 1984, 216 p.

2. Bromberg A.A. and others. Mechanical and power equipment of airports. -M.: Engineering, 1968, 336 p.

3. Grodnev I.I., Schwartzman V.O. Theory of directing communication systems. -M .: Communication, 1978, 296 p.

4. Ipatov A.M. Operation of storage tanks for fuels and lubricants. -M .: Transport, 1985, 176 p.

5. Ishanin G.G. et al. Sources and receivers of radiation. -SPb .: Polytechnic, 1991, 240 pp.

6. Litvinov A.A. Fundamentals of the use of fuels and lubricants in civil aviation. -M .: Transport, 1987, 312 p.

7. Mukhitdinov M., Musaev E.S. Light emitting diodes and their application. -M .: Radio and communications, 1988, 80 p.

8. Nosov Yu.R. Optoelectronics -M .: Soviet Radio, 1977, 232 p.

9. Level sensor capacitive DUE-1. Technical description and operating instructions for Ca 2.834.002 TO.

10. TSB. T. 7, p. 519. -M .: Soviet Encyclopedia, 1972, 608 p.

11. Chemical encyclopedia. T. 4, p. 24. -M.: Big Russian Encyclopedia, 1995, 640 p.

Claims (3)

1. The method of determining the height of the liquid level, which consists in directing optical radiation through a sufficiently transparent extended body (tape, rod, fiber) immersed in the liquid, and registering the radiation coming out of the body, which is used to judge the height of the liquid level, characterized in that the radiation in the body is directed at angles of incidence to the side surface of the body, providing the greatest output of radiation into the liquid from the side surface, and a burst of radiation output from the side surface to the liquid is recorded near level coordinate photodetector located along the body. 2. The method according to claim 1, characterized in that an additional burst of radiation output from the side surface of the body is created by incorporating transforming and / or scattering additives, for example, phosphors, into the body. 3. The method of determining the height of the liquid level, which consists in directing optical radiation along a sufficiently transparent extended body (tape, rod, fiber) immersed in the liquid, and registering the radiation emerging from the side surface of the body with an indicator on the float, by which (radiation) they judge the height of the liquid level, characterized in that they create a predetermined depth regularity of the intensity of radiation from the side surface, achieving this by incorporating additives that convert and / or scatter radiation into the body for example, phosphors, with their specify the length of the body change the bulk density.

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