RU2680159C1 - Method for determining volumes of closed cavities - Google Patents

Abstract

FIELD: measuring equipment.SUBSTANCE: invention relates to a measuring technique, in particular to measurements of the capacities of closed sealed volumes in various complex systems and installations related to vacuum technology, with the possibility of placing inside their volumes of porous materials and/or structural elements of them. In the method of determining the volumes of closed cavities, which are sealed rigid structures, consisting in transferring gas of a known portion from a calibrated container to a measuring one and then carrying out a reverse flow of gas from the measuring container to a calibrated one, in which the pressure in the measuring tank before the return bypass is maintained equal to the pressure in the calibrated container before the direct bypass, the combined volume, consisting of successively connected measured volume, is evacuated, one or more auxiliary volumes and a calibrated volume, separated by control valves, in a calibrated volume, a control gas pressure is created at a relatively high pressure of a given value, measured by an absolute pressure transducer, on the basis of the condition that an equilibrium pressure should be established in the combined volume after the gas bypass.EFFECT: technical result of the invention is to improve the accuracy and reliability in determining the volumes of closed cavities, especially relatively small capacities of arbitrary shape, which are part of complex vacuum systems and installations.1 cl, 2 tbl, 4 dwg

Description

The invention relates to measuring equipment and can be used to measure volumes of closed sealed cavities in various complex systems used in vacuum technology, including installations with placement of porous materials and / or structural elements from them inside their volumes.

A known method for determining the volume of containers, in which before direct and reverse bypasses of gas, the pressure in the tank from which the gas is bypassed is set above the bypass pressure, and the tank into which is bypassed is set below the bypass pressure with subsequent identical time delays at these pressures. In this case, the temperature of the gas in the tank from which the gas is bypassed decreases relative to the average temperature of the gas in the system, and in the tank into which the gas flows, it rises. Copyright certificate No. 939945, IPC G01F 17/00, 06/30/1982.

It is believed that this circumstance, with the same time spent on determining the volume of the tank, can improve the accuracy of measurements. However, the time delay at its incorrectly set (selected) values can negatively affect the results, since the times necessary for the gas to come to equilibrium when the containers are combined, i.e. when the pressures in them are equal, they depend 1) on the ratio of the volumes involved (participating) in the measurements, and in addition, 2) on the throughput of the channels through which the gas flows, 3) the conductivity of the shut-off regulating body (valve), which separates the capacities at bypass and 4) from the time of its opening. This, in turn, can introduce uncertainty into the measurement result due to differences in the conditions of heat exchange.

Similar disadvantages can occur in the case of using the method of determining the volume of containers, in which an overpressure is created in the calibrated volume with respect to the measured capacitance, recorded by pressure measuring instruments, after which the gas is transferred from the calibrated container to the measured one with the help of a valve, followed by pressure registration tanks, then in the measuring tank create excess pressure in relation to the calibrated and carry out the reverse bypass of gas from the measured tank the pressure in the measured tank and the pressure drop between the tanks before the bypass are maintained equal to the pressure in the calibrated tank and the pressure drop between the tanks before the direct bypass, and the determination of the desired value is carried out according to the well-known formula (1):

- давление в калиброванной и измеряемой емкостях перед обратным перепуском,

- установившееся давление в системе после обратного перепуска. Авторское свидетельство №714156, МПК G01F 17/00, 05.02.1980. Данное техническое решение принято в качестве прототипа.where V _MOD, V _k - volumes measured and calibrated containers, P _k, P is the pressure in the _edited calibrated and measured capacitance to a direct bypass, P - steady pressure in the system after the direct bypass,

- pressure in calibrated and measured containers before return bypass,

- steady-state pressure in the system after the return bypass. Copyright certificate No. 714156, IPC G01F 17/00, 02/05/1980. This technical solution was made as a prototype.

In addition, this method is not accurate enough in view of the error introduced due to the lack of consideration of the possible manifestation of the thermal effect associated with temperature changes during bypasses, when the gas outflow is not a quasistatic isothermal process. The occurrence of possible disturbances in the thermodynamic balance between containers during gas expansion caused by the establishment of different flow regimes at different pressure levels without providing the necessary control over temperature changes in the system under consideration causes systematic errors in determining the desired volume, the elimination of which requires the introduction of appropriate correlation corrections.

Moreover, in both cases side effects associated with adsorption-desorption processes are not taken into account, the manifestation of which is most pronounced in systems at relatively low pressures (under high vacuum), especially when determining the volume of microporous materials in technological containers (copyright certificate No. 1818540, IPC G01F 17/00, 05/30/1993).

Certain difficulties associated with the appearance of relatively large errors arise in determining the volumes included in complex multiblock vacuum systems consisting of the nth number of volumes. Particularly problematic in the practical aspect are measurements carried out with free volumes for structures with relatively small capacities adjacent to volumes that can be several orders of magnitude larger. Moreover, in individual structures from among these volumes, due to their specificity (shape and characteristic dimensions), communication with other volumes, often placed in the working technical space of the system at a relatively large distance from each other, can be provided only through elements of low conductivity, for example, through narrow channels of variable cross-section, which greatly complicates obtaining adequate measurement results, due to the relatively long duration required to achieve maximum conditions of clarity (isotropic) gas distribution volumes. In addition to this, with the appearance of temperature differences between the volumes connected by narrow pipelines, during the bypasses under the conditions of the molecular regime, the phenomenon of thermal effusion (thermomolecular flow) occurs [G. Levin. Fundamentals of vacuum technology, trans. from English - M .: Energia, 1969. - P. 15], in which the gas pressures in these volumes are redistributed in proportion to the square root of the temperature. In this case, a change in gas temperature can occur directly in the measurement process, for example, due to adiabatic expansion of real gas in thermally insulated vacuum systems, leading to a decrease in temperature on the rarefaction side.

It should also be noted that the increase in bypass timing can result in an increase in the concentration of accumulated by-products from gas evolution and leakage in volumes, the identification and corresponding accounting of which without special tools and methodological techniques makes it difficult to solve this problem.

The low accuracy of the results of indirect measurements can also be due to either insufficient information or inaccurate data on individual characteristics of heat and mass transfer, on which the gas flow regime in the system under consideration depends. In addition to the previously mentioned temperature of the gas and the walls surrounding it, they include:

1) the pressure differential at the ends of individual sections and the system as a whole, 2) absolute pressure, 3) viscosity (internal friction of the gas), 4) the interaction of gas molecules with a solid surface.

The technical result of the invention is to increase the accuracy and reliability in determining the volume of closed cavities, especially relatively small containers of arbitrary shape, which are part of complex vacuum systems and installations.

The technical result is achieved by the fact that in the method for determining the volume of closed cavities, which are sealed by rigid structures, which consists in transferring gas from a calibrated tank to a measuring tank and measuring pressure in these tanks, followed by a reverse gas bypass from the measuring tank to a calibrated one, at which the pressure in the measuring tanks before reverse bypass are maintained equal to the pressure in the calibrated tank before direct bypass, vacuum the combined volume m, consisting of a series-connected measuring volume, one or more auxiliary volumes and a calibrated volume, separated by control valves, in a calibrated volume create a pressure of a comparative gas of relatively high pressure of a given value, measured by an absolute pressure transducer with a measurement range in the region of relatively high pressure, proceeding from the condition that, after gas bypass, an equilibrium pressure corresponding to under the conditions of the molecular flow regime (Kn> 0.33 is the Knudsen number), the created portion of gas is kept in a calibrated volume with a given exposure, and in parallel with the exposure, the auxiliary and measuring volumes are subjected to evacuation, the vacuum depth in which is determined by the specified level of rarefaction recorded by a wide-range gauge the transducer, at the same time as conducting pressure measurements, record the temperature of the gas in these volumes, until the expiration of the exposure period evacuation of the auxiliary and measuring volumes is simultaneously stopped and, at a given moment, a portion of the gas is directly transferred from the calibrated volume to the above volumes, the pressure and temperature are controlled after the bypass until a linear increase in the output signal from the absolute pressure gauge with a measurement range in the relatively low pressure, then produce joint evacuation of the combined volume to a value not exceeding at a predetermined vacuum level, upon reaching it, the evacuation of the calibrated volume is blocked for a time equal to the duration of the soaking interval in the given volume of the portion with the control gas pressure of the previously set value and the operations are repeated that are directly related to the gas bypass, but already accumulated as a result of desorption processes and leaks carried out in the same time interval, then re-carry out all the above operations, using the measuring volume when creating the portions, in preparation gas portions to the return bypass in the last one provide the establishment of the control gas pressure equal to the pressure prepared in the calibrated volume for direct bypass, with the auxiliary and calibrated volumes being evacuated in the intervals between bypasses until the background concentration level does not exceed the previously set value, at this, if at a selected level of pressure of the control gas in the created portions in the calibrated and measuring volumes, the achieved pressure after direct and reverse bypasses will not correspond to the rarefaction level characteristic of the occurrence of molecular conditions and / or it will not exceed the minimum reliable level of the output signal of the absolute pressure gauge with the measuring range in the region of relatively low pressure, then by appropriate selection or calculation of the control gas pressure in portions correlate to the required level with the repetition of all the above operations, according to the results build a graph of concentration As a function of time, the least squares are used to approximate the sections of the graph characterized by "background" and "useful" signals, find the corresponding increments or difference in gas pressures attributed by extrapolation to the time of the beginning of the forward and reverse bypass portions, and determine the desired value by following formula:

where ΔР ₁ , ΔP ₁ ^' are the increment values of the signals of the absolute pressure gauge with a measurement range in the region of relatively low pressure after the expansion of the control and background side gas, respectively, from the calibrated to the auxiliary volume, including the measured one, obtained by correlation and regression methods analysis attributed to the beginning of direct portion bypass, Pa; ΔР ₂ , ΔР ₂ ^' are the increment values of the signals of the absolute pressure gauge with a measurement range in the region of relatively low pressure after the expansion of the control and background side gas, respectively, from the measurement to the auxiliary volume, including the calibrated one, obtained by correlation and regression analysis attributed to the beginning of the reverse bypass of portions, Pa; V _to - the capacity of the calibrated volume, m ³ , V _ISM - the capacity of the measuring volume, m ³ ; T ₁ - temperature of the gas and the walls of the calibrated volume before the direct bypass, K, T ₂ - temperature of the gas and the walls of the measuring volume before the direct bypass, K.

The invention is illustrated by drawings.

In FIG. 1 shows a diagram of a device for implementing the method for determining the volume of closed cavities, where: 1 - measuring volume; 2 - high vacuum control valve; 3 - absolute pressure gauge with a measuring range in the field of relatively low pressure; 4 - auxiliary (technological) volume; 5 - absolute pressure gauge with a measuring range in the field of relatively high pressure; 6 - high vacuum control valve; 7 - calibrated volume; 8 - gas dispenser (leak); 9 - high vacuum control valve; 10 - wide-range pressure gauge; 11 - turbomolecular high vacuum pump; 12 - control valve; 13 - control valve; 14 - low vacuum sensor; 15 - foreline pump.

In FIG. Figure 2 shows the theoretical curves of the pressure change of the control gas with an increase in its volume over time, taking into account the specified integral flows from gas evolution and leakage in auxiliary volumes — curves a) and c) with flows of 1.0⋅10 ^-7 Pa⋅m ³ / s on the graph and 5.0⋅10 ^-9 Pa⋅m ³ / s, respectively, and the measuring volume - curves b) and d) with integral flows 1.0⋅10 ^-7 Pa⋅m ³ / s and 1.0⋅10 ^{- 9} Pa⋅m ³ / s with a holding time of 1 minute at a given value of conductivity of the channel between the volumes 2,304⋅10 ^-6 m ³ / s (corresponding to conduction tube molecular conditional minutes with an inner diameter of 1.5 mm and a linear dimension of 26.7 mm at room temperature (T = 293 K) for the control-neon gas (mass number M / z = 20)).

In FIG. Figure 3 shows a superposition of typical graphs of the pressure change dynamics characterizing the main processes in the system associated with direct and reverse bypasses of the control gas recorded using an absolute pressure transducer (curve e) and an ionization full pressure sensor (curve f) when determining the value of the measuring volume in the vacuum system of the UVKG installation.

In FIG. Figure 4 presents an illustration of a typical graph of the change in the total pressure values characterizing the main processes in the system associated with direct and reverse gas bypasses with a holding time of 5 min during the procedure for determining the volume of the measuring capacitance at the UFKG unit with a graphical representation of the results of approximation and extrapolation of the pressure function P = F (t) at the initial time (t = 0), obtained by mathematical methods using a linear model (P = F (t) = a⋅X + B).

A distinctive feature of the proposed method in comparison with the prototype is that it is the pressure increments determined using the methods of regression analysis and the subsequent extrapolation of the obtained functional dependencies beyond the specified time interval, the boundary of which is the beginning of the forward and reverse bypass portions, t. e. when the gas has not yet begun to spread through the combined volume. In this case, the pressure drops between the tanks, when the pressure of the control gas in the calibrated and measuring tanks is equal, before the direct and reverse bypasses can differ (be not equal) with each other. The use of the above increment at the time of bypass leads to an increase in the reliability of the results due to the fact that the proposed method takes into account the factor of the presence of temperature fields of gas in volumes during measurements. Since possible differences in gas temperatures in volumes before and after the bypass introduce additional errors into the measurement results, the value of the determined pressure increment, referred to the initial moment of bypass (forward / reverse), improves the accuracy of the desired result, since the volumes involved in the measurement process for a given period of time for the correct implementation of the method should be in conditions of thermodynamic balance, that is, with the equality of their temperatures, determined by the temperature of the non-thermal atirovannoy environment.

The reliability of the obtained results is also ensured due to the fact that under the additivity of processes occurring over time, the pressure increment values are free from additional errors due to the influence of desorption and leakage flows in calibrated and measuring volumes, since they are reproducible processes that are mutually compensated in the calculations . At the same time, the influence of non-excluded distortions of the background characteristics due to the possible manifestation of sorption phenomena, especially if microporous materials are placed in the system, is methodically taken into account.

In addition to the foregoing, the accuracy is enhanced by taking into account the influence of gas side components due to the tribological effect exerted locally on the surface areas of the valve-saddle friction pair, in which tribostimulated desorption of previously adsorbed molecules formed during bypass at the moment of valve opening is observed from rubbing surfaces , as well as gases released from the bound state from the surface of the bellows input of the valve during compression (tension) of the bellows.

Moreover, the circuit diagram of the installation for determining the volumes of closed cavities has been changed: instead of two volumes, the system has auxiliary (technological) containers available in almost all vacuum systems, while not exerting any negative effect on the accuracy of measurements under certain conditions. It should be noted that under molecular conditions the pressure ratio does not depend on the number of connected containers. Moreover, if additional containers having a different temperature are installed between the measured and calibrated volumes that are in the same temperature conditions, this will not affect the pressure in the measured and calibrated volumes [Rozanov L.N. Vacuum technology. Textbook for high schools in the specialty "Vacuum technology". - M .: Higher school, 1990. - S. 56], taking into account the fact that the pressure sensor should be outside the zone of auxiliary volumes. However, the presence of additional gas medium (load) in auxiliary volumes imposes certain additional restrictions on the implementation of the procedures of the proposed method (see below).

The proposed method for determining volumes allows to increase the accuracy and reliability of the measurement results, especially when implementing the method on unheated vacuum systems of complex design.

To implement the proposed method, in order to avoid introducing additional unaccounted for errors, measurements must be carried out either under relatively slowly changing environmental conditions, or with constant parameters, in particular, temperature (within room indicators) and atmospheric pressure. The temperatures of the control gas, which are close in value to the conditions of room temperatures for direct and reverse bypasses, may differ, but in each of these cases the temperature of the gas before each such bypass should be equal to the temperature of the walls surrounding it, as well as the walls of the volumes and pipelines along its route, including temperature gauge absolute pressure transmitters. In addition, the temperature parameters of the system with the corresponding bypasses (discharges) of the accumulated portions from the same volumes with the gaseous medium, the appearance of which is caused by the presence of flows from desorption and leakage, must also be exactly correlated with the temperature of the control gas for direct and reverse bypasses, respectively.

In the molecular regime, the equilibrium pressure of the gaseous medium in the volumes established after the bypass taking into account gas evolution and leakage due to the inflow of air components through the leaks is determined by the temperature of the surrounding walls, since there are no intermolecular collisions, i.e. There is a thermodynamic balance of gas and the environment. With the rapid (quick) opening of the shut-off and control valve, the gas expansion process is quick and gas exchange with the environment is practically absent, i.e. Q = 0, while in the unchanged combined volume the gas does not perform any work (A = 0). In accordance with the first law of thermodynamics (Q = ΔU + A), for Q = 0 and A = 0 there is no change in the internal energy of the gas in such a system, and with respect to an ideal gas in such a system there is no change in temperature. For a real gas during adiabatic expansion into vacuum (as previously noted), the gas temperature changes in the direction of its cooling; therefore, the equilibrium pressure in the system also changes. In this regard, as a control gas in the implementation of the proposed method, it is recommended to use monatomic - noble (inert) gases (He, Ne, Ar), which are as close as possible to the state of an ideal gas. But since, as previously noted, the wall temperatures of the above chambers before direct and reverse bypasses may differ (for example, due to the relatively long measurement cycle and the drift of the ambient temperature parameters in this case), and, in addition, changes in gas temperature can follow directly in the process of bypasses under the action of a pressure drop during gas flow through a narrow channel (Joule-Thomson effect), in the proposed method, these inconsistencies are taken into account and, moreover, partially are excluded on the basis of the presented analytical calculations and the following reasoning.

The generated gas portions of the control gas in the measuring V _ISM and calibrated V _to volumes under viscous conditions (the creation of gas portions in molecular conditions is impractical due to the increase in measurement error), respectively, can be expressed as the following formulas:

where P ₁ , P ₂ / n ₁ , n ₂ - pressure / concentration of the molecules of the control gas in the calibrated and measuring volumes, respectively; k is the Boltzmann constant.

It should be noted that under the viscous regime at different gas temperatures, the pressure in the volumes can be equal, and the concentrations can be different [I. Groshkovsky. High vacuum technology, trans. V.L. Bulat, - M.: Mir, 1975, - S. 56].

After the bypass, based on the Clapeyron equation (excluding secondary gas processes), we can write

- равновесное давление контрольного газа после прямого и обратного перепуска, соответственно;

- температура стенок (газа) объединенного объема после обратного перепуска из калиброванного и измерительного объемов, соответственно; - вместимость одного или более вспомогательных объемов.where ceteris paribus

- the equilibrium pressure of the control gas after direct and reverse bypass, respectively;

- the temperature of the walls (gas) of the combined volume after the bypass from the calibrated and measuring volumes, respectively; - the capacity of one or more auxiliary volumes.

Тем не менее, необходимо отметить следующее: если проводимости трубопроводов между объемами достаточно велики, то тепловая эффузия практически отсутствует.It should be emphasized that in the proposed method, in the volumes before the direct and reverse bypasses, the presence of temperature gradients is not provided. In this regard, it is necessary that all parts of the system from the combined volume before the bypasses are in the same temperature conditions. However, in the presence of an element of low conductivity between the volumes, if the pressure is relatively low, a molecular flow occurs in the latter and thermal effusion occurs due to cooling of the gas when it expands (especially when using real gases), therefore its temperature (T ₁ , T ₂ ) changes

Nevertheless, the following should be noted: if the conductivity of the pipelines between the volumes is quite large, then the thermal effusion is practically absent.

When solving formulas (4a) and (4b) together, with equal pressures P ₁ = P _{2, the} desired volume can be determined from the following expression:

и

для данного способа в действительности определяются в виде приращений давления, полученных путем аппрокимации с последующей экстраполяцией их значений на момент времени начала перепуска, то в выражении (5) частное

можно не учитывать, т.е. температурный градиент газа (при его наличии) допускается исключить из расчетов, без внесения погрешности в конечные результаты.Insofar as

and

for this method are actually determined in the form of pressure increments obtained by approximation with subsequent extrapolation of their values at the time of the start of the bypass, then in expression (5) the particular

can be ignored, i.e. the temperature gradient of the gas (if any) can be excluded from the calculations, without introducing an error in the final results.

Thus, the measurement equation is converted to the following form:

This equality (6) is a simplified expression of the measurement equation (2), with which the desired volume is determined.

The necessity of introducing temperature corrections in the calculations can be reduced to zero if the absolute pressure transducer, in addition to its own thermal stabilization (this hardware function of absolute pressure gauges is quite common in modern conditions), there is a possibility

interactive / automatic setting as one of the tuning parameters the values of the temperature of the measured gas, as well as the kind of gas, affecting, along with other parameters, the accuracy of the measurements.

In order to ensure the reliability of the results obtained, it is necessary to take into account the characteristics of gas evolution and leakage. These processes are indispensable "attributes" of all vacuum systems. Moreover, for the implementation of the method it is necessary that the gas flows Q _ISM , Q _K and Q _Σ , characterized by the values of the background concentration of the control gas (background) taking into account the incidental surface gas emission from the walls and leakage in the sealed measuring and calibrated closed volumes, as well as the background in the volume of auxiliary tanks, taking into account the influence of the above-mentioned similar side processes, respectively, should be relatively small. It should be noted that in a small volume, an increase in pressure, with a constant value of the gas flows noted above, proceeds faster than in a relatively large volume. The maximum increase in total pressure ( _Pmeas , P _to , P _Σ ) in the above volumes, due to flows from gas evolution and leakage, to obtain reliable measurement results, should not exceed half the partial pressure of the control gas, i.e. P _ism , P _to , P _Σ ≤ (P _0/2 ). For calibrated and measuring volumes (Р _к , Р _ISM ), this condition must be fulfilled for the entire specified exposure time (before bypass), and for the volume of auxiliary tanks Р _Σ , taking into account the combination with one of the volumes measured / calibrated with its gas component of the flow - immediately before the bypass and during the entire time allotted for measurements. This ensures the necessary signal-to-background ratio equal to two, which is set as the main coefficient in the calculations to ensure the reliability of the results obtained if we draw a certain parallel with leak detection (see OST 11 0808-90. Non-destructive testing. Leak detection methods).

To ensure this method, it is important to solve the question of how much the values of the calibrated and measuring volumes can differ from the volumes of auxiliary capacities used in the measurements, taking into account the differences in the values of integral side flows in volumes. Theoretical studies have shown that with the following given parameters: 1) the ratio of volumes, for example, V _ISM / V _Σ = 2,3166⋅10 ^-2 (V _to / V _Σ = 1,1202⋅10 ^-2 ), 2) the conductivity calibrated channel U _k and U _edited measuring volumes (e.g., U _a = U _rev = 2.30417⋅10 ^-6 m ³ / s) connecting them with auxiliary capacitances, under normal conditions, 3) flows in the measuring / auxiliaries / calibrated volumes on levels 10 ^-9 / 10 ^-9 / 10 ^-9 Pa⋅m ³ / s, respectively, with the established discrepancy in the values of the total pressure of not more than 0.1%, the time required for leveling after a bypass will be 1.1 s. At flows of 10 ^-8 / 10 ^-8 / 10 ^-8 Pa⋅m ³ / s with this discrepancy of not more than 0.1% (1%), the time interval for equalization will increase to 80.81 s (3.09 s), and at flows of 10 ^-7 / 10 ^-7 / 10 ^-7 Pa⋅m ³ / s - up to 149.3 s with a divergence of 0.1% and 25.56 s with a divergence of pressure by 1%. When the ratio of volumes V _ISM / V _Σ = 2,3166⋅10 ^-3 (V _k / V _Σ = 1,1202⋅10 ^-3 ) and the values of the above flows 10 ^-9 / 10 ^-9 / 10 ^-9 Pa⋅m ³ / s, the time when the pressure difference is not more than 0.1% will increase already to 68.93 s; at flows of 10 ^-8 / 10 ^-8 / 10 ^-8 Pa⋅m ³ / s, respectively, the discrepancy will reach a value of no more than 0.1% after 151.04 s. If we take the value equal to V _ISM / V _Σ = 2,3166 =10 ^-4 (V _k / V _Σ = 1,1202⋅10 ^-4 ) as the initial parameter (these relations hold for the installation of UFKG; see below), then at flows of 10 ^-10 / 10 ^-10 / 10 ^-10 Pa⋅m ³ / s the difference in pressure will not exceed 0.14% (1%) for 0.153 s (0.122 s), and at 10 ^-9 / 10 ^{- 9/10 -9} Pa⋅m ³ / s discrepancy does not exceed 0.1% (1%) due to 45.521 (8.392 c) with the established pressure value 0.4342 Pa. For reference, in the absence of side processes, the time required to equalize the pressure to 0.1% in volumes will be no more than 0.132 s. Changing the integral flows in the direction of their increase to values, for example, 10 ^-8 / 10 ^-8 / 10 ^-8 Pa⋅m ³ / s, will cause a pressure difference of not more than 0.1% (1%) at the level of 4.3426 / 4, 3383 Pa (Kn = 0.721 / 0.729) after 156.34 s (47.83 s).

It follows that the integral characteristics of flows from side processes, as noted earlier, can have a significant impact on the accuracy of calculation results. Therefore, for implementation of the proposed method requires that the ratio of the subsidiary capacitances and measuring / calibrated volumes for given values of integral flows (Q _edited, Q _a, Q _Σ) in them, which values should not exceed the value 1⋅10 ^-7 Pa⋅m ³ / c satisfies the following inequality

An analytical assessment of this complex (7) shows that it is necessary to achieve such a combination of all factors so that the result of the product on the left side of the expression is greater than or equal to unity. It should be noted that the difference in pressure in volumes, subject to this ratio, will not exceed 0.1%.

The upper limit value of the integral flows in volumes indicated above is a practically achievable parameter for pressurized systems, since leaks at the level of> 1⋅10 ^-7 Pa⋅m ³ / s are eliminated even at the stage of assembly and commissioning, and gas evolution flows with this level can be significantly reduced either by evacuation with simultaneous high-temperature heating of all structural elements involved in the measurements, or (if the latter is impossible) due to the relatively long high vacuum pumping system.

To reduce the effects of sorption by surfaces forming vacuum volumes, it is necessary to “flush” the vacuum system several times with a control gas flow using a gas meter (leak) at pressure levels close to the upper limits at which a transitional (molecular-viscosity) ) the gas flow regime in the vacuum system during continuous pumping by a high-vacuum pump with subsequent pumping of residues to a predetermined vacuum level.

The method is as follows.

A vacuum system consisting of measuring volume 1, auxiliary volumes 4 and calibrated volume 7 is evacuated to a predetermined pressure level using turbomolecular 11 and 15 forevacuum pumps with open valves 2, 6, 9, 12 and closed shut-off and regulating elements - gas metering (leakage ) 8 and valve 13, while the pressure in the system is measured using absolute pressure transmitters 3, 5 and a wide-range pressure gauge 10, as well as a low pressure sensor kuuma 14.

Having closed the control valve 9, they shut off the pumping system of the vacuum system and inflate the test gas into it to a predetermined pressure value measured by a pressure transducer 5 by means of a gas dispenser 8 using a gas inlet system (not shown in Fig. 1). The valve 6 is shut off with soaking in the calibrated volume of the created portion of the control gas during simultaneous unregulated processes of gas evolution from the surface of the volume wall and from the flows of leakage of air components through its leaks for a given time t _in , the value of which should not be less than the value of the minimum holding time t _{in_min} (see below).

Prior to the expiration of the exposure period, the evacuation of the auxiliary and measuring volumes with the closing of valve 9, when the control valve 2 is open, simultaneously stops (the interval is determined by the time from several seconds to several tens of seconds). By the time the soaking time t _in expires, at which the condition t _in ≥t _{in_min} must be satisfied, by opening the high-vacuum control valve 6, a direct portion of the gas is calibrated from the calibrated volume into the auxiliary and measuring volumes of the vacuum system and the output signal increment of the absolute pressure transmitter 3 is determined 3 characterizing the presence of a control gas content with a content of “products” in a portion of the action of sorption-desorption processes, as well as from leakage flows through leaks (see Fig. 2). Simultaneously with the measurement of pressure in the indicated volumes, the gas temperature is also measured. At the end of the time that is maintained until a linear increase in pressure is established in the same time interval, starting from the moment the portion is relieved, the high-vacuum control valve 9 is opened and the controlled gas medium is evacuated by means of a turbomolecular pump (TMP) 11 until the background indicators preceding the discharge are restored. Upon reaching a vacuum level not exceeding the previously set value, the evacuation of the calibrated volume is shut off using valve 6 for a time equal to the duration of the soaking interval t _{in the} portion with the control gas pressure of the previously set value in this volume, and the operations directly related to gas bypass are repeated , but already accumulated as a result of desorption and leakage processes, with the determination of the increment (difference) in pressure at the time of portion bypass, carried out in the same time interval.

After that, all the above operations are repeated, starting with the closure of the control valve 9 and the inlet of the control gas into the vacuum system through the gas dispenser 8, using a measurement instead of a calibrated volume when creating portions. When preparing a gas portion for a return bypass, the latter ensures the establishment of a control gas pressure equal to the pressure created earlier in a calibrated volume when preparing a portion of a control gas for direct bypass. Fixing a portion of gas of a given value in the measuring volume is carried out by closing valve 2.

If at the selected pressure level of the control gas in the generated portions in calibrated and measuring volumes, the achieved pressure after direct and reverse bypasses does not correspond to the rarefaction level characteristic of the occurrence of molecular conditions, and / or it does not exceed the minimum reliable level of the output signal of the absolute pressure transmitter with a range measurements in the field of relatively low pressure, then by appropriate selection (calculation) the pressure of the control gas in portions of the correlated rises to the required level with the repetition of all the above operations.

Next, measurement graphs are constructed (see Fig. 3), and based on the results obtained by regression analysis methods, for example, the least-squares method approximates the registered areas characterized by “background” and “useful” signals (see Fig. 4).

производят и с данными по «фону», полученными при перепуске контрольной газовой среды из измерительного объема в объединенный объем вакуумной системы с учетом сторонних (побочных) газов, накопленных в нем за счет вышеуказанных побочных процессов.The dependence on the “useful” signal obtained by mathematical methods is extrapolated to the point corresponding to the time moment of the start of the bypass. As a result, the difference ΔP ₁ is determined between the pressure of the portion of the control gas in the combined volume of the vacuum system after direct bypass, taking into account the background, leakage, gas evolution and the tribological gas component from the surfaces of the materials of the calibrated volume, and the residual pressure in the combined volume of the vacuum system, taking into account the secondary gas impurities (desorption, leakage, etc.) assigned to the time point of bypass, respectively. The same mathematical processing associated with the determination of the pressure difference

they also produce data on the “background” obtained when the control gas medium was bypassed from the measuring volume into the combined volume of the vacuum system, taking into account external (secondary) gases accumulated in it due to the above side processes.

полностью повторяет операции, связанные с поиском «наилучших» линий регрессии на основе корреляционно-регрессионного анализа, в результате которых были определены приращения давлений ΔP₁ и

The data obtained during reverse bypass are subjected to similar mathematical processing: the algorithm for constructing the calculations of the determined increments ΔP ₂ and

completely repeats the operations associated with the search for the “best” regression lines based on correlation and regression analysis, as a result of which pressure increments ΔP ₁ and

The calculation of the measuring volume capacity is estimated by the relation (2) taking into account the temperature indicators of gases in the measuring and calibrated volumes immediately before the direct and reverse bypasses.

It should be noted that the previously indicated minimum holding time is characterized by the time during which the residues of the control gas are evacuated from the volumes of the vacuum system involved in the creation of the control portion until the pressure is set below a predetermined vacuum level. In this case, a certain role is played by the effective pumping rate and the value of the integrated residual gas flow from the surface of the volume, as well as the porous material placed (placed) in it (if any) together with the air components penetrating the pumped combined volume of the vacuum system through the leaks.

The estimate of the minimum holding time t _{in_min is} calculated by the formula (8) taking into account the value of the allowable integrated flow Q _Σ and the effective pumping speed S _{0 of the} vacuum system (in a given section):

where P _k , P _o is the initial and final target pressure of the control gas, recorded by absolute pressure transducers (it is possible to use indirect transducers, for example, a wide-range pressure gauge) for measuring ranges of relatively large and, accordingly, low pressures, Pa;

V _Σ is the combined (total) volume of free space from auxiliary volumes together with adjacent sections of the vacuum system with connected absolute pressure transducers and one of the volumes (calibrated or measuring) depending on where the created portion of the control gas is contained at the time of pumping, m ³ ;

Q _Σ - integrated flow, characterized by leakage flows from leaks and the total component from the desorption flows from the walls of the vacuum system, Pa⋅m ³ / s;

S ₀ - effective pumping speed of the vacuum system, m ³ / s.

An estimate of the value of the integrated flow can be obtained using well-known methods of tightness control, for example, gauge (vacuum gauge). As an estimate of the effective pumping speed of the vacuum system for engineering calculations, we can take the value of the volume of gas flowing per unit time through the cross section of a high-vacuum control valve, the throughput of which, as a rule, through air or nitrogen, is one of the main technical parameters indicated in it support materials (passport, instruction manual, etc.). Finally, in the absence of the above-mentioned parameter values for calculation, the value of a given time interval t _{in_min} can be estimated previously by direct measurements.

The proposed method is implemented in practice using a vacuum system of an automated heated mass spectrometric installation for finishing the tightness control of gas-filled UFKG arresters [S.A. Bushin, S.S. Galkin. The results of the pilot operation of a vacuum automated installation for monitoring the tightness of arresters // Vacuum Equipment and Technology, vol. 23 (issue 1), St. Petersburg, 2014. - P. 39-41].

As a calibrated volume, we used a microcavity formed from voids based on the design of a high-vacuum angular controlled valve, one of the joints of which (on the side of the valve-seat pair) is sealed with a special all-metal flange having a cylindrical protrusion along the axis of rotation with a linear dimension (26.7 mm) and a diameter close to the inner diameter of its surface, acting as a displacer of free volume. The capacity of a portioned calibrated microvolume V _m , determined by the weight method under normal conditions, is estimated at 0.63553⋅10 ^-6 m ³ ; the value of the mean square error σ ₀ (V _mk ) of this microvolume is 1.14% (± 0.0072⋅10 ^-6 m ³ ).

The counter flange of the angle valve is connected to an expansion tank with a capacity of 2.0899 dm ³ , which is in communication with the vacuum path line, consisting of a series of several process volumes connected to each other, separated by similar ultrahigh-vacuum controlled valves with large nominal diameters. A collector with 8 welded nozzles in the form of beams arranged symmetrically with a sweep angle of 360 degrees is docked to the last of them; to each branch pipe one measuring volume of the same design is connected.

The measuring volume is a cylindrical composite cavity, formed from two joints, in which on one side there is a split collet with a miniature gas-filled device that enters along a sliding fit. The collet has special internal grooves for the protruding disk electrodes of the device with axially arranged rings made of vacuum-tight ceramics between them (it should be noted that, despite the mention of the vacuum density of the ceramics used, the surface of the instrument shell is a porous structure), forming a multiblock design body; the casing of the device is 60% made of Al ₂ O ₃ . On the other hand, there is a volume displacer in the form of a cylindrical rod of variable diameter with an axial through hole of 1.5 mm, mating one end with a collet through a groove-undercut, and the second end located at a distance of ≈ 1 mm opposite the end of the plate (seal ) controlled ultra-high vacuum valve with pneumatic actuator (normal state of the valve is closed). Opening time (closing) of a pneumatic valve ≈ 1 s [Catalog of vacuum valves of the company VAT, ser. 57, 2012].

All ultrahigh-vacuum all-metal valves with pneumatic actuator are controlled based on the developed hardware-software interface.

To maintain the vacuum in the vacuum system during the measurement process, a HiPace 80 series turbomolecular pump is used, which works in conjunction with a fore-vacuum non-paint spiral pump of the ISP-90 type.

It is worth noting that measurements in the areas of comparative high and low pressures can be carried out by one absolute pressure gauge with a measurement range that extends to these pressure areas (overlapping ranges).

When preparing gas portions in calibrated and measuring volumes, an absolute pressure transducer of the type 690A01TRA was used, working in conjunction with a vacuum meter (controller) type MKS "Baratron" type 670В. The main relative error in measuring the pressure of a portion of the test gas for a range from 1.0⋅10 ^-3 Pa to 1.333⋅10 ² Pa, measured by an exemplary MKS "Baratron" vacuum gauge with a membrane-capacitive type sensor 690A01TRA, does not exceed ± (2 ... 0.05 )%.

For a comparative analysis and assessment of the reliability of the results (as a rule, it is recommended to use at least two gauge converters, the operation of which is based on various principles [see: G. Eshbach. Practical information on vacuum technology. Obtaining and measuring low pressures. Transl. From German B.I. Koroleva, - M.-L.: Energy, 1966. - P. 90]) during the tests, the pressure was also measured using a wide-range combined pressure sensor PBR260 (Bayard-Alpert type), which is controlled via the I / O220 I / O module integrated into the Pfieffer QMG220 PrismaPlus quadrupole mass spectrometer; The main relative measurement error for this transducer is ± 15%.

The throughput of valve 9 with a nominal diameter of 40 mm (DN 40), docked by means of an adapter to the inlet flange of the ТМН, through which the vacuum system is pumped out (S ₀ ), according to the passport data, is 5⋅10 ^-2 m ³ / s (by air )

The estimated value of the measuring volume V _ISM before the tests was approximately (1 ... 2) ⋅ 10 ^-6 m ³ (1 ... 2 cm ³ ).

Taking into account the initial parameters and resistance of the pipelines to the location of the wide-range pressure transmitter PBR260 (S ₀ ≈ 2.5 ... 5 dm ³ / s), the calculated value of the minimum holding time t _{in_min} (see formula 3) was estimated at ≈ (10 ... 20) s . It should be noted that the tests were carried out with measuring volumes, the total number of which was 8 pieces.

The measuring, calibrated, auxiliary volumes (in the amount of four) and the gas inlet system were evacuated to a pressure of ≤ 5⋅10 ^-5 Pa (5⋅10 ^-7 mbar), achieved taking into account the commissioning of oil and gas in isothermal conditions in about 30 minutes.

The average time for obtaining a pressure and temperature readout is about 1 s, while the time allotted for carrying out control measurements taking into account the linear increase in the pressure gauge signals was more than 2 min (i.e., this approximately corresponds to the removal of ≈ 120 sample readings used for approximation).

To control the temperature, three chromel-alumel type thermocouples were used by means of MINITERM 400.31.11 SI thermocontrollers equipped with a device for compensating cold junctions KHS-M.

The tests were carried out in two stages, with repetition on different days. First, preliminary tests were carried out to determine the values of the pressure range at which the conditions of the molecular flow regime (stage I) are maintained after trial bypasses into the volumes of the vacuum system. As a result of the control, it was found that an acceptable value in the prepared portion is a pressure not exceeding 133.32 Pa (1 mmHg).

The initial set pressure P _k and the final pressure P _{0 of the} control gas were set at 133.32 Pa (upper measurement level of the MKS "Baratron" 690 A01TRA) and not more than 1.5⋅10 ^-4 Pa, respectively. In this case, the pressure of the control gas when creating the batches was a reproducible value from batch to batch (the spread in the values was not more than ± 0.015 Pa) due to the feedback electrical connection provided by the joint interaction of the MKS "Baratron" type 670 V vacuum meter and dispenser gases (leakage) RRG 248A, controlled by a controller type 250E.

и обратного

перепусков, соответственно

Stage II of the test included alternating operations associated with simultaneous control of temperature and evacuation of the above volumes to a vacuum level not exceeding 1⋅10 ^-4 Pa, with predetermined holding times of 1 and 5 min (see Fig. 4), followed by sequential filling and bypasses from the calibrated volume to auxiliary volumes together with the measuring one and vice versa. Similar operations were carried out for measuring microvolume. Table 1 presents the temperatures (K) (with an error within the experiment) “before” and “after” direct

and vice versa

bypasses, respectively

The results of statistical calculations of measuring volumes in the amount of 8 pcs., Determined by the formula (2) taking into account the background characteristics and temperature coefficients (corrections), are presented in table. 2.

It should be noted that part of the measurement chambers actually has some differences in design from the requirements of technical documentation (assembly drawing) due to additional mechanical work that was necessary at the end of welding operations with the camera bodies in order to achieve interchangeability of the elements included in their composition.

According to the results of the tests, the average (of 8 pieces) of the free volume of a closed measuring cavity with a volume displacer and a miniature device, measured using an absolute transducer of the MKS "Baratron" type and a wide-range transducer PBR260 amounted to: V _{meas ._baratron} = (1.279750 ± 0 , 0433351) ⋅10 ^-6 m ³ and V _{meas_pbr260} = (1.293375 ± 0.0445868) ⋅10 ^-6 m ³ , respectively. Obviously, both values have almost similar indicators, and this gives reason to believe that the results obtained on the basis of the commonality of the data of two different types of converters confirm the correctness of the results. At the same time, despite the fact that for non-absolute gauges for converting readings (readings) into pressure values for a particular gas, it is necessary to know its relative sensitivity coefficients, in the proposed method this information is not necessary: the coefficients are ultimately reduced when determining the desired volume value. This, in turn, makes it possible, if necessary, to use in practice, in order to determine the required volumes, the vacuum gauges available on vacuum systems and installations.

Despite the relative insignificance of the values of the ratios T ₂ / T ₁ obtained during the tests, due to their smallness, for the correct calculation of the volumes, they should be taken into account in the calculations in the form of the corresponding factor, since the correctness of the results is determined by the tendency of the systematic error to zero. It should be noted that the difference in temperature values of the walls of the volumes of the vacuum system before direct and reverse bypasses by 1 K causes the appearance of a relative error in the calculations of 0.3%.

For comparison, tests were carried out to determine the volume of cavities in all eight measuring chambers, evaluated as a single volume (all 8 valves connected through flange connections to the chambers were in the "open" state). The obtained value of the average value of the measurement volume was 1.23723 cm ³ . The measurement error relative to the average of eight values obtained on the basis of sample data from the MKS "Baratron" pressure sensor is estimated at minus 3.2%.

The calculations of the values of the integral flux leakage and outgassing in the auxiliary volume with the expansion coefficients of V _MOD / _Σ V, (V _in / V _Σ), obtained after definition of known volumes of test gas at a predetermined pressure value in the serving (133.32 Pa), considering the requirement that after the bypass the signal-to-background ratio (as previously noted) should be at least 2 at a set exposure time, for example, 15 minutes, showed that its value should not exceed 9.73⋅10 ^{- 8} Pa⋅m ³ / s with bypass e from the measuring and 4.71-10 ^-8 Pa⋅m ³ / s from the calibrated volumes. At a shutter speed of 5 minutes, similar parameters should have the following values: 1.46 × 10 ^-7 Pa⋅m ³ / s and 7.06⋅10 ^-8 Pa⋅m ³ / s, respectively.

и

, входящих в формулу измерения (2), при проведении испытаний с учетом значений фоновых побочных интегральных потоков не превысили 1,45%. Здесь следует отметить, что на непрогреваемых вакуумных системах аналогичные показатели побочных процессов превышают реально полученные при испытаниях численные значения потоков на порядки. При этом отсутствие необходимого учета фоновых характеристик может значительно исказить конечные результаты из-за возникновения погрешностей, которые могут достигать нескольких десятков процентов.The actual flow values integrated _MOD Q, Q _a, Q _Σ from the gas emission and leakage through the leak in the respective vacuum system volumes were assessed (using a manometric method) following meanings: Q _izm≈ 2,83⋅10 ^-9 Pa⋅m ³ / s , Q _to ≈1.21⋅10 ^-9 Pa⋅m ³ / s and QΣ≈2.55⋅10 ^-8 Pa⋅m ³ / s, while the values of the coefficients from the quotients

and

included in the measurement formula (2), when testing taking into account the values of the background side integral flows did not exceed 1.45%. It should be noted that on unheated vacuum systems, similar indicators of side processes exceed the numerical values of flows actually obtained during the tests by orders of magnitude. In this case, the lack of the necessary accounting for background characteristics can significantly distort the final results due to errors that can reach several tens of percent.

Thus, on the basis of the proposed method for determining the volume of closed cavities, it is possible to increase the accuracy and reliability of the obtained results, which allows, without resorting (without necessity) to other methods and methods, including weight, which can not always be used in practice, to carry out fairly accurate measurements based on the available technical equipment that are part of most vacuum systems and complex systems where rarefied media are obtained and used.

Claims (3)

A method for determining the volume of closed cavities, which consists in transferring gas from a calibrated tank to a measuring one and measuring the pressure in these tanks, followed by a reverse gas bypass from the measuring tank to a calibrated one, in which the pressure in the measuring tank before the backflow is maintained equal to the pressure in the calibrated tank before direct bypass, characterized in that the combined volume is evacuated, consisting of a series-connected measuring volume, one sludge and more auxiliary volumes and calibrated volume, which are sealed by rigid structures, separated by means of control valves, in the calibrated volume create a pressure of a control gas of relatively high pressure of a given value, measured by an absolute pressure transducer with a measurement range in the region of relatively high pressure, based on the condition that after gas bypass in the combined volume, an equilibrium pressure corresponding to the conditions of the molecule should be established Nogo flow regime created by the gas portion is maintained in a calibrated screen at a predetermined exposure, and in parallel with the delay and the auxiliary measuring volume is evacuated, the vacuum depth is determined predetermined level vacuum gauge wide-range detected transducer; simultaneously with the pressure measurements, the temperature of the gas is recorded in these volumes, before the expiration of the exposed exposure period, the auxiliary and measuring volumes are evacuated at the same time they are stopped and, at a given moment, a direct portion of the gas is transferred from the calibrated volume to the above volumes, pressure and temperature are controlled after the bypass until establishing a linear increase in the output signal coming from the absolute pressure gauge about the action with the measurement range in the region of relatively low pressure, then the combined volume is evacuated to a value not exceeding the specified vacuum level; upon reaching it, the evacuation of the calibrated volume is closed for a time equal to the duration of the soaking interval in the given volume of the portion with the control gas pressure of the specified previously values, and repeat operations that are directly related to gas bypass, but already accumulated as a result of desorption and leakage processes, wire of them in the same time interval, then repeatedly perform all the above operations, using the measuring volume when creating the portions, while preparing the gas portion for the return bypass, the latter ensures the establishment of the control gas pressure equal to the pressure of the gas prepared in the calibrated volume for direct bypass , with the auxiliary and calibrated volumes being evacuated in the intervals between bypasses until the level of background concentrations does not exceed the previously specified about the value, while if, at a selected level of control gas pressure in the generated portions in calibrated and measuring volumes, the achieved pressure after direct and reverse bypasses will not correspond to the rarefaction level characteristic of the occurrence of molecular conditions and / or it will not exceed the minimum reliable level of the gauge output signal absolute action transducer with a measurement range in the field of relatively low pressure, then by appropriate selection or calculation of pressure The control gas in portions correlates to the required level with the repetition of all the above operations, according to the obtained results, a graph of the concentration dependence of time is constructed, the least squares are approximated by the sections of the graph characterized by “background” and “useful” signals, and the corresponding increments or difference in gas pressure are found attributed by extrapolation to a point in time of the beginning of the forward and reverse portion bypasses, and the determination of the desired value is carried out according to the following form le:

, where ΔP ₁ ,

- the values of the increment of the signals of the absolute pressure gauge with a measurement range in the region of relatively low pressure after the expansion of the control and background side gas, respectively, from the calibrated to the auxiliary volume, including also the measurement obtained by the methods of correlation and regression analysis, assigned to the beginning of the direct bypass of portions, Pa ; ΔP ₂

- the values of the increment of the signals of the absolute pressure gauge with a measurement range in the region of relatively low pressure after the expansion of the control and background side gas, respectively, from the measuring to the auxiliary volume, including also calibrated, obtained by the methods of correlation and regression analysis, assigned to the beginning of the bypass portions, Pa ; V _to - the capacity of the calibrated volume, m ³ ; V _ISM - the capacity of the measuring volume, m ³ ; T ₁ - temperature of the gas and the walls of the calibrated volume before the direct bypass, K, T ₂ - temperature of the gas and the walls of the measuring volume before the direct bypass, K.

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