RU2680159C9 - Method for determining volumes of closed cavities - Google Patents

Abstract

FIELD: measuring equipment.SUBSTANCE: invention relates to measurement equipment, in particular to measurements of capacity of closed sealed volumes in various complex systems and installations related to vacuum equipment, with possibility of arrangement of porous materials and / or structural elements therein. 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, at which pressure in measuring container before reverse bypass is maintained equal to pressure in calibrated capacity before direct bypass, evacuating the combined volume consisting of series-connected measured volume, one or more auxiliary volumes and 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. Corresponding to conditions of molecular flow mode, created portion of gas is held in calibrated volume with specified exposure, and in parallel with holding auxiliary and measuring volumes are subjected to evacuation, wherein depth of vacuum is determined by a given level of rarefaction, recorded by a wide-range manometric converter; simultaneously with pressure measurements, recording gas temperature in said volumes, before expiration of the exposure period, evacuation of the auxiliary and measuring volumes is simultaneously terminated and at a given moment, the portion is directly bypassed from the calibrated volume into the above volumes, control of pressure and temperature after bypass is performed until linear increase of output signal received from absolute pressure manometric converter with measurement range in relatively low pressure region, then combined evacuation of the combined volume to a value not exceeding a given level of rarefaction, when it reaches pumping of calibrated volume is closed for a time equal to duration of holding interval in a given volume of a portion with pressure of control gas of a predetermined value.EFFECT: high accuracy and reliability when determining volumes of closed cavities, especially relatively small capacities of arbitrary shape, which are part of complex vacuum systems and plants.1 cl, 2 tbl, 4 dwg

Description

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

There is a method for determining the volumes of tanks in which the pressure in the tank from which gas is bypassed is set above the bypass pressure before the direct and reverse gas by-passes, and the capacity into which the bypass is below the bypass pressure followed by the same time delay 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 №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 vessel, allows to increase the accuracy of measurements. However, the time delay with its incorrectly set (matched) values may negatively affect the results, since the times necessary for the onset of gas equilibrium when combining the tanks, i.e. when the pressure in them is equal, they depend 1) on the ratio of the volumes involved (involved) in the measurements, and in addition, 2) on the capacity of the channels through which the gas flows, 3) the conductivity of the shut-off regulator (valve) separating the vessels at bypass and 4) from the time of its opening. This, in turn, may introduce uncertainty in the measurement result due to the appearance of differences in the conditions of heat exchange.

Similar drawbacks can also appear in the case of using the method of determining the volumes of tanks, in which an overpressure is applied to the measured capacitance in a calibrated volume, recorded by pressure measurement devices, and then the valve is used to transfer gas from the calibrated reservoir to the measured one, followed by recording the pressure in tanks, then in the measuring tank create an overpressure with respect to the calibrated and reverse gas bypass from the measured capacitance while the pressure in the measured capacity and the pressure drop between the tanks before the reverse bypass is maintained equal to the pressure in the calibrated container and the pressure difference between the tanks before the direct bypass, and the determination of the desired value is carried out using 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 _meas , V _k - volumes of measured and calibrated tanks, P _k , P _meas - pressure in calibrated and measured containers before direct bypass, P - steady state pressure in the system after direct bypass,

- pressure in calibrated and measured containers before reverse transfer,

- the established pressure in the system after the reverse bypass. Author's certificate number 714156, IPC G01F 17/00, 02/05/1980. This technical solution was made as a prototype.

In addition, this method is not sufficiently accurate due to the error introduced due to the lack of consideration for the possible manifestation of the thermal effect associated with temperature changes during by-passes, when the outflow of gas is not a quasistatic isothermal process. The occurrence of possible disturbances in the thermodynamic balance between the capacities in the process of gas expansion due to 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, for which elimination 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 conditions), especially when determining the volume of microporous materials in process tanks (certificate of authorship No. 1818540, IPC G01F 17/00, 05/30/1993).

Certain difficulties associated with the appearance of relatively large errors arise in determining the volumes that make up complex polyblock vacuum systems consisting of the n-th number of volumes. Especially problematic in practical terms are measurements carried out with free volumes for structures with relatively small capacities, adjacent to volumes, which may be larger in magnitude by several orders of magnitude. Moreover, for 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 only be provided through elements of low conductivity, for example, through narrow channels of variable cross-section, which makes it difficult to obtain adequate measurement results, due to the relatively long duration required to achieve the maximum one-way conditions of clarity (isotropic) gas distribution volumes. In addition to this, with the appearance of temperature differences between the volumes connected by narrow pipelines, in the process of by-passes under the conditions of the molecular regime, the phenomenon of thermal effusivity (thermomolecular flow) occurs [G. Levin Fundamentals of vacuum technology, trans. from English - M .: Energy, 1969. - P. 15], at which the gas pressure in these volumes is redistributed in proportion to the square root of temperature. In this case, a change in gas temperature can occur directly in the measurement process, for example, due to the 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 an increase in the timing of the bypass may be an increase in the concentration of accumulated side gases from gas release and leakage in volumes, the identification and appropriate accounting of which without special tools and methodical techniques makes it difficult to solve this problem.

The low accuracy of the results of indirect measurements may also be due to either insufficient information or unreliability of data on individual characteristics of heat and mass transfer, on which the flow of gas in the system under consideration depends. In addition to the previously mentioned temperatures of the gas and its surrounding walls, these include:

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

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

The technical result is achieved in that the method of determining the volumes of closed cavities, which are sealed rigid structures, consists in passing gas from a calibrated tank to the measuring tank and measuring the pressure in these tanks with subsequent implementation of the reverse bypass of the gas from the measuring tank to the calibrated one, at which the pressure in the measuring tank before the return bypass, the tanks are kept equal to the pressure in the calibrated tank before the bypass, the combined volume is evacuated m, consisting of successively connected measuring volume, one or more auxiliary volumes and calibrated volume separated by control valves, in a calibrated volume create a control gas pressure of a relatively high pressure of a given value measured by an absolute pressure pressure transmitter with a measuring range in the relatively high pressure region, based on the condition that after the bypass of the gas in the combined volume should be established equilibrium pressure, corresponding to the molecular flow conditions (Kn> 0.33 is the Knudsen number), the created portion of the gas is maintained in a calibrated volume with a given exposure, and in parallel with the exposure the auxiliary and measuring volumes are subjected to vacuuming, the depth of the vacuum in which is determined by a given level of vacuum recorded by a wide-range gauge the converter, simultaneously with the measurement of the pressure, records the gas temperature in these volumes, before the exposure time lasts evacuating the auxiliary and measuring volumes simultaneously stop and at a given moment carry out a direct bypass of the gas portion from the calibrated volume to the above volumes, control over pressure and temperature after the bypass is carried out until a linear increase in the output signal from the absolute pressure gauge converter is established with a measurement range in the region of relatively low pressure, then produce a joint evacuation of the combined volume to a value not exceeding breathing a specified level of dilution, upon reaching it, pumping out a calibrated volume is covered for a time equal to the duration of the exposure interval in a given portion of the portion with the control gas pressure of a previously specified value and repeat operations that are directly related to gas transfer, but already accumulated as a result of desorption and overflows carried out in the same time interval, then re-perform all the above operations, using the measurement volume during the creation of portions e gas portion to reverse the bypass in the latter ensures the establishment of a control gas pressure equal to the pressure prepared in a calibrated volume for a direct bypass, with auxiliary and calibrated volumes being evacuated in the intervals between the bypasses until the background concentrations not exceeding the previously specified value are obtained herewith, if at the selected pressure level of the control gas in the generated portions in the calibrated and measuring volumes, the pressure reached after direct and reverse by-passes will not correspond to the level of rarefaction typical of molecular conditions and / or it does not exceed the minimum reliable level of the output signal of an absolute absolute gauge transducer with a measuring range in the relatively low pressure region, then by appropriate selection or calculation of the control gas pressure in batches correlate to the required level with the repetition of all the above-mentioned operations; based on the results obtained, a concentration graph is plotted Ion dependence on time, using the least squares method, approximates sections of the graph characterized by “background” and “useful” signals, find the corresponding increments or difference of gas pressures, referred by extrapolation at the time of the beginning of the forward and reverse flow transfers, and determine the desired value by the following formula:

where ΔP ₁ , ΔP ₁ ^' are the increment values of the signals of the absolute pressure gauge converter with a measurement range in the relatively low pressure region after the expansion of the control and background secondary gas, respectively, from the calibrated to the auxiliary volume, including the measuring volume, obtained by the correlation-regression methods analysis related to the beginning of the direct bypass portions, Pa; ΔP ₂ , ΔP ₂ ^' are the increment values of the signals of the absolute pressure gauge converter with a measurement range in the relatively low pressure region after the expansion of the control and background secondary gas, respectively, from the measuring volume to the auxiliary volume, including the calibrated one, obtained by the methods of correlation and regression analysis referred to the beginning of the reverse bypass portions, Pa; V _k - capacity calibrated volume, ^m3, V _MOD - capacity measuring volume, ^m3; T ₁ is the temperature of the gas and the walls of the calibrated volume before the direct bypass, K, T ₂ is the temperature of the gas and the walls of the measuring volume before the reverse bypass, K.

The invention is illustrated by drawings.

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

FIG. 2 shows the theoretical curves of the change in pressure of the control gas with an increase in its volume over time, taking into account the specified integral flows from gas release and leakage in auxiliary volumes — the graph shows curves a) and c) with flows of 1.0⋅10 ^-7 Pa ³ / s and 5.0⋅10 ^-9 Pa⋅m ³ / s, respectively, and the measuring volume - curves b) and d) with integral flows of 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)).

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

FIG. 4 is an illustration of a typical graph of changes in total pressure values characterizing the main processes in the system associated with direct and reverse gas by-passes with a dwell time of 5 minutes during the procedure for determining the volume of the measuring capacitance at the UFG unit with a graphical representation of the results of the approximation and extrapolation of the pressure function P = F (t) at the initial time reference point (t = 0), obtained by mathematical methods using the linear model (P = F (t) = a ⋅ X + B).

A distinctive feature of the proposed method in comparison with the prototype is that the calculations use exactly the pressure increments determined using regression analysis methods and subsequent extrapolation of the resulting functional dependencies beyond the specified time interval, the boundary of which is the beginning of the forward and reverse flow transfers, t. e. when the gas has not yet begun to spread over the combined volume. In this case, the pressure drops between the tanks, with equal pressures of the control gas in the calibrated and measuring tanks, before the forward and reverse by-passes may differ (be not equal) between themselves. The use of the above increment at the time of the 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 gas temperature fields in the volumes during the measurements. Since possible differences in gas temperatures in the 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 the bypass (forward / reverse) contributes to 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 terms of thermodynamic balance, that is, when their temperatures are equal, determined by the temperature of the non-thermos atirovannoy environment.

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

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

Moreover, the schematic diagram of the installation for determining the volumes of closed cavities has been changed: instead of two volumes in the system, there is auxiliary (technological) tanks, which are present in almost all vacuum systems, while not having any negative effect on the measurement accuracy under certain conditions. It should be noted that under molecular conditions, the pressure ratio does not depend on the number of containers to be connected. Moreover, if between the measuring and calibrated volumes, in the same temperature conditions, install additional tanks with a different temperature, then this will not affect the pressure in the measuring and calibrated volumes [Rozanov L.N. Vacuum technology. Textbook for universities in the specialty "Vacuum Technology". - M .: Higher School, 1990. - P. 56] taking into account the fact that the pressure sensor must be located outside the area of auxiliary volumes. However, the presence of additional gaseous medium (load) in auxiliary volumes imposes certain additional restrictions on the performance of the procedures of the proposed method (see below).

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

To implement the proposed method, in order to avoid introducing additional unaccounted error, measurements should be carried out either under relatively slowly changing environmental conditions, or with the constancy of their parameters, in particular, temperature (within room indices) and atmospheric pressure. The temperatures of the control gas close in values to the conditions of room temperature at direct and reverse bypasses may differ, but in each of these cases, the gas temperature before each such bypass must be equal to the temperature of the surrounding walls, as well as the walls of the volumes and pipelines along its path, including absolute pressure gauge temperature transducers. In addition, the temperature parameters of the system with appropriate overlaps (discharges) of accumulated portions of the same volume with the gas medium, the appearance of which is caused by the presence of flows from desorption and leakage, must also be exactly correlated with the value of the control gas temperature during the forward and reverse overshoots, respectively.

In the molecular mode, the equilibrium pressure of the gaseous medium in the volumes, established after the bypass, taking into account gas evolution and leakage, due to the influx of air components through leaks, is determined by the temperature of the surrounding walls, since there are no intermolecular collisions, i.e. There is a thermodynamic equilibrium of gas and the environment. In case of operative (fast) opening of the control valve, the process of gas expansion is fast-flowing, and there is practically no heat exchange between the gas and the environment, 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), when Q = 0 and A = 0, there is no change in the internal energy of a gas in such a system, and in relation to an ideal gas there is no change in temperature in such a system. For real gas during adiabatic expansion into vacuum (as previously noted), the gas temperature changes in the direction of its cooling, so 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 ideal gas state. But since, as previously noted, the temperature of the walls of the above chambers before the forward and reverse by-passes may differ (for example, due to the relatively long duration of the measurement cycle and the drift of environmental temperature parameters), and, in addition, changes in gas temperature may follow directly in the process of by-passes under the action of a pressure differential when a gas flows through a narrow channel (Joule-Thomson effect), in the proposed method, these discrepancies are taken into account and, moreover, partially and are based on the analytical calculations and the following reasoning.

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

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

You should pay attention to the fact that under the conditions of the viscous regime at different gas temperatures, the pressures in the volumes can be equal, and the concentrations - different [I Groshkovsky. High Vacuum Technology, trans. V.L. Bulat, - M .: Mir, 1975, - p. 56].

After the bypass, based on the Clapeyron equation (without taking into account side 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 other things being equal

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

- the temperature of the walls (gas) of the combined volume after the return 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 forward and reverse by-pass no temperature gradients are provided. In this regard, it is necessary that all parts of the system from the combined volume before the overshoots are in the same temperature conditions. However, if there is an element of low conductivity between the volumes, if the pressure is relatively low, a molecular flow occurs in the latter and thermal effusions take place, caused by gas cooling as it expands (especially when using real gases), therefore its temperature (T ₁ , T ₂ ) changes

However, it is necessary to note the following: if the conduction of pipelines between the volumes is large enough, there is practically no thermal effusivity.

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

и

для данного способа в действительности определяются в виде приращений давления, полученных путем аппроксимации 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, in reality, they are defined as pressure increments obtained by approximation followed by extrapolation of their values at the time of the start of the bypass, then in expression (5) the quotient

can be ignored i. temperature gradient of gas (if available) 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 equation of measurement (2), which is used to determine the desired volume.

The need to introduce temperature corrections in the calculations can be reduced to zero, if the absolute pressure transducer, apart from its own thermal stabilization (this hardware function is quite common for absolute gauges in modern conditions), there is an opportunity

interactive / automatic setting as one of the tuning parameters of the temperature of the measured gas, as well as the type of this gas, affecting along with other parameters on the accuracy of 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. At the same time to implement the method, it is necessary that the gas flows Q _meas , Q _c and Q _Σ , characterized by the values of the background concentration of the control gas (background) taking into account the secondary surface gas evolution from the walls and leakage in the sealed measuring and calibrated closed volumes, as well as the background in the volume of the 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 the increase in pressure, with the constancy of the values of the gas flows noted above, flows faster than in a relatively large volume. The limiting increase in the total pressure (Р _meas , Р _к , Р _Σ ) in the above volumes, caused by gas emission and leakage, in order to obtain reliable results of measurements, should not exceed half of the partial pressure of the control gas, i.e. Р _ism , Р _к , Р _Σ ≤ (Р _0/2 ). For calibrated and measuring volumes (P _k, P _meas), this condition must be satisfied throughout the predetermined time delay (to bypass) and for the volume of the auxiliary capacitances P _Σ considering associations with a volume measuring / calibrated with its gas component flow - immediately before the bypass and during the entire time spent on measurements. This provides 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. Nondestructive control. Leak detection methods).

To ensure this method, it is important to decide how much the values of the calibrated and measuring volumes may differ from the volumes of auxiliary tanks used in the measurements, taking into account the differences in the values of the integral side flows in the volumes. Theoretical studies have shown that with the following parameters: 1) volume ratio, for example, V _meas / V _Σ = 2.3166⋅10 ^-2 (V _к / V _Σ = 1.1202⋅10 ^-2 ), 2) conductivities 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 of 10 ^-9 / 10 ^-9 / 10 ^-9 Pasem ³ / s, respectively, with the established difference of the total pressure not more than 0.1%, the time required for equalization after the bypass will be 1.1 s. With 10 ^-8 / 10 ^-8 / 10 ^-8 Pa -m ³ / s streams with this discrepancy no more than 0.1% (1%), the time interval for alignment will increase to 80.81 s (3.09 s), and with streams of 10 ^-7 / 10 ^-7 / 10 ^-7 Pa- ³ / s - up to 149.3 s with a difference of 0.1% and 25.56 s with a difference of pressure of 1%. When the ratio of the volume V _{MOD ^{/ V Σ = 2,3166⋅10 -3 (V}} k / V Σ = 1,1202⋅10 -3) and the values expressed above streams 10 ^-9 / 10 ^-9 / 10 ^-9 Pa⋅m ³ / s the time when the pressure difference will not exceed 0.1% will increase to 68.93 s; with streams of 10 ^-8 / 10 ^-8 / 10 ^-8 Pa⋅m ³ / s, respectively, the discrepancy will reach no more than 0.1% in 151.04 s. If in the initial setting to take a value equal to V _{MOD ^{/ V Σ = 2,3166⋅10 -4 (V}} k / V Σ = 1,1202⋅10 -4) ( data relations hold for installation UFKG; see below.) then with the flow of 10 ^-10 / 10 ^-10 / 10 ^-10 Pa⋅m ³ / s, the difference in pressure will be no more than 0.14% (1%) in 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 for pressure equalization to 0.1% in volumes will be no more than 0.132 s. A change in the integral flows in the direction of their increase to values, for example, 10 ^-8 / 10 ^-8 / 10 ^-8 Pasem ³ / s will cause a pressure difference of no 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 the calculation results. Therefore, for the implementation of the proposed method, it is necessary that the ratio of auxiliary tanks and measuring / calibrated volumes for given values of integral flows (Q _meas , Q _c , Q _Σ ) in them, the values of which should not exceed 1⋅10 ^-7 Pa⋅m ³ / c, satisfies the following inequality

Analytical evaluation 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 one. It should be noted that the discrepancy of pressures in volumes, while respecting this ratio, will not exceed 0.1%.

The above upper limit value of the integral flows in volumes is a practically achievable parameter for sealed systems, since leaks of> 1 >10 ^-7 Pa-s ³ / s are eliminated even at the stage of assembly and commissioning works, and gas outflows with such a level can to be significantly reduced either by evacuation with simultaneous high-temperature heating of all structural elements involved in the measurements, or (if the latter is not possible) due to a relatively long high vacuum pumping system.

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

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 a turbomolecular 11 and fore-vacuum 15 pumps with valves 2, 6, 9, 12 open and closed stop-regulating elements - a gas meter (leak) ) 8 and valve 13, while the pressure in the system is measured using absolute pressure transducers 3, 5 and a wide-range pressure gauge pressure transducer 10, as well as a low pressure sensor akuuma 14.

Closing the control valve 9, shut off the evacuation of the vacuum system and inject the control gas into it to a predetermined pressure value, measured by the pressure gauge converter 5, by means of the gas metering unit 8 using the gas injection system (not shown in Fig. 1). The valve 6 is shut off with an exposure in a calibrated volume of the created portion of the test gas while simultaneously unregulated processes of gas evolution from the surface of the volume wall and from the flow of air components flowing through its leaks during a specified time t _in , the value of which should not be less than the minimum exposure time t _{in_min} (see below).

Prior to the expiration of the maintained exposure period, the evacuation of the auxiliary and measuring volumes with valve 9 closed, with the regulating valve open 2, is simultaneously stopped (the interval is determined from a few seconds to several tens of seconds). By the time of expiration of the exposure time t _in , at which the condition t _in ≥t _{v_min is to} be _fulfilled , opening the high-vacuum control valve 6, a direct passage of gas from the calibrated volume to the auxiliary and measuring volumes of the vacuum system is made and the increment of the output signal of the absolute pressure converter 3 is determined characterizing the presence of the content of the control gas with the content in a portion of the “products” of the action of sorption-desorption processes, as well as from leakage flows through (see Fig. 2). Simultaneously with the measurement of pressure in the indicated volumes, the temperature of the gas is also measured. At the end of time, maintained to establish a linear pressure build-up in the same time interval, starting from the moment of discharge of the batch, open the high-vacuum control valve 9 and evacuate the controlled gaseous medium by means of a turbomolecular pump (MTH) 11 until the batch indicators preceding the discharge are restored. Upon reaching the level of vacuum not exceeding the values pumping calibrated volume previously specified overlap via the valve 6 at the time equal to the duration of the interval shutter t _in a given volume portion with the pressure control gas values given previously, and repeating the operation directly related to the bypass gas , but already accumulated as a result of the processes of desorption and leakage, with the determination of the increment (difference) of pressure at the moment of transfer of the portion, carried out in the same time interval.

After that, all the above operations are repeated, starting with the closing of the regulating valve 9 and the inlet of the test gas into the vacuum system through the gas metering unit 8, using measurement to create portions instead of the calibrated volume. When preparing the gas portion for the reverse bypass in the latter, the control gas pressure is established to be equal to the pressure created earlier in the calibrated volume during the preparation of the pilot gas portion for the direct bypass. Fixing portions 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 the calibrated and measuring volumes, the pressure reached after the forward and backward bypasses does not correspond to the vacuum level characteristic of the molecular conditions, and / or it does not exceed the minimum reliable output level of the absolute pressure transducer with a range measurements in the field of relatively low pressure, then by appropriate selection (calculation) of the control gas pressure in correlated portions rushes to the required level with the repetition of all the above operations.

Next, graphs of measurements are constructed (see Fig. 3), and on the basis of the results obtained by means of regression analysis, for example, using the least squares method, the registered areas characterized by "background" and "useful" signals are approximated (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 a “useful” signal, obtained by mathematical methods, is extrapolated to a point corresponding to the instant of the start of the bypass. As a result, the difference ΔP ₁ is determined between the pressure value of the portion of the control gas in the combined volume of the vacuum system after a direct bypass taking into account the background, leakage, gas evolution and the tribological gas component from the surfaces of the calibrated volume materials, and the residual pressure in the combined volume of the vacuum system impurities (desorption, leakage, etc.), referred to the time point of the bypass, respectively. The same mathematical processing associated with the determination of the pressure difference

produce and with data on the "background", obtained by bypassing the control gaseous medium from the measuring volume into the combined volume of the vacuum system, taking into account third-party (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 the reverse bypass are subjected to analogous mathematical processing: an algorithm for constructing calculations of the defined increments ΔP ₂ and

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

The calculation of the capacity of the measuring volume is estimated by the ratio (2) taking into account the temperature indices of gases in the measuring and calibrated volumes immediately before the forward and reverse overshoots.

It should be noted that the previously indicated minimum exposure time is characterized by the time during which evacuation of the residual control gas from the volumes of the vacuum system involved in the creation of the control portion to the establishment of a pressure below a specified vacuum level takes place. In this case, a certain role is played here by the effective pumping rate and the value of the integrated residual gas flow from the surfaces of the volume, as well as the porous material placed (placed) in it together with air components penetrating the evacuated combined volume of the vacuum system through leaks.

The estimate of the minimum holding time t _{b_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 - the initial and final setpoint pressure of the control gas recorded by absolute pressure transducers (use of indirect action transducers, such as a wide-range pressure gauge sensor, is acceptable) for measuring ranges of relatively large and, accordingly, low pressures, Pa;

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

Q _Σ is the integrated flow, characterized by leakage flows from the leaks and the total component from the desorption flows from the walls of the vacuum system, Pa 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 for monitoring tightness, for example, gauge (vacuum). As an estimated value of the effective pumping speed of a vacuum system for engineering calculations, it is possible to take the value of the volume of gas flowing per unit of time through the cross section of a high-vacuum control valve, whose throughput, usually by air or nitrogen, is one of the main technical parameters specified in its maintenance materials (passport, instruction manual, etc.). Finally, in the absence of the above-mentioned values of the parameters for the calculation, the value of this time interval t _{in_min} can be estimated in advance by direct measurements.

The proposed method is implemented in practice with the use of a vacuum system of an automated heated mass spectrometry installation of the final control of the tightness of gas-filled dischargers UFKG [S. Bushin, S.S. Galkin. The results of the trial operation of the 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, a microcavity was used, formed from voids based on the design of a high-vacuum controlled angular valve, one of the joints of which (from the side of the valve-saddle pair) is plugged with a special all-metal flange having a cylindrical protrusion with a linear dimension (26.7 mm) and a diameter close to the internal diameter of the surface surrounding it, acting as a free-volume propellant. The capacity of the batch calibrated microvolume V _microns , determined by the gravimetric method under normal conditions, is estimated to be 0.63553⋅10 ^-6 m ³ ; the value of the mean square error σ ₀ (V _microns ) of this microvolume is 1.14% (± 0.0072⋅10 ^-6 m ³ ).

The angular valve counter flange is docked to an expansion tank with a capacity of 2.0899 dm ³ , which has a connection with the vacuum path line, consisting of a sequence of several process volumes connected to each other, separated by similar ultrahigh-vacuum controlled valves with large diameters of the nominal passage. A collector with welded 8 nozzles in the form of beams located symmetrically with a 360 degree sweep angle is docked to the last of them; A single measuring volume of the same construction is connected to each nozzle.

The measuring volume is a cylindrical composite cavity formed from two joints, in which a split collet with a miniature gas-filled device is located on one side. The collet has special internal grooves for protruding disk electrodes of the device with axially arranged rings made of vacuum-tight ceramics (it should be noted that, despite the mention of the vacuum density of the ceramics used, the surface of the device envelope is a porous structure); The shell 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 mated by one of its ends with a collet through a groove undercut, and a second end located at a distance of ≈ 1 mm opposite the end of the plate (seal a) controlled pneumatically actuated ultrahigh vacuum valve (the normal state of the valve is closed). Opening time (closing) of the 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 on the basis of the developed hardware-software interface.

To maintain the vacuum in the volumes of the vacuum system in the process of measurement, a HiPace 80 series turbomolecular pump is used, which works in conjunction with an ISP-90 type oil-free spiral pump.

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

In preparing gas portions in a calibrated and measuring volume, an absolute pressure transducer of type 690A01TRA was used, working in conjunction with a vacuum gauge (controller) of the type MKS "Baratron" type 670V. The basic relative error of measuring the portion of the test gas for the range from 1.0⋅10 ^-3 Pa to 1.333⋅10 ² Pa, measured by an exemplary MKS Baratron vacuum gauge with a 690A01TRA membrane-capacitive sensor, 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 transducers, whose work is based on different principles [see: G. Eschbach. Practical information on vacuum technology. Obtaining and measuring low pressures. Transl. From German B. Korolev, - M.-L .: Energia, 1966. - P. 90]) during testing, pressure measurement was also carried out using a wide-range combined pressure sensor PBR260 (Bayard-Alpert type), which is controlled by means of an I / O220 I / O module built into the QMG220 PrismaPlus QMG220 PrismaPlus quadrupole mass spectrometer; The basic relative measurement error for this transducer is ± 15%.

The capacity of the valve 9 with a nominal diameter of 40 mm (DN 40), docked through an adapter to the TMN inlet flange through which the vacuum system is pumped out (S ₀ ), according to the passport data is 5 -210 ^-2 m ³ / s (by air ).

The estimated values of the measuring volume V _edited before testing was approximately (1 ... 2) ⋅10 ^-6 m ³ (1 ... 2 cm ^3).

Taking into account the initial parameters and resistances of pipelines to the location of the wide-range pressure transducer PBR260 (S ₀ ≈ 2.5 ... 5 dm ³ / s), the calculated value of the minimum exposure time t _{b_min} (see formula 3) is estimated at ≈ (10 ... 20) . 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 injection system were evacuated to a pressure of ≤ 5⋅10 ^-5 Pa (5⋅10 ^-7 mbar), achieved taking into account the start of the TMS under isothermal conditions in about 30 minutes.

The average time for obtaining the pressure and temperature readings is about 1 s, while the time allowed for control measurements, taking into account the linear increase in the signals of the manometric sensors, was more than 2 minutes (i.e., this corresponds approximately to removal ≈ 120 sample points used for approximation).

To control the temperature, three chromel-alumel 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. Initially, 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 runs into the volumes of the vacuum system. As a result of the control, it was established that an acceptable value in the prepared portion is a pressure not exceeding 133.32 Pa (1 mm Hg).

The initial set pressure P _k and the final - P ₀ control gas were set at 133.32 Pa (the upper level of the MKS Baratron measurement 690 A01TRA) and not more than 1.510 ^-4 Pa, respectively. The pressure of the control gas when creating portions was reproducible from portion to portion (the spread in values was no more than ± 0.015 Pa) due to the backward electrical communication provided by the MKS Baratron type 670 V vacuum gauge (controller) and the metering unit gases (leak) RRG 248A, controlled by a controller type 250E.

и обратного

перепусков, соответственно

Phase II of the tests included alternating operations associated with simultaneous temperature control and evacuation of the above-mentioned volumes to a vacuum level not exceeding 1⋅10 ^-4 Pa, with specified exposure times of 1 and 5 minutes (see Fig. 4), followed by successive filling and by-passes from the calibrated volume to the auxiliary volumes together with the measuring volume and vice versa. Similar operations were carried out for the measuring microvolume. Table 1 presents the temperature values (K) (with an error within the experiment) “before” and “after” direct

and reverse

restarts, respectively

The results of statistical calculations of measuring volumes in the amount of 8 pieces, determined by the formula (2) taking into account the background characteristics and temperature coefficients (amendments), 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 the additional work on their mechanical refinement, which were necessary after the end of the welding operations with the camera bodies, to achieve the interchangeability conditions of their constituent elements.

According to the results of the tests, the average (out of 8 units) values of the free volume of a closed measuring cavity with a displacer of volume and a miniature device, measured using an absolute action transducer of the MKS Baratron type and a wide-range PBR260 transducer, 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. It is obvious that both values have practically similar indicators, and this gives reason to believe that the results obtained, based on the commonality of data from two different types of converters, confirm the correctness of the results obtained. At the same time, despite the fact that vacuum gauges of non-absolute action for translating signal-readings (readings) into pressure values for a particular gas require knowledge of its coefficients of relative sensitivity, in the proposed method this information is not necessary: the coefficients ultimately reduce determining the desired volume value. This, in turn, makes it possible, if necessary in practice, to use the vacuum gauge hardware available on vacuum systems and installations to determine the volumes sought for.

Despite the comparative insignificance of the T ₂ / T ₁ ratios obtained during testing due to their smallness, for correctness of calculating the volume values they should be taken into account in the calculations in the form of the corresponding multiplier, since the correctness of the results is determined by the systematic error tending to zero. It should be noted that the difference in the temperature values of the walls of the volumes of the vacuum system before the forward and reverse by-pass by 1 K causes a relative error in the calculations of 0.3%.

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

The calculations of the values of the integral flow of leakage and gas evolution in auxiliary volumes, taking into account the expansion coefficients V _meas / V _Σ , (V _c / V _Σ ), obtained after determining the known volumes with a given value of the control gas pressure in a batch (133.32 Pa), taking into account the requirement that after a bypass the signal / background ratio (as previously noted) should be at least 2 at the set exposure time, for example, 15 min, showed that its value should not exceed the values of 9.73⋅10 ^{- 8} Pa ³ / s bypass e from measuring and 4.71⋅10 ^-8 Pa⋅m ³ / s from calibrated volumes. With an exposure of 5 min, the same parameters should have the following values: 1.46⋅10 ^-7 Pa⋅m ³ / s and 7.0610 ^-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 real values of the integral flows Q _meas , Q _c , Q _Σ from gas evolution and leakage through leaks in the corresponding volumes of the vacuum system were estimated (using the manometric method) with the following values: Q _meas ≈ 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 private

and

included in the measurement formula (2), when testing, taking into account the values of background collateral integral fluxes, did not exceed 1.45%. It should be noted here that on unheated vacuum systems, similar indicators of side processes exceed the numerical values of the flows by orders of magnitude actually obtained during testing. At the same time, the absence of the necessary consideration of the background characteristics can significantly distort the final results due to the occurrence of errors that can reach several tens of percent.

Thus, on the basis of the proposed method of determining the volumes of closed cavities, the accuracy and reliability of the results obtained is improved, which allows, without resorting (unnecessarily) to other methods and methods, including weight methods, which can not always be used in practice, sufficiently accurate measurements on the basis of available technical means, which are part of most vacuum installations and complex systems, where rarefied media are obtained and used.

Claims (3)

The method of determining the volumes of closed cavities, which consists in passing gas from a calibrated tank to a measuring tank and measuring the pressure in these tanks with subsequent backflow of gas from the measuring tank to a calibrated one, at which the pressure in the measuring tank is maintained before the reverse bypass equal to the pressure in the calibrated tank bypass, characterized in that they vacuum the combined volume consisting of successively connected measuring volume, one or more and more auxiliary volumes and a calibrated volume, which are sealed rigid structures separated by control valves, in a calibrated volume create a control gas pressure of a relatively high pressure of a given value measured by an absolute pressure pressure transmitter with a measurement range in the relatively high pressure region, based on the condition that after the bypass of the gas in the combined volume should establish an equilibrium pressure corresponding to the conditions of the molecule 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 measurement of the pressure, the gas temperature is recorded in these volumes, before the expiration of the withstand exposure period the evacuation of the auxiliary and measurement volumes is simultaneously stopped and at a given moment a portion of the gas is directly bypassed from the calibrated volume into the above volumes, the pressure and temperature control is performed establish a linear increase in the output signal from the absolute pressure gauge converter About actions with a range of measurements in a relatively low pressure area, then the combined volume is co-evacuated to a value not exceeding a predetermined level of rarefaction; when it reaches the pumped out, the calibrated volume is covered for a time equal in duration to the exposure interval in the given portion of the portion with the control gas pressure given earlier values, and repeat operations that are directly related to the transfer of gas, but already accumulated as a result of the processes of desorption and leakage, the wire in the same time interval, then all the above operations are repeated, using the measurement volume when creating portions, in preparing the gas portion for the reverse bypass in the latter ensure that the reference gas pressure is equal to the pressure of the gas prepared in the calibrated volume for direct bypass , with vacuuming auxiliary and calibrated volumes in the intervals between overshoots to achieve a level of background concentrations not exceeding the previously specified о values, if at the selected pressure level of the control gas in the created portions in the calibrated and measuring volumes, the pressure reached after the forward and backward bypasses will not correspond to the level of rarefaction characteristic of the molecular conditions and / or it will not exceed the minimum reliable level of the gauge output signal absolute transducer with a range of measurements in a relatively low pressure region, then by appropriate selection or calculation in the portions, the control gas correlates to the required level with the repetition of all the above-mentioned operations; according to the results obtained, a graph of concentration dependence on time is plotted; by the least squares method, the sections of the graph are characterized by the “background” and “useful” signals, the corresponding increments or pressure difference of the gas are found , referred by extrapolation at the time of the beginning of the direct and reverse bypass portions, and the determination of the desired value is carried out according to the following forms le:

, where ΔP ₁ ,

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

- the increment values of the signals of the absolute pressure gauge transducer with a measurement range in the relatively low pressure region after expansion of the control and background by-product gas, respectively, from the measuring to the auxiliary volume, including also calibrated, obtained by correlation and regression analysis methods, referred to the beginning of the reverse portion bypass, Pa ; V _to - the capacity of the calibrated volume, m ³ ; V _MOD - capacity measuring volume, ^m3; T ₁ is the temperature of the gas and the walls of the calibrated volume before the direct bypass, K, T ₂ is the temperature of the gas and the walls of the measuring volume before the reverse bypass, K.

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