Материал: Стандарт MIL-STD-202G

MIL-STD-202G

5.4 Procedures. This test condition consists of five procedures (see 5.4.1 to 5.4.4, inclusive), the choice of which must be specified in the individual specification. Procedure IV is the preferred method of performing this test for parts that are not evacuated.

5.4.1Procedure I. The mass-spectrometer-type leak detector shall be coupled to the unsealed evacuation tube of the specimen, and a vacuum created within the specimen. It is extremely important that the coupling connections between the specimen and the leak detector be perfectly sealed. The specimen shall then be subjected to a gas atmosphere either by surrounding the specimen with the gas or by spraying the specimen thoroughly with a jet of the gas. If a defect exists in the specimen, an amount of gas that depends upon the size of the defect will be drawn through it and passed into the leak detector, which will read the leakage rate. When this portion of the procedure is completed, the specimen shall be filled with air or gas at a specified pressure and having a known percentage of tracer gas. The evacuation tube shall then be pinched off and sealed. After sealing, the seal of the tube shall be verified by either procedure IIIa, IIIb, IIIc or IV, if filled with tracer gas.

5.4.2Procedure II. The mass-spectrometer-type leak detector shall be coupled to the chamber. The specimen shall be placed in the chamber, and a tube leading from the gas source and having a known percentage of tracer gas shall be connected to the unsealed evacuation tube of the specimen. It is extremely important that the coupling connections between the specimen and the leak detector be perfectly sealed. The chamber shall then be evacuated and the gas forced under pressure into the specimen. If a leak exists, the gas passes through the specimen into the chamber and then into the leak detector, which will read the leakage rate. If pressure cycling is required, the individual specification shall specify the number of times that a pressure cycle shall be applied. When this portion of the procedure is completed, the specimen shall be filled with air or gas at a specified pressure and having a known percentage of tracer gas. The evacuation tube shall then be pinched off and sealed. After sealing, the seal of the tube shall be verified by either procedure IIIa, IIIb, IIIc or IV, if filled with a tracer gas.

5.4.3Procedure III. The tests shall be conducted in accordance with procedure IIIa, or IIIc. When bomb pressure specified exceeds the device capability, alternate pressure, exposure time and dwell time may be used provided they satisfy the leak rate pressure, time relationships which apply and provided no less than 30 psig bomb pressure is applied in any case.

5.4.3.1 Testing precautions.

a.These precautions are applicable to procedures IIIa, IIIb, and IIIc. To avoid spurious indications, contaminants and surface coatings that may conceal existing leaks, and tracer gas that may be absorbed or adsorbed on the surfaces of the specimen, shall be removed. These test procedures shall not apply to organic-coated parts.

b.Test procedure IIIa is a "fixed" method with specified conditions per table I that will insure the test sensitivity necessary to detect the required leak rate (R1).

c.Test procedure IIIc is a "flexible" method that allows the variance of test conditions in accordance with the formula of 5.4.3.2.3 to detect the specified equivalent standard leak rate at a predetermined leak rate (R1).

5.4.3.2General procedure. For IIIa and IIIc the completed device(s) shall be placed in a sealed chamber which is then pressurized with a tracer gas of 100 +0, -5 percent helium for the required time and pressure. The pressure shall then be relieved and each specimen transferred to another chamber or chambers which are connected to the evacuating system and a mass-spectrometer-type leak detector. When the chamber(s) is evacuated, any tracer gas which was previously forced into the specimen will thus be drawn out and indicated by the leak detector as a measured leak rate (R1). The number of devices removed from pressurization for leak testing shall be limited such that the test of the last device can be completed within 60 minutes for procedure IIIa or within the chosen value of dwell time (tz) for procedure IIIc.

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MIL-STD-202G

5.4.3.2.1 Procedure IIIa. The device(s) shall be tested using the appropriate conditions specified in table I for the internal cavity volume of the package under test. The time (t) is the time under pressure and time (tz) is the maximum time allowed after release of pressure before the device(s) shall be read. This method shall not be used if the maximum equivalent standard leak rate limit given in the procurement document is less than the limits specified herein for procedure IIIc. Upon completion of this procedure, the specimen shall be checked for gross leaks by subjecting the specimen either to test condition A, B, or D. Water, at room ambient temperature and a pressure of 2.5 inches (63.5 mm) of mercury, may be used in place of silicone oil, if test condition B is used to verify gross leaks.

TABLE I. Fixed conditions procedure IIIa.

5.4.3.2.2 Procedure IIIb.

5.4.3.2.2.1 Activation parameters. The activation pressure and soak time shall be determined in accordance with the following equation:

R Q =

s skT Pt

The parameters of equation (1) are defined as follows:

QS = The maximum calculated leak rate allowable, in atm cm3/sKr, for the devices to be tested.

R= Counts per minute above the ambient background after activation if the device leak rate were exactly equal to QS. This is the reject count above the background of both the counting equipment and the component, if it has been through prior radioactive leak tests.

s= The specific activity, in microcuries per atmospheric cubic centimeter, of the krypton 85 tracer gas in the activation system.

k= The overall counting efficiency of the scintillation crystal in counts per minute per microcurie of krypton 85 in the internal void of the specific component being evaluated. This factor depends upon component configuration and dimensions of the scintillation crystal. The counting efficiency shall be determined in accordance with 5.4.3.2.2.2.

T = Soak time, in hours, that the devices are to be activated.

_

P= Pe 2 - Pi 2, where Pe is the activation pressure in atmospheres absolute and Pi is the original internal pressure of the devices in atmospheres absolute. The activation pressure (Pe) may be established by specification or if a convenient soak time (T) has been established, the activation pressure (Pe) can be adjusted to satisfy equation (1).

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MIL-STD-202G

5.4.3.2.2.2 Determination of counting efficiency (k). The counting efficiency (k) of equation in 5.4.3.2.2.1 shall be determined as follows:

a.Five representative units of the device type being tested shall be tubulated and the internal void of the device shall be backfilled through the tubulation with a known volume and known specific activity of krypton 85 tracer gas and the tubulation shall be sealed off.

b.The counts per minute shall be directly read in the shielded scintillation crystal of the counting station in which the devices are read. From this value, the counting efficiency, in counts per minute per microcurie, shall be calculated.

5.4.3.2.2.3Evaluation of surface sorption. All device encapsulations consisting of glass, metal, and ceramic or combinations thereof, including coatings and external sealants, shall be evaluated for surface sorption of krypton 85 before establishing the leak test parameters. Representative samples of the questionable material shall be subjected to the predetermined pressure and time conditions established for the device configuration as specified by

5.4.3.2.2.1.The samples shall then be counted every 10 minutes, with count rate noted, until the count rate becomes asymptotic with time. (This is the point in time at which surface sorption is no longer a problem.) This time lapse shall be noted and shall determine the "wait time" specified in 5.4.3.2.2.4.

5.4.3.2.2.4Specific procedure IIIb. The devices shall be placed in radioactive tracer gas activation tank. The activation chamber may be partially filled with inert material to reduce pumpdown time. The tank shall be evacuated to 0.5 torr. The devices shall be subjected to a minimum of 2 atmospheres absolute pressure of krypton 85/dry nitrogen mixture for the time necessary to satisfy the equation. Actual pressure and soak time shall be determined in accordance with 5.4.3.2.2.1. The R value in counts per minute shall be not less than 600 above ambient background. The krypton 85/dry nitrogen gas mixture shall be evacuated to storage until 0.5 torr vacuum exists in the activation tank. This evacuation shall be completed within 3 minutes maximum. The activation tank shall then be backfilled with air (air wash). The devices shall then be removed from the activation tank and leak tested within 1 hour after gas exposure with a scintillation-crystal-equipped counting station. Device encapsulations that come under the requirements of 5.4.3.2.2.3 shall be exposed to ambient air for a time not less than the "wait time" determined by

5.4.3.2.2.3.In no case will the time between removal from the activation chamber and test exceed 1 hour. This exposure shall be performed after gas exposure but before determining leak rate with the counting station. Device encapsulations that do not come under the requirements of 5.4.3.2.2.3 may be tested without a "wait time". (The number of devices removed from pressurization for leak testing shall be limited such that the test of the last device can be completed within 1 hour.) The actual leak rate of the component shall be calculated with the following equation:

Q = ( ACTUAL READOUT IN NET COUNTS PER MINUTE ) X QS

R

Where Q = Actual leak rate in atm cm3/s, and QS and R are defined in 5.4.3.2.2.1.

Unless otherwise specified, devices that exhibit a leak rate equal to or greater than 1 x 10-8 atmospheric cubic centimeters of krypton 85 per second shall be considered a failure.

Upon completion of this procedure, the specimen shall be checked for gross leaks by subjecting the specimen either to test condition A, B, or D. Water, at room ambient temperature and a pressure of 2.5 inches (63.5 mm) of mercury, may be used in place of silicone oil, if test condition B is used to verify gross leaks.

5.4.3.2.2.5 Personnel precautions. A Nuclear Regulatory Commission (NRC) license is necessary for possession and use of the krypton 85 leak-est equipment. In the use of gas, code of Federal regulations Nuclear Regulatory Commission Rules and Regulations, Title 10, Chapters 1, 20, 30, 31, and 32 should be followed and the maximum permissible tolerance levels prescribed by the National Committee on Radiological Protection should be observed.

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5.4.3.2.3 Procedure IIIc. Values for bomb pressure exposure time and dwell time shall be chosen such that actual measured tracer gas leak rate (R1) reading obtained for the device under test (if defective) will be greater than the minimum detection sensitivity capability of the mass spectrometer. The devices shall be subjected to a minimum of 2 atmospheres absolute of helium atmosphere. If the chosen dwell time (tz) is greater than 60 minutes, graphs shall be plotted to determine an R1 value which will assure overlap with the selected gross leak test condition. The chosen values, in conjunction with the value of the internal volume of the device package to be tested and the maximum equivalent standard leak rate (L) limit (as shown below or as specified in the applicable procurement document), shall be used to calculate the measured leak rate (R1) limit using the following formula:

Unless otherwise specified, devices with an internal cavity volume of 0.01 cm3 or less shall be rejected if the equivalent standard leak rate (L) exceeds 5 x 10-8 atm cm3/s. Devices with an internal cavity volume greater than 0.01 cm3 and equal to or less than 0.4 cm3 shall be rejected if the equivalent standard leak rate (L) exceeds 1 x 10-7 atm cm3/s. Devices with an internal cavity volume greater than 0.4 cm3 shall be rejected if the equivalent standard leak rate (L) exceeds 1 x 10-6 atm cm3/s.

Upon completion of this procedure, the specimen shall be checked for gross leaks by subjecting the specimen either to test condition A, B, or D. Water, at room ambient temperature and a pressure of 2.5 inches of mercury, may be used in place of silicone oil, if test condition B is used to verify gross leaks.

5.4.4 Procedure IV. The specimen, which is backfilled and sealed during production with a known percentage of tracer gas, shall be placed in a chamber connected to the mass-spectrometer-type leak detector, and the chamber evacuated. The internal pressure of the specimen shall be one atmosphere or greater. If a leak exists, the gas passes through the specimen into the chamber and then into the leak detector which will read the leakage rate. If specified, the specimen shall be perforated to determine if a gas is actually present. Upon completion of this procedure, the specimen shall be checked for gross leaks by subjecting the specimen either to test condition A, B, or D. Water, at room ambient temperature and a pressure of 2.5 inches (63.5 mm) of mercury, may be used in place of silicone oil, if test condition B is used to verify gross leaks.

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MIL-STD-202G

6. TEST CONDITION D.

6.1 Materials.

6.1.1 Fluorocarbon liquid. The fluid shall be D02, D02-TS, D03, FC-40 1/ or FC-43 2/. 6.2 Apparatus.

6.2.1Heated fluid container. The container for the fluid shall be made of pyrex glass and shall be sufficient size to hold the test specimen in the fluid and to maintain a temperature of 125°C ±5°C (257°F ±9°F).

6.2.2Filter. A filter shall be used which is capable of removing particles greater than 1 micron in size from the

fluid.

6.2.3Magnifier. A 3X minimum magnifier or a stereo zoom microscope shall be used for observation of bubbles emanating from the test specimens when immersed in the indicator fluid.

6.3Precautions. The following precautions shall be observed:

a.Fluorocarbon liquids shall be filtered as specified in 6.2.2. Bulk filtering and storage is permissible. Leak indicator fluids, which have accumulated observable quantities of particulate matter during use, shall be discarded or reclaimed by filtration for reuse. Leak detecting fluids shall not be used for more than one eight-hour shift without being refiltered.

b.The observation container shall contain sufficient fluid to assure coverage of the test specimen to a minimum depth of two inches.

c.Illumination shall be a lighting source capable of providing at least 15,000 foot candles at the test specimen position. The lighting source shall not require calibration but the light level shall be verified at the test specimen position. The background behind the glass observation container should be a dull, non-reflective black material.

d.The observation container should be covered at all times when not in use to minimize evaporation losses and moisture adsorption.

e.Test specimens to be tested should be free from foreign material on the surface, including conformal coatings and markings which may contribute to erroneous test results.

6.4Procedure. The test specimens, mounted on specified fixtures to hold them in the fluid, shall be immersed, with the uppermost portion at the minimum depth of 2 inches below the surface of the fluid, lid downward, one at a time (or in such a configuration that a single bubble from a single specimen out of a group under observation may be clearly observed as to its occurrence and source). The fluid shall be maintained at a temperature of 125°C ±5°C (257°F ±9°F). The specimens shall be observed against a dull non-reflective black background through the magnifier (see 6.2.3) from the instant of immersion until 20 seconds after immersion. Leakers will be identified by a single bubble or a stream of bubbles. Specimens from which a single bubble is observed is considered to be a reject.

1/ D02, D02-TS, and D03 are the registered trade mark of Ausimont (Division of Montedison). 2/ Minnesota Mining Co. (3M) registered trade name.

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