REFERENCE LEAKS D Mari M Bergoglio INRIM Istituto
REFERENCE LEAKS D. Mari, M. Bergoglio INRIM: Istituto Nazionale di Ricerca Metrologica, Strada delle Cacce 91, 10135 Torino, Italy 1
Secondary gas flow standards. . . need calibration by means of Primary standard systems Currently, two types of secondary gas flow standards are primarily available: • Geometrical leaks (crimped capillaries. . . ) - wide range of gas flow rate is provided, depending on the gas pressure inlet - temperature is not a critical factor; temperature coefficient is lower than 5· 10 -3 C-1 - solidity of leak element Crimped capillaries • Permeation leaks - gas flow rates down to 10 -12 mbar L/s - only one value of gas flow is generated - strong temperature dependence - fragility of the leak element - only gases for which permeating materials are available Permeation leaks 2
Realization of leak elements 1. Glass capillaries Five glass capillaries were supplied by Danfoss: nominal gas flow rate values in the range 1 x 10 -6 mbar L/s - 1 x 10 -5 mbar L/s at 1 bar referred to vacuum. - The glass capillary is connected to a leak detector unit and used, in principle, as a "sniffer" leak detecting device. - The glass is then heated up uniformly using a special burner device. As the glass is heated at a specific point (on and off to prevent closure of the element), the capillary slowly contracts forming the restriction (leak element). - During this procedure helium is blown on top of the glass leak element and nominal leak rate is measured in the leak detector narrower part of glass capillary Section of narrower part of glass capillary 3
2. Metal leaks 2. 1 Micro-hole in Stainless steel double knife 16 CF flange - Six double CF 16 flanges lowered to 0. 8 mm by electro-erosion 2. 2 Micro-hole in cupper disk having the same dimension of a CF 16 gasket - Three cupper disks having the same dimension of the CF 16 gasket lowered to 0. 4 mm by mechanical process 2. 3 Micro-hole in aluminium disk - five aluminium disks lowered to 0. 4 mm by mechanical process 4
- In the lowered area of flanges/disks, micro-holes having diameters between 5 μm and 20 μm were drilled by laser technology, irradiating the metal surface with a laser beam focused in very small spot able to melt and vaporize the material. - RTM Sp. A, an Italian company leader in the laser technology, realized the micro holes by a Diode-Pumped Solid State laser: short wavelength (532 nm), focused beam diameter of 15 mm, pulse energy in the order of 1 m. J, short pulse duration (nanosecond regime), and a radiation flux density in the processing zone of about 1 GW/cm 2. 5
2. 1 Micro-hole in Stainless steel double knife 16 CF flange Laser beam outlet Laser beam inlet Hole after FIB cleaning (inlet) on the side where the laser beam pierces, a deposition of material has been observed. At INRIM nanofacilities a Focused Ion Beam (FIB) instrument having micromachining capabilities at the nanometer–micrometer scale was used to remove the material around the hole 6
3. Rectangular channel (supplied by Université de Toulouse) The microsystem consists of series of parallel rectangular microchannels etched by deep reactive ion etching (DRIE) in silicon wafers, and covered with Pyrex plates by anodic bonding Micro-system characteristics : Depth, h= (0. 545 ± 0. 10) μm Width, W= (50. 0 ± 0. 3) μm Length, L= (5000 ± 10) μm Number of Microchannels: 575 7
4. Nano-holes The nano-holes were supplied by Nano. Med labs of Genova University They were drilled by FIB in a silicon nitrate membrane The dimensions of the membranes are: frame dimensions of 5 mm, window size 100 m, thickness 200 nm. The membrane is able to stand up to 1 bar of differential pressure 8
5. Fibers Photonic Bandgap Fiber guide light was used as small capillary. The fiber has a hollow core, surrounded by a microstructured cladding formed by a periodic arrangement of 286 holes in silica in a coating of acrylate At INRIM a methodology to cut the fiber was developed: the acrylate coating was removed and the silica was cut by a diamond blade. 9
Geometry characterization of the leak artifacts (micro-holes and fibers) Grating calibration A 2 -D grating was used as reference for the diameters characterization. The pitch p of the grating was measured at INRIM length section (the traceability to SI was obtained through INRIM standards of angle and length) The pitch p of the grating used in our measurements is: p = 462. 92 nm; U(p)=0. 023 nm A SEM image of the grating by using a magnification x 5 k and having size 1280 x 1040 pixel was taken; starting from grating calibration one pixel was determined as equal to (26. 17± 0. 26) nm 10
Hole diameters: A SEM image of each hole was taken, with the same magnification x 5 k used for SEM image of the grating The diameter of each hole (for both sides of the flange /disk) was determined, starting from 10 profiles obtained by means of an image processing software Thickness of the leak artifacts The thickness of the micro holes was measured by a 1 D linear comparator (Moore M 3) equipped with a laser interferometer. Measurements were carried out in eight different positions and repeated two times in the lowered area of the flanges/disks. 11
Fittings Each leak element was encased in a suitable fitting to ensure a good tightness (at least lower than 10 -10 mbar. L/s) and to be useful for mounting on the various NMIs primary flowmeters. 1. Glass capillary The glass tube 4. 2 was sealed by Torr Seal resin in a copper spacer 4. 1. At the end of the holder 1 a gasket seat was realized in which the indium gasket is allocated and pressed by screw 3. The indium gasket tightness in the piping was checked using a blind spacer (like 4. 1) purposebuilt and an helium leak detector. The helium signal from detector was lower than 10 -11 mbar. L/s…. 12
2. Micro-hole in Stainless steel double knife 16 CF flange The fitting was realized using commercial CF 16 and VCR components The tightness of the pipe was checked by a helium leak detector. One side of the artifact was connected directly to helium detector and the other side was closed by a plug. The helium was spread around the piece and the signal from the detector lower than 10 -11 mbar. L/s was recorded. 3. Micro-hole in cupper disk having the same dimension of a CF 16 gasket The fitting was realized using commercial CF 16 and VCR components. The tightness ensured a leak lower than 10 -11 mbar. L/s. 13
4. Micro-hole in aluminum disk The fitting was realized using a custom INRIM system in conjunction with commercial CF 16 and VCR components. The tightness of the pipe was checked: the signal from the helium detector was lower than 10 -11 mbar. L/s. 5. Nano hole The membrane was mounted on a copper disk compatible with a copper gasket CF 16 and sealed with Torr Seal resin. The fitting was realized using commercial CF 16 and KF 16 components The tightness ensured a leak lower than 10 -11 mbar. L/s. 14
April 2012 to January 2013: The realized leak elements were measured for many gas species and pressures in several National Metrological Laboratories and Universities in a large measurement campaign, both against vacuum and atmosphere. Participating laboratories performed two measurement cycles in the following conditions: measurements referred to vacuum - inlet pressure: 50, 100, 300, 500, 700, 1000 Pa, 3, 5, 7, 10, 30, 50, 70, 100 k. Pa (or maximum throughput measurable). -gases: He, N 2, Ar, H 2 or mixture H 2/N 2, SF 6 measurements referred to atmosphere - inlet pressure (relative) from 2, 5, 10, 50, 100, 150, 200, 250, 300, 350, 400 k. Pa or more when possible. - gases: He, N 2, Ar, R 134 a, if possible CO 2, 1234 y, H 2 or mixture H 2/N 2 and SF 6. 15
Glass capillary G 1 10 -6 mbar L/s @ 1 bar He LNE → INRIM no sealing Glass capillary H 1 10 -5 mbar L/s @ 1 bar He CMI Flange (stainless steel) A 1 11 mm IMT B 1 15 mm LNE B 2 15 mm Giessen University C 1 19 mm CMI A 2 11 mm Giessen University Al disk E 1 8 mm PTB Al disk E 2 10 mm INRIM/LNE Cu disk D 1 11 mm INRIM Fiber INRIM Nanohole INRIM /Università di Genova Micro-system (rectangular channels) INRIM/ Université de Toulouse RILSAN PA 12 INRIM Polycarbonate foils IMT clogged 16
Cupper leak D 1 - INRIM Glass capillary H 1 - CMI 17
Aluminium leak E 1 - PTB Aluminium leak E 2 - INRIM-LNE 18
Stainless steel leak B 1 - LNE Stainless steel leak C 1 - CMI 19
Micro-system (rectangular channels) 20
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